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. For example, it can grow long and narrow to rip and tear meat or flat and wide to grind vegetation. The human jaw system combined features of  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 on the top of an upright spinal column. Balancing it there required incorporating the jaw muscles into the network of postural muscles that pull down all around the head to create an upright resting posture with a uniform low muscle tonus throughout. The mandible became 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. 

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 head up and 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 bite forces. Strong bite forces drive the sides of the face more upward, forward, and outward; while weak bite forces allow the inherent eruption forces in and around the teeth to drive the lower jawbone down and back in various proportions, usually followed by the upper jawbone, except when the tongue intervenes to protect the airway.

This adaptive growth occurs in the upper and lower jawbones 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 has to function like a maxillo-mandibular suture to coordinate and harmonize their independent growth patterns, which requires some slippage and adaptation at the surface of the bite table. 

In natural growth, this adaptive growth created jaw systems that exhibited a remarkable functional harmony. In whatever central location the mandible braced or shifted, the three mandibular joints (the TMJs and bite table) shared support for the mandible.  As the bite surfaces wore down, bite table stability was maintained by continual eruption of the teeth and continual growth 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. After maturity, the natural human jaw system was designed to maintain its functional harmony with age by gradually transforming in a manner that made it progressively more easily operable by aging tissues. As the respiratory muscles weakened about 5% per decade, maxillary expansion reduced nasal airway resistance, and mandibular advancement reduced pharyngeal airway resistance.  The mandible usually advanced to an edge-to-edge bite with age, even in tribes who ate soft non-abrasive foods like tree fruit and did not encounter rapid tooth wear. At the same time, the bite table narrowed due to wear on the sides of the teeth, while wear between the teeth (interproximal wear) made the bite table shorter and more stable so it required less strength to operate. Also, as jaw movements became smoother and less ballistic, neuromuscular reflexes became quieter. The system could maintain its structural integrity and functional harmony - even as the teeth wore down to their root tips.

This human jaw system could create and maintain a functional harmony that fit almost any type and degree of chewing, but not insufficient chewing. Human chewing has 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; typical facial growth patterns produce 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 destabilizes the bite table and displaces the mandible, generally more downward and backward, and often to one side. Without sufficient bite forces, 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. At the same time, 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 the mandibular rotation has significantly lengthened the average modern face, so it no longer remains proportional to body height but 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 only structure in there is the tongue base. 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 can 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 reduces the potential for slippage and adaptive shifting in the bite. 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.