Summary

SUMMARY 

Jaw systems led the way in evolution.  The mandible was the first bone to be attached to the body by a flexible joint, and the mechanism for attaching it led the way for attaching arms, legs, and other appendages.  Reptiles and amphibians could use their mandibles to crush and tear, but they could not really chew.  In mammals, the development of TMJs enabled mandibles to develop a horizontal component of movement, which enabled the rubbing of teeth, which enabled chewing.  The mammalian jaw system could be modified to fit almost any type of chewing task required of it.  The human jaw system combined features of previous mammalian jaw systems for species adaptability with added individual adaptability due to a delayed growth process that was stimulated by function.  This formula thereby enabled each jaw system to acquire a form that best fit whatever chewing tasks were required of it.  It enabled humans to process all types of food and to spread out all over the surface of the earth. 

The feature that made the human jaw system most remarkable was its location at the top of an upright spinal column.   Upright posture incorporated the jaw system with an airway passage through its center into the head posture mechanism; which was incorporated with the rest of the postural components into a physically balanced upright stance maintained by a network of low level muscle tonus that enabled the muscles to maintain good resting circulation throughout their capillary beds and thereby kept them ready for action.  Postural variations requiring freedom of movement of the mandible were accommodated on the bite table by a flat central area, which was produced in the primary dentition by a wide lateral thrust during mouth opening and then preserved by a power-crushing pattern in which the mandible pivoted on the bolus.  Developing together to fit the same functional and postural forces, the jaw and postural systems of our recent ancestors acquired a functional harmony that maintained the health of all the involved components as well as a perfect fit between them.

This harmoniously functioning human jaw system was the product of a sophisticated growth process in which the components grow to fit the biting forces to which they are exposed.  Upper and lower jawbones both employ growth mechanisms that are stimulated by biting forces, and they maintain harmony with each other by growing in parallel to fit those biting forces.  In between them, functioning like a maxillo-mandibular suture, the bite table coordinates their growth to compensate for normal variations in the supporting bones and teeth while accommodating the diverse growth patterns above and below it to maintain its steadiness and orientation as one of the central architectural landmarks in the facial growth process.  As a result, the face grows by shifting smoothly steadily down and forward from the cranial base and the underside of the front of the cranial vault, creating a steady and slowly transforming overall craniofacial matrix. 

Buried within this apparently steady craniofacial growth matrix, there are some surprisingly diverse growth processes that were built into the jawbones and their supporting tissues to maintain the steadiness of the bite table throughout life, despite whatever rate of tooth wear occurred.  Because they are stimulated by chewing forces, these growth processes occur as fast as needed to compensate for the loss of bite table height that occurs due to chewing.  People who chew very forcefully wear their teeth down fast and undergo faster growth to compensate for that wear. 

However, these compensatory growth processes are motivated by entirely different processes in the upper and lower jawbones.  The upper jawbone expands on a horizontal plane like a box that is opening.  Meanwhile, the mandibular corpus gets pushed from growth behind it that advances it while also rotating it upward in front to keep bringing the lower teeth upward and forward into the upper teeth.  Between these two differentially growing jawbones, the bite functions like a maxillo-mandibular suture able to compensate for variations in growth above and below it.  For example, an injury that alters growth on one side of the bite table (upper or lower jawbone) also alters growth on the other side of the bite table.

This compensatory jaw growth was designed to persist, even in the few people who lived to be very old, by altering its form and function with age in a manner which fit the natural changes that take place in all aging tissues.  As muscles weaken at the rate of about 5% per decade and neuromuscular reflexes become slower, chewing becomes easier and steadier.  The bite table becomes narrower due to wear on the sides of the teeth and shorter due to wear between the teeth; jaw movements become smoother and less ballistic due to the loss of steep cusps until the triggering of protective reflexes became rare; the cross-sectional area of the nasal airway enlarges due to continual maxillary expansion, and the cross-sectional area of the  oropharyngeal airway enlarges due to continual mandibular advancement.  At the same time, adaptive mechanisms such as continual eruption of the teeth and mesial drift maintain the orientation and stability of the bite table, whether tooth wear is fast or slow, while a relatively constant overall facial form is maintained by steady tonus in the craniofacial and postural muscles throughout life.

This adaptable craniofacial growth process enabled our jaw systems to grow to fit almost any type of functional forces, but not a lack of functional forces. Until the last couple of centuries, chewing in humans had been weakening for tens of thousands of years, making our heads grow rounder, our faces grow longer and narrower, and our mandibles grow less protrusively; but our jaw systems still maintained a functional harmony due to a perfect fit between form and function.  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, making human jaw muscles too weak to create and maintain a stable bite table or to regulate craniofacial growth well enough to maintain a good fit among its components. 

As a result of these changes due to the softening of the human diet, instead of contributing to the overall steady growth pattern of the face, intramatrix jawbone growth now often displaces it; resulting in increased irregularity and asymmetry of the craniofacial skeleton and an average redirection of the growth of the human face downward and backward.  Chewing forces no longer expand most upper jawbones enough to fit around the tongue or advance the mandibular corpus.  While the corpus used to rotate slightly upward and forward with age, it now rotates slightly backward on average, and the rest of the face usually follows, except when the tongue intervenes to protect the airway.  As a result, 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.  At the same time, horizontal growth is restricted by the bite, because the deep overbites and steeply interdigitated teeth that have become common today lock together the upper and lower jawbones.  The upper jawbone cannot expand, because it is locked to a mandible that can only translate; and the mandible cannot translate, because it is locked to an upper jawbone that can only expand. 

As a result of these interferences to normal jawbone growth, mechanical strains are continuously produced between upper and lower jawbones that can never acquire a perfect fit.  The strained craniofacial growth pattern requires rapid adaptation during the teenage years and continues to require adaptation during the slow growth of adulthood. The strains due to impaired facial growth only produce TMJ disorder 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, chronically elevated central nervous system stress contributes to increased jaw muscle tonus and further decreases adaptive capacity.  

THE SOLUTION - To eliminate TMJ and related disorders 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 acquire and maintain a new more delicate and carefully engineered functional harmony that suits our modern environment and life styles.