CH 3 – GROWTH OF THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
This adaptable and gradually transforming hominid masticatory system was not simply created by genetics, it grew by a complex multistaged postnatal growth process resulting from an interaction between its functional environment and its genetics. 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. Such growth adaptation was one of the keys to our success. Genetics and epigenetics interacted with functional stimuli to produce structures which perfectly fit those functional stimuli.
FORM AND FUNCTION
One of the reasons for the evolutionary success of hominids was the adaptability we gained from having much of our growth and development take place postnatally under the influence of functional stimuli. In many ways, humans seem to have evolved from an ape born in an embryonic stage. 49 Delaying growth until the organism was exposed to functional forces allowed each individual's growth to be customized to fit its particular needs.
Functional stimuli affect form during growth, because bones are constantly growing or remodeling under the influence of functional forces. 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, muscle activity causes the bones at their origins and insertions to strengthen just enough to be able to withstand whatever forces they can apply, you cannot grow your muscles strong enough to break your bones, and weakness or paralysis of muscles produces bones that are extremely thin and mechanically deficient. 50 51
There is evidence that functional forces produce 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 increased 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
The masticatory system forms especially late postnatally and therefore is more shaped by functional forces than most other areas. At birth, the TMJs have not yet formed, the squamous portion of the temporal bone is essentially flat, and the mandibular corpus is just a bulbous tube enclosing a collection of tooth crowns with a midline that is still unfused, making it not yet capable of receiving or transmitting significant forces. As these structures grow and develop under the influence of functional stimuli, they are shaped by functional forces to produce a masticatory system optimally suited to withstand those functional stimuli.
The whole craniofacial area is affected. In front, 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 masticatory 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 masticatory forces to the rear portions of the zygomatic arches. The zygomatic arches in turn transferred masticatory forces to the zygomatic sections of the maxillary and temporal bones. Medially masticatory forces were transferred to the cranial base via the walls of the maxillary sinuses and the wings of the sphenoid bones.
These forces affect the entire head. In monkeys, a new layer of bone forms on the supraorbital ridge just after the arrival of each new molar 68, and forceful biting bends the whole cranium separating the two sides at the sagittal suture running along the top of the head. In pre-industrial eskimos, who had very strong jaw muscles, powerful chewing produced a distinct thickening along the sagittal suture. 69 In modern humans, the jaw muscles are weaker but the cranial bones are thinner, and they are probably affected as well. 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.
The effect of masticatory forces on growth has been demonstrated experimentally. In animals, weakening masticatory forces by softening the diet produces thin craniofacial bones, strengthening masticatory forces by hardening the diet produces thick craniofacial bones 67 , and 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 humans, the alveolar processes do not form if there are no teeth to receive forces, and the articular eminentia and glenoid fossae do not form when there is no condyle 76 77 Even after they are fully formed, the articular eminentia lose their contours if the condyles are removed or fractured. 78 79 80
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 Growing under the influence of masticatory forces, the craniofacial structures grow to achieve a mandibular complex optimally designed to deliver masticatory forces and a maxillary complex optimally designed to withstand masticatory forces.
Before the teeth erupt, the mandibular elevator muscles brace the mandible by squeezing it against the tongue or nipple resting between the gum pads, and the masticatory system structures begin to grow in response to rhythmic compression. 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 establishes a whole new set of neuromuscular reflexes that change the behavior of all the muscles in the craniofacial area. After the occlusal table replaces the tongue as the functional 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. People who retain an anterior open bite often retain an infantile swallowing pattern.
Subsequently, the structural components of the masticatory system keep growing in response to the complex gradient of craniofacial strains produced by masticatory forces - with highest strains experienced near the occlusal table, moderate strains in the middle face, and very low strains in the upper face. 93 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 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 TEETH
Functional forces are needed to guide the teeth into place, because their eruption paths cannot be controlled very precisely by intrinsic mechanisms. The first functional forces to guide the eruption of the teeth are the light steady soft tissue pressures directed inward from the lips and cheeks and outward from the tongue; along with the eruption forces pushing the teeth out of their basal bones and the occlusal forces pushing the teeth back into their basal bones. Between these opposing forces (shown below) is a neutral zone into which the teeth tend to drift.
Within the neutral zone, the teeth are designed to line up where uppers and lowers meet to form a stable occlusal table at the location used most frequently by the mandible in bracing. Since each mandibular tooth (except the central incisors) meet two maxillary teeth, and each maxillary tooth (except for the terminal molars) meets two mandibular teeth; the teeth align to form smoothly curved arches. The anterior teeth meet with some overbite and overjet. The posterior teeth meet with an interdigitation that refines their alignment bucco-lingually by means of the so-called cone and funnel mechanism illustrated below:
The lingual cusps of the mandibular teeth help ensure a functional alignment of teeth by preventing those mandibular teeth from erupting outside of the maxillary dental arch. 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 occlusal table achieves stability, because the eruption of each tooth stops when the axially directed occlusal forces that push the teeth into their basal bones during bracing and chewing counterbalance the eruption forces that are continuously pushing the teeth out of their basal bones and into the occlusal table. The consistency and forcefulness of mandibular bracing aligns the occlusal surfaces to form an occlusal table at a height determined by the activity of the mandibular elevator muscles, where it produces a indented area in the center of the occlusal table to house the bracing of the mandible.
At the same time, the 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 aligns the teeth in a manner that does not restrict the mandibular range of motion. 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 on the mandible maintains 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 In humans, because functional forces have such an important impact on occlusal stability, dental arch dimensions show very low heretability and decrease with age while most craniofacial dimensions show high hereditability and increase with age. 86 87 88
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 erupting in front and in back of the primary occlusal table 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. Occlusal stability then continues to affect craniofacial growth, because the occlusal table serves as the exercise template for the jaw elevator muscles and functions as an articular surface between the mandibular corpus and the maxilla. Stable occlusions evoke stronger jaw muscle firings and therefore also better jaw muscle development. Asymmetrical occlusions evoke asymmetrical jaw muscle firing, which in turn causes asymmetrical craniofacial growth. However, to understand the effect of occlusion on craniofacial growth requires looking at each of the various growth processes involved. These growth processes include neurocranial expansion, cranial base elongation, and facial translation.
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 cranium. 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 cranium 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 cranium, the orbits behave similarly. Each orbit contains an enclosed expanding neural mass, and it grows by proliferation at its surrounding sutures just enough to perfectly enclose its expanding neural mass. Several bones of the midface form portions of the inferior and medical walls of the orbits, and they are affecgted by orbital expansion. Removal of the eyeball during growth results in deficiencies in the anterior and lateral growth of the midface.
While the expansion of the cranium is motivated by enlargement of the enclosed neural contents, the shape ultimately acquired by the cranium is at least partly determined by externally imposed forces. Limiting cranial growth in one direction causes it to expand in other directions. Some pre-industrial human tribes successfully altered head shapes by binding their infants' heads with cloths or boards, apparently without impairing brain development. Similarly, the cranium can be shaped at least partially by the postural and functional forces applied to it by the jaw and skeletal muscles.84 85 Compression of the cranium by the vertically arranged mandibular elevator muscles produces compensatory growth horizontally. As a result, pre-industrial humans with relatively strong overall musculature had crania and faces that were relatively short and wide, while pre-industrial humans with relatively weak overall musculature developed crania and faces that were relatively tall and narrow.
As the expansion of the cranium slows to a stop in childhood, 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 below. 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 connected by 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 section while the mandible grows from the back section, 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 vertical elongating of the face during postnatal growth dramatically changes its proportions between childhood and adulthood, as can be seen below.
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 more lateral portions of the face are less directly affected by cranial base elongation and 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. It continues to push the front of the nasal airway anteriorly. One researcher suggested that the growth of the nasal septum causes the anterior growth of the maxilla because of traction from a septo-maxillary ligament.
The protrusion in the center of the face is needed to increase airway space in the growing body. 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 need for airway passage, relatively rapid central facial protrusion must continuously diminish airway resistance in the face during the active growth of the first two decades.
From the underside of the front portion of the cranial base, the midface translates downward and forward to form a maxillary platform ideally constructed to absorb masticatory forces and distribute them as widely as possible around the craniofacial structure.
The midface grows this maxillary bite receiving platform downward and forward by adding bone to the sutures behind it. As shown below, these sutures are all generally aligned in the parallel, at a perfect angle to fill in bone 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 circum-maxillary sutures stay open throughout life, because chewing activity helps maintain their patency. 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 (Giblin N, Alley A. Studies in skull growth. Coronal fixation. Anat Rec. 1944;8:143-153.) Studies have also shown that muscular forces can cause relative movement of the adjacent bones and delay or prevent closure of the suture joining them. (Buckland-Wright JC. The shock-absorbing effect of craniofacial sutures in certain mammals. (Abstr.) J Dent Res 1972;51:124 ) Softening the diet has been shown to produce some premature obliteration of facial sutures in rats. Open sutures help absorb shocks.
The structure of the maxilla enables it to absorb masticatory forces and distribute them as widely as possible around the cranium, because it is supported by flying buttresses extending widely around 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. 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.
Beneath the translating midface, the mandibular corpus translates faster and further than the midface. It grows by elongating from the posterior portion of the cranial base to form a tool that is situated directly below the midface and is ideally suited for delivering masticatory forces. The front of the mandible is a horseshoe shaped corpus that functions as a hammer head. It maintains its genetically determined shape while thickening as much as needed to be able to deliver large compressive forces upward, inward, and slightly forward against the underside of the maxillary bite platform. Because of this structural adaptation to functional 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.
Behind the corpus, the rami and condyles grow as an elongating handle that holds the hammer head out under the midface. They grow new bone at their posterior and superior borders and thereby thrust the corpus anteriorly and inferiorly.
The condylar cartilage acts as a growth center 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.
The condylar cartilage is also highly adaptive throughout life due to a proliferative layer of undifferentiated mesenchymal cells that grow as much and wherever needed to hold the mandibular corpus out under the maxillary dentition. Because the condylar cartilage has periosteum-like properties, its growth is highly adaptive, especially to mechanical loads. 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 maxilla. As a result of this adaptaive growth, 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 rotates the condyles anteriorly and thereby redirects condylar growth posteriorly 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.”
Between the condyle and corpus, the ramus contributes to the elongation of the handle by steadily growing at its back end to shift the corpus at its from end forward. Enlow described this growth of the ramus by an illustration in which a brick wall is moved continually by pulling bricks off one side and adding them to its other side.
These facial structures comprising upper and lower jawbones 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 structural components of the masticatory system 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 facial mask appears to grows by swinging out from the underside of the front of the cranium as if it were hinged at the forehead, and the profile flattens between infancy and adulthood as shown below.
THE MAXILLO-MANDIBULAR SUTURE
Coordinating the diverse growth patterns in the midface and the lower face, the dental occlusion functions as a maxillo-mandibular suture by connecting and thereby coordinating the growth patterns of the bones on each side of the suture. 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 The mandibular corpus leads 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.
At the anterior aspect of this maxillo-mandibular suture, the overbite relationship that the maxillary and mandibular anterior teeth acquire when their eruption first brings them into contact serves to couple the growth of the mandible with the growth of the maxilla and thereby keep them relatively close together for effective mastication in spite of their diverse growth mechanisms. At the posterior aspect of this maxillo-mandibular suture, the interdigitation of the posterior teeth also serves to maintain the proximity of maxillary and mandibular dental arches.
As a maxillo-mandibular suture, the dental occlusion also provides whatever fill-in growth is necessary to maintain structural continuity between the maxilla and the mandibular corpus when growth carries the bones away from each other. 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. These structures can erupt the teeth as well as the surrounding alveolar bone as far as needed to fill in any spaces created by growth. Primate experiments show that anything that lowers the postural position of the mandible promotes additional tooth eruption.
MUSCLE RESTING POSTURES
While the maxillo-mandibular suture coordinates growth of the maxilla and the mandible, the overall facial shape is controlled by 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 fascia hold each bone in a kind of neutral zone determined by their passive tension. 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.
THE FACIAL MASK
While the mandible and maxilla move down and forward motivated by very different growth processes, the anterior border of the face at their front end translates down and forward as a mask that generally maintains its form. The 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. This mask shifts downward and forward due to upward and backward growth of the structural components behind it.
CONTROL OF MUSCLE RESTING POSTURES
Controlling craniofacial and craniocervical resting muscle tensions is a hierarchy of neuromuscular reflexes with airway protection reflexes on top. 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 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.
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 - primarily expansion of the maxilla and translation and forward rotation of the mandibular corpus. Instead of ceasing by the end of the second decade like other growth patterns, intramatrix facial growth slows to adult levels at about ten percent of its previous rate and then continues throughout life. Because this intramatrix growth is stimulated by the forces of mastication and because occlusal wear is also a result of mastication, the amount of intramatrix growth that occurred was proportional to the rate of occlusal wear, and intramatrix growth occurred about as much as it was needed. In this manner, intramatrix growth was effective at compensating for occlusal wear.
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 maxilla 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.
Expansion of the maxilla also shifted the maxillary buttresses outward. 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.
These functional forces that expand maxillary growth horizontally also limit maxillary growth vertically. 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 enlarges the nasal airway by widening the floor of the nasal airway as well as the buttresses that carry masticatory forces around the nasal airway. As the structural components of the maxilla and its buttresses shift laterally, their medial aspects resorb, leaving room for expansion of the sinuses 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 anteriorly due to growth in the condyles and rami just behind it.
The continuous anterior translation of the mandibular corpus was designed to continuously 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. These were also the people who experienced the most occlusal wear and thereby needed the increased anterior translation to compensate for that occlusal wear.
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 corpus shifted anteriorly, 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. Mandibular growth was correlated with maxillary growth. People with strong jaw muscles usually developed bimaxillary protrusion.
FORWARD ROTATION OF THE MANDIBULAR CORPUS
While the mandibular corpus shifts forward under the rest of the face, it also rotates upward in front, carrying the roots of the lower teeth further upward and into the occlusal table while maintaining the height of the ramus. A similar growth rotation can be seen in other primates. In pre-industrial humans, this forward growth rotation 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, an anterior bracing location, the center of the rotation was usually located near the front of the dentition. Thus it too effectively compensated for occlusal wear by correlating its activity with the activity that causes occlusal wear.
The rotation of the corpus can be seen most clearly relative to the two rami. The rami are stable architectural landmarks, because their positions are controlled by the steady postural tensions of the temporalis muscles. The forward rotation of the corpus that occurred relative to the ramus resulted in a sharper gonial angle and faces that were shorter 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 acquire a growth pattern which ensured 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. Within this steady growth pattern, a separate growth pattern in the jawbones was designed to supply the occlusal table with as much tooth structure as needed to compensate for whatever rate and pattern of occlusal wear resulted from mastication. As a result of this and other growth adaptation mechanisms, 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.
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