Summary

BACKGROUND - In mammals, the bite table is the stable architectural feature around which the rest of the face grows, and its shape reflects the function of the whole craniofacial area and jaw system. For example, it grew long and narrow to rip and tear meat or flat and wide to grind vegetation. The human jaw system combined features of the previous mammalian jaw systems for adaptability that enabled it to deal with almost any food source. Its most distinctive feature is its location in a cranium balanced on the top of an upright spinal column, which required employing the mandible as a shield to protect the delicate facial structures from postural forces by 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 human jaw muscles were incorporated into the network of postural muscles that maintain a balanced upright stance with a uniform low muscle tonus throughout.

An important reason for the success of this human jaw system was that it grows and develops much later than in other mammals, enabling it to build 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 in a direction which is largely determined by the amount of stimulation they receive from bite forces. Strong bite forces drive the sides of the face more forward and outward, while weak bite forces allow the inherent eruption forces in and around the teeth to push the lower jawbone down and back, usually followed by the upper jawbone, except when the tongue intervenes to protect the airway. However, the adaptive growth in the upper and lower jawbones occurs 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 works like a maxillo-mandibular suture to coordinate and harmonize their independent growth patterns, which requires some slippage and adaptation at the surfaces of the bite table. 

This growth of the human jaw system in adaptation to functional forces in the upper and lower jawbones created jaw systems that exhibited a remarkable functional harmony. The three mandibular joints, (the TMJs and bite table), shared support for the mandible in every direction of motion from their fully braced or close packed locations.  As the teeth were exposed to functional forces and tooth wear, 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. People with strong bite forces experienced more tooth wear and therefore needed and achieved more shifting of the teeth and growth of the basal bones to compensate for it.

Also, after maturity, the human jaw system was designed to maintain its functional harmony with age by transforming in a manner that made it progressively more easily operable by aging tissues. After the second decade, jawbone growth slowed down about 90% and then continued slowly. As our respiratory muscles weakened at about 5% per decade, maxillary expansion reduced nasal airway resistance, and mandibular advancement reduced pharyngeal airway resistance, while face height remained steady in proportion to body height. Mandibular advancement to an edge-to-edge bite with age was almost universal, even in tribes who ate mostly tree fruit and did not encounter extreme tooth wear. At the same time, the bite table narrowed due to wear on the buccal surfaces, while wear between the teeth (interproximal wear) made the bite table shorter and more stable. As neuromuscular reflexes slowed, jaw movements became smoother and less ballistic, so those reflexes were rarely triggered. The system could maintain its structural integrity and functional harmony - even as the teeth wore down to their root tips.

This human jaw system was able to produce and maintain a functional harmony that fit almost any type and degree of chewing forces, but not to a lack of chewing forces. Our chewing forces have 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 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. Without sufficient functional forces to coordinate and harmonize the growth of the upper and lower jawbones; growth produces strains that require continuous adaptation.

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 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 the tongue and mandible 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 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. The backward component of our mandibular rotation frequently 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 are not growing harmoniously and thus can never acquire a perfect fit. 

Further inhibiting healthy horizontal 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 so-called freeway space between the teeth, the maxillo-mandibular joint space, which can cause non-functional tooth contacts which 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, 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.