Summary
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 before developing a model of how they should work. Without understanding bites, we avoid treating them, except esthetically. We allow strained bites to warp the faces and craniofacial structures of our children, while we straighten their teeth. To understand how bites work requires understanding their role in the growth and physiology of the jaw and postural systems.
BACKGROUND - In mammals, the bite table is the stable architectural feature around which the rest of the face grows. In different types of mammals, the shapes of the bite table and the rest of the face have been customized to fit the chewing tasks required of them - narrow and sharp to rip and tear meat, flat and wide to grind vegetation, or long antero-posteriorly to gnaw. The human jaw system and facial structure combined all these features for adaptability that enabled it to deal with almost any food source.
The most distinctive feature of the human jaw system was its location suspended from a cranium balanced on the top of an upright spinal column. That change from quadruped to biped required reshaping the cranium to move its center of mass back to a position directly above 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, the mandible formed the top of the chain, where it provided a shield that transferred postural forces away from the delicate facial structures and around to the sides of the cranium, where the zygomatic arches and temporal bones provide plenty of supporting bone. The jaw muscles were incorporated into a network of postural muscles that maintained low tonus at rest and therefore readiness for action.
One reason for the success of this human jaw system is that it was able to establish a remarkable functional harmony which maintained the health and goodness of fit among all its components; and also to transform that functional harmony with age in a manner that made it more easily operable by aging tissues. The TMJs and bite table shared support for the mandible in its direction of motion throughout its range of motion while chewing forces pumped it forcefully and rhythmically up against the underside of the cranium to pump circulation through all the affected tissues. In the jaw muscles, alternating firings between opposing muscle groups drove out waste products during each firing and allowed arterial resupply between firings. During adulthood, as muscles weakened, the bite table became smaller due to wear on the biting surfaces and more stable due to wear between the teeth (interproximal). 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 other reason for the success of this human jaw system is that each individual jaw system customized its growth process to grow 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 that is determined by bite forces. That growth to fit bite forces enabled the system to acquire a form that fit all kinds of bite forces.
However, the growth processes in the upper and lower jawbones respond to bite forces by completely different mechanisms, in slightly different directions, and on overlapping but somewhat different time scales; so they need coordination. 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 and translates forward (advances). Harmonizing these diverse growth processes requires sufficient bite forces to coordinate their growth. In between the upper and lower jawbones, the bite table functions like a maxillo-mandibular suture to coordinate the growth patterns and compensate for variations in growth above and below it.
After the second decade of life, when other growth processes stop, this jawbone growth slows down about 90%, but then continues slowly in a pattern that was built into our genetics in order to ensure our longevity. 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 to replace every micron of lost tooth structure. 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 growth to compensate for it. In this manner, the human jaw system maintains an adequate airway and a stable bite table, even as the teeth wear down to their root tips.
This human jaw system was able to grow to fit almost any type and degree of bite forces, but not to a lack of bite forces. Chewing 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. Then, a couple of centuries ago, 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.
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. The change in bite force inhibits normal facial growth, resulting in irregular and asymmetrical craniofacial skeletons and an average redirection of facial growth downward and backward. The modern human facial growth pattern locates the mandibular corpus (and chin) too far posteriorly, where it impinges on airway passage. Mechanical strains are continuously produced between upper and lower jawbones that can never acquire a perfect fit. 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. The tongue is crowded against the cervical spine.
Further inhibiting healthy facial growth, the 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 cannot expand, and the mandibular corpus cannot translate, because it is locked to an upper jawbone that grows by expanding rather than translating.
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.