Summary

BACKGROUND - In mammals, the bite table is the stable architectural feature around which the face grows, and its shape reflects the function of the whole jaw system. The human jaw system combined features of previous mammalian jaw systems and bite tables to maximize adaptability, and then incorporated the postural system, requiring major structural changes. The jaw muscles became members of the network of postural muscles that stabilize the head by maintaining a light downward tension all around it, and the mandible became a shield to protect the delicate sense organs of the face from postural forces by transferring all those forces around to the sides of the cranium, where the zygomatic arches and temporal bones provide plenty of supporting bone. 

The most important reason for the evolutionary success of this human jaw system was its growth adaptability. A sophisticated system of coordinated postnatal growth mechanisms enabled the human jaw system to grow to fit whatever 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, after the jaw muscles have become functional, to carry the sides of the face forward in a direction which is largely determined by bite forces.

Both upper and lower jawbones grow in the same general direction in response to the same bite forces; but they grow by different mechanisms, in slightly different directions, and on overlapping but somewhat different time scales; requiring coordination at the bite table. 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 functions like a maxillo-mandibular suture to coordinate and harmonize those independent growth patterns above and below it, which requires some adaptative slippage at the surface of the bite table. 

In our ancestors, the jaw systems produced by this sophisticated adaptive growth system exhibited a remarkable functional harmony that persisted into old age by transforming in a manner that made them progressively more easily operable by aging tissues. As the bite surfaces wore down, bite table stability was maintained by continual eruption of the teeth and gradual shifting of their basal bones to replace every micron of lost tooth structure by new tooth structure moving into the bite table so face height remained steady in proportion to body height. At the same time, the biting tables narrowed due to wear on the sides of the teeth, while they shortened and stabilized due to wear between the teeth (interproximal wear), as jaw movements became smoother and less ballistic, so neuromuscular reflexes were rarely triggered. To compensate for the loss of respiratory muscle strength with age, maxillary expansion reduced nasal airway resistance, and mandibular advancement reduced pharyngeal airway resistance. 

This human jaw system could grow and maintain a functional harmony that fit almost any type and degree of chewing, but not a lack of chewing. Human chewing forces had been weakening ever since we began cooking food, making our heads grow rounder, our faces grow longer and narrower, and our mandibles less protrusive; but our jaw systems still maintained a functional harmony that kept them healthy until a couple of centuries ago, when our jaw muscles became too weak to coordinate and harmonize growth in the upper and lower jawbones and to stimulate the horizontal jawbone growth needed to create and maintain an adequate nasopharyngeal airway. Instead of contributing to the overall steady growth pattern of the face by maintaining a steady bite table, jawbone growth now produces mechanical strains between upper and lower jawbones that are not growing harmoniously and thus can never acquire a perfect fit. The average upper jawbone doesn't expand enough, leaving the underside of the palate too narrow to accommodate the tongue in a healthy resting posture and the nasal cavity on the top of the palate too narrow to accomodate an adequate nasal airway passage. The average mandibular corpus now rotates downward and backward (clockwise) to crowd the oropharynx by squeezing it against the cervical spine, instead of upward and forward (counter-clockwise) to steadily increase the size of the airway passage, like it did in our ancestors. The downward component of our mandibular rotation has significantly lengthened the average modern face. 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.   

Further inhibiting healthy horizontal facial growth, steeply interdigitating unworn teeth and steep or deep overbites can lock together the upper and lower jawbones in a manner that prevents their independent growth by preventing adaptation at the maxillo-mandibular suture, the bite table. The upper jawbone cannot expand, because it is locked by the bite to a mandibular corpus that can only translate, and the mandibular corpus cannot translate, because it is locked by the bitre to an upper jawbone that 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 reduces adaptive capacity in the tissues supporting the teeth. At the same time, because of the imbalance in size between jaw opening and closing muscles, chronically elevated central nervous system stress reduces the freeway space, the maxillo-mandibular joint space, which can cause non-functional tooth contacts and chronically 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, 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 and ensure adequate airway space. 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, which is the goal of the following manuscript.