Chapter 2

CHAPTER 2 – FUNCTIONAL HARMONY IN NATURAL HUMAN JAW SYSTEMS

Human jaw systems adapted successfully in all these environments by producing and maintaining a remarkable functional harmony in which all the components of the system worked together in a way that produced a goodness of fit with each other in almost any functional environment; and they gradually changed character with age in ways that maintained a functional harmony which also gradually changed in ways that better fit the tissues as they underwent the natural declines of aging.  The system only failed when its structural components, the bite table and TMJs, had worn out completely.  

THE MANDIBULAR JOINTS

The lower jawbone is supported by three joints functioning in perfect harmony - the two TMJs and the dental joint formed by the fitting together of the upper and lower dentitions.  With all three of these mandibular joints continuously adapting their contours to the same functional jaw movements, they also maintain a perfect fit with each other.  Studies of pre-industrial humans show that the form and surface changes of the TMJs are well correlated with the shape of the bite table; while in modern humans they are better correlated with age or sex. 37 38 39 40

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 MANDIBULAR BRACING

The mandible is especially well supported at these three joints in their close-packed positions when the mandible is clamped up forcefully against the underside of the cranium for bracing, and all three mandibular joints are simultaneously loaded within a relatively small centrally located area. Mandibular bracing facilitates postural stability by giving the postural muscles in the front of the body a handle by which to pull down on the front of the cranium.  Mandibular bracing is needed during swallowing to first provide a stable platform against which the tongue can push its tip forward to collect the food at the front of the palate to form a bolus and then later to provide a fixed base from which the suprahyoid muscles can pull the hyoid bone upward and forward to allow the food to pass behind it.  

The most critical need for mandibular bracing is for protecting the vital systems just behind the condyles.  A blow to the chin could drive the condyles like little hammers into the hearing and balance centers.   In carnivores and simians, the mandible was protected from such backwardly directed blows, even at postural rest, by the overlapping canines.  In hominids, the canines withdrew into the bite table with the other teeth to increase adaptability in the range of motion of the mandible, and the mandible is left only protected from backwardly directed blows by neuromuscular reflexes that brace the mandible as soon as danger is sensed. 

During mandibular bracing, the TMJs exhibit the resiliency typical of pressure-bearing synovial joints, with progressive resistance in the microstructure of the tissues.  On the surfaces of the articulating bones, proteoglycans within the load-bearing areas respond to compressive force by expressing water molecules and deforming to spread the compressive forces onto a larger surface area.   When the compressive force moves to a different area, these proteoglycans from the previous area of compression reabsorb water molecules and return to their resting contours.  Larger compressive forces are absorbed by increasingly stiff layers - the plastic lubricating film and fibrous articular covering, the calcified cartilage, the thin subchondral bone, multiple stiffer and more vertically oriented bony trabeculae in the spongiosa, and finally the cortical bone.  

During mandibular bracing, the bite tables, (the articular surfaces of the dental joint) exhibit similar resiliency by absorbing compressive force with the viscoelastic and hydrostatic properties of the tooth sockets.  Each tooth is suspended in the middle of its socket by a circumferential fibrous sling embedded in a layer of collagenous ground substance with interstitial fluid and extensive vascular plexes that cushion its movements in response to bite forces.  Biting pressure intrudes the roots into this structure, tugs on the surrounding fibrous sling, drives fluids into nearby vessels, bends out the walls of the socket, and thereby encounters resistance that increases steadily as the tooth moves further from the center of the socket. Release of biting pressure permits rebound of the tooth, first by elastic recoil and then by a slow hydrodynamic phase with superimposed pulsation. 19  20  21  22   At the same time, the whole articular surface of this big dental joint absorbs compressive forces by rapid shifting of the teeth over small distances within their sockets in order to spread them out onto more teeth.  For example, during light closure in the central bracing area, the load falls on the teeth in the middle of the dentition – primarily the first molars and the premolars. More forceful closure compresses or otherwise shifts those teeth until the load spreads out to include the canines and the second molars.  

At the same time, biting forces stabilize the upper and lower bite tables by closing the interproximal contact areas connecting the members of each dental arch. Adjacent teeth at rest are separated by small (interproximal) gaps of about ten microns.  Biting forces tip the teeth inward and forward until these interproximal gaps close and the adjacent teeth make physical contact, which allows groups of neighboring teeth to absorb forces as a single structural unit, like one long tooth supported by many roots.  

CHEWING

The mandible is also supported at all three joints throughout its functional range of motion in chewing.  Whenever the ball (the condyle) moves in any direction away from the center of the socket, it rides up on the walls of the socket in that direction, with the teeth that are most directly in the path of the movement supporting the mandible along its pathway.   For example, when the mandible moves forward, the lower front teeth ride up onto the upper front teeth.  When the mandible moves forward and laterally, the lower canine and neighboring teeth of that side ride up onto the upper canine of that side. When the mandible moves laterally, the lower premolars and molars of that side ride up onto the upper premolars and first molars of that side.  When the mandible moves backward, the last lower molars of both sides ride up onto the last upper molars of both sides.  

In the bites of our ancestors, this type of group dental function was even more universal than the presence of a single central bracing area. 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

occlusion_fig_2.low_res.jpg

PROTECTING THE SPECIALIZED ARTICULAR SURFACES

The dental and temporomandibular joints each function in their own ways to protect their specialized articular surfaces from the violent movements of the mandible during chewing,– whether those surfaces are composed of cartilage or teeth. 

The TMJs are well designed to absorb shocks.    "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. Also condylar movements are damped by the rapid filling and emptying of the vascular retrodiscal plexus at the back of the TMJs.  Each time a condyle moves backward, the loose vascular plexus behind it is compressed and emptied.  Then, each time the condyle moves forward, the retrodiscal plexus have to fill as fast as the condyle can move.  During chewing, these hydraulic forces help to cushion condylar movements.

In the TMJs, the articular surfaces are prevented from separating and then coming back together with an impact that could damage them by maintaining a point of contact due to the shape of the disk.  The disk is a biconcave structure composed of a central thin zone surrounded by peripheral wedges and bathed in lubricant, and therefore compressive forces from biting continuously center the disk between the articulating bones.  When the mandible is braced centrally, the thin central zones of the disks are located between the bones. When one condyle is distracted, as when forcefully biting on a resistant bolus placed between the second or third molars and therefore behind (posterior to) the center of mass of the jaw closing muscles, the disk of that TMJ is pulled forward by traction of the superior lateral pterygoid muscle on the front 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 the glenoid fossa above it.   

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A second mechanism that protects the articular surfaces by maintaining contact is the collateral ligaments (composed of medial and lateral thickenings of the TMJ capsule) that hold the articular disk down on the head of the condyle like chin straps holding a hat down on your head.  Because these ligaments do not stretch, the disk can roll forward and backward on the rounded condylar head, but it cannot be pulled laterally or superiorly out of contact with the condyle.  Another pair of ligaments that protects the articular surfaces are the outer oblique fibers of the temporomandibular ligaments, which function like the radius of a circle to keep the condyle pressed firmly against the slope of the articular eminence just above it whenever a condyle translates forward - as seen from left to right below.

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 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 occur during chewing requires more flexibility than the disks in other articular joints; therefore TMJ articular disks are made of fibrous connective tissue rather than the brittle hyaline cartilage that comprises the articular disks in other synovial joints.

In the dental joint, the protected articular surfaces are the teeth, and the dental arches work in parallel with the temporomandibular joints.  While the mandibular condyle on the chewing side (the ball) is driven upward, inward, and forward through the socket on the underside of the skull; the outer cusps of the lower teeth are driven upward, inward, and forward through the fossae (and marginal ridge areas) of the upper teeth.  On the chewing side, the point of compression moves from the back teeth forward in a wavelike fashion, just as the working side condyle rides the same wave forward through the glenoid fossa.    

Then, as the mandible follows through to the other (non-chewing) side, the condyle of that side moves backward until it almost reaches the nonarticular facets at the back of the TMJ (on the backward facing slope of the condyle and the forward facing wall of the postglenoid process).34 35   These non-chewing side TMJ facets are border areas rather than functional areas, consequently they remain highly vascular and do not participate in the passive mechanical guidance of the mandible. However, remodeling of the bone beneath the specialized articular surfaces creates an obvious congruence between facets on the back of the condyles and the front of the postglenoid processes; and the condyle passes by those facets while, in a similar fashion, the outer cusps of the lower teeth on the chewing side pass under the inner cusps of the upper teeth.

THE JAW CLOSING MUSCLES

Providing the forces of bracing and chewing are the powerful jaw closing muscles, which converge onto the lower jawbone from origins dispersed widely around the cranium.

The temporal muscles are the postural muscles for the mandible. Right and left temporal muscles form a bilateral sling that suspends the mandible by its coronoid processes from attachments all over the sides of the upper portion of the cranium. The temporal muscles are 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, dominate the deep portions of the muscle.11   During chewing, these muscle fibers act mainly as stabilizers. The tension they exert depends largely on the position of the mandible.12

The medial pterygoid and masseter muscles of each side form a sling that generates unilateral power for chewing.  Because their origins are so widely spread apart on the underside of the front of the skull, they can pull the mandible strongly up and to either side at a wide variety of angles. Their type 2 muscle fibers are capable of generating powerful chewing forces. Gradually increasing activity leading to primary discharge during the final power-crush stroke suggests that these muscles are used for developing large loads between the teeth and are not particularly sensitive to the position of the mandible.13

The superior lateral pterygoid muscles control the horizontal location of the mandible when the jaw closing muscles applied their compressive forces to the bolus.  By pulling inward and forward on the condyles, the superior lateral pterygoid muscles control the position of the mandible like steering a bicycle by its handlebars.     

bicycle.jpg 

When incising, both superior lateral pterygoids fire 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 relax 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 is used as a crushing tool rather than a lever arm, thereby minimizing the compressive forces at the TMJs.  

During power-crushing, the superior lateral pterygoid muscle of the working side, pulling the working side condyle foward and inward; and the posterior temporalis of the balancing side fire, pulling the non-working side condyle backward, fire together to form a force couple that creates a twisting of the mandible on top of the bolus to crush food much like crushing out a cigarette butt by twisting on it with the ball of the foot.  

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Finally the mandible continues onto the non-working side in a follow-through phase that lengthens the power-crush stroke as the non-working side condyle reaches its rearward (posterior) limit and starts moving forward along with the working side condyle. With both condyles moving forward, the lower jawbone smoothly transitions into its opening phase.

JAW OPENING MUSCLES
Jaw opening is the responsibility of two groups of muscles that act as a force couple to rotate the mandible down and back, placing its center of rotation near the mandibular foramen, where the neurovascular bundle enters it.14   The inferior lateral pterygoid muscles, pulling the condyles down and forward, are relatively short and strong and therefore useful for fast opening.  However, the inferior lateral pterygoid muscles are also angled inward (medially) and therefore lose much of their jaw opening force when the lower jawbone is open fairly wide.  As these muscles experience decreased leverage, the long and thin digastric muscles, which pull the chin down and back, experienced increased leverage. 

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 NEUROMUSCULAR CONTROLS

Controlling the jaw muscles is a vast array of neuromuscular reflexes that are designed to protect the articular components and maintain the functional harmony. 

Chewing is initiated by a central pattern generator which strums a constant background of repetitive firings.  Each firing is 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 track the position of the lower jawbone enable the system to rapidly detect and respond to unpredictable changes such as sudden fracturing or shifting of the bolus.

Positive feedback loops protect the teeth and their supporting tissues by only allowing the jaw closing muscles to shower down their full forces after the periodontal receptors have signaled multiple bite contacts. 26  Because of these loops, maximal voluntary bite force is positively correlated with bite stability.27 28. Even simply anesthetizing the teeth causes a significant reduction of maximal voluntary bite force. 29

Negative feedback loops protect the system by rapidly interrupting jaw closing muscle activity in response to nociceptive afferent feedback 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 jaw closing muscle forces fast enough to prevent a stone in a bolus from damaging a tooth. 31 32

There are so many protective mechanisms able to shut down the jaw closing muscles that the central nervous system has to be able to limit their influence or they could prevent the necessary application of power during chewing. To prevent such central nervous system gridlock, inhibitory neurons diminish the responses to sensory stimulation on the working side during chewing. 33  If protective reflexes triggered by periodontal receptors surrounding misplaced teeth threaten to disrupt chewing pathways and thereby hinder functional capacity, the neuromuscular system is able to alter chewing pathways to incorporate a protective pattern of jaw movements that restored functional capacity.  These altered patterns of movement to accomodate bite 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 chewing is experimentally 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 lower jawbone during function actually experiences surprisingly small forces during function.

Group function of the teeth during chewing minimizes the need for protective reflexes.  Thinking back to the hallway analogy used to describe engrams, functional harmony is acquired when no boards were left sticking out, and the hallway acquires a shape that perfectly fits your body as you walk through it.  During chewing, a cadence of strong, smoothly alternating, and relatively uninterrupted firings corresponds with a steady repetitive pattern of sensory feedback as control shifts smoothly back and forth between muscles and articular structures.  Each closing is guided by the jaw muscles and the TMJs until the dentition or the food bolus is engaged.  Then, as the jaw closing muscles shower down their power-crushing forces, guidance is smoothly transferred to the teeth and food bolus.  Finally, as the power-crush stroke follows through and merges with the opening stroke, guidance is smoothly transferred back to the muscles and joints.  Agonists and antagonists alternate smoothly without overlapping their firings, which would cause the jaw muscles to pull against each other (co-contraction of antagonists).

The smooth alternating muscle function produces accessory circulation to all the tissues.  The long lever arm formed by the mandible functions much like a pump handle driven up against the skull thousands of times each day, and condylar movements during chewing act like a piston to pump waste products from the synovial tissues into venous circulation.    

WEAR ON THE BITING SURFACES

The bite tables are designed to encounter wear, whether it is fast or slow. In rapid wear, attrition quickly transforms their biting surfaces of cusps and fossae into wear facets composed of cupped out areas of dentin surrounded by protruding rings of enamel that form effective grating surfaces.  In front, the overbite and overjet are steadily reduced and then eliminated, as illustrated below.  

anterior wear.jpg

The overbite reduction can be seen in the illustration below showing the shift of the horizontal line marking the incisal edges of the lower incisors in Australian aborigines chewing patterns as they continue to approach the incisal edges of the upper incisors with age.

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In back, the tall sharp cusp tips become rounded and then flattened until the opposing teeth meet along large wear facets that maintain functional edges much like in herbivores, as illustrated below. 46   

  NEWLY ERUPTED TEETH        MATURE DENTITION          ELDER DENTITION  

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To compensate for the continual shortening of the teeth due to this wear on the biting surfaces, the teeth continually erupt just quickly enough to maintain the height of the face.  In most pre-industrial 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 or in those who got significant amounts of their nutrition from animal blood and milk, wear occurred slowly.  In tribes who ate food with lots of grit, such as meat cooked in a sand pit or grains milled on coarse stones, wear occurred rapidly. By spreading out chewing forces among a large number of contacting surfaces, 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.  

WEAR ON THE INTERPROXIMAL SURFACES

The proximal surfaces of the teeth were also designed to withstand whatever rate of wear occurred between the teeth.  Typically, interproximal contact contact areas enlarged from the small round concavities, much like those found in carnivores, to broader interproximal contact areas more like those of herbivores.  In many of the skeletal human remains in museums, the teeth have lost 3 mm of mesiodistal dimension and the interproximal contacts have become 6 mm wide.23   Below are photographs of the upper and lower dental arches from a moderately worn dentition (rural India 1950’s).  The goodness of fit between the interproximal contact areas and the way they function to allow a range of motion interproximally and interproximal bracing when squeezed together is not difficult to imagine.     

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AGING

Wear on the teeth was one of the ingrediants that enabled the whole jaw system to change its form and function gradually in the few of our ancestors who lived into old age in concert with the changes that naturally take place in all aging tissues, generally transforming from more carnivorous features in youth to more herbivorous features in older years.  This change was not simply an unfortunate side effect of using the jaw system, but an integral part of the environment for which it was designed.

During childhood and early adulthood, a steep vertically oriented 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 across a jagged bite table and negotiate around steep interlocking cusps while still avoiding traumatic collisions between opposing teeth. The jaw muscles could perform intricate dances to avoid bite interferences.  Extensive blood supply was readily available where it was needed.  With maturity, the system reached its maximum power.

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 changes in the bite table that made the system require less strength and adaptability.   The bite table became smaller in area due to loss of tooth structure from wear on the sides of the teeth, smoother due to wear on the tops of the teeth, and more stable due to the combination of interproximal wear and mesial drift. 

Mature dentitions acquired a bite table that integrated all the front and back teeth into one smooth curve with contours similar to the surface of a sphere with a radius of about four inches.  This same size sphere was described by all the early dental researchers - in an antero-posterior plane by Spee, in a lateral plane by Wilson, and as a larger sphere 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.  

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More careful analysis showed that the smooth flattened curve eventually acquired by the human bite table has helicoidal longitudinal twists like a propeller blade.48   The plane of the bite table relative to a horizontal plane is relatively steep at the central incisors and flat at the second molars.  At the first molars, the plane of the upper teeth slopes lingually, and at the third molars it slopes buccally.  Contours in the lower dental arch are opposite and complementary. 47

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occlusion_fig_10.low_res.jpg 

 As the end product of a shaping process motivated by chewing, the helicoidal curve is the imprint of the functional range of motion of the mandible in the biting surfaces of the teeth.  Tooth structure that is in the way of these pathways is removed by attrition or minor tooth movement, and areas of the biting surfaces of teeth that are too low to support those pathways erupt further into the bite table.  As a result, the bite table becomes a three dimensional registration of the envelope of motion acquired by the lower jawbone after the jaw muscles have overcome the resistance posed by the dentition. Even the few tribes of our ancestors who experienced minimal wear still developed a bite table with helicoidal curves.

That helicoidally curved bite platform facilitates operation of the aging jaw system.  It enables the aging jaw muscles to fire in long steady strokes that gradually build momentum and come to a smooth end, uninterrupted by protective reflexes or co-contraction of antagonists, making fluid pressure rise and fall more gradually with each stroke.  The wider and steadier chewing movements also maintain health at aging articular surfaces by facilitating weeping circulation and minimizing the need for extensive shock absorbing capacity.  The more smoothly curved bite table allows a relatively quiet and smooth neuromuscular system by feeding it sensory stimuli in a uniform manner that allows easy tracking of lower jawbone position.  Pressure passes evenly down the arch from each tooth to its neighbor, applying compressive forces that gradually rises and falls on each sensory receptor bed and allowing a simple and repetitive pattern of motor impulses to drive the system.  As the bite and the mandibular postural position shift anteriorly with age, they lower airway resistance; just as the natural weakening of muscles at the rate of about 5% per decade makes them progressively less able to strongly pull in air.

This effective human jaw system is able to maintain its functional harmony into old age.  The articular disks usually stay in place but wear out along with the other TMJ components and the dentition.48  Adaptive movement of the teeth and their supporting bones maintain the proximity of the upper and lower bite surfaces and thereby also effective chewing ability despite whatever rate of tooth wear is encountered.   In people who encountered rapid wear, such as people who eat food cooked in sand pits, wear on the biting surfaces completely eliminates the crowns of the back teeth and reduces them to individual roots functioning as little teeth - each with its own bite surface and interproximal contact facets connecting it to the neighboring roots.  Even these roots enable effective chewing.  The system only fails when there were no tooth surfaces or condylar heads left on which to chew.  

 

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48. Tobias PV. The natural history of the helicoidal occlusal plane and its evolution in early Homo. Am J Phys Anthropol 1980;53:173-187.