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 the 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. This repositioning of the cranium required reorganizing the skeletal muscles into chains pulling down all around the cranium to stabilize it and also required completely reshaping the cranium to move its pivot point with the spinal column to a location under its center of mass. The face moved all the way back to leave a small space for airway flow in front of the cervical spine. The mandible became a shield embedded in the chain of muscles that held down the front of the head, where it protected the delicate 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 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. It achieved such a harmony by a delayed growth process (compared to the other mammals) that enabled each individual jaw system to grow to fit the functional forces and the functional range of motion it was exposed to. By aligning themselves in response to the same forces, the components also maintained a good fit with each other. This process of form adapting to function was complex and sophisticated, because the upper and lower jawbones grow by completely different mechanisms, in slightly different directions, and on overlapping but different time scales. The membrane bone structure of the upper jawbone expands, while the thick cortical bone holding the lower teeth (the mandibular corups) gets pushed forward by growth behind it. These diverse growth processes meet at the bite table, which functions like a maxillo-mandibular suture to compensate for variations in growth above and below it. 

The other reason this human jaw system was so successful is its longevity. Its functional harmony slowly transformed during adulthood in a manner which fit the natural changes that take place in all aging tissues and thereby made the system more easily operable. As weakening of the jaw muscles reduced biting forces, the bite table became smaller due to wear. As neuromuscular reflexes slowed, jaw movements became smoother and less ballistic due to the loss of steep cusps, making the triggering of protective reflexes rare. At the same time, slow continuous jawbone growth maintained the stability of the bite table and the patency of the airway. Advancement and upward rotation of the front of the mandible kept supplying tooth structure at the bite table as the existing tooth structure wore away, and that growth was stimulated by biting forces so it occurred about as much as needed. People with strong bite forces generally had more tooth wear and therefore needed more growth. The advancement of the lower jawbone reduced pharyngeal airway resistance, while the slower expansion of the upper jawbone reduced nasal airway resistance.

This sophisticated craniofacial growth process was designed to grow and maintain a jaw system suitable for almost any type of functional forces, but not for a lack of 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. Also both of these horizontal jawbone growth processes are often blocked by retained 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 cannot expand but can only translate; and the corpus of the mandible cannot advance, because it is locked to an upper jawbone that cannot translate but 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, making them more V-shaped than U-shaped. Many people have palatal surfaces that no longer fit the tongue, eliminating the use of natural suction to prevent the tongue from dropping back into the pharynx. Also, the mandibular corpus no longer advances so far or so fast. It has become significantly more posteriorly positioned, which is part of the cause of the forward head posture that has accompanied it. Also the mandible it now rotates slightly downward and backward on average instead of slightly upward and forward. The rest of the face usually follows the mandible in its downward and backward rotation, 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 adaptation, and 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 disorders, forward head posture, and obstructive sleep apnea 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. Instead of counting on stsrong chewin forces to regulate our facial growth, we can employ tools such as orthodontics, implants, dental reconstruction, and exercise gum to shape our faces to protect our airways, dentitions, and TMJs.