Dental occlusion, facial growth, and TMJ disorders
The following manuscript grew out of anthropologic research motivated by a desire to better understand dental occlusion. Dental school had taught me guidelines for treating dental occlusions, but the rationales provided to justify those guidelines just didn't make sense. My dental education had largely ignored the medical implications of the way teeth fit together, even though such implications seemed important for my patients. Research on bites had been mostly concerned with tracking variables that seem unrelated to patient's overall health or even dental health. I felt like dentistry had failed to see the forest through the trees. What follows is an in-depth look at the forest. The first half of the text explains how our masticatory systems and occlusions were designed to work, designed to grow, and how they grow and work differently today as a result of the softening of the human diet in the last couple of centuries. The second half of the text (treatment) explains how we can re-establish functional harmony in the modern human masticatory system in order to restore healthy facial growth and thereby prevent some of the TMJ disorders, headaches, and other symptoms that have become common in modern populations. This second half of the text will be posted in late 2015.
TMJ disorders are a product of modern civilization. They arise all over the world within one generation whenever societies abandon their traditional life styles and adopt a diet of sweetened refined foods. Before the industrialization of food, there was no evidence of the types of TMJ disorders that are common in modern societies today. In the thousands of intact human skulls in museums as well as in the few tribes still living traditional life styles, we can see evidence of arthritic degeneration of the TMJs associated with injuries and extreme tooth wear, but we see no evidence of the dislocated disks, strained and unstable dental occlusions, or asymmetrical retrognathic verticalized facial growth found in many modern TMJ disorder patients.
To understand why TMJ disorders have become endemic in modern society, they must be seen in the context of the evolution of the human masticatory system. Humans were so successful in evolution partly because our masticatory systems were designed to grow adaptively in response to functional stimuli. With upper and lower jaws both adapting their growth to fit the same set of normal functional forces applied during chewing, the jaws also grew to fit each other. The convex portions of the mandible (the condyles and the buccal cusps of the mandibular teeth) developed a goodness of fit in the concave portions of the upper jawbone (the glenoid fossae and the central fossae and marginal ridges on the maxillary teeth). At the same time, the basal bones supporting the upper and lower jawbones aligned themselves to best withstand and deliver functional forces and thereby became aligned with each other in a way that maintained their proximity and parallelism. The resulting goodness of fit among all the components of the masticatory system during rest, bracing, and normal functional movements constitutes a remarkable functional harmony.
This functional harmony in the human masticatory system was the product of a uniquely complex and sophisticated craniofacial growth process. From the underside of an expanding cranial vault and an elongating cranial base, the upper and lower jawbones translate down and forward by very different growth mechanisms. The lower jawbone grows as a hammer designed to deliver masticatory forces, while the maxilla grows to become a platform suited to withstand masticatory forces. The lower jawbone grows due to the mandibular ramus and condyle pushing the mandibular corpus steadily forward. The upper jawbone grows by expanding due to the repetitive pounding of the mandibular corpus. Coordinating these diverse growth processes, the resting tensions of the muscles maintain a steady overall growth pattern that functions as a craniofacial growth matrix.
Within that steady craniofacial growth matrix, a surprisingly dynamic intramatrix growth pattern occurs in the structures of the masticatory system throughout life in order to compensate for the occlusal wear that also continued throughout life. This intramatrix growth involves primarily expansion of the maxilla and anterior translation as well as forward rotation of the mandibular corpus. Remodeling at the borders of this intramatrix growth masks much of its effects on the overall growth pattern.
In our ancestors, intramatrix growth provided adaptability. Because it was stimulated and shaped by masticatory function, it could adapt the structures of the masticatory system to whatever masticatory function was required of them - from cracking nuts to chewing frozen boots to soften them. It could achieve functional harmony even in the presence of genetically diverse parts and missing or crooked teeth. Because intramatrix growth was stimulated by mastication, it served to compensate for the rate of occlusal wear, which was a result of mastication.
However the human masticatory system was never designed to adapt to a condition of insufficient functional stimuli. In modern humans, without a consistent set of functional forces to which all the parts can mutually adapt, the parts never get worn in. They do not develop the remarkable goodness of fit that is seen in the masticatory systems of even our recent ancestors.
While the weakening of our jaw muscles has weakened the stimulus for intramatrix growth, the narrowing of our chewing movements has also narrowed the structural components of the mandibular articulation, which has in turn inhibited facial growth horizontally and redirected it vertically. As a result, the average modern human face is significantly longer, narrower, more convex, and more retrusive (compared to the rest of the skull) than the average ancestral human face. Anthropologists have noted the same changes in a wide variety of diverse populations in various parts of the world, and very similar changes have been experimentally induced in several species of animals by simply softening their food.
At the same time, weakening of the craniofacial muscles has diminished their ability to regulate facial growth. Without consistent symmetrical resting and functional muscle forces to ensure regularity and symmetry of form, modern craniofacial structures have significantly more average asymmetry and irregularity than was seen in the craniofacial structures of our ancestors.
Facial growth still continues throughout life, but it no longer serves to compensate for tooth wear. Instead, it makes the face longer, makes the teeth crooked, and creates mechanical strains that must be continuously accomodated by the body's adaptive mechanisms. As a result, adaptation must keep trying to hit a moving target.
Symptoms only occur when adaptation fails, but the loss of functional jaw muscle forces has also diminished adaptive capacities. Less vigorous rhythmic pumping of the mandible against the underside of the skull diminishes the potential for adaptive growth in areas such as the craniofacial sutures and the periodontium. Diminished interproximal wear has reduced the ability of the teeth to form a continuous line of stable interproximal contacts which act like a line of joints connecting all the members of each dental arch in a way that maintains the range of motion, spacing, and alignment of the teeth. Diminished occlusal wear has reduced the ability of the dentition to accomodate the diversity of facial growth patterns.
The way to eliminate these disorders in society does not require returning to hard diets like those of our ancestors, but using an understanding of how the masticatory system was designed to work in order to learn how we can establish a new functional harmony that suits our modern human masticatory systems.
Chapter 1 – THE NATURAL HISTORY OF THE HUMAN MASTICATORY SYSTEM
Chapter 2 – THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
Chapter 3 – GROWTH OF THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
Chapter 4 – GROWTH OF THE MODERN HUMAN MASTICATORY SYSTEM
Chapter 5 – DYSFUNCTION OF THE MODERN HUMAN MASTICATORY SYSTEM
CH 1) NATURAL HISTORY OF THE HUMAN MASTICATORY SYSTEM
THE ROLE OF THE MASTICATORY SYSTEM IN EVOLUTION
Masticatory systems generally led the way in evolution. Improvements in mastication provided new sources of nutrition that made possible whole new lines of species.
"Feeding is a function of such paramount importance that natural selection seldom acts with such decisive vigour as when a process of such fundamental nature is involved. Thus it is postulated that even the speed which the primitive vertebrate predator gained by the development of a flexible backbone and large paired fins was largely complementary to the improvement in the functional efficiency of the jaws in procuring food."1
“Without the predatory powers of jaws and teeth and the possibility of swift and accurate pursuit of prey there would have been no evolution of the sense organs of smell, sight and hearing, of elaborate muscular coordination, of prevision of how to get from here to there and the possible consequences of the transit - in short, there would have been no centralization of the nervous system such as ultimately produced the brain, and the earth would never have known the phenomenon of consciousness, at least of an order superior to that of the lobster, scorpion, or butterfly."2
Mouths became important in evolution shortly after organisms developed an internal tube through which portions of the external environment could pass. The mouths of early unicellular protozoans were just gashes in their sides. The mouths of jellyfish became surrounded by tentacle-like folds to help engulf food. The mouths of early fish were used to suck food off the ocean floor.
One of the first big steps in the evolution of the masticatory system took place roughly 300,000,000 years ago when the space between the lips and the throat widened to accomodate moveable jaws.
“Plankton gathering is neither the quickest nor easiest way of obtaining food and, therefore, it is not surprising that in the succeeding vertebrates a number of evolutionary experiments occurred, aimed at changing from a microphagous to a macrophagous habit. Such experiments included, for example, the development of a horny-toothed oral sucker and tongue for rasping away the flesh of other animals which still survives today in the lampreys. However, the one really successful experiment was the modification of the skeleton supporting the two anterior gill arches to form opposable jaws."3
The mandible was the primary moveable part in this system. It was the first bone to be attached to the body by a flexible joint, and the mechanism created for that joint paved the way for attaching arms, legs, and all other appendages.
"It seems that other joints in the fishes' body are never as highly developed as the jaw joint... When jaws were devised, they were the first speedy, wide swinging, vigorous appendages to be attached to the body. The true diarthrodial joint was first formed here in fishes, and its basic structure remained essentially unchanged when it was appropriated by the limbs of land animals... Thus, the jaws led the way in all joint evolution."4
The property which made moveable jaws especially valuable was their ability to bear teeth which could be used as tools to crush or incise food. Teeth arose almost simultaneously with moveable jaws by the simple modification of the dermal denticles surrounding the mouth. The first teeth were epithelial outgrowths in the skin or mouth lining. Soon afterwards teeth became attached to the underlying bones - appearing on all the jaw bones, several palatal bones, and the rod-like tongue in many primitive fish. The teeth were specialized for different food sources. Thus sharp teeth for cutting were found in sharks, and flat teeth for crushing were found in rays and skates.
These early masticatory systems of fishes, reptiles, and amphibians could grasp, rip, and tear; but they could not really chew. The simple, slightly recurved, identical, conical teeth were easily broken off or blunted. During function they acted as grasping or restraining devices rather than food processing devices. The mandible was moved straight up and down and used in combination with the neck muscles to break off and swallow.
Life on land provided impetus for major new evolutionary developments, because vast nutritional sources were stored there in nuts and seeds locked up inside cell walls and still unavailable to the digestive process. With the increase in metabolic activity needed to hold the body out of water and transport it around on land, these nutritional sources had to be efficiently utilized. Breaking them down in the beginning of the gut was a way to greatly increase the surfaces accessible to the digestive juices, and consuming smaller quantities of more thoroughly masticated food provided more nutrients with less energy spent procuring it.
THE TEMPOROMANDIBULAR JOINT
The breakthrough that made real chewing possible was the development of a whole new type of joint (the TMJ) about 70 million years ago when the back ends of the mandible retruded far enough to make direct contact with the temporal bone of the skull. A joint formed in this location where a synovial bursa appeared between two layers of rubbed periosteum and an intercepted muscle tendon. This TMJ allowed much better processing of food than was possible previously. The improved chewing efficiency enabled the development of species that were able to maintain a constant high body temperature. In such manner, the new TMJ ushered in the age of mammals.
"In the later part of the Triassic epoch, some 200 million years ago, mammals came into existence. This was a gradual process of evolution from a group of reptiles called the Therapsida or mammal-like reptiles. The earliest representatives were probably unlike any modern mammal. They were very small creatures - smaller even than the tiny pigmy shrew. We do not know if they were viviparous or if they suckled their young. We do not even know if they were warm-blooded (they could probably keep their body temperature above that of their surroundings, but not maintain it constant). We do, however, know that they had developed the mammalian jaw joint between the dentary and squamosal bones - our temporo-mandibular joint, and, in zoological terms, this classifies them as mammals... At this stage we see, also for the first time, attrition of upper and lower teeth."5
The mammalian TMJ was an entirely new creation and not an adaptation of a previously existing structure. The reptilian jaw joint had supported both the hearing and chewing mechanisms, and a stapes bone that was large enough to support vigorous chewing limited the sensitivity of the auditory mechanism. In the mammal-like reptiles, the lower jawbone increased in size, developing a coronoid process for muscle insertions and a temporal fossa for muscle origins; but the quadrato-articulate joint continued to operate the sound conducting mechanism. In the true mammals, support for the lower jawbone became entirely the responsibility of the new TMJ, and the old reptilian articular-quadrate joint was completely removed from any role in jaw support. The quadrate and articular bones became incorporated into the middle ear where they joined a much reduced stapes to produce a chain of three tiny ear ossicles. Thus a new mammalian jaw joint developed right beside the old reptilian jaw joint, completely separating the jaw and auditory systems and thereby giving mammals the advantage of being able to chew and hear simultaneously.
One advantage of the new TMJ was that it could withstand significant compressive forces. Using it as a fulcrum allowed the rigid lower jawbone to function like a class 3 lever and transfer strong compressive forces from muscle masses near the middle of the skull to the front of the lower jawbone where they were needed to bite things off.
A second advantage of the new TMJ was its mobility. The joint was divided into two compartments – an upper compartment that allowed sliding of the condyle to produce a horizontal range of movement of the mandible and a lower compartment that allowed rotation of the condyle for opening and closing the mandible from a great variety of condylar positions. The two compartments were separated by a tough articular disk held in place by a surrounding ligamentous sleeve which served to limit movement and bind the bones and disk into a single working unit. The combination of increased compressive forces and a new wide range of mandibular movement permitted much more precise and powerful jaw movements.
As permanent structures, mammalian teeth acquired highly differentiated shapes which were individualized to fit their locations. Generally the front teeth became incisors, the back teeth became molars, and the teeth between became canines and premolars. Teeth also could be adapted for digging, fighting, and other functions.
Specialized structures were required to create a seal between the tooth roots and the gingiva to prevent infection from the microbes which were omnipresent in the mouth. Unlike a continuous sheet of skin or mucous membrane, the junction between tooth structure and soft tissue was difficult to protect from microbial ingress. Thus the top of each socket was covered by a flexible gingival attachment forming a narrow sulcus through which fluid could flow outward to carry away food and bacteria. In most species, a slight overhang of tooth structure just above the sulcus prevented food impaction.
THE BITE TABLE
Together the teeth provided a platform that functioned like a table against which the jaw muscles could work the mandible. That platform had a central bracing area within which at least a little horizontal movement of the mandible was possible without an increase in vertical dimension.
THE MAMMALIAN SKULL
The mammalian skull was designed around a simple recurrent architectural theme, with viscerocranium, neurocranium, and the top of the vertebral column all aligned sequentially. In front was the viscerocranium, an elongated pyramid which housed the upper air and food passages and supported the sense organs. Its upper surface was formed by the nasal bones, its sides by the premaxillae and maxillae, and its underside by the premaxillae, maxillae, palatine, and pterygoid bones. Behind the viscerocranium, the neurocranium housed the brain. Reinforcing the connection between the viscerocranium and the neurocranium, the zygomatic arches formed wide laterally placed braces. At the back of the neurocranium, where the spinal column continued straight backward from the brain, a flat occipital plane faced squarely backward to connect with the neck. The cranial base, as a forward extension of the vertebral axis, determines the shape of the cranium. The brain sits on its upper surface, and the organs of the face and neck hang from the lower basal surface. The mandible acts as a curved shield that fits around the face and neck.6
At the front of the cranium, the face was designed to deliver and withstand compression by the mandible in chewing. A rigid mandible enabled forces from the jaw closing muscles on both sides to apply force to a bolus on either side, and the midface was comprised of membrane bones aligned to withstand the compressive pressures delivered by the mandible. To absorb the antero-medially directed forces of power-crushing on the working side, the maxillary teeth were embedded in alveolar bone that was well buttressed palatally, and the mandibular teeth were embedded in alveolar bone that was well buttressed buccally.
In order to absorb masticatory forces, the maxillary bite table was braced by pillars of bone extending in many directions to areas dispersed widely around the skull. At the front of the face, compressive forces were transferred directly up to the roof of the neurocranium via the triangular nasal septum as well as around the nasal cavity and the orbit, necessitating the presence of horizontal connectors. The sense organs, areas for airway passage, and any other structures not concerned with chewing were fit in the remaining spaces.
The maxillary and mandibular dentitions were supported by bony bases that were capable of adaptive growth to maintain their alignment and thereby preserve the capacity for effective mastication. Petrovic said, “The responsiveness of condylar cartilage growth to local factors may account for the evolutionary success of the phylogenetically new mammalian joint between the skull and the lower jaw... The regulatory possibility for the mammalian lower jaw to adjust in length to the upper jaw during growth certainly favored the selection of genetic variations resulting in facial posteroanterior shortening, in molarization of post-canine teeth, and subsequently, in mastication.”7
THE MAMMALIAN JAW MUSCLES
Nearly all the muscles of the mammalian craniocervical area were involved in eating. Flexible cheek and lip muscles, innervated by the facial nerve, helped pick up food and manipulate it in the mouth. The tongue muscles worked together with these cheek and lip muscles to keep pushing the food back onto the bite table. The neck muscles pulled the head straight backward to help tear food off, rotated the head back to help open the mouth, and braced the head during chewing and swallowing.
The single reptilian adductor muscle was separated into distinct units (temporals, masseters, and pterygoids), which were arranged in pairs that formed slings around the mandible. With these slings converging onto the mandible from widely separated origins, they were able to exert fine control of the position of the mandible. This control permitted chewing strokes which could be customized to process each of the many different kinds of food available.
The jaw closing muscles dominated the front of the cranium. The temporalis muscles were especially useful for vertical chopping. With their long straight fibers they could perform fast snapping jaw closures from wide opening. The masseters and medial pterygoid muscles were shorter, thicker, and more horizontally oriented. From the bottom of the lower jawbone, the masseter muscles extended upward and outward (laterally), while the medial pterygoid muscles extended upward and inward (medially). Working in combination, these muscles formed a pair of slings that could pull the mandible forcefully from side to side for power-crushing of resistant foods.
The jaw opening muscles were much smaller. The suprahyoid and infrahyoid muscles opened the mouth by pulling downward on the chin. In some species, the lateral pterygoid muscles also helped to open the jaw by pulling forward on the mandibular condyles.
During chewing, a central pattern generator initiated rhythmically alternating firings to the jaw opening and closing muscles, and these firings were then modified by neuromuscular reflexes to create a characteristic mammalian chewing pattern with species specific variations. The mammalian chewing pattern moved the mandible along a pathway that was oval shaped as seen from the front. The oval was very wide in some mammals and very narrow - almost a teardrop - in others. However its consistent lateral component, even if small, distinguished it from the truly straight vertical jaw movements of reptiles and amphibians.
DIVERSIFICATION OF MAMMALIAN MASTICATORY SYSTEMS
The biggest advantage of the basic mammalian masticatory system design was its adaptability. By minor alterations in tooth shape, joint contours, and musculoskeletal features; a wealth of different chewing systems could be differentiated from the same basic musculoskeletal plan. For each available food source, a species developed masticatory system components which made the best use of it – including its own unique arrangement, angulation, and proportional development of the same basic mammalian jaw muscles in order to enhance the vectors required for chewing the food it utilized most commonly, a skull shape which provided advantageous points of attachment for those jaw muscles, tooth shapes uniquely designed to best masticate those specific foods, and a facial framework tailored perfectly to resist the forces normally applied while chewing those foods.
Adaptation led to evolutionary success. A new mammalian species could develop a mouth suitable for grinding plant matter, grasping and holding live prey, gnawing bones, shredding roots, cracking nutshells, or crushing insects. Animals that needed grinding mandibular movements developed wide skulls with large molars, animals that needed chopping mandibular movements developed long skulls with pointed incisors, insect eaters developed molars suited for puncture-crushing of insect shells, and animals that nibbled food developed incisors surrounded by extensive sensorineural structures and neuromuscular reflexes for fine control. With proprioception able to direct chewing function, the consistency of the food available determined the mandibular movements that would be used for chewing it. Given the same food, two different species will handle it in the same way irrespective of differences in tooth form; and a single species will show greater variation in the way it handles two different types of food than occurs between members of different species.
In time, a few divergent developments of the same fundamental mammalian masticatory system characteristics proved most successful. Carnivores developed mandibular articulations (TMJs and dentitions) that were vertically arranged, herbivores developed mandibular articulations that were laterally arranged, rodents developed mandibular articulations that were antero-posteriorly arranged, and omnivores developed mandibular articulations that were arranged in all these directions.
Carnivore masticatory systems had vertically arranged structural components to accommodate mandibular movements that were almost straight up and down. Meat was such a rich food source that it didn't need much preparation before digestion. Of prime importance was wide opening and fast snapping closure, requiring long sharp canines for grasping prey and securely locked-in temporomandibular joints for protection during violent predatory action.
Since these actions required long fibers, the temporalis muscles became especially well developed. An expanded temporal fossa and an enlarged coronoid process provided horizontal space for more muscle mass at the temporalis origins and insertions, and a long narrow skull provided vertical length to accomodate long temporalis fibers. Extensive development of type 2B muscle fibers provided the rapid forceful contractions that were useful in the capture of prey.
In the TMJs, the cylindrical condyles could not translate except for a lateral shifting as they shifted sideways in the tubular slots of the glenoid fossae. Protecting the condyles from being displaced in other directions helped provide protection from the trauma that presents a real danger when animals being preyed upon have only seconds to fight for their lives. Mandibular movements consisted primarily of opening and closing the mandible by rotating around its condyles.
Because it was important to protect the mandible from retrusive forces which could drive its condyles into the vital structures just behind it, the interdigitation of the lower canines in front of the uppers with each closure served to lock the mandible forward. Since the canines were so much taller than the other teeth, this canine guided protection against retrusion functioned even with the mouth part way open, such as when the mandible received a distally directed blow like a kick.
The lateral shifting at the top of the long thin oval pattern of mandibular movement was small but also very important, because it created a shearing action between the upper and lower teeth that kept them sharp. In the molar region, the lower dental arch fit completely inside the upper dental arch so that the buccal surfaces of the lower molars faced the palatal surfaces of the upper molars. These two facing surfaces were convex, both anteroposteriorly and superoinferiorly, and hence could not be brought into contact at more than one point at a time. As the mandible shifted laterally during function, these surfaces worked together like two shears which fed each other and thereby maintained a marked mesiodistal cutting edge.
Brodie likens the situation to a pair of scissors, explaining, "Scissors have two blades with faintly concave surfaces facing each other but only one edge of these surfaces can be brought into contact with the other. The blades, when viewed from their edges, are also faintly concave in their length dimension so that only one point of the edge can be in contact with the other at one time. Thus, when the scissors are closed, the edges of their blades are in contact only at their very ends. As the scissors are opened, this point travels backward until, at full opening, it can be seen that the cutting edges cross each other. Upon closing, the contacting point travels forward and it is only at this precise point that work is done. Since the cutting edges of the blades are beveled and their facing surfaces are concave, the scissors sharpen themselves while they work."8
In almost direct contrast to carnivores, herbivores developed masticatory systems with structural components that were flat and wide to fit their wide lateral chewing movements. Herbivore chewing was characterized by leisurely prehension followed by prolonged forceful milling of relatively hard resistant plant substance, so herbivorous masticatory systems were suited for extended periods of slow powerful grinding rather than short bursts of quick chopping. Maximum gape was limited, and lateral movements were wide and free.
To power such function, the pterygoid and masseter muscles rather than the temporal muscles became well developed. The zygomatic arches and pterygoid plates where they originated were set far apart in short wide skulls so the pterygomasseteric slings converging onto the mandible could pull the mandible strongly from side to side. Fatigue resistant type 1 muscle fibers predominated. There was only a short, slim, coronoid process and small temporal fossa for the attachment of the much diminished temporalis muscle. The lateral pterygoid muscles were well developed to assist in grinding.
For the TMJs to accommodate such a wide range of movement, both the condyles and the glenoid fossae were flat enough to allow the condyles to freely slide around during function; and the surrounding joint capsules were loosely arranged to allow a wide range of mandibular movements.
The back teeth rather than the front teeth were well developed. At the front of the mouth, the incisors and canines were diminutive or absent where no vigorous prehension was required. In contrast, the back teeth developed extensive chewing surfaces for milling large masses of plant matter and long roots to support extensive chewing. The molars were reinforced with vertical plates of enamel to resist wear. The infolding of enamel interlayered with cementum left ridges of enamel that acted as grinding flutes after the cementum wore away. In most cases, the grooves on the bite surfaces were oriented antero-posteriorly so they would grind efficiently when the mandible moved in laterally directed power strokes. In a few herbivores, like elephants, the grooves were oriented buccolingually, and the mandible moved mesiodistally. To compensate for continual occlusal wear, the teeth of large herbivores such as horses continued to erupt at the rate of about 3 mm per year.
During masticatory tasks, the mandible rocked from side to side, alternating chewing activity between right and left sides and maintaining functional edges on the teeth by disarticulating the condyle on the non-working side each time the mandible was swung to the working side.
Rodents developed masticatory systems with structural components that were arranged antero- posteriorly to fit an antero-posteriorly oriented mandibular range of motion. The mandible could function in an anterior incisive position for gnawing with the front teeth, or it could function in a posterior power-crushing position for chewing very resistant vegetation, including nuts and seeds, with the back teeth.
Rodent TMJs contained elongated temporal bone fossae with a tubular shape that was oriented antero-posteriorly to prevent sideways dislocation and enable the mandible to move easily forward and backward between its two functional locations. To support the antero-posterior range of mandibular movement, rodent skulls were also long antero-posteriorly and narrow mediolaterally.
Rodent jaw muscles were also designed for providing mandibular elevator forces in the two mandibular functional positions. The lateral pterygoid muscles were extremely well developed in order to power the extreme anterior movements which placed the incisors in an edge-to-edge position for gnawing. The masseter and medial pterygoid muscles were also well developed to permit application of large closing forces when the mandible was held forward for gnawing.
The rodent dentition also was designed to fit its functional demands. Instead of canines, there were long spaces between the front and the back teeth. The front and back teeth could not both be engaged simultaneously, but the mandible could move forward to engage the front teeth or backward to engage the back teeth. The molars had grooves running buccolingually so they could grind food effectively as the mandible moved antero-posteriorly. Because of the shape of the back teeth, occlusal wear produces sharp ridges of enamel that increase their ability to grind up tough foods like seeds.
Gnawing produced rapid wear of the incisors by abrasion and also continued sharpening due to honing the teeth together. Because the incisors have enamel only on their front sides, working the teeth against each other creates and maintains a sharp chisel-like shape.
Brodie explains, “rodent incisors at eruption are cone-like and covered with enamel on only their labial surfaces. Chisel sharpness is imparted to this enamel and maintained by an alternate passing of the lower tooth against the lingual and then the labial surfaces of the upper - the lower sharpening the upper during one stroke, the upper sharpening the lower during the next. The rodent must engage in this tooth sharpening activity continually to adjust for the rapid and continuous growth of these teeth.”9
Because the combination of abrasion and sharpening eliminates tooth structure quickly, rodent incisors do not close off at the base after they have finished growing like the teeth of other mammals. Instead, the base of the tooth remains wide open, allowing the tooth to continue growing throughout life at a rate of up to 4 mm per month. Rabbit incisors erupt similarly, but are fully encased in enamel on both sides and therefore function more like herbivore teeth.
Although these highly specialized carnivore, herbivore, and rodent masticatory systems were very effective at dealing with the types of foods for which they were designed; eventually environments shifted and overspecialization became a disadvantage. Generally species that had become too dependent on the continued presence of a very specific type of environmental condition or food source were sooner or later replaced by more adaptable designs. When one particular food source became no longer available, these more adaptable organisms were able to switch to a different one.
Pigs, bears, badgers, and other mammals developed masticatory systems that were able to chew a wide variety of foods. Generally, skulls became shorter and rounder, and the masticatory system components became located deeper in the face where they could exert more power. Locating the working portion of the mandible closer to the jaw closing muscles and relocating the mandibular condyles well above the level of the rest of the mandible allowed functional movements that were better controlled and could be more easily tailored to fit different food sources.
Most omnivores had jaw muscles that were present in balanced proportions. The horizontal grinding muscles (medial pterygoids and masseters) were roughly the same size as the vertical chopping muscles (the temporals). The lateral pterygoids were well developed for unilateral grinding or protruding the mandible for incising.
Omnivore temporomandibular joints incorporated both sliding and hinging movements. The articular eminentia formed long gradual slopes which allowed a range of antero-posterior and lateral movements.
Omnivore teeth were all-purpose chewing utensils which combined features of previous mammalian teeth. In bears, a basically carnivorous system lost the carnissal blades and enlarged the distal cheek teeth to form flattened crushing structures.
In primates, the facial muscles became differentiated and well developed to allow communication through facial expression. The arms and hands acquired a greater role in food preparation and thereby diminished the burden on incisors for ripping and tearing just to get food into the mouth. In apes, the entire dental arcade retruded on its osseous base to a location closer to the center of mass of the jaw closing muscles, and the bite tables flattened to allow a wider range of mandibular movements.
The only range of mandibular motion that was still limited by the dental occlusion was posteriorly. Generally the canines were shorter than in carnivores, but they still interlocked during closing to protect the TMJs from retrusive forces, as can be seen in the frontal views of ape dentitions below:
The teeth could shear. “The upper occlusal plane of most primates is arranged in a long anteroposterior convexity, while the lower is arranged along a concave curve. In this arrangement, as the jaw begins to close, the first parts to occlude are the medial posterior cusp of the lower third molar and the posterior border of the upper third molar. As the jaws close, interlocking proceeds forward like the teeth in a cog wheel. At the same time a lateral component becomes conspicuous causing the lateral elevations of the lower teeth to shear across the lingual surfaces of the outer cusps of the upper molars.”10
The teeth could also grind. During a lateral excursion, in the follow-through phase, when canine contact on the non-working side stopped the front of the mandible from moving laterally, the mandible pivoted around that canine contact and drove the working side molars further medially. The side to side movements of the molars pivoting around the canine contacts enhanced the ability to crush food in the posterior part of the dentition.
The next big change in the masticatory system involved incorporating upright posture. Upright posture fundamentally changed the masticatory system by involving many of its components in the mechanics needed to keep the tower erect. In such a manner, the dental occlusion became involved in the head posture mechanism. DuBrul frequently commented, "To understand the teeth, you need to look at the feet."
Upright posture provided significant survival advantages. Perched on top of a double condyle articulation with the vertebral column, the head could pivot on its base and thereby move around quite freely in relation to the rest of the body. With enlarged sternocleidomastoid muscles and elongated mastoid processes to facilitate head rotation, the eyes could come close together in the midline and develop stereoscopic vision without losing a wide field of view.
However upright posture also required profound structural changes throughout the skeleton. These changes incorporated the mandible and the jaw muscles into the dynamic musculoskeletal framework that achieved an alignment with sufficient physical balance to maintain an upright resting posture.
In quadrupeds, the spinal vertebrae had been arranged in a simple linear sequence parallel to the ground and supported by four widely spaced vertical struts. Functional components simply hung from these struts and the beam connecting them. The head hung from the shoulders by the thick postcervical muscles attached to the prominent occipital bone at its back end, and the neurovascular connections to the rest of the body through the occipital foramen were conveniently located at the back of the skull where it was connected with the rest of the body. The forward ends of the food and air channels, hanging in sequence from the forward portion of that beam, were easily kept separate and scarcely affected by movements of the head or mandible.
Simply uprighting one of these quadrupeds would create an unstable tower, as can be seen below left. Balancing a quadruped skull on the top of an upright vertebral column would require tremendous traction forces downward at the back of the skull in order to prevent it from rolling down onto the chest. In contrast, the bipedalism (shown below right) enabled upright posture by aligning all of the body's structural components along a single axis in which the center of mass is located generally on a plumb line through the center of the pelvic girdle and over the feet.
To balance the hominid head on top of the vertebral column, its center of mass had to move closer to its pivot point on the top of the vertebral column. Thus the long snout disappeared, improving the visual field and allowing better manipulation of close objects. The thermoregulatory function of the reduced snout was replaced by a nearly hairless skin containing many sweat glands and an elaborate vasomotor temperature control system. The face moved back as far as possible, until it ran into the airway.
A radical change in the shape of the head was also required to prevent the face from rotating upward with the rest of the cranium or the eyes would be aimed uselessly at the sky and the semicircular canals of the inner ear would lose their ability to maintain balance. Thus, for the viscerocranium to maintain its orientation with the horizon, the skull was bent sharply in its midsection.
"Bending the skull produced an effect similar to bending a bar of taffy. The top curved and the bottom buckled. The cranial effects were therefore: (1) curving of the cranial roof, (2) buckling of the cranial floor (at sella turcica), (3) extreme retrusion of snout and jaws, (4) deepening of the mandible with outward flaring of its lower border, especially at the chin, and (5) downward and forward swing of the nuchal plane carrying the foramen magnum and occipital condyles forward toward the center of the skull."24
The bending of the cranial base opened up the top of the skull. Once the brain no longer had to fit in between the viscerocranium and the occiput, it could expand upward and outward. The resulting dome shaped cranial vault provided strategic points of origin for the temporal muscles.
The bending of the cranial base also compressed the underside of the skull. It forced together many of the structures of the face. A sharp bend was created where the oro-nasal airway met the pharyngeal airway, as can be seen in the above illustration.
Numerous changes had to be made to accommodate this sharp bend in the upper airway. The tongue became balled up and crowded back into the pharynx. A secondary palate was developed to separate the nasal passage from the mouth. An elaborate mechanism was developed for safeguarding the entrance to the pharynx.
The mandible developed a chin to provide rigidity without impinging on airway space. “To accommodate vital structures in the anterior of the neck during retrusion of the lower border of the mandible, the posterior ends diverged creating a change in angulation of the external pterygoids. When this angle increased, greater stresses were placed on the symphysis, such as pressing the ends of a wishbone. As foreshortening of the mandible proceeded through the primate order in response to upright posture and vertical positioning of the head over the vertebral column, a reinforcement of bone developed in the midline. In the apes, shortening of the muzzle and further retrusion of the mandible within the hominids everted this addition of bone anteriorly through remodeling to form a chin.” 25
To maintain a habitual erect stance, the skeleton had to acquire a weight bearing alignment in which all the skeletal muscles could stay relatively relaxed and ready for action. Therefore, to distribute the forces generated from weight bearing evenly among the skeletal supporting structures, the chains of muscles running up the front of the body counterbalanced those running up the back of the body, and those on the right side counterbalanced those on the left side. Acting together, these myofascial chains surrounding the vertebral column formed a reciprocal tension mechanism that held the vertebral column erect much like stays hold the mast of a sailboat erect. During function, skeletal alignment moved smoothly away from this alignment and then back to it, guided by an almost perfectly simultaneous reciprocal inhibition of agonists and antagonists.
On the sides, this reciprocal tension mechanism had inherent stability. The mastoid processes, shoulders, and hips extended way out to the sides to provide convenient locations for muscle attachments. Still lower, the two feet placed side by side created a stable foundation to resist sideways tipping of the whole structure. Thus, in the frontal plane, design was symmetrical, and little energy was required for maintaining a static equilibrium.
To resist tipping to the front or back was far more difficult for a body so tall and flat. In the dorso-ventral direction, symmetry was lost right from the top. At the back of the head, the occiput was anchored to the vertebral column and back of the shoulders by a thick post-cervical muscle mass that had already been well developed in quadrupeds, where it prevented the snout from dragging on the ground. However, at the front of the head, a much more elaborate mechanism was needed to maintain downward traction. The front of the neck required flexibility for functions such as swallowing, rotation of the head, coughing, vomiting, spitting and speech – each of which depended on independent movement of parts within the total framework. Speech required mobility of the larynx and elimination of the rigid support of the hyoid bone that is present in most mammals. Thus, instead of a small number of thick muscles, a large number of smaller pre-cervical muscles were attached at various angles between a series of small bones (including clavicles and hyoid) arranged generally in parallel and stretching like links in a chain from the sternum to the mandible.
Up a little higher, at the front of the head, the postural muscles could not be directly attached to the face, because the face housed delicate and vital sense organs as well as a web of small muscles that were important for communication and would be impaired by the presence of thick bone required to anchor strong muscles. Thus, to avoid impinging on the face, the long rigid mandible acted as an architectural strut and transferred the downward traction of the anterior kinetic chain around to the sides of the head where it could be controlled by the powerful jaw closing muscles. For the mandible to provide sufficient anchorage to enable the anterior kinetic chain to hold down on the front of the head, the dental occlusion had to provide a stable bracing platform against which the mandible could be clamped immovably against the underside of the skull by the powerful jaw closing muscles.
The jaw muscles had become part of the postural system. The postural muscles and the jaw muscles recruited each other frequently. The cranio-cervical muscles contributed to chewing, and the jaw muscles contributed to postural stability. The post-cervical muscles stabilized the head by pulling down on its back end during swallowing when the anterior kinetic chain was pulling down on its front end and by alternating firing with the mandibular elevator muscles to prevent the head from rocking during chewing.
THE HOMINID MASTICATORY SYSTEM
On the top of this tower and structurally integrated with it was a hominid masticatory system that was unique for its adaptabiity. Retruding the dentition all the way back to a location directly under a bulging forehead at the front of the cranium enabled mastication with maximal power and control. Withdrawing the occlusal surfaces of the canines into the occlusal table formed by the rest of the teeth removed the protection from mandibular retrusion that was created by the simian canine interdigitation, but it also prevented the canines from limiting the mandibular range of motion. The teeth could support a range of mandibular movement in any direction needed for effective mastication. Combining the vertical mandibular movements of carnivores with the lateral mandibular movements of herbivores and antero-posterior mandibular movements of rodents provided a range of mandibular movements that was versatile and complex. The jaw closing muscles could deliver power-crushing forces in a wide variety of locations and directions, and each stroke could be altered to fit the mechanical requirements of the particular chewing task at hand.
JAW CLOSING MUSCLES
The temporal muscles were the postural muscles for the mandible. Right and left temporal muscles formed a bilateral sling that suspended the mandible by its coronoid processes from attachments all over the sides of the skull. The temporal muscles were multipennate; with fibers at continuously varying angles that enabled precise postural control. Type one fibers, resistant to fatigue and generally active in postural control throughout the body, dominated the deep portions of the muscle.11 During chewing, these muscle fibers acted mainly as stabilizers. The tension they exerted depended largely on the position of the mandible.12
The medial pterygoid and masseter muscles of each side formed a sling that generated power for chewing. Because their origins were so widely spead apart on the underside of the skull, they could pull the mandible strongly up and to either side at a wide variety of angles. Their type 2 muscle fibers were capable of generating powerful chewing forces. Gradually increasing activity leading to primary discharge during the final power-crush stroke suggests that these muscles were used for developing large loads between the teeth and were not particularly sensitive to the position of the mandible.13
The superior lateral pterygoid muscles controlled the horizontal location of the mandible at which the mandibular elevator muscles applied their compressive forces to the bolus. By pulling medially and anteriorly on the condyles, they could control the position of the mandible like steering a bicycle by its handlebars. Firing the right superior lateral pterygoid shifted the mandibular midline to the left, and firing the left superior pterygoid shifted the mandibular midline to the right. When food was located anteriorly for incising, both superior lateral pterygoids fired to bring the condyles down and forward on the slopes of the articular eminentia, enabling the protruded mandible to function as a class 3 lever. When food was moved to the molar area for power crushing, they relaxed to allow the condyles to slide all the way up the articular eminentia into their central bracing areas in the glenoid fossae and thereby locate the elevator forces directly over the bolus. In this position, the mandible was used as a crushing tool rather than a lever arm, thereby minimizing the compressive forces at the TMJs.
During the final stages of chewing, the superior lateral pterygoid muscles fired independently to give the mandible a twisting motion that helped crush food much like one might crush a cigarette butt with the ball of the foot. As the mandible approached the midline from the working side and the jaw closing muscles applied large compressive forces to the bolus, the nonworking side posterior temporalis muscle pulled backward and upward on the non-working side condyle while the working side superior lateral pterygoid muscle pulled forward and inward on the working side condyle. In such a manner, the posterior temporalis of the non-working side and the superior lateral pterygoid of the working side formed a force couple which pulled the working side condyle forward, inward and downward; while it pulled the non-working side condyle backward, outward, and upward. This action rotated the mandible around the bolus while also crushing it, as seen below.
After crossing through or over the central bracing position (the intercuspal area), the mandible continued its path through the midline and onto the non-working side in a follow-through phase that lengthened the power-crush stroke by bringing the buccal cusps of the lower teeth in approximation with the palatal cusps of the upper teeth on the working side while the cusps on the non-working side were separated by the thickness of the food bolus. Finally, at the end of this follow-through phase of the power-crush stroke, the non-working side condyle reached its posterior limit and started moving forward along with the working side condyle. With both condyles moving forward, the mandible smoothly transitioned into its opening phase.
JAW OPENING MUSCLES
Jaw opening was the responsibility of two groups of muscles that acted as a force couple to rotate the body of the mandible down and back. The inferior lateral pterygoid muscles, pulling the condyles down and forward, were relatively short and strong and therefore useful for fast opening. The digastric and suprahyoid muscles, pulling the chin down and back, were relatively long and thin and therefore especially useful at wide opening when their leverage began to increase just as that of the inferior lateral pterygoid muscles began to decrease. The vectors in jaw opening are shown below.
With the inferior lateral pterygoid muscles pulling the condyles down and forward at the same time the digastric and circumhyoid muscles pulled the front of the mandible down and back, jaw opening came to consist of a cocking action with the center of rotation in the ramus. This rotation of the mandible around the middle of the ramus protected the neurovascular bundle where it entered the mandible at the mandibular foramen.14
Hominid TMJs were also a combination of preceding mammalian designs. The articular eminentia had an inclination of about 45 degrees to the plane of the bite table - less than the steep vertical temporal bone walls of carnivores and more than the flat temporal bone surfaces of herbivores. As the condyles slid around the inclined articular eminence slopes during function, they rotated like carnivore condyles while sliding laterally like herbivore condyles and antero-posteriorly like rodent condyles.
RODENT TMJ HERBIVORE TMJ CARNIVORE TMJ HOMONID TMJ
The TMJs were well designed to absorb shocks. Proteoglycans within the load-bearing areas respond to compressive force by expressing water molecules and deforming. Then, when the compressive force is released, they reabsorb water molecules and expand to return to their resting contours. As a result, the resistance of the tissues increases progressively as the forces applied to them increases. "Vigorous chewing sends sudden waves of impacts from the articular surfaces to the bodies of the bony components. These waves are effectively dissipated by a graduated increase in resistance provided by the elegant design of the underlying architecture. Thus thrusts are first met by layers of maximum flexibility in the plastic lubricating film and fibrous articular covering. The wave then flows through increasingly more rigid regions, - the calcified cartilage, the thin subchondral bone, multiple, stiffer, vertically oriented, bony trabeculae in the spongiosa - finally to end in the hard cortex of the body of the bone. This action can be likened to the softening of impact by the studied recoil of the athlete's arm when catching a fast ball with a bare hand."15
Cushioning of the joint components was also provided by the rapid filling and emptying of the vascular retrodiscal plexus. Each time a mandibular condyle moved backward, the loose vascular plexus behind the condyle was compressed and emptied. Then, each time the condyle moved forward, the retrodiscal plexus had to fill as fast as the condyle could move so that a vacuum could not be created. During chewing, the rapid anteroposterior movements of the condyle pumped the retrodiscal plexus like a piston in a cylinder, producing hydraulic forces which helped to cushion the violent movements of the condyle during chewing.
Cushioning of the joint components was also provided by disk and ligament mechanism of the TMJs. The network of temporomandibular ligaments that suspended each condyle up against the articular eminence functioned to prevent the possibility of the parts separating and then coming back together with an impact that could damage the specialized articular surfaces. Thus, when a condyle translated forward, as seen in the middle and right side portions of the illustration below, the outer oblique fibers of each temporomandibular ligament functioned like the radius of a circle to keep the condyle pressed firmly against the slope of the articular eminence.
During this process, intrajoint contact was maintained by the positioning of the articular disks. When the mandible was braced centrally, the disks filled most of the space between the condyles and the surrounding glenoid fossae to provide effective cushioning of compressive forces. As each condyle moved around during function, a set of collateral ligaments (composed of medial and lateral thickenings of the TMJ capsule) held its disk down on the head of the condyle like chin straps holding a hat down on your head. As a result, the disk could roll forward and backward on the rounded condylar head, but it could not be pulled laterally or superiorly out of contact with the condyle without stretching or tearing these ligaments. Because the disk was a biconcave structure composed of a central thin zone surrounded by peripheral wedges and bathed in lubricant, compressive forces from biting continuously centered the disk between the bones of the TMJ.
During function, when forcefully biting on a resistant bolus placed between the second or third molars caused the working side condyle to be mechanically distracted as shown above, the forward traction exerted by the superior lateral pterygoid muscle on the front aspect of the disk and capsule maintained intrajoint contact by ensuring that the disk occupied a position on the condyle which was rotated as far forward as was permitted by the width of the articular disk space, as shown below. In this manner, any space created in the TMJ by separation of the condyle and temporal bone was immediately filled in by as much of the posterior band of the disk as could fit in there, and the joint was still able to maintain a point of contact that protected it from impacts.
For the disks to be able to move their wedge-like contours rapidly back and forth on the condyles and to repeatedly change shape between concave up and concave down in response to the many different degrees of loading and the rapid fluctuations in the width of the joint spaces that occurred during chewing required more flexibility than the disks in other articular joints. To provide such flexibility, TMJ articular disks were made of fibrous connective tissue rather than the more brittle cartilage that comprises the articular disks in other joints.
THE HOMINID DENTITION
Hominid teeth also combined elements of the preceding mammalian teeth to form a new and highly adaptable hybrid dentition. Incisors, canines, premolars, and molars were all moderately well developed. One important change was that the canines no longer projected beyond the plane of the occlusal table as they did in primates.
Like in other mammals, the teeth needed to wear in before they could chew efficiently. After the teeth were fully erupted and interdigitation was no longer needed to align them, attrition quickly reduced the cusp tips and created closely fitting irregular contours suitable for grinding. Because dentin wears more quickly than enamel, occlusal wear transformed a contour of dentin cusps and fossae covered by a layer of enamel into cupped out areas of dentin surrounded by protruding rings of enamel that formed effective grating surfaces. The way the enamel, cementum, and dentin are dispersed in the tooth structure, such as in rings or flutes, determines the shapes of the cutting, crushing, or grinding surfaces that will appear on the occlusal surfaces after they begin to wear due to the abrasives that were omnipresent in mammalian evolution.
Occlusal wear also progressively shortened the teeth, requiring mechanisms to continuously compensate for the loss of structure in the framework of the face. In most hominids, the teeth wore down at a rate of at least a half millimeter every year. In tribes who ate relatively clean food like tree fruit, it occurred more slowly. In tribes who ate good with a lot of grit, such as meat cooked in the ground or grains milled on coarse stones, it occurred more rapidly. If shortening of the teeth from continuous wear allowed the bite platform to continually become shorter, the upper and lower jawbones would continuously move closer together, and the jaw muscles would have to keep changing their working lengths to maintain chewing efficiency.
One mechanism that evolved to compensate for continuous occlusal wear was continuous eruption. By means that we don't yet fully understand, mammalian teeth (and to some extent the supporting portions of the surrounding bones) developed a tendency to continually rise up and away from their bony bases into the bite table, as if they were spring-loaded with a force of 6 to 8 grams. 16 17 As a result, every micron of tooth structure lost to occlusal wear was replaced by a micron of new tooth structure brought up into the bite table. As a result, the framework of bones and teeth that supported the face was able to maintain a stable height, allowing the jaw elevator muscles to maintain constant resting and working lengths. With continual eruption and gingival recession in balance, the distance between the alveolar crests and the occlusal surfaces stayed constant, the gingiva could maintain its architecture, and the periodontium could receive a constant source of stimulation. 18
The combination of continual wear and continual eruption required continual pulpal recession to keep the tooth pulps protected from exposure to the bacteria-laden oral environment. Therefore tooth pulps responded to temperature changes and mechanical vibration by receding down into the roots and leaving behind layers of secondary dentin that mineralized to become new chewing surface. If attrition on any tooth proceeded faster than the pulp was able to recede, sensitive thermal and tactile receptors in the dentin overlying the pulp produced pain so that chewing on that tooth was avoided until secondary dentin had mineralized enough to recreate sufficient pulpal protection.
Continual eruption was so common in our ancestors that it may have become necessary to maintain the health of the periodontal structures by allowing the old cementum that had accumulated bacterial toxins at the bottom of the sulcus to be continually replaced by new sterile cementum on the erupting root surfaces. DuBrul explains, "Cementum, like bone, ages and finally degenerates. In bone this process leads to resorption of the old and its replacement by new bone. In the cementum such turnover is impossible. Instead, the aging cementum is covered by the formation of an additional young layer of cementum. This continuous apposition of new cementum occurs, in all probability, in waves separated by periods of rest. Growth of cementum is evidently indispensable for the integrity of the dentition. Continued growth of the cementum, however, needs space, and space is provided by the continued active eruption of the teeth. The latter in turn depends on continued occlusal and incisal wear. Thus attrition as the prerequisite of compensatory active eruption is itself a necessary factor for the health of the teeth."
The dentition was designed to absorb shocks. Each tooth was suspended in the middle of its socket by a circumferential fibrous sling embedded in a layer of collagenous ground substance with interstitial fluid and a system of vascular plexes so elaborate that a vascular pulse can be detected in each tooth. Biting pressure intruded each tooth root into this structure, tugged on the surrounding fibrous sling, drove fluids into nearby vessels, bent out the walls of the socket, and thereby encountered resistance that increased steadily as the tooth moved further from its rest position in the center of the socket. Release of biting pressure permitted rebound of the tooth, first by elastic recoil and then by a slow hydrodynamic phase with superimposed pulsation. 19 20 21 22
The bones supporting the dentition were also designed to absorb shocks.23 At the front of the mouth, the nasal processes of the maxillae carried incisal biting forces to the medial aspects of the supraorbital ridge. At the canine and premolar areas, the walls of the maxillary sinuses and nasal cavity transferred chewing forces up to the sides of the supraorbital ridge and the front portion of the zygomatic arch. At the molars, the bony prominences over the buccal roots transferred chewing forces to the rear portions of the zygomatic arches. The zygomatic arches in turn transferred chewing forces to the zygomatic sections of the maxillary and temporal bones. Medially chewing forces were transferred to the cranial base via the walls of the maxillary sinuses and the wings of the sphenoid bones. The bending of the paired midfacial membrane bones, the intrusion of the maxillary bite table, and even the bioelastic pressure-bearing synchondroses of the cranial base and the neurocranial sutures helped to dissipate the high-magnitude strains produced during chewing. Finally, this composite structure of membrane bones supporting the upper jawbone was surrounded by flexible sutures that do not close like other cranial sutures.
In the succeeding lines of hominids, advances in the use of fire and food preparation progressively diminished the need for more extensive chewing and larger digestive capacity. Evolution selected masticatory systems that were lightweight and adaptable. The jaws became smaller and the teeth became fewer. A large number of neural pathways to protect vital functions and more sophisticated means for acquiring food obviated the need for massive structural components and created space for brain expansion. The superficial head of the temporalis muscle and the protruding supraorbital region diminished. The "puffed out” lateral walls of the maxillary sinuses provided support for the teeth without impinging on space needed for respiratory and sensory functions. The muscles of facial expression became highly developed for better communication which enabled community protection, improved child care, and cooperative hunting.
Eventually a species known as homo sapiens sapiens was so successful that it was able to spread out all over the surface of the earth. Humans could live in caves, deserts, or snow; and they could acquire nutrients from an enormous variety of food sources. They provided the genes we acquire now.
CHAPTER 2 – FUNCTIONAL HARMONY IN THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
Pre-industrial human masticatory systems developed a remarkable functional harmony by a process that involved mutual growth adaptation of its individual components to a pattern of normal daily function. Recently we have changed the pattern of normal daily function. By learning about how the masticatory systems of our ancestors from even just a couple of centuries ago adapted their masticatory system components to fit their functional environment, we can learn how to modify our masticatory system components to fit the new functional environment we have created for our masticatory systems with our modern life styles and more delicate masticatory function.
A central feature in this process of achieving a functional harmony in the masticatory system is the dental occlusion. The occlusal table functions as the exercise template for the jaw muscles and the articulation between the mandibular corpus and the midface. To understand how dental occlusion should function now in our modern environment requires first understanding how it functioned in harmony with the other masticatory system components in our ancestors.
Controlling and coordinating muscle activity was a uniquely complex neural network. Chewing was initiated by a central pattern generator which strummed a constant background of repetitive firings. Each firing was then customized by complex arrangements of facilitating and inhibiting effects and reflex responses to receptors in order to enable the musculature to control the chewing pathways according to the feel of the food. Receptors in the jaw muscles that constantly tracked the position of the mandible enabled the system to rapidly detect and respond to unpredictable changes such as sudden fracturing or shifting of the bolus.
Positive feedback loops protected the teeth and their supporting tissues by preventing the mandibular elevator muscles from being able to shower down their full forces unless the periodontal receptors surrounding the roots of the teeth signaled widespread stable occlusal contacts. 26 For that reason, maximal voluntary bite force is positively correlated with occlusal stability. 27 28 Even simply anesthetizing the teeth causes a significant reduction of maximal voluntary bite force. 29
Negative feedback loops protected all the tissues of the masticatory system with neuromuscular reflexes. The sensory input to these reflexes was supplied by mechanoreceptors distributed abundantly throughout the tissues. Periodontal mechanoreceptors cover the roots of the teeth like the fingers of a glove and are sensitive enough to detect a force of 1.5 grams or an occlusal discrepancy of .02 millimeters. Even acoustic signals, mechanical vibration, electrical stimulation of pulpal nerves, and pain from almost any area of the face can activate protective reflexes. Once triggered, these reflexes evoke such a fast response by the jaw muscles that it can alter the path of the mandible within milliseconds to avoid an obstacle such as a small rock in a food bolus. 31 32
There were so many protective mechanisms able to inhibit the functional movement of the mandible that the central nervous system had to be able to limit their influence or they could prevent the necessary application of power on the working side during chewing. To prevent such central nervous system gridlock, inhibitory neurons diminished the responses to sensory stimulation on the working side during chewing. 33
If sore or damaged teeth threatened to trigger enough protective reflexes to hinder the functional capacity of the masticatory system, the neuromuscular system had a way of incorporating a protective pattern of mandibular movements that still allowed effective function. The altered patterns of movement are called engrams.
"Imagine that your job requires you to walk constantly through a very narrow hallway that has a series of boards protruding from the wall every two feet. These boards are so situated that the top of your right shoulder hits each one as you walk by. Your choice is then to allow your shoulder to hit the boards or learn to reposition your shoulder slightly lower so that it is below the level of the boards as you pass by. Of course, your choice would be to learn quickly to walk with your right shoulder slightly lower than your left to avoid any trauma from the boards."
Experimentally, when an obstacle to normal masticatory function is placed on the teeth, the firing pattern of the chewing muscles is very much altered initially, but eventually reestablishes itself almost completely in spite of the continued presence of the obstacle due to engrams. As a result, experiments have shown that a tooth made tall enough to theoretically receive the full force of the mandible actually experiences surprisingly small forces during function, and it's difficult to damage a TMJ by altering the occlusion.
THE MANDIBULAR ARTICULATION
The template against which the jaw muscles worked the mandible, (like the shape of the hallway in the previous analogy), was a structural tripod composed of the occlusal table and the two TMJs, as shown below.
Because all three of these articular surfaces were shaped by functional forces to fit the normal range of motion of the mandible, they also fit each other. Studies of pre-industrial humans show that form and surface changes of the TMJs were well correlated with dental attrition rather than with age or sex. 37 38 39 40
The process of altering the articular contours to fit the functional range of motion is different in the TMJs and in the dentition, but the effect is very similar. While the TMJs are shaped by remodelling of bone laregely due to osteoclastic activity in areas of high pressure and osteoblastic activity in areas of low pressure, the occlusal table is shaped by the positioning of teeth responding to masticatory bracing forces and then by the customization of the microcontours of each occlusal surface as a result of occlusal wear.
The most important source of functional pressures to which the components of the mandibular articulation adapted their shape was the forceful mandibular bracing that occurred when all the mandibular elevator muscles simultaneously fired to lock the mandible up against the underside of the front of the cranium. This powerful jaw muscle action occurred hundreds or thousands of times each day.
Bracing the mandible was needed for postural stability, because it allowed the postural muscles running up and down the front of the body to pull down securely on the front of the cranium. A mandible that was free floating could form a weak link in the anterior kinetic chain.
Bracing the mandible was also needed during swallowing. At the onset of each swallow, a braced mandible was needed to provide a stable platform against which the tongue could push to drive its tip forward and collect the food at the front of the palate. Then, later in the swallowing process, a braced mandible was needed to provide a fixed base against which the suprahyoid muscles could pull the hyoid bone upward and forward to allow the food to pass behind it.
Bracing of the mandible was also needed for protection. The heavy mandible hung freely below the skull, with its back ends close to the hearing and balance centers and the temporal lobes of the brain. A blow to the chin could impact a condyle like a hammer against these structures. In addition, the shortening of the canines which occurred in the transition from apes to hominids left the mandible vulnerable to retrusive blows. Thus, to protect vital functions, the first reaction to danger is to fire the jaw closing muscles and thereby immobilize the mandible by bracing it against the underside of the cranium simultaneously at the occlusal table and at the two TMJs.
Mandibular bracing was so frequent and forceful that it molded central concavities in each of the three pillars of the mandibular articulation. Each TMJ developed a glenoid fossa with a central concavity that perfectly fit a centrally braced condyle. The maxillary occlusal table developed multiple downward facing concavities (central grooves and marginal ridges) that perfectly fit each upward facing convexity (mandibular buccal cusp tip).
FUNCTIONAL MANDIBULAR MOVEMENTS
Another source of functional pressures to which the components of the mandibular articulation adapted their contours was the masticatory strokes that took place in pathways all around the mandibular bracing position. The teeth positioned themselves to accommodate those functional mandibular pathways, and the TMJs remodeled to accommodate those functional pathways. With all three areas of the mandibular articulation functioning cooperatively to provide continuous support for the mandible, the whole mandibular articulation functioned with the harmony of a single joint.
Wherever the mandible moved, it was continuously supported at all three pillars of the mandibular articulation by means of an upwardly facing ball (a condyle or a mandibular buccal cusp tip) articulating against a downwardly facing and slightly larger socket (a glenoid fossa or a maxillary central fossa or marginal ridge). When the mandible moved to the right, the right sides of the upward facing balls rubbed up against the right sides of the downward facing sockets - the mandibular buccal cusp tips of the right (working) side rode up on the inner inclines of the buccal cusps of the right side maxillary teeth and the right side condyle rode up on the lateral border of the right side TMJ glenoid fossa - while the teeth on the other (left) side separated. When the mandible moved to the left, all these same contact relationships occurred on the left side. When the mandible moved anteriorly, the mandibular incisors rode up on the palatal surfaces of the maxillary incisors and the front aspects of the condyles rode up on the backward facing slopes of the articular eminentia. When the mandible moved posteriorly, the mandibular buccal cusps rode up on the buccal slopes of the maxillary palatal cusps of the posterior teeth and the posterior aspects of the condyles rode up on the posterior slopes of the glenoid fossae.
During the power-crushing of food, on the working side, the mandibular buccal cusps were driven into the maxillary central fossae like a pestle against an upside down mortar. The working side mandibular buccal cusps rubbed powerfully against the lingual facing surfaces of the maxillary buccal cusps, through the habitual mandibular bracing position, and then in a follow-through against the buccal facing surfaces of the maxillary palatal cusps. During most power-crushing, there was a resistant bolus between the posterior teeth on the working side, and opposing teeth passed very close but did not actually rub together.
As the mandible followed through to the non-working side, the non-working side condyle moved backward until it almost reached the nonarticular facets on the backward facing slope of the condyle and the forward facing wall of the postglenoid process. 34 35 These non-working side facets in the TMJs were border areas rather than functional areas, consequently they did not become avascular and did not participate in the passive mechanical guidance of the mandible. However, as a result of remodeling of the bone beneath the specialized articular surfaces, they acquired shapes that fit the same normal range of mandibular motion, as indicated by the obvious congruence between facets on the posterior slopes of the condyles and those on the anterior slopes of the postglenoid processes. Thus, even though the congruence between opposing facets on the non-working side was remarkable, these were non-articular facets and did not participate in the guidance of the mandible during function, much the like non-articular facets in the TMJs on the posterior slopes of the condyles and the anterior slopes of the postglenoid processes.
Over time, the shape of the occlusal table adapted to fit the functional pathways of the mandible. Mastication with wide lateral slides made the shape of the occlusal table relatively flat in a transverse plane, and mastication with strong antero-posterior slides made the shape of the occlusal table relatively flat in a sagittal plane - with a curve of Spee that rose at a gradual angle anteriorly and posteriorly.
GROUP FUNCTION OCCLUSION
Eventually, the best fit of the occlusal surfaces to mandibular movement pathways created an arrangement of opposing teeth known as group function. Each dental arch operated like a single long curved wall composed of closely chinked rocks - teeth that fit perfectly together along broad interproximal contact facets. During function, this wall delivered, absorbed, and transmitted forces in a manner that protected each tooth from the danger of receiving the full impact of the moving mandible and thereby maximized functional capacity. The location of contact (or of extremely high compressive force on the bolus) usually began at the back of the arch and moved forward in a wavelike fashion. As the mandible rocked back and forth from side to side with this action, it created a shearing action that effectively chewed food while maintaining functional edges on the teeth.
Group function minimized the need for protective reflexes. Thinking back to the hallway analogy used to describe engrams, functional harmony was acquired when no boards were left sticking out, and the hallway acquired a shape that perfectly fit your body as you walked through it. During chewing function, a cadence of strong, smoothly alternating, and relatively uninterrupted firings corresponded with a steady repetitive pattern of sensory feedback as control shifted smoothly back and forth between muscles and articular structures. Each closing of the mandible was guided by the jaw muscles and the TMJs until the dentition or the food bolus was engaged. Then, as the jaw closing muscles showered down their power-crushing forces, guidance was smoothly transferred to the teeth and food bolus. Finally, as the power-crush stroke followed through and merged with the opening stroke, guidance was smoothly transferred back to the muscles and joints. Agonists and antagonists, firing in strokes that frequently used only slightly less force than was permitted by the physiology of the system, alternated smoothly. When one muscle group fired strongly, the antagonistic muscle group relaxed fully. There was no unnecessary overlapping of opposing muscle groups and thereby little energy wasted by muscles pulling against each other (co-contraction of antagonists).
By spreading out these forces among a large number of contacting surfaces, group function also maximized longevity of the masticatory system. In evolution, rapid attrition was the primary threat to the stability of the masticatory system. In group function, attrition was distributed evenly among all the teeth and thereby resisted for as long as possible. Teeth didn't wear out in one area until they were almost worn out everywhere.
Group function was so important for the health of the masticatory system that it was established even when a single intercuspal area was not. Australian aborigines with apparently very healthy masticatory systems sometimes lack a single position of bilateral maximal intercuspation but instead have separate stable unilateral intercuspal positions on each side. These two unilateral intercuspal positions cannot be used simultaneously, but alternate providing a solid platform for the mandible during function. This same type of bite, known as X occlusion, is found in some other mammals.
CHANGES IN THE MASTICATORY SYSTEM WITH AGE
The functional harmony in the human masticatory system gradually transformed with age in a manner that made it progressively more compatible with the changes that naturally take place in all aging tissues. The harmony remained stable, but its character changed.
During childhood, a vertically oriented and often irregular bite table was well tolerated. Newly erupting teeth had sharp lines and angles. The incisors came into the mouth with chisel-like edges and a steep overbite that quickly developed a cutting edge, and the newly erupting canines were pointed and sharp. Unworn premolars and molars had tall pointed cusps. The dentition was most effective at incising, puncturing, and ripping food.
Young tissues were well equipped for such function. They were full of water, enzymes, and elastic fibers which enabled them to withstand diverse articular stresses and unanticipated impacts. Bones easily bent, and teeth easily shifted. Fast and active protective neuromuscular reflexes could cope with frequent sudden changes in firing patterns or forces applied. Chewing strokes could bring the mandible laterally across a steep intercuspation and then in and out of a sharply determined intercuspal position while still avoiding traumatic collisions between tall overlapping cusps. The jaw muscles could perform intricate dances to avoid occlusal interferences. Extensive blood supply was readily available where it was needed.
As people got older, soft tissues became more like leather, and hard tissues became more like stone. Elasticity and flexibility diminished as water and enzymes got replaced by fibers and fat. Collagen fibers became denser and stiffer. Cartilage developed fatty inclusions within cells, thickening and clumping of fibers, dehydration, calcification by deposition of calcium salts, and increased mineralization of the extracellular matrix. Articular disks became dehydrated, fibrotic, and sometimes calcified. Bones underwent a decrease in water content and an increase in the amount and size of apatite crystals with blockage of nutrient canals. Vessel walls became more rigid, and collateral circulation became less abundant – anastomoses becoming confined to thick-walled less physiologically active vessels rather than capillary beds. In teeth, blood supply to the pulps decreased through reduction in numbers and internal diameter of arteries as a result of calcification, intimal thickening, and elastic hyperplasia. The body became less able to rapidly increase blood supply to any area where it was needed. In muscles the availability of ATP for fuel dwindled, the number of contractile fibers declined at the rate of about 5% per decade, and the number of noncontractile elements within them increased. In nerve tissue, there were age related declines in several different abilities. Neuromuscular reflexes became slower due to a delay in processing rate and conduction velocity. In the performance of tasks, there was a loss of coordination and precision of movement.
These progressive losses in strength and adaptability with increasing age were accompanied by concomitant changes in the form of the mandibular articulation and the range of motion that made the masticatory system require less strength and adaptability. The mandibular range of motion and the shape of the articular components shifted toward becoming more smooth, less resistant, and easier to operate.
Anteriorly the overbite was eliminated so that the incisors and canines no longer extended beyond the bite table. The overbite reduction can be seen in the shifting of the horizontal line marking the movement of the incisal edges of the mandibular incisors in the illustration below comparing Australian aborigines chewing patterns in youth, middle age, and old age.
Posteriorly tall sharp cusp tips that once fit together with pinpoint precision became rounded and then flattened until they met along wide areas with significant lateral freedom of movement. The enamel was completely worn off from the occlusal surface of the first molar by the time the second molar erupted, and the enamel was completely worn off from the occlusal surface of the second molar by the time the third molar erupted. With age, chewing strokes acquired progressively longer glides with a biphasic chewing pattern that maintained functional edges much like in herbivores. Evidence of that biphasic chewing pattern can be seen in the scratch marks oriented in two distinctly different directions on the molars of pre-industrial human skulls in museums . 46 The normal change in the dentition with age is illustrated below.
NEWLY ERUPTED TEETH MATURE DENTITION ELDER DENTITION
Mature dentitions acquired an occlusal table that integrates the anterior teeth and the posterior teeth into one smooth curve much like the surface of a sphere of approximately 4 inch diameter, as shown below.
Wider and steadier mandibular movements maintained the health of aging jaw muscles. The jaw muscles were able to fire in long steady strokes that gradually built momentum and came to a smooth end, uninterrupted by protective reflexes or co-contraction of antagonists. Fluid pressure rose and fell more gradually with each stroke. Interproximal tooth wear and mesial drift made the occlusal table shorter while bucco-lingual wear made the occlusal table narrower. As a result of the decrease in the area of the occlusal surface, chewing strokes met less resistance and required less force.
Wider and steadier mandibular movements also maintained health at aging articular surfaces by facilitating weeping circulation and minimizing the need for extensive shock absorbing capacity while circulatory ability and shock absorbing capacity naturally decreased with age. The longer and smoother functional strokes minimized the production of mechanical shocks in the temporomandibular joints and the basal bones supporting the maxillary bite table just as these areas were losing their natural shock absorbing capacity anyway due to loss of elasticity and circulation and also just as the aging nervous system became progressively less able to maintain a continual state of alert. The more smoothly curved occlusal table which resulted from years of function allowed a relatively quiet and smooth neuromuscular system by feeding it sensory stimuli in a uniform manner that allowed easy tracking of mandibular position. Pressure passed evenly down the arch from each tooth to its neighbor, applying compressive forces that gradually rose and fell on each sensory receptor bed and allowing a simple and repetitive pattern of motor impulses to drive the system.
The smooth flattened curve eventually acquired by the human occlusal table has been described as helicoidal, because the occlusal surfaces of the maxillary and mandibular dental arches formed long continuous walls with helicoidal longitudinal twists somewhat like a propeller blade. The plane of the occlusal table relative to a horizontal plane was relatively steep at the central incisors and flat at the second molars. At the first molars, the maxillary occlusal plane sloped lingually, and at the third molars the maxillary occlusal plane sloped buccally. Contours in the lower dental arch were opposite and complementary. 47
As the end product of modification of the occlusal table by function, the helicoidal curve was the imprint in tooth structure erupting all around the mandible of the functionally generated pathways of the mandible. Tooth structure that was in the way of these pathways was removed by attrition or minor tooth movement, and areas which were low erupted further into the occlusal table. As a result, the occlusal table became a three dimensional registration of the envelope of motion acquired by the mandible after the jaw muscles had overcome the resistance posed by the dentition.
The helicoidal curve was not at all in line with the curves along which the teeth first erupted. The steep curves of Spee and Wilson, present in newly erupting teeth, were always quickly eliminated by function. These curves were designed by evolution to continually supply working surfaces for chewing function, not to dictate the functional pathways of the mandible. Even the few tribes who experienced minimal occlusal wear still developed an occlusal table with a helicoidal curve.
The human masticatory system was able to maintain its functional harmony even in those who lived into old age. The articular disks stayed in place but wore out along with the other TMJ components and the dentition.48 Adaptive movement of the teeth in response to continual tooth wear maintained the proximity of the maxillary and mandibular occlusal surfaces and thereby also the effectiveness of mastication, even after occlusal wear had completely eliminated the crowns of the posterior teeth and reduced them to individual roots functioning as little teeth - each with an occlusal surface and an interproximal contact area. Facial height stayed constant, dental crown height stayed constant, and the condyles maintained their central positions in the glenoid fossae even as the mandibular corpus kept leading the rest of the face in anterior translation. As a result, most older dentitions were either end-to-end or class 3. If the system finally failed, it was usually because the articular components had exhausted their reserves, and there were no tooth surfaces or condylar heads left on which to chew.
CH 3 – GROWTH OF THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
These pre-industrial human masticatory systems did not just start out with a functional harmony. Their functional harmony was the result of a long and multistaged postnatal growth process in which genetics interacted with functional stimuli to build a masticatory system optimally suited to withstand those functional stimuli. Genetics provided the motive forces, the raw materials which were bound to cause proliferation of tissues according to a pre-determined sequence. However, the final form taken by the tissues was also very much determined by the system’s adaptation to the environment in which the genetic tendencies were expressed.
One of the reasons for our evolutionary success was the adaptability we gained from having much of our growth and development take place postnatally and therefore under the influence of functional stimuli. In many ways, humans seem to have evolved from an ape born in an embryonic stage. 49 In addition, the human masticatory system forms especially late in postnatal development and therefore is especially affected by functional stimuli. At birth many of its structures are either rudimentary or nonexistent. The TMJs have not yet formed. The squamous portion of the temporal bone is essentially flat. The mandibular corpus is just a bulbous tube of tooth crowns with a midline that is still unfused, making it not yet capable of receiving or transmitting significant forces. Delaying much of the growth and development of the masticatory system until after it begins functioning enabled each individual's masticatory system to acquire a form perfectly suited to apply and withstand those functional stimuli. Thus it was the interplay between genetics and environment that produced masticatory system structures the right size and shape needed for functional harmony. As a result of that interplay, the human masticatory system grew a form that perfectly fit the functional capacity it required.
FUNCTION AFFECTS FORM
It's easy to see how form affects function during growth, but it's also true that form adapts to function. Wherever muscles pull on bones, they create protuberances of bone for the attachments of the muscles. Wherever muscles produce bending stresses on bones, those bones develop an internal architecture almost perfectly aligned to withstand those bending forces. As a result, when muscles grow stronger, their activity causes the bones at their origins and insertions to strengthen just enough to be able to withstand the stronger forces, and weakness or paralysis of muscles produces bones that are extremely thin and mechanically deficient. 50 51
There is evidence that functional forces produce localized bone growth by enhancing local circulation. In bone growth, oxygen seems to be a "master controlling switch" – affecting osteoblast metabolism, osteoclast metabolism, and osteoid calcification.52 53 Oxygen supply is likely enhanced by the rhythmically alternating forces that occur during function. Stimuli that produce osteogenic activity are frequency-specific. Repetitive loading is osteogenic while constant loading is not. 54 55
In response to bone growth stimulated by functional forces, the masticatory system grows structural components to fit masticatory forces. For example, the articular eminentia and glenoid fossae do not form when there is no condyle 76 77 ; and, even after they are fully formed, will lose their contours if the condyles are removed or fractured. 78 79 80 The alveolar processes do not form if there are no teeth.Most of the functional forces in the craniofacial area are generated by the jaw muscles.
In addition, masticatory forces affect large potions of the cranium. In monkeys, a new layer of bone forms on the supraorbital ridge just after the arrival of each new molar, 68 and forceful biting opens the sagittal suture at the top of the head by separating the parietal bones. Pre-industrial eskimos, who had very strong jaw muscles, developed a distinct thickening along that sagittal suture. 69 The widespread distribution of masticatory forces around the craniofacial areas was demonstrated graphically when Benninghoff coated skulls with stress sensitive paint and then loaded them as in biting. His illustration of the distribution of bite forces is shown below.
As a result of the distribution of these forces, masticatory forces can affect the growth of even distant portions of the craniofacial structure. Experiments have shown that masticatory forces profoundly affect the growth of much of the cranium in animals. Weakening masticatory forces by softening the diet produces thin craniofacial bones, and strengthening masticatory forces by hardening the diet produces thick craniofacial bones.67 Unilaterally damaging jaw muscles or extracting posterior teeth can produce significant cranial scoliosis. 56 57 58 59 60 61 62 63 64 65 66
In fact, much of postnatal human craniofacial growth and development may be a process of adaptation to chewing forces. The density of all the bones of the cranial vault varies according to jaw muscle strength as well as cervical muscle strength. 81 Moss pointed out, "The external form of the human skull... is related directly to imposed loadings, a point verified experimentally in miniature swine skulls. It is known that unit strains are greatest in infant skulls, less in adolescent skulls and least in adult skulls, suggesting that the growing skull increasingly adapts its structure to masticatory loadings." 82
THE ROLE OF THE OCCLUSAL TABLE
A key architectural component in force distribution through the craniofacial area and thereby also in the growth of the craniofacial area is the occlusal table. It serves as the exercise template for the jaw elevator muscles and functions as an articular surface between the mandibular corpus and the maxilla. As a result, the occlusal table is one of the most stable features in the craniofacial growth of both humans and animals. Its stability is due to the constancy of functional and resting muscle forces rather than to genetics. As a result, while most craniofacial dimension show high hereditability and further increases with age, dental arch dimensions show very low heretability and decreases with age. 86 87 88
Before the teeth erupt, the structural stability of the occlusal table is provided by the soft tissues of the masticatory system. In sucking behavior, the tongue is the primary functional organ, working like a plunger against the strong lips tightly surrounding it, and the mandible is stabilized by squeezing it against the tongue resting between the gum pads. In response to rhythmic compression by the mandible against the tongue or nipple, the TMJs begin to form between the condyles and the temporal bones where opposing periosteal envelopes rubbing against each other produce secondary cartilage on the surfaces of the bones and a fibrocartilaginous pressure-bearing disc from the tendon of the lateral pterygoid muscle. The pressure-bearing articular surfaces become avascular. The disk acquires a thin central zone surrounded by peripheral wedges. An articular eminence forms in response to loading by the immature condyle against the temporal component of the joint.
The arrival of teeth radically changes the nature of the platform between the jawbones and establishes a whole new set of neuromuscular reflexes which profoundly changes the behavior of nearly all the muscles in the craniofacial area. As a bite table replaces the tongue as the platform between the jawbones, the jaw closing muscles replace the facial muscles as the source of stabilization for the mandible during swallowing. Freeing the facial muscles from their crude infantile suckling and swallowing functions allows them to take on the more delicate and complicated functions of speech and facial expression. Experiments have shown that the consistency of the food is at least partly responsible for the change in neuromuscular control. Older children given a bottle revert to infantile suckling, swallowing, and respiratory rhythms.
The structural components of the masticatory system keep growing in response to chewing. As the dentition acquires its functional capacity, the zygomatic processes thicken and begin to bow outward, the glenoid fossae deepen, and the articular eminentia begin to develop their characteristic S-shaped profiles. By the time the primary dentition is complete, the TMJs are well established and the articular eminentia have gained more than half of their adult form.
ALIGNMENT OF THE DENTITION
Functional forces guide the teeth into place. Eruption paths of the teeth cannot be controlled very precisely by intrinsic mechanisms. Interdigitation of steep opposing inclines provides much more control. In addition, allowing functional forces to position the teeth enhances the ability of the masticatory system to adapt to varied masticatory patterns.
The first functional forces to guide the eruption of the teeth are the light steady background tensions directed inward from the lips and cheeks and outward from the tongue. These opposing forces (shown below) create between them a neutral zone into which the teeth tend to drift.
In the anterior region, the overbite relationship that the maxillary and mandibular anterior teeth acquire when their eruption first brings them into occlusal contact serves to couple these regions to ensure that they stay relatively close together in spite of the fast anterior translation of the mandibular corpus. In the posterior region, as the maxillary and mandibular posterior teeth come into contact, their interdigitation refines their alignment. This so-called cone and funnel mechanism causes the teeth to shift as needed to acquire maximal interdigitation. The process is illustrated below:
Since each maxillary posterior tooth interdigitates with two mandibular posterior teeth and each mandibular posterior tooth interdigitates with two maxillary posterior teeth, the interdigitating arrangement of opposing teeth spreads up and down the arch.
The next functional forces to guide the eruption of the teeth are the persistent compressive forces produced by the mandibular elevator muscles in bracing. The frequent and forceful squeezing of the mandible against the maxilla each time the mandible is braced stops the eruption of each tooth when the axially directed occlusal forces that are pushing the teeth into their basal bones counterbalance the eruption forces that are continuously pushing the teeth out of their basal bones and into the occlusal table. The consistency of this bracing activity aligns the occlusal surfaces of the teeth to form an occlusal table at a height determined by the activity of the mandibular elevator muscles.
At the same time, the posterior teeth acquire positions that also fit the normal functional range of motion of the mandible. During youth, when the dentition is forming, chewing movements begin by opening with a wide lateral shift (as seen on the right side of the illustration below) which may have a role in aligning the teeth in a manner that does not restrict the mandibular range of motion laterally. After the occlusal table is established, the wide lateral mandibular shift on opening disappears. Subsequently chewing movements open near the midline and close from a more lateral position for maximally effective power-crushing (as seen on the left side of the illustration below).
Because the mandibular elevator muscles have such divergent origins on the cranium, their convergence at the occlusal table during bracing maintains the location and orientation of the occlusal table with great stability. Serial cephalograms show that the occlusal table is a central architectural landmark and a key structural component in the facial growth process. In growing rabbits, experimentally altering the plane of the occlusal table can produce a scoliosis of the whole craniofacial skeleton.89
The occlusal stability established in the primary dentition is maintained during the transition to the permanent dentition by the order of eruption of the permanent teeth. The first permanent teeth erupt in front and in back of the primary occlusal table in order to form a structural tripod that supports and extends the occlusal table. Once these three pillars of permanent teeth are stable, the primary teeth between them are replaced one or two at a time without disturbing the stability of the occlusal table.
To understand the role of the dental occlusion in craniofacial growth requires looking at the varied and diverse growth processes that all converge at the occlusal table. The response of the craniofacial area to masticatory forces is complex. The distribution of forces during chewing generates a gradient of strains in the face with highest strains experienced near the occlusal table, moderate strains in the middle face, and very low strains in the upper face.(Paschetta C, de Azevedo S, Castillo L, et al. The influence of masticatory loading on craniofacial morphology: A test case acrss technological transitions in the ohio valley. Am J Phys Anthrop 2010;141:297-314) The tooth bearing portions of the jawbones and their extensive frameworks of supporting bones experience compressive forces, regions of muscle attachment and insertion (such as the zygomatic arch and the coronoid process) experience tensile forces, and areas between these two portions experienced twisting, bending, and shearing forces.83 In addition, the functional activity of the jaw muscles changes throughout life. The jaw muscles of an infant behave very differently than the jaw muscles of a young adult which behave very differently than the jaw muscles of an elder. To understand how these growth processes were regulated and coordinated by functional stimuli in our pre-industrial ancestors to create masticatory systems that maintained a remarkable harmony of form and function with age requires looking at each of the various growth processes involved.
The earliest growth process to dominate the craniofacial area is neurocranial expansion. It's rapid growth stops at about one year of age, and it's growth is almost complete by the age of 7. Neurocranial expansion enlarges the cranial vault, which is a tabular structure of membrane bones that forms a continuous shell enclosing the expanding brain and eyes. The cranial vault enlarges as the bones drift away from the center, inner areas less rapidly than outer areas where extensive myelinization is occurring. Lateral and frontal serial X-ray tracings of a growing head look like a slow motion picture of an explosion, as seen on the right side of the illustration below.
Between the individual cranial bones, growth at the sutures occurs in sufficient amount to accommodate whatever expansion takes place in the cranial vault. Rapid proliferation of new bone at these growth centers is triggered by mechanical separation of the cranial bones bordering them. As a result of this growth, the cranial vault always expands just enough to perfectly fit the periphery of the brain. Even when the cranial contents expand much more quickly than normal in hydrocephaly, the sutures are still able to produce enough bone to maintain a continuous shell around the grossly enlarged cranium.
As subsections of the cranial vault, the orbits behave similarly. Each orbit contains an enclosed expanding neural mass, and each orbit grows by proliferation at its surrounding sutures just enough to perfectly enclose its expanding neural mass. Several bones of the midface contribute to the structure of the orbit. Removal of the eyeball during growth results in deficiencies in the anterior and lateral growth of the midface.
While the expansion of the cranial vault is motivated by enlargement of the enclosed neural contents, the shape ultimately acquired by the cranial vault is at least partly determined by externally imposed forces. In pre-industrial humans, there were tribes who successfully altered head shapes by binding their infants' heads with cloths or boards, apparently without impairing brain development. Similarly, the cranial vault can be shaped at least partially by the postural and functional forces applied to it by the jaw and skeletal muscles.84 Animals fed a hard diet develop thicker cranial vault bones. 85 Cutting the temporalis muscle leads to changes the length and width of the braincase.
Since the cranial vault expands as much as necessary, limiting its growth in one direction causes growth in other directions. Compression of the cranium by the vertically arranged jaw elevator muscles produces compensatory growth horizontally. As a result, pre-industrial humans with relatively strong overall musculature had cranial vaults and faces that were relatively short and wide, while pre-industrial humans with relatively weak overall musculature developed cranial vaults and faces that were relatively tall and narrow.
As the expansion of the cranium slows, the elongation of the cranial base becomes the dominant growth process in the craniofacial area. The cranial base is a thick spline of bone which extends from back to front along the midline of the floor of the cranium to form a stable structure on which the brain rests. Because the cranial base is thick, it does not burgeon out in response to neurocranial expansion as readily as the thin plates of membrane bone bordering the other sides of the vault. The cranial base holds its shape, much like the reinforced bottom of a box which contains an expanding mass, as seen in the illustration to the left. The relative independence of cranial base growth from neurocranial expansion can be seen in the way it is only slightly affected in microcephaly and hydrocephaly, while the rest of the cranial vault becomes grossly distorted.
While holding its shape, the cranial base elongates due to endochondral ossification - like the long bones of the limbs or the vertebrae. The two primary components of the cranial base, the basi-occiput and the baso-sphenoid, are phylogenetically cephalized vertebrae. The epiphyseal plates of the cranial base (the spheno-occipital synchondrosis, intersphenoid synchondrosis, and spheno-ethmoidal synchondrosis) are composed of growth plates arranged back to back. Growth at these sites is able to elongate the cranial base in a sagittal plane with a rate that peaks at puberty and ends after the second decade of life.
As seen below, the cranial base is actually comprised of two sections, a front portion and a back portion, connected at an angle of about 130 to 135 degrees. Because the maxilla grows from the front of the cranial base while the mandible grows from the back of the cranial base, the angle formed by the cranial base is an important determinant of jawbone relationships. Sharp cranial base angles produce faces with more retrusive maxillae and more protrusive mandibles, while obtuse cranial base angles produce faces with more protrusive maxillae and more retrusive mandibles.
Elongation of the more vertically oriented posterior portion of the cranial base causes increased facial height by pushing the cranium up and away from the shoulder girdle and chest. As a result, the face grows longer vertically as overall body height increases. This downward and forward elongating of the face during postnatal growth dramatically changes facial proportions between childhood and adulthood, as can be seen below. The changes above the line, which are primarily due to neurocranial expansion, are minimal compared to the changes below the line, which are due to vertical elongation of the face.
Elongation of the more horizontally oriented anterior portion of the cranial base causes increased facial length antero-posteriorly by pushing the central portion of the face forward relative to the rest of the cranium. People with a genetic defect that prevents the cartilage of the cranial base from expanding normally develop a face that looks as if it had been pushed in at its center. The lateral portions of the face are only indirectly affected by cranial base elongation. Their growth is more directly affected by jaw muscle forces as explained in the subsequent text.
The cartilage of the nasal septum functions like a final extension of the anterior cranial base. One researcher suggested that the forward growth of the nasal septum pulls the surrounding facial contours forward with it because of traction from a septo-maxillary ligament connecting the nasal septum with the surrounding tissues.
The protrusion in the center of the face is needed to increase airway space as the growing body needs more air. Significant increases in the cross sectional area of respiratory passage have to accompany increases in body size; because respiratory needs are a function of body volume which increases in proportion to the cube of any increment in linear dimension, while cross sectional area of the airway only increases as the square of any increment in linear dimension. Thus to meet the growing body's even faster growing increase in airway passage, relatively rapid central facial protrusion is needed to continuously diminish airway resistance in the face during the active growth of the first two decades.
From the underside of the cranial base, the facial mask maintains its shape while it translates downward and forward. The many superficial facial muscles of the face are weaved together to form a tight mat that maintains the underlying superficial bony contours of the face in a way that keeps each face unique and recognizable throughout life. Growth of the structural components of the face shifts this facial mask downward and forward.
The face descends structurally much like three shelves that wrap around the center of the face and move down and forward in parallel. From top to bottom these shelves are the infraorbital shelf, the maxilla, and the mandibular corpus.
These shelves all move in about the same direction downward and forward, but not at the same rate. Generally the mandibular corpus leads the way. The other shelves translate at a rate that is intermediate between the fast translation of the corpus and the steadiness of the cranial base. As a result, in a sagittal plane, the face appears to grows out from the underside of the front of the cranium as if it were swinging out from hinge located at the forehead. In this manner the profile flattens between infancy and adulthood as shown below.
The bottom of the three shelves, the mandibular corpus, grows as a tool, much like a hammer, for delivering masticatory forces. It is designed to deliver large compressive forces upward, inward, and slightly forward against the underside of the cranium.
Functioning as a hammer head, the mandibular corpus maintains its shape while thickening as much as needed to be able to deliver masticatory forces. The size of the chin is proportional to maximal bite force, the thickness of the condyles is determined by the functional loads they receive 70 71 72 73, the size of the gonial angle varies as a direct function of the size of the masseter and internal pterygoid muscles74 75 and the growth of the coronoid process depends on the presence of the temporal muscle,
Functioning as an elongating handle for the hammer, and the rami and condyles grow to keep pushing the corpus further forward in front of them in order to provide a continuous supply of tooth structure at the occusal table. Enlow described this growth as shifting a brick wall forward by pulling bricks off its front end and restacking them on its back end.
The condylar cartilage acts as a growth center, especially early in life. When ankylosis of a TMJ prevents condylar growth, the back of the mandible fails to descend and the back of the face becomes remarkably short. When a hormonal irregularity causes extreme elongation of cartilage, as in acromegaly, condylar growth pushes the mandibular corpus anteriorly beyond the rest of the face into a class 3 malocclusion.
Growth at the condyles is also highly adaptive. The condyles are covered by a proliferative layer of undifferentiated mesenchymal cells that gives them the capacity to grow as much and in whatever directions are necessary to hold the mandibular corpus out under the maxillary dentition. This condylar adaptability can maintain a connection between the glenoid fossae and the posterior ends of the rami even when the corpus becomes irregularly displaced due to disease, injury, extreme functional habits, functional orthodontic appliances, chin cup appliances, loss of teeth, or surgery to change the position of the maxillary occlusal table. Condylectomy is routinely followed by spontaneous regeneration of a new condyle with a functional articular head and condylar neck contained within a normal synovium and in some cases with a fibrocartilaginous cap. In rats, rotating the mandible open by fixing a block between the front teeth redirects condylar growth backward until contact with the glenoid fossae is re-established. In rabbits, experimentally shifting the glenoid fossae posteriorly triggers an increase in condylar growth until contact between condyles and fossae is re-established.
Many researchers have commented on the important role that condylar adaptability plays in human growth. Enlow says, "The variable capacity of condylar growth provides adaptation to different facial types, different articular patterns between the individually variable configuration and dimensions of the cranial floor and maxilla, different occlusal patterns, and normal structural changes occurring in conjunction with progressive growth. As the whole mandible becomes displaced in whatever vectors are involved at different ages and in whatever variations occur among different individuals, the condylar cartilage and the contiguous membranes forming the intramembranous bone of the condylar cortex and the condylar neck grow in whatever directions and in whatever amounts are required to sustain constant functional position and articulation with the cranial floor.” Petrovic says, “The condylar cartilage growth is integrated into an organized functional whole having the form of a servosystem, which is able to modulate the lengthening of the condyle in such a way that, through postnatal growth, the lengthening of the lower jaw adapts to the lengthening of the upper jaw.”
The maxilla grows very differently - it is designed to receive masticatory forces and distribute them as widely as possible around the cranium. it is designed to absorb shock and to adapt its shape to the forces it receives. It is comprised of membrane bones with long suture connections that allow them to swing apart both around their midline and around their anterior connection to modify the shape of the platform as needed to absorb masticatory forces. It is also surrounded by a series of circum-maxillary sutures that stay open throughout life. Strong chewing activity probably helps maintain the patency of these sutures. Experiments have shown that continued slight interosseous movements at sutures keeps them open, while experimental immobilization of sutures leads to ossification and suture closure. 90 91 Even merely softening the diet has been shown to produce some premature obliteration of facial sutures in rats.
Growth at the circum-maxillary sutures occurs by a process which is much like growth at the cranial sutures - except that these sutures are all generally aligned in the parallel, at a perfect angle to fill in behind a downward and forward translating maxillary platform. The persistent repetitive pounding of the maxillary bite table up against these downward and forward facing sutures may produce a net anterior growth vector that contributes to the anterior growth of the maxilla.
The upper shelf provides a structural platform for bracing the maxilla against masicatory forces, supported by flying buttresses originating all over the front half of the cranium. The premaxillary region transfers incisal forces up to the nasal region, the prominent bone around the canines transfers chewing forces to the anterior aspects of the zygomatic arches, the molar regions transfer masticatory forces to the posterior aspects of the zygomatic arches and around the maxillary sinuses, and the zygomatic region serves as a buttress to resist the twisting that results when forceful contraction of the masseter muscle on the working side draws the zygomatic arch downward and inward.
Because the sutural growth in the face is stimulated by mastication, over time the distribution of bone in the midface becomes virtually identical to the stress distribution experienced by that area during loading, and the midface becomes a lightweight and efficient honeycomb of thin membrane bones that form a platform almost perfectly designed to withstand and distribute the stresses produced by mastication.
THE MAXILLO-MANDIBULAR SUTURE
Between the lower facial shelf and the upper facial shelves, the dental occlusion functions like a maxillo-mandibular suture. It provides fill-in growth, because the teeth continually erupt as far as needed to fill in any spaces created between the maxilla and the mandibular corpus. Primate experiments show that any factor lowering the postural position of the mandible promotes additional tooth eruption. It functions as a growth center, because each tooth root is surrounded by a metabolically active periosteal layer on one side and a proliferating layer of cementoblasts with embedded Sharpey's fibers on the other side. It alsotransmits growth forces. In both the early primary and the early secondary dentitions, the jagged occlusal surfaces of the newly erupting teeth connect the maxillary and mandibular dentitions to coordinate their growth. Studies have shown that alterations in the growth of the maxilla affect growth of the mandible, and alterations in the growth of the mandible affect growth in the maxilla. 92
While the occlusal table is becoming established, the coupling of maxillary and mandibular teeth prevents the relatively rapid anterior translation of the mandibular corpus from bringing the mandibular dentition away from the maxillary dentition which has an expanding rather than an anteriorly translating base. Then later, the mandibular corpus continues to lead the way in facial growth, and the structures located between the mandibular corpus and the cranial base generally have growth patterns that are intermediate between the extreme growth of the mandibular corpus and the steadiness of the cranial base. While the corpus moves the farthest anteriorly; the mandibular alveolar bone moves less far anteriorly than the corpus (thereby shifting posteriorly on its bony base), the mandibular teeth move less far anteriorly than the mandibular alveolar bone (thereby tipping backward on their bony base), the maxillary teeth move less far anteriorly than the mandibular teeth (thereby reducing overjet), the maxillary alveolar bone moves anteriorly less far than the maxillary teeth, and the maxillae move anteriorly still less far than the maxillary alveolar bone. Similarly, the forward rotation of the mandibular corpus affects the midface in proportion to its distance from the occlusal table.
MUSCLE RESTING POSTURES
While the maxillo-mandibular suture helps to coordinate maxillary and mandibular growth patterns, the most important coordinator of maxillary and mandibular growth patterns is the resting postures of the craniofacial muscles. Light steady pressures move and shape bones very effectively, and the primary source of light steady forces are the resting postures of the muscles. The muscles and the fascia enclosing them in pockets hold each bone in a kind of neutral zone determined by resting muscle tensions. Any shift of a bone away from this neutral zone or any change in the shape of a bone creates an imbalance between opposing myofascial pulls and thereby causes a constant light tension pulling the bone back toward its neutral zone or returning it to its original shape. Surgeons find that rapid resorption occurs in any part of a bone graft which is located beyond the tension zone controlled by the musculature, and any change in the passive tension of the myofascial curtain can reshape the underlying bones. The passive tension of the myofascial curtains draped from the cranium down onto the shoulder girdle and sternum in rest posture maintains the resting positions of the mandible and the hyoid bone embedded within those myofascial curtains. In animals, intruding on the passive tension of the jaw closing muscles by increasing the height of the occlusal table causes intrusion of the underlying teeth until the mandibular elevator muscles have returned the mandible to its previous resting position.
CONTROL OF MUSCLE RESTING POSTURES
Controlling craniofacial and craniocervical resting muscle tensions is a hierarchy of neuromuscular reflexes, the most important of which are the airway protection reflexes. The pharyngeal airway is bounded in front and on both sides by the mandible and in back by the cervical spine, To maintain adequate airway passage through this area, neuromuscular reflexes control the resting tensions of the intrinsic musculature of the pharynx, the muscles able to pull the mandible forward (the lateral pterygoids, anterior temporals, and superficial masseters), the muscles able to pull the tongue forward (the genioglossus, geniohyoid, and transverse and vertical intrinsic muscles of the tongue), and the postcervical muscles. These muscles control both head and mandibular posture in the service of airway preservation.
"When we examine cephalometric landmarks in individuals affected by mongolism and achondroplasia, we see that respiratory function has been protected by different kinds of facial adaptation in each group. The adaptive changes in mongoloids have been described earlier as very localized effects on parts of the skull that spare the respiratory passages but reduce the size of the olfactory and masticatory components. In achondroplastics nasal airway volume is protected in spite of the mid-face deficiency and the increased cranial base flexure by an adaptive counter-clockwise rotation of the palatal plane. The biologic problem of respiratory survival is solved by a shortened palate in one group and by downward or counter-clockwise palatal tipping in the other."
Airway blockages trigger these airway protective reflexes. Blocking the nasal airway in monkeys produces a lowered mandibular posture, rhythmic activity of the geniohyoid and jaw closing muscles in synchrony with breathing, and a reshaping of the tongue to form an oral airway passage by lying low in the floor of the mouth, curling longitudinally, or protruding out between the teeth. Blocking the nasal airway in humans causes a change in the direction of facial growth downward and backward as can be seen in the serial cephalograms below taken of a child after complete nasal airway obstruction was created surgically.
The airway protective reflexes controlling the jaw muscles keep the mandible protruded as far as needed to permit breathing. When people are sleeping on their backs, the retrusive effect of gravity on the mandible evokes increased tonus in the superior lateral pterygoid muscles. After orthognathic surgery to retrude the mandibular corpus, muscle resting postures still maintain a constant minimal antero-posterior distance between the back of the hyoid bone and the posterior pharyngeal wall. In some instances, the base of the tongue actually moves forward as the mandible is moved back. Muscular adaptations occur all the way down to the level of the clavicle and often include a change of head posture.
THE CRANIOFACIAL GROWTH MATRIX
The resting postures of the craniofacial muscles impart a steadiness to the overall craniofacial structure. Longitudinal studies of children have shown that annual increments of individual facial bones are not evenly distributed among the various bony elements. Some grow faster one year and others grow faster the next year. However, diminished growth of any one bone is always compensated for by increased growth in neighboring bones and the steadiness of the overall matrix is preserved. Even after an experimental occlusal interference has caused a growth deformity, removal of the experimental occlusal interference is followed by compensatory growth which re-establishes symmetry. Similarly, after functional orthodontic appliances are used to alter growth, the treatment period is often followed by "catch up" growth that at least partially restores the previous facial growth pattern.
Looking at the stability of facial growth over time, Brodie remarked, "The only agent that could be responsible for such stability was the musculature that connected the mandible with surrounding parts. It was apparent that this musculature grew in the same orderly fashion as did the bony skeleton of the head and led to a stable relationship of the mandible in each person." The stability of a typical craniofacial growth pattern can be seen below.
INTRAMATRIX FACIAL GROWTH
Within the steady craniofacial growth matrix, a surprising amount of growth occurs in and around the jawbones in order to compensate for occlusal wear. Because this intramatrix growth is stimulated by the forces of mastication and occlusal wear is a result of mastication, the amount of intramatrix growth that occurred was proportional to the rate of occlusal wear, and intramatrix growth occurred as it was needed. As a result, intramatrix growth was effective at compensating for occlusal wear. In addition, both intramatrix growth and occlusal wear continued throughout life.
This intramatrix growth involved primarily expansion of the maxilla and translation and forward rotation of the mandibular corpus.
EXPANSION OF THE MAXILLA
The first facial growth process to slow to adult levels is the expansion of the maxilla. The maxilla is comprised of two paired membrane bones (right and left maxillae) that connect anteriorly at the pre-maxilla and superiorly along the midpalatal suture. Mastication spreads these two maxillary bones apart by swinging them out around both of these connections. As a result, the shape of the maxilla is altered by the pounding it receives. It responds to masticatory forces by widening.
The rotation of the two maxillae around an axis through the midpalatal suture can be seen from left to right in the illustration below. As chewing forces drive the lateral components of the maxillary bones upward and outward around their midline connection, they flatten the roof of the palate. In our ancestors, strong chewers had flat wide palates.
The rotation of the two maxillae around their anterior connection can be seen from left to right in the illustration below. In the presence of vigorous chewing, there is more swinging out of the paired membrane bones around the pre-maxilla.
Both of these rotations between the two upper jaw bones serve to enlarge the nasal airway by widening the palate which forms the floor of the nasal airway, as well as the buttresses which carry masticatory forces around the nasal airway. Because both of these maxillary rotations are at least partly motivated by jaw muscle forces, facial width was highly correlated to overall muscle strength and particularly to jaw muscle size in our ancestors. Pre-industrial humans who were strong chewers developed wide flat midfacial structures (including small zygomaxillary angles, large nasomalar and zygomaxillary angles, and flared zygomas), while pre-industrial humans with relatively weak jaw muscles developed narrower and vertically longer midfacial structures.
Functional forces limit maxillary growth vertically and expand maxillary growth horizontally. The effect is much like pushing downward on a lump of clay so it becomes wider. A few decades ago, people who used a Milwaukee brace to push the head up and back to align the spine found that the forces directed superiorly onto the maxilla from the mandible caused extreme expansion of the maxilla and a splaying of all the maxillary teeth.
The expansion of the maxilla also served to enlarge the nasal airway on its superior border. As the structural components of the maxilla and its buttresses shift laterally, their medial aspects resorb, leaving room for expansion of the sinus and the nasal airway.
ANTERIOR TRANSLATION OF THE MANDIBULAR CORPUS
The second facial growth process to slow to adult levels is anterior translation of the mandibular corpus. The rapid phase of anterior mandibular corpus translation continues at least 2 years longer than the rapid phase of maxillary expansion. The core of the mandibular corpus, which is comprised of the inferior alveolar neurovascular bundle and the surrounding bony canal, (shown in black below) maintains its shape while shifting its position anteriorly due to growth at the front of the rami just behind it functioning like elongating handles.
These elongating handles push the mandibular corpus downward and forward in front of them, as seen below.
As the mandibular corpus shifts anteriorly, it increases the space available in the pharyngeal airway. The corpus surrounds the airway in front and on both sides.
Also, in pre-industrial humans, the continuous anterior translation of the mandibular corpus helped to compensate for continuous occlusal wear by continually bringing the mandibular arch forward into the maxillary arch surrounding it. The gradual anterior translation of the corpus relative to the maxillary occlusal table usually eliminated overbite and overjet by early adulthood. Because this anterior translation of the corpus was at least partly driven by functional stimuli, pre-industrial humans with relatively strong chewing activity and strong jaw muscles experienced more anterior translation of the corpus and faster elimination of overjet and overbite.
The coincidence of anterior translation of the mandibular corpus and circumferential expansion of the maxillary occlusal table kept the lines of maxillary and mandibular teeth in close proximity. As the mandibular dental arch was drawn anteriorly by the translation of the corpus, a wider portion of the mandibular dental arch came to lie under the same area of the maxillary dental arch, which was simultaneously widening by direct expansion. The anterior translation of the corpus relative to the maxillary occlusal table thus brought the mandibular buccal cusp tips up the posteriorly and medially facing inclines of the buccal cusps of the maxillary teeth, even as expansion of the maxillary bite table moved the maxillary teeth further laterally. In such a manner, the mandibular molars shifted anteriorly relative to the maxillary molars while the maxillary molars moved laterally relative to the mandibular molars. The net effect was to keep the maxillary and mandibular buccal segments in close proximity for efficient chewing while occlusal slippage allowed them to move in slightly different directions.
FORWARD ROTATION OF THE MANDIBULAR CORPUS
While the mandibular corpus shifts forward under the rest of the face, it also rotates forward, carrying the roots of the lower teeth upward and into the occlusal table while maintaining the height of the ramus. This forward growth rotation was a normal feature in our pre-industrial ancestors. It occurred faster in the presence of relatively strong jaw muscles and slower in the presence of relatively weak jaw muscles. Because of the solid stop at the anterior end of the occlusal table, the center of the rotation is usually located near the front of the dentition. A similar growth rotation can be seen in other primates.
The rotation of the corpus can be seen most clearly relative to the two rami, which are stable architectural landmarks, because their positions are controlled by the steady postural tensions of the temporalis muscles. The extreme forward rotation of the corpus that occurred in ancestral humans with relatively strong muscles resulted in a sharper gonial angle and thus faces that were shorter in front, while the less extreme forward rotation of the corpus which occurred in ancestral humans with relatively weak jaw muscles resulted in more obtuse gonial angles and thus faces that were longer in front. Men, who have stronger average bite forces than women, have more acute gonial angles.
The same relationship between gonial angle and jaw elevator muscle strength can be found within a single individual. Longitudinal studies have shown that the gonial angle is obtuse in youth when chewing forces are still small, it becomes more acute with adulthood and increasing jaw muscle strength, and it becomes more obtuse again later in life when the jaw muscles become weak from the effects of aging.
In ancestral humans, forward rotation of the mandible helped to compensate for occlusal wear by continually bringing the mandibular dental arch further up into the maxillary dental arch. Much like in the case of anterior translation, because forward rotation of the corpus was at least partly driven by functional stimuli, pre-industrial humans with relatively strong chewing activity and strong jaw muscles experienced more forward rotation of the corpus and thereby more growth to compensate for continual occlusal wear.
Human facial growth was designed to ensure that the tooth containing portions of the upper and lower jaws maintained their parallel alignment and close proximity, the mandible grew forward sufficiently to make room for the pharyngeal airway, and the midface expanded sufficiently to make room for the nasal airway. The facial growth processes were designed to respond to functional stimulation in a manner that customized each masticatory system to best fit the functional demands required of it - even in the presence of extreme mismatches among facial features due to genetic diversity, injury to one or more components, or extremes of use and tooth wear. However, human facial growth was not designed to deal with a situation that was almost never encountered in evolution - insufficient functional stimulation.
CH 4) GROWTH OF THE MODERN HUMAN MASTICATORY SYSTEM
Within the last couple of centuries, in a development which was extremely sudden by evolutionary standards, the industrialization of our food made it so soft that the masticatory system stopped receiving adequate functional stimulation to coordinate and regulate facial growth. The human diet had been softening ever since the use of fire for cooking and softened further in the transition from hunting to farming. Then, with the rapid spread of industrialization in the nineteenth and twentieth centuries, food became so soft that it no longer provides a coherent resistant food bolus on which the mandible can pivot during power-crushing. The resulting change in functional mandibular range of motion and in jaw muscle development has had surprisingly important impacts on facial growth, and the resulting change in facial growth has had surprisingly important impacts on our health.
LOSS OF JAW MUSCLE STRENGTH
Softening of our diet has weakened our jaw muscles by at least half. Without tough resistant foods to stimulate the ideal type of exercise the jaw muscles get from healthy functional chewing, our jaw muscles no longer develop to nearly the extent they did in our ancestors. Bite forces no longer increase rapidly with age after the primary dentition. Similar losses of jaw muscle strength can be produced by raising various species of animals on soft food diets.
LOSS OF OCCLUSAL STABILITY
Accompanying the loss of jaw muscle strength has been a loss of occlusal stability. These two variables, jaw muscle strength and occlusal stability, are highly correlated in modern population studies. Even within individual masticatory systems, the jaw muscles are stronger on the side of the more stable posterior occlusion.
Pre-industrial humans always had stable occlusions. Their occlusal surfaces had large areas of opposing facets that fit almost perfectly throughout the functional range of movement of the mandible. Their jaw muscle strength was proportional to overall muscle strength.
In contrast, modern humans have both variable occlusal stability and variable jaw muscle strength. Their jaw muscle strength is better correlated with their occlusal stability than with their ability to build muscle tissue where it is needed.
Even modern humans with stable dental occlusions generally have an occlusal surface comprised of small facets that only fit simultaneously when the mandible is braced within a small central area. The occlusal interface has become a surface with irregular jagged peaks and valleys instead of a smoothly sloping bowl-like shape. An illustration of a typical modern occlusal interface can be seen below:
Occlusal instability can contribute to the loss of jaw muscle strength by inhibiting jaw muscle development due to frequent triggering of reflex protective mechanisms designed to prevent traumatic contacts between teeth. An irregular occlusal interface is a difficult template for the jaw elevator muscles to exercise against, because it continually activates protective reflexes that limit jaw muscle forces. The masticatory system functions more carefully, somewhat like the way your leg muscles would function when walking barefoot on gravel. Studies have shown that restrictive occlusal interferences inhibit jaw closing forces and cause narrowing of the range of movement of the mandible, while elimination of occlusal interferences leads to increased bite forces.
Conversely, jaw muscle weakness can cause occlusal instability by limiting the forces that maintain the alignment of the occlusal surfaces of the teeth. The bracing and masticatory forces of the jaw muscles produce and maintain occlusal stability. Diminishing those forces diminishes occlusal stability. Studies have shown that baboons raised on soft food have decreased occlusal stability.
Additional contributers to modern occlusal instability include the increased prevalence of caries and periodontal disease that have closely followed the spread of industrialized foods around the world. Caries can cause drifting of teeth by removing tooth structure that was maintaining interproximal contacts or occlusal contacts. Subsequent fillings can restore lost tooth structure, but they are rarely able to reverse any misalignment which may have occurred. Periodontal disease can cause drifting of teeth by changing the bony support around their roots.
NARROWING OF THE MANDIBULAR RANGE OF MOVEMENT
Softening of our diet has also narrowed our mandibular range of motion. Chewing pathways automatically widen in response to tough foods, while they stay close to the midline for simple mashing of softer foods. 93 Pre-industrial humans ripped, tore, and crushed food with the mandible operating in long gliding strokes that pass over the intercuspal area and follow through to the non-working side; while modern mastication involves primarily mashing the bolus by squeezing it between the teeth, while the mandible stays close to the midline and often stops for about 100 msec before the next cycle begins. The average change in masticatory pattern can be seen in the illustration below comparing an Australian aborigine chewing pattern (on top) with a modern european chewing pattern (below).
The dramatic change that has occurred in chewing pathways during the last couple of centuries can also be seen by comparing frontal tracings of the mandible during chewing in modern adults (left below) with those of young and middle aged Australian aborigines (center and right below). The chewing pathways of young aborigines are already flatter than those of modern adults, and adult aborigine chewing pathways are much flatter still. While the contact glide in Australian aborigines is about 3 to 4 mm long, the contact glide in modern Europeans is only about 1 mm long.94
The dramatic change that has occurred in chewing pathways during the last couple of centuries in a sagittal plane could be illustrated in Posselt's curve of the mandibular range of motion by drawing it with curves rather than sharp line angles at the superior surface of the functional range of motion, such as shown below:
NEWLY ERUPTED MATURE ELDERLY
VERTICALIZATION OF MASTICATORY SYSTEM COMPONENTS
The narrowing of the functional mandibular range of motion has caused a verticalization of the articular components of the masticatory system - a good example of function dictating form.
The verticalization of the TMJs can be seen in deeper glenoid fossae and steeper articular eminentia. Studies of rabbits raised on soft food show similar changes in the contours of the TMJs.
The verticalization of the occlusal table can be seen in the deep interdigitation and steep curves of Wilson and Spee which are now maintained throughout life instead of flattening with age as they did in our pre-industrial ancestors. Masticatory forces have so little influence on teeth positions that the contours of the occlusal table are now more strongly determined by the angles at which teeth erupt than by the functional range of motion of the mandible; even though those eruption paths were designed to continually supply tooth structure at the occlusal table – not to determine the functional range of motion of the mandible. When the functional forces are too weak to overcome the resistance provided by a form designed for aligning the newly erupting teeth, form dictates function.
The verticalization of the occlusal table has caused a relative locking together (partial synostosis) of the maxillo-mandibular suture. The translation of the mandibular corpus is restricted by connecting it to a maxilla that expands rather than translates, and the expansion of the maxilla is restricted by connecting it to a mandibular corpus that cannot expand but can only translate. Experimental synostosis of craniofacial sutures in animals restricts growth of the most directly involved bones and disturbs the growth pattern in the whole region. 95 A partial synostosis of the maxillo-mandibular suture may have similar effects.
RESTRICTED INTRAMATRIX GROWTH
The partial synostosis of the maxillo-mandibular suture and the loss of jaw muscle functional forces have acted synergistically to produce many of the same changes in craniofacial form. At times it may be difficult to determine which is cause and which is effect. Even in otherwise well controlled animal experiments, the effects of weaker chewing forces and a narrowed mandibular range of motion cannot be separated, because the animals automatically narrow the mandibular range of motion in response to the softening of the food that also evokes less forceful jaw muscle use and less jaw muscle development. Together, the partial synostosis of the maxillo-mandibular suture and the loss of jaw muscle functional forces have caused a significant change in the average shape acquired by the lateral and lower portions of the face. Comparing pre-industrial human skulls with modern human skulls makes them look like two different species.
To quantify the change in the shape of the modern human face, it must be seen against the background of other craniofacial changes that have also occurred during the same time period. For that reason, all these changes will be discussed below.
ROUNDING OF THE CRANIUM
The expansion of the neurocranium occurs about 90% prenatally, therefore most of its growth is unaffected by its functional environment. However, the average weakening of skeletal muscles which has gradually accompanied our change to a less physically demanding life style has had some effect on postnatal neurocranial expansion and thereby has slightly affected the shape of the cranial vault. On average, the cranial vault has become slightly rounder as a result of a growth pattern that is influenced less by muscle forces and more by the circumferential expansion of the brain. Long skulls have become shorter, and wide skulls have become narrower, with both dolichocephalics and brachycephalics normalizing to become more mesocephalic, much like infant skulls which have not yet been influenced by the pulls of the musculoskeletal system.96 97 98 99 100
INCREASED CARTILAGINOUS ELONGATION
At the same time, the rate and extent of elongation at cartilaginous growth centers has increased. The cause is thought to be an effect of increased sugar and carbohydrate consumption on growth hormone production. 101 102 103 104 105 106 107 108 109 110 111 112 113 Increased cartilage growth in the vertebrae has caused an average increase in overall height within a generation or two. The cranial base is an extension of the vertebral column into the cranium and has therefore affected craniofacial growth. Increased cartilage growth in the posterior cranial base has caused an increase in average vertical facial height by pulling the mandible further downward relative to the rest of the cranium. Increased cartilage growth in the anterior cranial base has caused an increased protrusion of the center of the face by pushing the nose and the medial portion of the infraorbital shelf further anteriorly relative to the rest of the cranium.
REDUCED LATERAL AND LOWER FACIAL GROWTH
Alongside the slightly increased protrusive and vertical growth in the center of the face and beneath the slightly rounder cranium, the lateral portions of the midface and the entire lower face have changed growth direction. On average these areas now grow more downward and less forward, especially at the front of the face. The intramatrix growth processes involving expansion of the maxilla, anterior translation of the mandibular corpus, and forward rotation of the mandibular corpus have been inhibited. Much of the inhibited facial growth has been redirected vertically. Similar changes in facial growth have been produced experimentally in rats, monkeys, rabbits, hyraxes, and minipigs by raising them on soft diets. Similar changes have also been recorded in humans with muscle disease.
DECREASED MAXILLARY EXPANSION
One of the most obvious effects of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been a decrease in the rate and extent of maxillary expansion. As a result, the average modern palate is much narrower than the average palate of our ancestors.123 Similar maxillary narrowing has also been produced in animals raised on soft diets124 125 126 , and in humans without masseter and pterygoid muscles.127 Monkeys raised on soft diets often develop crowding of the upper teeth much like that frequently seen in modern children.128
The decreased maxillary expansion seen when tribal people adopt a western diet has been noted for nearly a century. Anthropologists, such as Weston-Price illustrated the sudden narrowing of the palate that occurred when aborigines moved into settlements. Sir Arthur Keith observed, "Misplacements of the teeth, long narrow dental arches, high vaulted palates, and carious teeth, which are so common among Englishmen of today, were almost unknown amongst the British people of the Neolithic and Early Bronze periods; these conditions make a sporadic appearance as the Roman period is approached, becoming more frequent in this period. They are conditions which are rarely seen amongst the remains from Saxon graveyards. Indeed they do not assume anything approaching their present frequency until the eighteenth century is reached and England entered upon her life of industrialism."
The diminished maxillary expansion has affected the zygomatic processes that buttress the lateral portions of the maxilla against the cranium. They rotate more down than out, causing the appearance of “sunken” cheekbones that many people have observed when comparing modern humans to pre-industrial humans.
The diminished maxillary expansion has also diminished the widening of the nasal airway. The roof of the mouth is the floor of the nose. A narrower base to the triangular nasal cavity means less space available for airway passage between the bony pillars that form the lateral borders of the nasal cavity.
DECREASED PROTRUSION OF THE CORPUS
A second effect of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been a decrease in the rate and extent of anterior translation of the mandibular corpus. A comparison of late medieval and recent Finns (minimizing genetic mixing) showed a 6% decrase in mandibular length despite overall skull size increases.(Varrela J. Dimensional variation of craniofacial structures in relation to changing masticatory functional demands. Eur J Orthod 1992;14:31-36.) Similar decreases in size of the corpus and retrusion of the posture of the corpus also occur in patients with injury or disease that weakens the craniofacial muscles.
While prognathism was positively associated with jaw muscle strength in pre-industrial humans, it is only positively correlated with jaw muscle strength in modern humans when jaw muscle strength is high enough to exert a significant effect on growth. In many people there is so little anterior facial translation produced by intramatrix facial growth that total facial prognathism is determined more by cranial base shape than by functional forces or jaw muscle strength. When jaw muscle strength is so low that it doesn't stimulate anterior translation of the corpus, the position and posture of the corpus is determined primarily by the resting tensions in the surrounding myofascial curtains rather than by stimulation of intramatrix growth processes.
As a result of the retrusion of the mandibular corpus, class two malocclusions have become much more common today. In pre-industrial humans, class two malocclusions comprised only about ten percent of the population. In modern humans they comprise twice as many, and many people with class one malocclusions have both upper and lower jawbones in a retrusive position relative to the cranium. Studies comparing modern and ancestral populations of Japanese 114 , Egyptians, and Americans 115 have shown that the modern skulls have more retrusive midfaces than those of the recent past.116 Autopsy studies have also shown that tissue damage and regressive remodeling commonly occurs at the posterior aspects of the condyles, and progressive remodeling commonly occurs on the anterior aspects of the condyles.117 118 A similar pattern of remodeling has been produced in the TMJs of animals by using inclined planes or chin cups to retrude the corpus.
A MORE BACKWARD FACIAL ROTATION
A third effect of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been an average change in the direction of rotation of the mandibular corpus. Generally faces no longer rotate so strongly forward, and some rotate backward, a situation never seen in pre-industrial humans. The role of masticatory forces in the changed pattern of facial rotation has been demonstrated by a study in which children with backward rotating faces underwent a change in growth direction to a forward rotation while chewing exercise gum and then returned to backward facial rotation after they stopped chewing the exercise gum. 119 With the roots of the mandibular incisors carried down and back by their basal bone, their crowns don't upright as readily as they did previously. The long axes of upper and lower incisors form a smaller angle than in they did in pre-industrial humans.
Since much of the growth rotation of the mandible is masked by remodeling at the borders of the corpus, the change in growth direction of the mandibular corpus can be most easily seen relative to the mandibular ramus. The orientation of the ramus is controlled by the airway, and it maintains a relative stability during growth. The gonial angle, where the backwardly rotating corpus meets the very stable ramus, has become more obtuse. 120 At the border between these two divergently growing regions, just anterior to the gonial angle, it's become common to find an antegonial notch.
When remodeling at the gonial angle is not able to fully absorb the backward rotation of the mandibular corpus, that backward rotation may affect the whole mandible and can rotate the ramus and condyle around a pivot location somewhere in the molar area and thereby drive the anterior aspect of the condyle into the the eminence. One effect may be regressive remodeling on the anterior aspect of the condyle. If this adaptive growth occurs slowly, it may produce a hooked condyle shape. If it occurs rapidly, it may produce a condition that has been labeled idiopathic condylar resorption.
The facial shelves, in proportion to their distance from the cranial base, have followed the rotation of the corpus and fanned out anteriorly more than they used to. Even the angle of the cranial base has been affected, becoming slightly more acute in modern humans as the front portion of the cranial base has rotated backward following the rest of the face.121 122
VERTICAL INCREASES AT THE FRONT OF THE FACE
Much of the facial growth which is restricted horizontally has been redirected vertically, especially at the front of the face. Loss of mandibular elevator muscle forces has been shown to lead to dramatic increases in the vertical dimensions of the anterior face - whether the loss is caused by disease which impairs muscle development129, cutting or removing muscles or motor nerves, or natural trauma. When experimental impairment of the mandibular elevator muscles is carried out unilaterally in animals, increased dental height occurs on the side of impairment. In population studies, the height of the front of the face is inversely proportional to jaw muscle strength.130 131 In experimental studies, adolescents with a facial growth pattern charactertized by excessive vertical growth experienced improved their facial growth patterns when using an exercise gum to increase jaw muscle strength.( Ingervall B, Bitsanis E. A pilot study on the effect of masticatory muscle training on facial growth in long-face children. Eur J Orthodont 1987;9:15-23) Orthodontists have long recognized that many of the problems in their patients today are due to excess height at the front of the face.132
The maxilla follows the verticalized growth of the corpus. One result is an increase in the distance from the incisal edges of the upper central incisors to the nasal floor. Gummy smiles, never seen in pictures of tribal peoples, have become common. In some modern faces, the framework of bones and teeth has become so long that it impinges on facial muscle resting lengths. The perioral muscles may show visible strain when trying to maintain a lip seal, the freeway space may be obliterated, and the passive tension of stretched masseters may contribute to the narrowing of the maxillary arch.
The vertical increases at the front of the face now continue significantly during adulthood. While our pre-industrial ancestors had facial heights which remained steady during adulthood, many modern faces keep getting longer at a rate which averages .37 mm per year in the third and fourth decades of life. 133
One contributor to the increase in height at the front of the face may be the eruption force of the teeth and the surrounding alveolar bone. If their eruptive force is greater than the axial pressure on teeth produced by the jaw closing muscle forces that naturally limit tooth eruption, the teeth can supererupt – elongating the whole framework of bones and teeth at the front of the face. It's interesting that the rate at which teeth continually wore down and erupted in some pre-industrial cultures is very similar to the rate at which faces now continuously lengthen in modern post-industrial cultures where teeth no longer wear down significantly.
AVERAGE OVERALL CHANGE IN FACIAL MORPHOLOGY
The changed pattern of intramatrix growth is only partially absorbed by remodeling at the interfaces between the intramatrix growth processes and the rest of the craniofacial skeleton. As a result of the intramatrix facial growth that is not masked by remodeling, modern human faces grow, on average, longer vertically, narrower, and more retrusively than they did just a couple of centuries ago. This average change in facial shape has not affected everyone the same way, and it is not easy to quantify against a background of genetic mixing and normal variation which sometimes dwarf it. However, when large samples are compared and genetic mixing is minimized; the same tendency toward longer, narrower, and more retrusive facial features has occurred in all racial groups in all parts of the industrialized world.
The average change in profile can be seen in a superposition of the facial shape of modern Swedes (dotted lines) with those of Australian Aborigines (straight lines) in a sagittal plane as shown below.
The average change can also be seen in the increased convexity of the modern midface, as shown on the left below.
With the face becoming longer, narrower and more retrognathic - even in those with strong skeletal muscles and short wide cranial vaults, mismatches may be produced between the shapes of the vault and the face. Pre-industrial humans with strong overall musculature had shorter and wider vaults (brachycephalic) with shorter and wider faces (euryprosopic), and pre-industrial humans with weaker overall musculature had longer vaults (dolichocephalic) with longer faces (leptoprosopic). The correlation was so consistent that some researchers even refer to short wide faces as brachyfacial and to long narrow faces as dolichofacial. However, today some people with strong musculature may have relatively long narrow faces, because they failed to develop the jaw muscles as much as their other muscles. Thus, unlike in skeletal remains, leptoprosopic faces can be found on brachycephalic crania.
AVERAGE OVERALL CHANGE IN HEAD POSTURE
Accompanying the average change in facial shape has been a change in the average posture of the head in a sagittal plane. As the mandibular corpus has shifted posteriorly relative to the long axis of the spine, the center of mass of the head in normal standing posture has shifted anteriorly relative to the long axis of the spine. Forward posture of the head and backward posture of the mandible are highly correlated in population studies.
Forward head posture and backward mandibular posture are synergistic - either one can cause the other. Forward head posture can cause backward mandibular posture by stretching the muscles and fascia that attach the mandible to the clavicles and sternum and thereby preventing the mandible from shifting as far forward as the head. 134 Backward mandibular posture can cause forward head posture by evoking adaptations to protect the airway. The muscles of the head and neck are controlled by a hierarchy of neuromuscular reflexes, and airway protection is at the top of that hierarchy. All the muscles of the area acquire whatever resting postures are needed to hold the bones in whatever positions they need to maintain an adequate airway. Because the mandible surrounds the airway on three sides and the cervical spine borders its fourth side, backward mandibular posture can constrict the airway between the mandible and the cervical spine (middle illustration below) and thereby trigger these airway protective reflexes. The muscles respond by extending the head in order to rotate the mandible forward away from the cervical spine and out of the airway space.
However the head cannot just tip backward, because the visual orientation reflex keeps it level with the horizon. This reflex was able to bend the entire cranium in mice forced to live standing on their hind legs, and it causes extension of the head to maintain a useful visual field in spite of the drooping eyelids in people with palpebral ptosis. In people with mandibular retrusion, it causes extension of the head to be accompanied with forward shifting of the head in order to pull upward and forward on the mandible while maintaining its line of sight, as seen in the illustration on the right below.
While the causal relationship between backward mandibular posture and forward head posture can go both ways, there is good evidence that backward mandibular posture usually comes first. Growth studies have found much stronger associations between mandibular growth and subsequent body posture than between body posture and subsequent mandibular growth.140 141 142 The ability of backward mandibular posture to cause forward head posture can be seen in natural experiments when children who undergo TMJ ankylosis or other injuries or defects that prevent the mandible from translating forward with the rest of the face during growth acquire extreme forward head posture simply as a result of the extreme backward mandibular posture. The ability of pharyngeal airway blockage to cause extension of head posture has been demonstrated by studies which showed that many children with swollen tonsils have extended head posture135, which reverses quickly after surgery to remove the swollen tonsils.136 The ability of extension of the head to move the hyoid bone anteriorly137 and thereby increase pharyngeal airway space has been demonstrated by imaging.138 139
Backward mandibular posture can be caused by a retrognathic or distalizing dental occlusion because, within the boundaries needed to maintain an adequate airway, one of the most important determinants of mandibular posture is the location of the habitual mandibular bracing area (HMBA). The HMBA provides a home base for the masticatory system. The jaw muscles are programmed to brace the mandible there by clamping it up against the underside of the cranium immediately whenever danger is detected, when the postural system needs stabilizing for applying external forces, and at the beginning of each swallow. Because bracing of the mandible is so central to the function of the masticatory system, the jaw muscles are programmed to hold the mandible in a postural position located just below its HMBA in order to maintain fast easy access to that bracing.
The accomodation of mandibular posture to the location of the HMBA has been demonstrated in different planes. Vertically, an immediate increase in freeway space follows the first occlusal contact after placement of a bite raising appliance, and an immediate return to the pre-treatment freeway space follows the first occlusal contact after removal of the bite raising appliance.13 Laterally, children who develop unilateral cross-bite undergo a shifting of the mandible in both bracing and postural positions to the side of the cross-bite motivated by an increased resting tension in the posterior temporalis on the side of the cross-bite14 15 ; and their jaw muscle resting tensions return to symmetry after correction of the cross-bite.16 Antero-posteriorly, monkeys who experimentally receive a protrusive occlusal interference exhibit an immediate increase in the tonus of the ipsilateral superior lateral pterygoid muscle.17
THE INFLUENCE OF PERSONALITY
The way an individual craniofacial structure is affected by restricted intramatrix facial growth depends partly on the way that individual’s neuromuscular system responds to stresses and growth strains, and the response of each individual’s neuromuscular system depends partly on personality. Because the end plates of the motor nerves are anatomically and physiologically extensions of the brain, the state of tension in the skeletal muscles directly reflects the electrical state of the brain. Therefore personality affects muscle resting postures which affect growth. Studies have shown that individuals have relatively consistent, unique, physiological response patterns to a variety of stressors. For example, a "muscle responder" will respond repeatedly with tension in the same set of muscles to a wide range of emotional stimuli. Similarly provocation studies found that different types of people have different responses to a the same bite change. Some people seem to compulsively focus on an experimentally placed bite interference and develop a habit of grinding against it, thereby increasing their mandibular elevator muscle activity. Other people given the same bite interference will avoid it by decreasing their mandibular elevator muscle activity – even learning to swallow without touching teeth by inserting the tongue between the teeth to stabilize the mandible at the beginning of each swallow.
Because of the influences of personality, a different modification of the growth process occurs in each different personality type. The two most distinct personality responses, the aggressive responders and the passive responders, are illustrated below:
Aggressive responders seem to react to a growth restriction by fighting against it. They may develop unusually strong parafunctional habits (clenching or grinding) which can significantly alter the pattern of late craniofacial growth. Often they are able to limit verticalization of the anterior facial skeleton and maintain a forwardly rotating facial growth pattern, as shown on the left in the illustration above.
However these people often develop a deep overbite, because their anterior teeth do not provide a stable incisal platform against which the mandibular elevator muscles can brace and on which the mandibular corpus can rotate during growth. Instead, the mandibular corpus rotates around a center closer to the molar area. The posterior teeth may become intruded while the anterior teeth become extruded, leading to a deep overbite and a bilevel occlusal table in which the maxillary and mandibular anterior teeth are noticeably taller than the posterior teeth.
In some cases aggressive responders establish a dual bite, characterized by two distinct bracing positions for the mandible. In one bracing position, the mandible is locked back at the posterior end of its normal range of motion. In the other bracing position, the mandible rests further down and forward against an alternative occlusal platform. The presence of this second bracing position seems to protect the system, because it is not well associated with the presence of symptoms.
Bruxing is not isometric like clenching, and may even be rhythmic like chewing, therefore it is generally a healthier form of exercise than clenching. In addition, bruxing may occur with sufficient force to eliminate at least some aspects of the maxillo-mandibular synostosis. However, in many people with relatively deep overbites, bruxing only delivers significant axially directed bite forces to the posterior teeth, and it intrudes or reduces only the posterior segments of the dentition. Even when the bruxism does include the anterior and posterior teeth in a healthy proportion, the pattern of mandibular movements that gets worn in generally lacks the variability produced by healthy chewing function in which the mandible pivots around a resistant bolus.
Passive responders seem to react to a facial growth restriction by avoiding it as much as possible. A strained bite is usually avoided by lowering the posture of the mandible sufficiently to avoid frequent tooth contacts. The retrusive effect of the maxillo-mandibular synostosis may be minimized in the short term, but in the long term compensatory vertical growth is evoked by the loss of jaw muscle development and maxillary width is limited by the medially directed pressure of passively stretched masseter muscles. As a result, the face rotates backward, as shown in the right side of the illustration above. The face may grow extremely long anteriorly, with lips that are open at rest because they cannot stretch far enough to cover the framework of bones and teeth at the front of the mouth.
ADAPTIVE TONGUE POSTURES
Some of the responses to inhibited facial growth involve aberrant tongue postures. For example, the tongue may respond to airway demands by acquiring a posture very low in the mouth, as can be seen in some of the monkey experiments with blocked airways. Alternatively, the tongue may acquire a resting posture interposed between the teeth in order to provide a cushioned platform against which the mandible can rest. This results in the tongue scalloping that is frequently seen in people with narrow palates or TMJ disorders.
LOSS OF GROWTH COORDINATION
The loss of jaw muscle strength has also disrupted some of the regulatory mechanisms that are needed to ensure coordination of facial growth processes. These regulatory mechanisms relied on the jaw muscles to provide stable symmetrical restings forces which shape bones and jaw muscle functional forces to symmetrically stimulate the diverse growth processes in upper and lower jaws. Studies have shown that people with weaker jaw muscles show greater interindividual variation in their vertical facial dimensions.(Kiliaridis S. The importance of masticatory muscle function in dentofacial growth. 2006 Elsevier Inc.) Even mandibular movement pathways have become more irregular. In Aborigines chewing, opening and closing movements rarely cross, while in modern humans they often cross.143
A result of this increased irregularity and decreased muscle symmetry is that faces grow, on average, significantly more asymmetrically and irregularly. Animals raised on soft diets develop less symmetrical craniofacial structures than normals, and anthropologists have noted that symmetry of the craniofacial skeleton and the range of variation of several facial angles and dimensions have risen markedly in recent centuries. Weston Price observed that, in tribes still eating traditional diets, the people all looked like brothers and sisters; but when they switched to modern diets, they lost their resemblance.
Increased irregularity and asymmetry can be seen in the pattern of adaptive remodeling which occurs in the TMJs. While condylar remodeling in pre-industrial humans was generally consistent in direction and proportional to age, condylar remodeling in modern humans is more dependent on mechanical factors than on age and has become much more variable than it was in our ancestors.
Increased irregularity and asymmetry can also be seen in the prevalence of malocclusion, which has recently risen far beyond the 10 percent level found in earlier humans and in primates.144 Corruccini collected a great deal of cross-cultural bite data using a variety of native populations before and after adopting a modern diet and concluded that the increases in malocclusion occur directly in proportion to the change of diet. He noted, "Cross cultural data dispel the notion that considerable occlusal variation is inevitable or normal. Rather it is an aberrancy of modern urbanized populations. Furthermore, the transition from predominantly good to predominantly bad occlusion repeatedly occurs within one or two generations' time in these (and other) populations, weakening arguments that explain high malocclusion prevalence genetically. Cumulatively, over these study samples, there is no chance for consistent inbreeding, racial mixing, or genetic change accounting for the transition."145
As a result of the combination of lost growth coordination and average directional changes in craniofacial growth, there are now extreme variants in craniofacial structures, even among populations relatively sheltered from genetic mixing. Certain craniofacial skeletal structures that have become fairly common today are rarely or never found in skeletal material from pre-industrial societies.
CHAPTER 5) DYSFUNCTION OF THE MODERN HUMAN MASTICATORY SYSTEM
The change to a more vertical, retrusive, narrow, and asymmetrical facial growth pattern and the diminution of some regulatory facial growth mechanisms do not necessarily cause symptoms. Human facial growth is designed to be able to maintain functional capacity in spite of incorporating a wide variety of genetically varied facial components and after many different types of tooth and jaw injuries. Symptoms only occur when adaptation to the dysfunctional facial growth pattern is unable to establish and maintain adequate functional harmony in the masticatory system.
The disorder that comprises the symptoms that result from dysfunction of the masticatory system is best described as a functional disorder. A functional disorder is a medical condition that impairs the normal function of a bodily process, but where the body looks normal under examination. Unlike in a structural disorder or a psychosomatic disorder, a single cause of the symptoms often cannot be identified. The mechanical stimuli produced by normal function evoke responses that are not conducive to health, and a variety of tissues may be affected. Whatmore and Kohli describe how signaling errors can work to the detriment of the system. Germs and injuries can have additional detrimental effects, and they often act as the straw that broke the camel's back.
Unfortunately, in trying to establish a diagnosis, we too often blame the straw. Dental researchers trying to determine the cause of symptoms in TMJ disorders compare subtle changes in anatomy between those who report symptoms and those who don't report symptoms. In many of these cases, the differences between these two groups is not the particular type of facial growth strain but the diminished adaptive capacity in the symptomatic group.
THE ROLE OF ADAPTATION
Symptoms occur when adaptation fails to prevent damage to tissues. Almost any tissues can be affected. Usually the damage occurs in tissues that can be seen as a weak link, - an area that is particularly vulnerable to the effects of the dysfunction. In the masticatory system, this most commonly includes the muscles, joints, or dentition.
Because of the important role of adaptation in TMJ disorders, anything that diminishes adaptive capacity (such as stress) can increase symptoms, and anything that enhances adaptive capacity (including nutritional support, relaxation, aerobic exercise, etc,) can eliminate symptoms. Better enabling the patient to adapt to a strained jaw system can relieve the effects of the strain without ever eliminating the strain.
LOSS OF ADAPTIVE CAPACITY
Unfortunately, the same loss of jaw muscle strength and narrowing of the mandibular range of motion that have caused dysfunctional facial growth in the last couple of centuries have also caused a coincident loss of adaptive capacity. Many of the adaptive properties of the masticatory system rely at least partially the presence of strong rhythmic chewing forces to provide accessory circulation to the tissues undergoing adaptation. The long lever arm formed by the mandible functioned much like a pump handle driven up against the skull thousands of times each day. In the vascular and metabolically active retrodiskal plexus, condylar movements acted like a piston to pump waste products from the synovial tissues into venous circulation while allowing ingress of fresh oxygen and nutrients between each pumping action. 41 In the midfacial buttresses supporting the articular areas, functional forces bent membrane bones rhythmically back and forth. In tendons and ligaments, functional forces provided rhythmic tugs that stimulated the periosteum.
In the articular areas, long smooth chewing strokes with sufficient variability to provide alternate compression and release in all portions of the articular zone kept the avascular tissues supplied with oxygen and nutrients while helping them eliminate waste products by weeping circulation. Studies have shown that loss of weeping circulation due to insufficient jaw function leads to a thinning of the condylar cartilage 42, immobilizing a synovial joint produces atrophic degenerative changes in that joint characterized by reduced proteoglycan content and alteration of proteoglycan structure 43 44, and remobilizing an injured joint with continuous passive motion dramatically reduces the time needed for healing. 45 The loss of chewing forces in modern humans has very likely diminished some of that accessory circulation.
A loss of the rhythmic compressive loading across the facial sutures has decreased suture widths and increased suture ossification. Similar effects have been demonstrated experimentally by gluing sutures together, pinning them together with metal plates, or softening the diet. 146 Sutures are important sources of adaptive growth, and functional forces maintain the adaptability of sutures by keeping them open and metabolically active. Ossification and closure of sutures deprives these areas of their adaptive potential.
In the temporomandibular joints, diminished strength, range and variability of the functional movements of the mandible has caused a decrease in weeping circulation that may limit the potential for remodeling activity. Monkeys raised on soft diets have less dense fibrous tissue in the articular zone of the temporomandibular joints. One researcher points out, "Experimental studies in mice, rats, rabbits, and non-human primates have shown that mechanical loads are vital for maintaining normal growth, morphology, and function of the secondary cartilage of the temporomandibular joint... In vitro studies confirm that normal mechanical loading stimulates cell division, matrix synthesis, and enzyme activity in the tissues of the TMJ."
In the dentition, diminished functional forces and occlusal wear have both limited the ability of opposing teeth to shift relative positions and thereby accommodate the diverse growth patterns in the upper and jaws. Opposing teeth with deeply interdigitating occlusal surfaces cannot shift relative to each other as easily as opposing teeth with rounded or flattened occlusal surfaces. In addition, periodontium that receive inadequate functional forces may experience diminished accessory circulation by depriving them of the rhythmically alternating compressions and releases that have enough variability and range of motion to provide accessory circulation throughout the periodontal ligament spaces during chewing. In healthy masticatory function, the teeth are loaded primarily in an axial direction, with a small and variable range of movement horizontally. During each loading event, some fluids in the direction of compression were forced through the bony socket out of the surrounding periodontal ligament space and into venous circulation, followed by a subsequent rebound of the tooth which brought in new fluids from arterial circulation. Diminished function is likely to diminish such accessory circulation, and diminished accessory circulation in turn is likely to limit the ability of the teeth to adaptively shift positions.
THE ROLE OF STRESS
Stress is an important source of diminished adaptive capacity. For that reason, a stressful episode is a common trigger for TMJ disorder symptoms, and relaxation is frequently successful at eliminating the symptoms, at least temporarily. Diminishing overall stress has been shown so effective at reducing symptoms that a few decades ago Laskin and other researchers concluded that stress was the primary cause of TMJ disorders.
Stress also can trigger TMJ disorder symptoms by increasing the resting background tonus in the jaw muscles from a level at which they receive just enough resting circulation to a level at which they receive insufficient circulation to prevent cell damage. Stress increases background tonus in all of the body's muscles. If one group of muscles is already operating at bordeline resting circulatory capacity, even a slight elevation in its resting tension can cause it to become symptomatic by lowering resting circulation in at least some areas below a threshold level.
Because of the mass of jaw closing muscles dwarfs the mass of jaw opening muscles, stress can alter mandibular posture by holding the mandible further closed at rest - sometimes so far that the teeth maintain contact. In other parts of the body, stress does not alter the positions of bones, - it just holds them more tightly between equally strong muscles pulling in opposite directions. However, the jaw muscles are not similarly balanced between openers and closers. Because the powerful jaw closing muscles are all vertically arranged and dwarf the jaw opening muscles, an increase in stress holds the mandible further closed.
THE INCREASE IN CHRONIC STRESS
During the industrialization of our diet in the last couple of centuries, chronic stress has increased. Evolution equipped our bodies with reflex responses to deal with acute stress. When these reflex responses are triggered chronically, they are ineffective and often cause other health problems.
CHANGES WITH AGE
As a product of the interaction between two variables that change with age, adaptation and continuous facial growth; the natural course of a TMJ disorder shows age related trends. Facial growth continually produces mechanical stresses and strains which must be accomodated by adaptation mechanisms, and adaptation is continually trying to catch up with the effects of the dysfunctional facial growth. The rate at which adaptation wins that race determines the subsequent symptomatology.
There are few signs or symptoms of TMJ disorders during childhood when tremendous adaptive capacity prevents damage to tissues. Even in the presence of injuries or genetic defects that cause extreme structural abnormalities, the tissues grow in a manner that prevents localized damage. The signs and symptoms that occasionally appear are usually fleeting and seem to affect boys and girls about equally.
Symptoms generally begin to appear in the teenage years, especially in post-pubescent females. After puberty, female growth patterns and male growth patterns diverge, with females developing more of the tendency toward backward facial rotation, narrow midfaces, and retrusive mandibles. In one study, Behrents found that, in post-pubescent facial growth, the mandible grows more retrognathically in females. Relative to the rest of the cranium, the mandible grows straight downward in females, while it grows downward and forward in males, as shown in his illustration below.
Another study found that the same growth trend towards mandibular retrognathia on average characterizes teens who develop TMJ disorder signs and symptoms (dotted line) compared with normals (solid line) on the left below, teens who show evidence of degenerative osseous remodeling of one or both TMJs (dotted line) compared with normals (solid line) in the middle illustration below, and also the one of two identical twins who developed TMJ disorder symptoms (dotted line) compared to her sister (solid line) on the right below.147
The same retrognathic growth tendency can be seen in profile photographs of the two identical twins whose X-ray tracings are seen in the right side illustration above.
A similar growth trend characterizes modern european faces (dashed lines) when compared to aborigine faces (solid lines) as shown in the figure on the right below; and also modern myotonic dystrophy patients (dashed lines) when compared to normals (solid lines) as shown on the left and in the center below.
While there are distinct facial growth patterns that are more likely than others to lead to TMJ disorders, the occurrence of various symptoms fluctuates a lot in this rapidly growing population. In any group of teenagers, there will be a significant percentage who will report that they are currently suffering from TMJ disorder symptoms, but it may be different ones who are suffering in different years.
Distinct groups of chronic TMJ disorder patients appear after the second decade, primarily in women. Facial growth slows at this age to adult levels, but adaptive growth and other adaptive capabilities may slow even more. The adaptive systems are constantly trying to adapt to stresses and strains which are constantly being created by the slow dysfunctional facial growth pattern that continues through most of adulthood. Women continue to grow more retrognathically than men, and women continue to represent the vast majority of TMD disorder patients.
Most TMJ disorder patients initially develop symptoms as a result of the dislocation of the articular disk from its normal intra-articular position in at least one TMJ and the subsequent bruising of the vascular retrodiskal tissues which got pulled into the joint space following the disk. The loss of the disk from the articular zone deprives the involved TMJ of cushioning and lubrication, making it susceptible to damage by triggering events such as minor trauma, a period of increased central nervous system stress, a long dental appointment, or destabilization of the bite. Even normal jaw function can damage the vulnerable retrodiskal tissues. Eventually the soft tissues of the articular eminence thicken to provide cushioning and adaptation of the retrodiskal tissues creates a pseudo-disk that can restore functional capacity and reduce or eliminate symptoms, but the internally deranged TMJ remains more vulnerable to injury than a normal TMJ.
The jaw muscles become involved, because muscles are responding organs. In response to an inflamed joint, they automatically acquire a state best described as protective bracing. Reflex protective bracing changes jaw muscle behavior in three ways. 1) It causes increased resting tension, because the muscles hold themselves tightly at rest as if they are constantly on guard. 2) It causes decreased functional tensions, because the muscles fire weakly during function as if they are afraid of damaging the articular structures. 3) It causes overlap of firing activity (co-contraction) of jaw opening and closing muscles because the muscles work against each other in order to more tightly control mandibular movements.
This protective bracing state was designed by evolution to help us cope with acute conditions. When maintained chronically, it can contribute to self-sustaining cycles of tissue damage and muscle tension. Over time, muscles that are held tight often undergo contracture and develop trigger points. Thus many of the symptoms found in TMJ disorders at this stage are most directly produced by the muscles reacting to the joint conditions.
At midlife and beyond, the symptoms dissipate due to a decrease in muscle reactivity. Arthrokinetic reflexes play key roles in maintaining the cycles of pain and dysfunction (tissue damage and muscles tensions) that often perpetuate TMJ disorders. With age these reflexes become less easily triggered and their muscle tightening becomes less intense. As a result, while signs of TMJ disorders such as joints noises and degenerative changes on imaging usually increase in severity, symptoms of TMJ disorders usually disappear during the same time period. The body may have been designed to accept some arthritic degeneration in old age. The TMJ disorder symptoms that occasionally arise in midlife are usually brief and less myogenous than the symptoms found in younger people.
Finally, although their TMJs keep undergoing more arthritic damage every year as seen on imaging like X-rays or MRI, the TMJ disorder symptoms almost completely disappear in the elderly. They may have difficulty with mechanical operation of the joints, shifting of the bite, occasional pain, and some ear problems like dizziness or difficulty hearing, but those symptoms almost always just need minimal treatment to resolve. There are also occasionally neurologic symptoms such as tics, neuralgias, and orofacial dyskinesia and also otic symptoms such as tinnitus and hearing loss. In most cases, if dysfunction of the masticatory system is producing symptoms, the dysfunction involves unusual stressors such as extreme loss of vertical dimension in dentures or tooth loss that has completely eliminated the stability of the dental occlusion.
The symptoms that are localized to the TMJs usually just involve a misfit between the stable central bracing positions of the dental occlusion and those of the TMJs. The misfit can arise due to changes in the dental occlusion or by rapid degenerative remodeling in one or both joints. The symptoms usually just involve inability to masticate effectively.
THE ROLE OF CENTRAL SENSITIZATION
Recent research has shown that, in some TMJ disorder patients, the state of neurotransmitters in the brain causes a hypersensitivity that can make even normal functional stimuli painful. This condition of centrally caused hypersensitivity is usually associated with sleep disorders and depression as well. Treatment that is solely directed at the peripheral condition (the TMJs or the jaw muscles) cannot relieve the symptoms unless that treatment is combined with more centrally directed treatment modalities such as antidepressants, cognitive behavioral therapy, or meditation.
When adaptation fails to protect all the tissues adequately, symptoms occur. The tissue damage may occur in any component of the masticatory system, and careful analysis usually reveals some degree of response in the muscles, bones, joints, and dentition. Clinicians describe a constellation of symptoms including dental problems, facial pain, headaches, dizziness, eustachian tube blockage, subjective hearing loss, postural strains, visual problems, and pain from muscles extending all over the body. However, usually the one component which forms the weakest link in the chain undergoes most of the damage and produces the clinical symptoms. The particular process which produces the damage in any one individual is variable and depends on many factors.
Symptoms produced directly by the degenerative process in one or both TMJs go through stages of inflammation and adaptation. The inflammation may be brief and clinically insignificant or may become chronic and persist for years. Even if the inflammation persists for decades, eventually adaptation occurs.
Many of the symptoms are produced by the tightening of the jaw muscles in response to inflammation in a TMJ. Inflammation in any joint triggers reflex protective bracing in the muscles which cross that joint. Protective bracing involves decreased functional forces and increased resting tensions. Reflex protective bracing was designed to protect acutely injured joints in evolution. When increased resting tensions are maintained chronically, they cause an anatomical shortening of the muscle fibers known as contracture. Contracture inhibits resting circulation and may cause build-up of waste products in little pockets known as trigger points. Trigger points can cause pain in areas far from their location (referred pain).
Muscle reactivity seems to play a role in producing symptoms and maintaining the disorder associated with the jaw system. The big jaw closing muscles are all oriented vertically, so increased resting tensions in the jaw closing muscles produced increased compressive force in the TMJs. This increased compressive force can exacerbate the degenerative process in the TMJs, and the increased degeneration produces increased inflammation which then triggers more tightening of the jaw closing muscles, causing a vicious cycle of pain and dysfunction.
The narrowing of the sides of the midface and the increased protrusion of the center of the midface has affected the shapes acquired by the bony orbits and thereby also very likely the shapes of the eyeballs themselves. The orbits are comprised on their lower and inner (medial) aspects by membrane bones of the midface and on their roofs by a plate of membrane bones that gets pushed forward by growth of the anterior part of the cranial base. The intimate relationship between orbit shape and eyeball shape is suggested by the correlation between their relative volumes. One researcher commented, “Analysis of the orbit, eye, and spherical equivalent refractive error (SER) reveals a strong relationship between relative size of the eye within the orbit and the severity of myoptic refractive error. An orbit/eye ratio of 3 for females and 3.5 for males (or an eye that occupies approximately 34% and 29% of the orbit, respectively), designates a clear threshold at which myopia develops, and becomes progressively worse as the eye continues to occupy a greater proportion of the orbital cavity. These results indicate that relative size of the eye within the orbit is an important factor in the development of myopia, and suggests that individuals with large eyes in small orbits lack space for adequate development of ocular tissues, leading to compression and distortion of the lithesome globe within the confines of the orbital walls.” (97)
The longer narrower orbits which have been produced by the average change in facial shape could be responsible for the epidemic of myopia in modern societies. In almost all animals which use vision, a process called emmetropization shapes the eyeball to fit the focal length of the lens by controlling growth of the dense connective tissue of the sclera enveloping the eyeball. Animal studies have shows that poor image quality on the retina can cause the scleral tissues to strengthen or weaken in an attempt to move the retina to the best location for a clear image. However in humans this emmetropization process slows at about age 6. After that age, emmetropization may not be able to keep compensating for the rapid facial growth of adolescent and teenage years. In myopic eyes, the growth in length of the eyeball far exceeds the growth in height and width of the eyeball. Myopia develops due to the increase in prolate to oblate proportions of the eyeball that occur during the period from 7 to 19 years of age. (98) At the end of the second decade, the development of myopia commonly stops when facial growth slows enough to allow emmetropization to compensate for any small subsequent changes in orbit shape.
The increase in asymmetry and irregularity in facial growth could be responsible for the rise of astigmatism and other irregularities of the shape of the eyeball which also develop in concert with myopia and rapid facial growth.
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