Chapter 2
CHAPTER 2 – FUNCTIONAL HARMONY IN NATURAL HUMAN JAW SYSTEMS
One reason for the success of this human jaw system was that it was able to establish a remarkable functional harmony in which all its components worked together in a way that maintained a goodness of fit with each other and with the other components of the postural system, and it could maintain functional harmony in the few people who got old by changing its character and form in a way that made it progressively more easily operated by aging tissues. The system only failed when its articular components, the bite tables and TMJs, had worn out completely.
THE MANDIBULAR JOINTS
The mandible articulates with three joints functioning in perfect harmony - the two TMJs and the dental joint, the maxillo-mandibular joint. They all have ball-and-socket joint dynamics. In the tooth-lined maxillo-mandibular joint, the ball is the lower bite table; and the socket is the upper bite table. With all three 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.10-12
BRACING
The close-packed positions of these three mandibular joints is reached when the mandible is braced by clamping it up forcefully against the underside of the cranium. Mandibular bracing was critically important during evolution for protecting the vital systems just behind the condyles, because a blow to the prominent chin could drive the condyles like little hammers into the hearing and balance centers. In hominids, the canines had withdrawn into the bite table to increase the mandibular range of motion, leaving the mandible protected only by neuromuscular reflexes that brace the mandible as soon as danger is sensed. Bracing also stabilized the mandible during swallowing by providing a platform against which the tongue can push its tip forward to collect the food at the front of the palate into a bolus at the beginning of the swallow; and then providing a fixed base from which the suprahyoid muscles can pull the hyoid bone upward and forward to allow the food to pass behind it later in the swallow.
SHOCK ABSORBING
The maxillomandibular and temporomandibular joints absorb shocks to the same effect by different mechanisms. All three mandibular joints protect their specialized articular surfaces from the violent movements of the mandible during chewing by absorbing shocks in parallel, but the anatomy and physiology by which that occurs is different when teeth are involved.
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 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.
At the same time, 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. 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. At the same time, the whole articular surface of this big dental joint absorbs compressive forces, because the first teeth to contact shift rapidly over small distances in order to spread the forces 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.
The bite tables also stiffen in the area receiving compressive force by squeezing the teeth together. Adjacent teeth at rest are separated by small (interproximal) gaps of 3-21 microns, which allows each tooth to have an independent arterial pulse in its resting position, which is surrounded by a range of motion within its socket. Biting on teeth tips them inward and forward until these interproximal gaps close and the adjacent teeth temporarily make solid physical contact, which allows groups of neighboring teeth to absorb forces as a single structural unit, like one long tooth supported by many roots. Then, after the bite force is released, the teeth slowly rebound toward the middle of their sockets.
CHEWING
The mandible is also supported at each of the three joints throughout its functional range of motion in chewing like a ball in a socket, riding up on the wall of the socket in the direction of movement. The dental joint works in parallel with the temporomandibular joints. While the mandibular condyle on the chewing side (the ball) is driven upward, inward, and forward through the downward facing socket on the underside of the skull; the outer cusps of the lower teeth are driven upward, inward, and forward through the downward facing fossae (and marginal ridge areas) of the upper teeth. When the mandible moves forward, the lower front teeth ride up onto the upper front teeth and the condyles ride up onto the anterior band of the disk and the slope of the articular eminence. When the mandible moves to one side, the buccal cusps of the lower molars and premolars ride up onto the central fossa of the upper molars and premolars of that side, while the condyles of that side ride up onto the lateral borders of the articular disks and the lateral socket wall. When the mandible moves backward, the last lower molars of both sides ride up onto the last upper molars of both sides and the condyles ride up onto the posterior bands of the disks. During chewing, 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.
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, it can only move as fast as the loose vascular plexus behind can be compressed and emptied. Then, it can only move forward as fast as the retrodiscal plexus can refill. During chewing, these hydraulic forces help to cushion condylar movements.
PREVENTING LOSS OF ARTICULAR CONTACT DURING CHEWING
When forceful chewing on a terminal molar distracts the ipsilateral condyle, the articular surfaces must be prevented from separating and then coming back together with an impact that could damage them, so they maintain a point of contact by pulling a thicker part of the disk between the separated bones. The disk has a central thin zone surrounded by peripheral wedges and bathed in lubricant, so compressive forces continuously center the disk between the articulating bones. When one condyle is distracted, the disk of that TMJ is pulled forward by traction of the superior lateral pterygoid muscle until its thick anterior band fills in the increased intrajoint space and thereby maintains a line of contact between condyle below and the glenoid fossa above.
The joint surfaces are also held in contact by a network of ligaments. The TMJs collateral ligaments ( medial and lateral thickenings of the TMJ capsule) hold the articular disk down on the head of the condyle like chin straps holding a hat down on a head. These ligaments allow the disk to roll forward and backward on the rounded condylar head, but not to move laterally or superiorly out of contact with the condyle. At the same time, the outer oblique fibers of the temporomandibular ligaments 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.
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 of other synovial joints.
THE JAW CLOSING MUSCLES
The human jaw closing muscles transfer pressure smoothly back and forth between the three joints.
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. During chewing, these muscle fibers act mainly as stabilizers. The tension they exert depends largely on the position of the mandible.
The medial pterygoid and masseter muscles are the chewing muscles. On each side, they 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 up at a wide variety of angles. 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.
The superior lateral pterygoid muscles control the horizontal location of the mandible. By pulling inward and forward on the condyles, they control the position of the mandible like steering a bicycle by its handlebars. They are the only muscles in position to pull the mandible anteriorly.
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. When power crushing, the mandible pivots on the bolus, powered by a force couple that includes 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), to twist the mandible on top of the bolus like crushing out a cigarette butt by twisting on it with the ball of the foot.
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 mandible 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. 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 mandible 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.
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. The postural muscles and the jaw muscles have coordinated firing patterns. The cranio-cervical muscles contribute to chewing, and the jaw muscles contribute to postural stability. The post-cervical muscles stabilize the head by pulling down on its back end during swallowing when the anterior kinetic chain pulls down on its front end and by alternating firing with the mandibular elevator muscles to prevent the head from rocking during chewing.
The postural muscles and the jaw muscles have also become integrated harmoniously into the habitual upright stance. The mandible functions as an integral component of the chain of bones and muscles on the front of the body that pull down on the front of the head, which balances the chain of muscles on the back of the body that pull down on the back of the head. In the habitual upright stance, the jaw and postural muscles share in a "tensegrity" that holds all the bones together in a neutral zone with a small uniform background muscle tonus.
The twisting of the mandible during chewing maintains a small flat area in the middle of the central bite table, which provides enough freedom of mandibular movement horizontally to accomodate different postural positions. The mandible can brace rapidly on the bite table from a variety of body and head postures.
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.
During functional activities, 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 a stable bite. As a result, maximal voluntary bite force is positively correlated with bite stability. Even simply anesthetizing the teeth causes a significant reduction of maximal voluntary bite force.13
Negative feedback loops protect the three mandibular joints by rapidly interrupting jaw closing muscle activity in response to nociceptive afferent feedback supplied by mechanoreceptors distributed abundantly throughout the joint 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.14-
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. 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 accommodate 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 supplies 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. Condylar movements during chewing act like a piston to pump waste products from the synovial tissues into venous circulation and alternating compression and release of the teeth during chewing pumps out the waste products from the tooth sockets.
AGING
This functional harmony was designed for longevity by transforming with age. Not many of our ancestors lived into old age, but the few that did made important contributions to the success of our villages. To ensure their longevity, the jaw system changed its form and function gradually 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.
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. The newly erupting canines, 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 while still avoiding traumatic collisions between steep overlapping cusps. 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 stiffer, and hard tissues became more calcified. 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 of 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 narrower due to loss of tooth structure from wear on the sides of the teeth and smoother due to wear on the tops of the teeth. Smooth steady chewing strokes rarely triggered protective neuromuscular reflexes. At the same time, the bite table also became shorter and more stable due to the combination of interproximal wear and mesial drift, as described below.
INTERPROXIMAL WEAR
Typically, as interproximal contact areas enlarged, the dental arches became shorter. 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. Below are photographs of the upper and lower dental arches from a moderately worn dentition (rural India 1950’s).
During this process, the interproximal contact areas enlarge from the small round concavities, much like those found in carnivores, to large flattened areas, more like those of herbivores. The interproximal contacts function like joints between adjacent teeth. The goodness of fit between the interproximal contact areas in their central (fully braced) positions is surrounded by a distinct range of motion, which can be seen most clearly in the upper second premolar in the lower of the two photographs. The central bracing areas are surrounded by a range of motion like a ball in a socket of an interproximal joint.
OCCLUSAL WEAR
Wear on the bite surfaces transforms them into 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 while the mandible slowly advances, as illustrated below.
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.
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.
NEWLY ERUPTED TEETH MATURE DENTITION ELDER DENTITION
To compensate for the continual shortening of the teeth due to this wear on the biting surfaces, the teeth and their supporting ridges of alveolar bone continually erupt just enough to maintain the height of the face, in proportion to the height of the body. In most pre-industrial human dentitions, the teeth wore down at a rate of at least a half millimeter every year and also erupted at the same rate. 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, and the teeth erupted 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, and the teeth erupted rapidly.
By spreading out chewing forces among all the teeth, the effects of extreme wear were resisted for as long as possible. Teeth didn't wear out in one area until they wore out everywhere. In many museum skulls, wear on the biting surfaces has completely eliminated the crowns and left the person chewing on the tooth roots, with the molars functioning as four separate roots which each have a bite surface and interproximal contact areas between them.
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 four inch radius. This same size sphere was described by all the early dental researchers who looked at healthy natural dentitions - 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.
The smooth flattened curve eventually acquired by the bite table in older natural dentitions has helicoidal longitudinal twists like a propeller blade.15-16 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.
As the end product of a shaping process motivated by chewing, the helicoidally shaped bite table is the imprint in dentin of the functional range of motion of the mandible. 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. The bite table becomes a three dimensional registration of the envelope of motion acquired by the mandible 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.
The helicoidally curved bite platform 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 gradually with each stroke. The wider and steadier chewing movements 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 maintains a quiet and smooth neuromuscular system by feeding it sensory stimuli that allow easy tracking of mandibular position. Pressure passes evenly down the arch from each tooth to its neighbor, applying compressive forces that gradually rise and fall on each sensory receptor bed and allowing a simple and repetitive pattern of motor impulses to drive the system.
With advancing age, face height reduced along with body height. The bite table perfectly fit the functional range of motion of the mandible. Chewing required little effort. The natural human jaw system was able to maintain its functional harmony until there were no tooth surfaces or condylar heads left to chew on.
FOOTNOTES
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