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 move their mandible vertically, opening and closing it to crush and hold, and then with the neck muscles to tear, but they could not really chew. The mammalian TMJ enabled mandibles to move horizontally, which enabled rubbing teeth together, which enabled chewing. The chewing pattern and the shapes of the jaws and teeth could be modified to fit almost any type of chewing task. They could rip and tear for carnivores, grind for herbivores, and gnaw for rodents. The human jaw system 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 in a cranium balanced on the top of an upright spinal column by chains of muscles pulling down all around it. The mandible formed an essential member of the anterior kinetic chain while also protecting the delilcate structures of the face from postural forces by transferring them around to the sides of the head. 

One reason this human jaw system was so successful is that, in all types of functional environments, it was able to establish a remarkable functional harmony in which all its components worked together in a way that maintained a goodness of fit with each other and with the other components of the postural system; and that functional harmony slowly transformed during adulthood in a manner which fit the natural changes that take place in all aging tissues. In our evolution, old people were important for the success of our villages. They used to be wise, and they knew where things were hidden. As a result, the jaw system is designed to maintain a steady bite platform even as the teeth wear down to root tips, and the whole system is designed for longevity. As muscles weaken at the rate of about 5% per decade, the bite table becomes smaller due to wear on the sides of the teeth and between the teeth. As neuromuscular reflexes become slower, jaw movements become smoother and less ballistic due to the loss of steep cusps, and the triggering of protective reflexes becomes rare. At the same time, adaptive mechanisms such as continual eruption and mesial drift of the teeth maintain the orientation and stability of the bite table, whether tooth wear is fast or slow. The system only failed when the its articular components, the teeth and TMJs, had worn out completely. 

The other reason the human jaw system was so successful is that a delayed growth process enabled each individual jaw system to grow to fit the functional forces it was exposed to. The mechanism by which this growth adapts its form to fit its function is sophisticated and complex. The upper and lower jaws both grow to fit bite forces, however they grow very differently. The growth in the upper and lower jawbones occurs by completely different mechanisms, in slightly different directions, and on overlapping but different time scales. The membrane bone structure of the upper jawbone expands by unfolding while drifting down and forward and laying down new bone behind it in the circum-maxillary sutures which do not close like other cranial sutures. Meanwhile, the cortical bone holding the lower teeth (the mandibular corups) maintains its shape and gets pushed by growth behind it anteriorly while also rotating it slightly upward in front. These diverse growth processes require coordination. By growing to fit the same chewing forces, the upper and lower jawbones also grow to achieve the necessary goodness of fit with each other. Between them, the bite functions like a maxillo-mandibular suture able to compensate for variations in growth above and below it. 

These jawbone growth processes slow about 90% after the second decade of life when other bones stop elongating; but then they continue slowly during adulthood. One reason for the importance of this slow continual adult jawbone growth was to compensate for tooth wear, which also continued slowly during adulthood. To ensure that this compensatory adult jawbone growth occurred about as much as needed, it was stimulated by biting forces. People with strong bite forces generally had more tooth wear and therefore needed more growth. The other reason for the importance of this slow continual adult jawbone growth was to reduce airway resistance as muscle strength diminished about 5% per decade, so about the same amount of effort was needed to maintain adequate resting airway flow. while the cross-sectional area of the nasal airway continues to enlarge slowly due to continual maxillary expansion, and the cross-sectional area of the oropharyngeal airway enlarges due to continual mandibular advancement

This adaptable craniofacial growth process was designed to grow and maintain a jaw system suitable for almost any type of functional forces, but not for insufficient functional forces. Chewing in humans has 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 good functional harmony 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. Instead of contributing to the overall steady growth pattern of the face by maintaining a steady bite table, jawbone growth now often displaces the bite table; resulting in more irregular and asymmetrical craniofacial skeletons and an average redirection of facial growth downward and backward. Chewing forces no longer expand most upper jawbones enough to fit around the tongue or advance the mandibular corpus. At the same time, this horizontal jawbone growth is often blocked by retained deep overbites and steeply interdigitated teeth that 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 corpus of the mandible cannot advance, because it is locked to an upper jawbone that can only expand.

As a result, our faces have dramatically changed their average shape in the last few centuries. Upper jawbones and palates are much narrower, especially in the premolar region under the anterior portion of the nasal cavity. Also, the mandibular corpus no longer advances so far or so fast. It now rotates slightly downward and backward on average instead of slightly upward and forward with age. The rest of the face usually follows, except when the tongue intervenes to protect the airway and limit the displacement of the upper jawbone, as seen in anterior open bites and facial growth in the presence of muscle disease. 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. 

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.