Summary

All biology has to be understood in the context of evolution. Dentistry has never understood bites, because dental researchers did not develop an understanding of how bites were designed to work in our ancestors before developing a model of how they should work in our patients today. Without understanding bites, dental authorities consider all bite treatments to be risky, and they advise dentists to avoid changing, adjusting, or even stabilizing bites. As a result, we allow strained bites to warp the faces and craniofacial structures of our children, even while we straighten their teeth esthetically. Unlike other joint and orthopedic structures, bites can be easily reshaped; but knowing how and when to reshape them requires first understanding how the bite table works together with the rest of the body in function and dysfunction, which requires understanding its role in the growth and physiology of the jaw and postural systems, which is the goal of the following chapters under ETIOLOGY.

BACKGROUND - In mammals, the bite table is the stable architectural feature around which the rest of the face grows. The shape of the bite table reflects the shape and function of the whole craniofacial area and jaw system - growing long and narrow to rip and tear meat or flat and wide to grind vegetation. The human jaw system and facial structure combined features of the previous mammalian jaw systems for adaptability that enabled it to deal with almost any food source. 

The most distinctive feature of the human jaw system was its location in a cranium balanced on the top of an upright spinal column. The change from quadruped to biped required balancing the cranium on the top of the spinal column by moving its center of mass back to a position directly above the top of the spinal column and its connection with the spinal column to a location almost directly under its center of mass. Stabilizing the cranium there required reorganizing the skeletal muscles into chains pulling down all around it. In front, at the top of the chain, the mandible it provided a shield that protected the delicate facial structures by absorbing postural forces and transferring them around to the sides of the cranium, where the zygomatic arches and temporal bones provide plenty of supporting bone. In this manner, the jaw muscles were incorporated into the network of postural muscles that maintained a balanced upright stance with a low tonus throughout the postural musculature.

One reason for the success of this human jaw system is that it was able to maintain a remarkable functional harmony which ensured the goodness of fit among all its components. The TMJs and bite table shared support for the mandible in its direction of motion throughout its range of motion while chewing forces drove it forcefully and rhythmically up against the underside of the cranium to pump circulation through all the affected tissues. In the jaw muscles, each muscle firing drove out waste products, and then each relaxation, while the antagonistic muscles fire, allowed arterial resupply.

A second reason for the success of the human jaw system is that it was able to transform that functional harmony with age during adulthood in a manner that made it more easily operable by aging tissues. During adulthood, as muscles weakened, the jaw systemed transformed to fit the smaller forces. The bite table became smaller due to wear on the biting surfaces while wear between the teeth (interproximal wear) made the dental arches shorter and more stable.  As neuromuscular reflexes slowed, jaw movements became smoother and less ballistic, so those reflexes were rarely triggered. Meanwhile, stable face height was maintained by continual eruption and mesial drift of the teeth, no matter how fast the teeth wore down. The system maintained its structural integrity and functional harmony, - even as the teeth wore down to their root tips.

The third reason for the success of this human jaw system is that each individual jaw system was able to create the perfect conditions for functional harmony by customizing its growth processes to create a jaw system that perfectly fit the functional forces it was exposed to. Beneath a cranial vault that grows very early in life by expanding circumferentially and beside a cranial base that grows slightly later by elongating in the midline to push the middle of the face forward; the jawbones grow much later, to carry the sides of the face forward. The direction in which they carry the sides of the face forward during growth is largely determined by the amount of stimulation they receive from bite forces. Strong bite forces makes the face grow more forward and outward (increasing width), while weak bite forces allow the inherent eruption forces in and around the teeth to push the lower jawbone more down and back, usually followed by the upper jawbone, except when the tongue intervenes to protect the airway. 

However, growth in the upper and lower jawbones occurs by different mechanisms, in slightly different directions, and on overlapping but somewhat different time scales. The membrane bones that form the upper jawbone unfold and expand, while the thick cortical bone holding the lower teeth (the mandibular corpus) maintains its shape while advancing and rotating forward (up in front) as much as necessary to compensate for tooth wear.  Connecting the upper and lower jawbones, the bite table has to coordinate these diverse growth patterns above and below it, functioning like a maxillo-mandibular suture. 

Jawbone growth and therefore also the need for the bite table to coordinate upper and lower jawbone growth patterns continues during adulthood, because we were built for longevity. After the second decade of life, when other growth processes stop, jawbone growth slows down about 90%, but then continues slowly in a pattern that was built into our genetics in order to ensure that the few who were lucky enough to become old lived as long as possible, because the system transformed smoothly with age to make it more easily operable. As our respiratory muscles weaken during adulthood at about 5% per decade, maxillary expansion reduces nasal airway resistance, and mandibular advancement reduces pharyngeal airway resistance. At the same time, bite table stability is maintained by continual eruption of the teeth and gradual shifting of their basal bones, as if they were all spring-loaded, to replace every micron of lost tooth structure by new tooth strucure moving into the bite table. This compensatory jawbone growth is stimulated by bite forces, so it occurs about as much as needed. People with strong bite forces generally have more tooth wear and therefore need and achieve more shifting of the teeth and growth of the basal bones to compensate for it. In this manner, human jaw systems were able to maintain an adequate airway and a stable bite table, whether there was little wear or enough wear to reduce the teeth to root tips.

Mandibular advancement is the most prominent and rapid dynamic in adult craniofacial growth. Researchers who have studied the jaws of our recent ancestors describe their bites as "attritional" occlusion, which results in an end-to-end bite. However, it is not the wear that causes the mandibular advancement. The wear is one of the mechanisms designed to accommodate the advancement. The mandible needs to keep advancing slowly during adulthood, while the palate expands even more slowly; and the teeth need to allow some slippage between the dental arches to accommodate that growth. In tribes that had no significant abrasives in their food, continual jawbone growth was accommodated by the gradual shifting of the teeth. The few people who lived to be old still developed end-to-end bites.

This human jaw system was adaptive enough to enable it to achieve a functional harmony that fit almost any type and degree of bite forces generated by chewing, but not to a lack of bite forces or a lack of chewing. Chewing forces in hominids has been weakening ever since they began cooking food, making heads grow rounder, faces grow longer and narrower, and mandibles less protrusive; but the jaw systems still maintained a functional harmony that kept them healthy until a couple of centuries ago, when the average human diet became too soft to stimulate the horizontal jawbone growth needed to create and maintain an adequate nasopharyngeal airway, and our jaw muscles became too weak to coordinate and harmonize growth in the upper and lower jawbones. While our ancestors often wore out their jaw systems, we more often fail to wear them in. They don't acquire the harmony of form and function they need for optimum health. They have become more irregular and asymmetrical.

Instead of contributing to the overall steady growth pattern of the face by maintaining a steady bite table, jawbone growth now often displaces and destabilizes the bite table, which alters the growth pattern at the sides of the face, generally redirecting it downward and backward and often to one side. The average upper jawbone doesn't expand enough, leaving many palates too narrow to accommodate the tongue in a healthy resting posture, which forces it down and back. The average mandibular corpus rotates downward and backward (clockwise), instead of upward and forward (counter-clockwise), like it did in our ancestors. The downward component of our mandibular rotation lengthens our faces. While face height remained proportional to body height in our ancestors, today it increases during adulthood at about the same rate that our teeth used to wear down. The backward component of our mandibular rotation squeezes the airway passage between the mandible, which surrounds it in front and on both sides, and the cervical spine behind it, crowding the tongue back into the pharyngeal wall and against the cervical spine, where it impinges on airway passage.  Mechanical strains are continuously produced between upper and lower jawbones that can never acquire a perfect fit. 

Further inhibiting healthy facial growth, steeply interdigitating unworn teeth and steep or deep overbites frequently lock together the upper and lower jawbones in a manner that prevents their independent growth. The upper jawbone cannot expand, because it is locked to a mandibular corpus that can only translate, and the mandibular corpus cannot translate, because it is locked to an upper jawbone can only expand.

The strained facial growth pattern only produces symptoms when adaptation fails, but the loss of functional jaw muscle forces has also lowered adaptive capacities. Less vigorous pumping of the mandible against the underside of the cranium, less potential for slippage in the bite, and less bite stability all diminish the potential for adaptation. At the same time, because of the imbalance in size between jaw opening and closing muscles, chronically elevated central nervous system stress reduces the space between the teeth, the maxillo-mandibular joint space, which can cause tooth contacts that trigger increased jaw muscle tonus, which further reduces adaptive capacity.  

THE SOLUTION - To eliminate TMJ disorders, forward head posture, and obstructive sleep apnea in our population does not require returning to hard diets like those of our ancestors or doing daily jaw muscle exercises, it just requires understanding how the human jaw system was designed to acquire and maintain functional harmony in its natural environment so we can learn how to create a new more delicate functional harmony that suits our modern life styles. For example, we can use prosthetic tools instead of strong chewing forces to regulate facial growth. However, to employ those tools effectively, we first need to understand how bites were designed to function and why they so frequently become dysfunctional today, the goal of the following manuscript.