CHAPTER 2 – FUNCTIONAL HARMONY IN THE PRE-INDUSTRIAL HUMAN MASTICATORY SYSTEM
A striking feature of the human masticatory system was its ability to achieve a remarkable functional harmony in a wide variety of different types of functional environments. That functional harmony integrated all the components of the masticatory system in a manner that kept them healthy and maintained an extreme goodness of fit among them. The occlusal table played a key role in that functional harmony.
THE OCCLUSAL TABLE
The occlusal table was a platform for receiving the forces generated by the mandibular elevator muscles. These forces were generated primarily by mastication and mandibular bracing. Some parafunctional activities, such as shaping sticks or softening frozen boots, played a minor role in shaping the occlusal table in some cultures.
The occlusal table was shaped by functional forces triggering dentoalveolar remodeling activity. Teeth erupted in areas where occlusal forces were low, and they became depressed in areas where occlusal forces were high. As a result, the occlusal table became indented where it experienced large occlusal forces, and it elongated in the other areas. As long as functional forces were strong and consistent enough, the occlusal table acquired the contours of a platform that fit its functional needs.
One major source of occlusal forces was mandibular bracing, which occurred at least hundreds of times each day when all the mandibular elevator muscles simultaneously fired with relatively strong forces to lock the mandible immoveably up against the underside of the front of the cranium. Mandibular bracing is needed for postural stability, because locking togetehr the mandible and the cranium allows that anterior cervical muscles to pull down on the front of the cranium by pulling down on the mandible. Mandibular bracing is needed during swallowing to provide a stable platform against which the tongue can push to drive its tip forward to collect the food at the front of the palate and form it into a bolus and then to provide a fixed base against which the suprahyoid muscles can pull the hyoid bone upward and forward to allow the food to pass behind it. Mandibular bracing is also needed for protection, because the heavy mandible at rest hangs freely below the skull with a protruding chin that is easily struck and with its back ends close to the hearing and balance centers and the temporal lobes of the brain. A blow to the chin can impact a condyle like a hammer into these vital structures. In canine and simian masticatory systems, the resting mandible is protected from such distal thrust by the overlapping canines. In hominid masticatory systems, the canines have withdrawn into the occlusal plane like in herbivores, and the resting mandible is only protected from distal thrust by neuromuscular reflexes that readily trigger mandibular bracing to immobilize the mandible by bracing it as soon as danger is sensed.
THE HABITUAL MANDIBULAR BRACING AREA (HMBA)
Bracing indented the middle of the occlusal table to form a mandibular bracing platform that is commonly known as the intercuspal position (ICP) or the position of maximal intercuspation (MI), but it is best described as an HMBA. It is an indented central portion of the occlusal table that provides the perfect platform for mandibular bracing. The word habitual is included, because some people also have other bracing positions (dual bite). The word area is included, because the HMBA allows the mandible to move around horizontally at least a small distance without encountering any increase in vertical dimension.
The other compressive force that shaped the occlusal table was mastication. The range of movement of the mandible in mastication extended the slightly indented contours of the platform in the middle of the occlusal table in all directions, in effect extending the maxillary socket that housed the mandibular ball by shaping the tooth inclines surrounding the HMBA. Relatively vertical mastication made the occlusal table relatively steep vertically, mastication with wide lateral slides made the occlusal table relatively flat in a transverse plane, and mastication with strong antero-posterior slides made the occlusal table relatively flat in a sagittal plane.
In the mandible's normal functional range of motion, the teeth that were most directly in the path of mandibular movement supported the excursion. When the mandible moved in any direction, that side of the lower dentition (as ball) rode up onto the surrounding overbite of the maxillary teeth (as socket wall) in that direction, and the resulting increase in mandibular vertical dimension separated the teeth in other directions. For example, when the mandible moved anteriorly, it rode up onto the maxillary anterior teeth. When the mandible moved antero-laterally, it rode up onto the working side maxillary canine. When the mandible moved laterally, it rode up onto the working side maxillary premolars and first molars. When the mandible moved postero-laterally, it rode up onto the maxillary second and third molars. When the mandible moved posteriorly, the distal slopes of the buccal cusps of the mandibular third molars rode up onto the anterior slopes of the palatal cusps of the maxillary third molars - posterior guidance.
In our ancestors, this universal group function was even more common than bilateral centric stops. In some apparently healthy Aboriginal dentitions, group function provided adequate support for bracing the mandible unilaterally on either side, but the mandible could not be braced bilaterally, as shown in figure 2 below.3
ABSORBING FUNCTIONAL FORCES
The occlusal surfaces of the maxillo-mandibular joint absorbed compressive forces much like synovial joints, with progressive resistance in the microstructure of the tissues. Joint structures were designed to protect the specialized articular surfaces – whether cartilage or teeth. Synovial joints resist forces with increasingly stiff layers - the plastic lubricating film and fibrous articular covering, then the calcified cartilage, then the thin subchondral bone, then multiple stiffer, and more vertically oriented bony trabeculae in the spongiosa, and finally the cortical bone. In a similar manner but by a different mechanism, the maxillo-mandibular joint microstructure resisted forces by the viscoelastic and hydrostatic properties of the periodontal ligament spaces. 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 occlusal surfaces of the dentitions also absorbed compressive forces with macrostructure dynamics that resembed those of synovial joints. Instead of deforming by expressing water molecules from proteoglycans in the area of compression as in synovial joints, the maxillo-mandibular joint macrostructure deforms by shifting of the teeth to spread the forces onto more teeth. Teeth move easily over short distances. During light closure in the center of the maxillo-mandibular joint, the load falls on the teeth in the middle of the dentition – primarily the first molars and the premolars. More forceful closure compresses those teeth until the load is spread out to include the canines and the second molars.
The occlusal surfaces also had a unique manner of resisting larger compressive forces. When the teeth were at rest, they were separated by small interproximal gaps of about ten microns. Occlusal forces tipped the teeth mesially and lingually until these interproximal gaps closed and the adjacent teeth made physical contact. That contact allowed groups of neighboring teeth to absorb forces as a single structural unit like one long tooth supported by many roots.
The group function in the masticatory system included the other articular components that supported the mandible, the TMJs. During power-crushing, on the working side, the mandibular condyle was driven superiorly, medially, and anteriorly through the depths of the glenoid fossa; while the mandibular buccal cusps were driven superiorly, medially, and anteriorly across the maxillary central fossae and marginal ridge areas. The location at which the bolus was compressed on the working side usually began at the posterior end of the arch and moved anteriorly in a wavelike fashion, just as the working side condyle moved anteriorly through the glenoid fossa. Then, 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 while the working side mandibular buccal cusps crossed the working side maxillary palatal cusps. The non-working side TMJ facets were border areas rather than functional areas, consequently they remained highly vascular and did not fibrose or participate in the passive mechanical guidance of the mandible. However, remodeling of the bone beneath the specialized articular surfaces created an obvious congruence between facets on the posterior slopes of the condyles and those on the anterior slopes of the postglenoid processes. Similarly the dentition on the balancing side was congruent but did not actively guide the mandible, because the mandibular buccal cusps passed beneath the maxillary palatal cusps.
To support the violent movements of the mandible in mastication, 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
Condylar movements were damped by the rapid filling and emptying of the vascular retrodiscal plexus. Each time a mandibular condyle moved posteriorly, the loose vascular plexus behind the condyle was compressed and emptied. Then, each time the condyle moved anteriorly, the retrodiscal plexus had to fill as fast as the condyle could move. During chewing, the rapid anteroposterior movements of the condyle produced hydraulic forces which helped to cushion the violent movements of the condyle.
Ligaments prevented the articular surfaces from separating and then coming back together with an impact that could damage the specialized articular surfaces. 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. At the same time, 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 whenever a condyle translated forward - as seen from left to right below.
To further protect the specialized articular surfaces from potential impacts that could damage them, the articular disks moved around constantly to maintain a point of contact. 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 articulating bones. When the mandible was braced centrally, the thin central zones of the disks were located between the bones. When the condyle was distracted, as when forcefully biting on a resistant bolus placed between the second or third molars, the disk was pulled anteriorly by traction of the superior lateral pterygoid muscle on the anterior aspect of the disk and capsule just far enough to fit the larger space between the bones and thereby maintain an area of contact between condyle and temporal bone through the medium of the disk.
For the disks to be able to rapidly move around to maintain a point of contact and 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 hyaline cartilage that comprises the articular disks in other synovial joints.
HARMONY AMONG THE MANDIBULAR JOINTS
The mechanism by which the articular contours were constantly molded to best fit functional demands was different in the TMJs and in the dentition, but the effect was very similar. The TMJs were shaped by remodeling of bone. The occlusal table was shaped by the repositioning of teeth and then fine tuned by occlusal wear. As a result of these shaping processes, the TMJs and the occlusal table acquired contours that were perfectly shaped to fit the same functional forces and thereby ended up fitting each other. They operated as a triad of support for the mandible, shown in the figure below. Wherever the mandible moved, it remained supported in these three areas of the mandibular articulation simultaneously. 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. Studies of pre-industrial humans show that, unlike in modern humans, the form and surface changes of the TMJs were well correlated with dental attrition rather than with age or sex. 37 38 39 40
The rest of the craniofacial area was involved in the same functional harmony. The maxillary platform, which received masticatory forces, was also designed to absorb shocks.23 It was surrounded by sutures which, unlike other cranial sutures, stay open throughout life. These sutures were connected to bony features that functioned like flying buttresses. 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.
THE JAW CLOSING MUSCLES
The jaw closing muscles powered the functional movements that created the functional harmony.
The temporal muscles were the postural muscles for the mandible, and they operated as integral members of the postural system. 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 upper portion of the skull. The temporal muscles were multipennate; with fibers at continuously varying angles for 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 unilateral power for chewing. Because their origins were so widely spread apart on the underside of the front 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, the superior lateral pterygoid muscles could control the position of the mandible like steering a bicycle by its handlebars.
When 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, as shown on the left below. When power crushing between the molars, the lateral pterygoids 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. When the bolus is located posterior to the center of mass of the mandibular elevator muscles, the condyle is distracted slightly, as shown on the right below.
During power-crushing, the superior lateral pterygoid muscle of the working side and the posterior temporalis of the balancing side fired together to create a twisting of the mandible on top of the bolus - an action that helped crush food much like crushing out a cigarette butt by twisting on it with the ball of the foot. As the mandible approached the midline from the working side and the powerful elevator 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 anteriorly and medially; while it pulled the non-working side condyle posteriorly. The mandible rotated around the bolus as seen below.
BALANCING SIDE CONDYLE
WORKING SIDE CONDYLE
After crossing the HMBA, the mandible continued onto the non-working side in a follow-through phase that lengthened the power-crush stroke. In this phase, the mandibular buccal cusps approximated the maxillary palatal cusps on the working side while the occlusal surfaces on the non-working side were separated by the thickness of the bolus and the inferior movement of the balancing side condyle as it moved toward the lateral border of the TMJ. Finally, at the end of this follow-through phase, 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, placing the center of rotation of the mandible near the mandibular foramen and thereby protecting the area where the neurovascular bundle enters the mandible.14 The inferior lateral pterygoid muscles, pulling the condyles down and forward, were relatively short and strong and therefore useful for fast opening. However, the inferior lateral pterygoid muscles were also angled medially and therefore lost much of their jaw opening force when the mandible was open fairly wide and the orientation of the lateral pterygoid muscles became mostly medial rather than anterior. At that partly open mandibular position, the long and thin digastric muscles, pulling the chin down and back, experienced increased leverage just as that of the inferior lateral pterygoid muscles began to decrease.
Controlling the jaw muscles was a vast array of neuromuscular reflexes that were designed to protect the articular components and maintain the functional harmony.
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 a vast network of receptors in and around the mouth 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 only allowing the mandibular elevator muscles to shower down their full forces if the periodontal receptors signaled multiple occlusal contacts. 26 Because of these loops, maximal voluntary bite force is positively correlated with occlusal stability 27 28 , and even simply anesthetizing the posterior teeth causes a significant reduction of maximal voluntary bite force. 29
Negative feedback loops protected all the tissues of the masticatory system by rapidly interrupting mandibular elevator muscle activity in response to nociceptive afferent feedback. 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 can shut down elevator forces fast enough to prevent a stone in a bolus from damaging a tooth. 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 protective reflexes triggered by periodontal receptors surrounding misplaced teeth threatened to disrupt chewing pathways and thereby hinder the functional capacity of the masticatory system, the neuromuscular system was able to alter chewing pathways to incorporate a protective pattern of mandibular movements that restored functional capacity. These altered patterns of movement to accomodate occlusal irregularities 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."
Because of engrams, 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. Also because of engrams, a tooth made tall enough to theoretically receive the full force of the mandible actually experiences surprisingly small forces during function.
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 mastication, 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 between the firings of opposing muscle groups and thereby little energy wasted by muscles pulling against each other (co-contraction of antagonists).
Our masticatory systems were designed for occlusal wear. Shortly after the teeth were fully erupted, attrition quickly transformed their occlusal surfaces of cusps and fossae into wear facets composed of cupped out areas of dentin surrounded by protruding rings of enamel that formed effective grating surfaces. To compensate for the continual shortening of the teeth due to occlusal wear, the teeth continually erupted just quickly enough to maintain the height of the face. In most human dentitions, 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 food with a lot of grit, such as meat cooked in a sand pit or grains milled on coarse stones, it occurred more rapidly. By spreading out masticatory forces among a large number of contacting surfaces, group function maximized longevity of the masticatory system. In evolution, rapid occlusal wear was the primary threat to the stability of the masticatory system. In group function, occlusal wear 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.
Continual eruption was one of the mechanisms that continually compensated for occlusal wear. 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. In describing the natural human dentition, 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."
CHANGES IN THE MASTICATORY SYSTEM WITH AGE
The functional harmony in the human masticatory system acquired longevity by gradually transforming 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 from a generally carnivorous one to a more herbivorous one.
During childhood, a steep vertically oriented and often irregular bite table was well tolerated by a fast and agile neuromuscular system. 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 supporting tissues were full of water, enzymes, and elastic fibers which enabled them to withstand diverse articular stresses and unanticipated impacts and to accomodate a large variety of functional movement pathways. Fast and active protective neuromuscular reflexes could cope with frequent sudden changes in firing patterns or forces applied. Chewing strokes could deftly work the mandible laterally across jagged occlusal table and then in and out of a steeply locked-in 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 became drier while undergoing 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 mandibular range of motion that made the masticatory system require less strength and adaptability. These changes included reduction in the size of the articulating components (the dentition became shorter from the combination of interproximal wear and mesial drift while it also become narrower bucco-lingually from wear on the buccal and lingual surfaces), increase in the stability of the articulating components, and smoother and steadier functional mandibular pathways which are easier for the jaw muscles to negotiate.
DENTAL ARCH STABILITY
Once the teeth had fully erupted and stabilized in an arch form, their alignment was maintained at least partly by the fit among interproximal facets. Teeth rubbed against their neighbors forcefully enough to eliminate mesial and distal tooth structure. To prevent interproximal wear from opening the interproximal spaces until they trapped food and undermined arch integrity, mammalian teeth developed mesial drift. The posterior teeth acquired an ongoing tendency to shift as far mesially as interproximal forces allowed. Eventually they formed a continuous line of facets that perfectly fit each tooth’s range of motion and thereby provided stability by resisting a change in the range of motion of any single tooth. Frequently teeth lost 3 mm of mesiodistal dimension and proximal contacts became 6 mm wide.(23) In many of our ancestors, by the age of 18, mesial drift had reduced arch length sufficiently to make room for the wisdom teeth.(24, 25)
The combination of interproximal wear and mesial drift provided important adaptive capacity. If a tooth erupted into a rotated or otherwise displaced position, these two processes kept reshaping its proximal surfaces until a good functional facet was produced. The result was often unusually shaped facets like that seen on the mesial surface of the first molar in figure 7.
SHALLOWING OF THE MANDIBULAR RANGE OF MOTION
Anteriorly, overbite and overjet were steadily reduced and then eliminated as illustrated below.
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 the tall sharp cusp tips became rounded and then flattened until the opposing teeth articulated along large wear facets that maintained functional edges much like in herbivores. 46 These normal changes in the posterior region of the dentition with age are illustrated below.
NEWLY ERUPTED TEETH MATURE DENTITION ELDER DENTITION
THE HELICOIDAL CURVE
Mature dentitions acquired an occlusal table that integrates the anterior teeth and the posterior teeth into one smooth curve. This curve acquired by the occlusal table has a contour that is well approximated by the surface of a sphere with a radius of about four inches. This same size sphere was described by all the early dental researchers - as the arc of a circle in an antero-posterior plane by Spee, in a lateral plane by Wilson, and in the plane of the occlusal table as well as extending up to the condyles by Bonwill. Monson portrayed a portion of the same sphere as a cone with all the teeth aligned in contact at its base and with the center of the sphere located at the glabella.
Actually the smooth flattened curve eventually acquired by the human occlusal table has been described as helicoidal, because it has helicoidal longitudinal twists like a propeller blade.48 The plane of the occlusal table relative to a horizontal plane is relatively steep at the central incisors and flat at the second molars. At the first molars, the maxillary occlusal plane slopes lingually, and at the third molars the maxillary occlusal plane slopes buccally. Contours in the lower dental arch are 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, although the curve is less obvious when the teeth retain their cuspal anatomy.
TRANSITION WITH AGE
The wider and steadier mandibular movements operating against a helicoidal curve 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.
The wider and steadier mandibular movements operating against a helicoidal curve also maintained health at aging articular surfaces by facilitating weeping circulation and minimizing the need for extensive shock absorbing capacity. The more smoothly curved occlusal table 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 human masticatory system was able to maintain its functional harmony 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 and their supporting bones in response to continual tooth wear maintained the proximity of the maxillary and mandibular occlusal surfaces and thereby also the effectiveness of mastication. 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. Eventually, occlusal wear 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. Even these roots functioning as teeth still maintained masticatory efficiency. 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.
23. Mehta JD, Evans CC. A study of attrition of teeth in the Arkansas Indian skulls. Angle Orthod 1966;36(3):248-57
24. Lysell L. Qualitative and quantitative determination of attrition and the ensuring tooth migration. Acta Odontol Scand 1958;16:267-292.
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