Summary

SUMMARY

Humans were successful in evolution partly because our masticatory systems were designed by combining the previous mammalian masticatory systems into a new and highly adaptable hybrid that was able to acquire a remarkable functional harmony in a wide variety of conditions and using a wide variety of food sources.  That functional harmony involved all the masticatory system components in generating and absorbing a pattern of functional forces that maintained their health and their goodness of fit.  The occlusal table played an important role in that functional harmony, working like one big ball-and-socket joint in parallel with the two TMJs to absorb the forces applied by the mandible in bracing and chewing. In the middle of the occlusal table was a central bracing area that provided a platform against which the jaw muscles could immobilize the mandible by bracing it against the maxilla.  During chewing, waves of compression passed from the distal end of the working side mesially across the dentition, the TMJs, and the facial structures.

With age, the occlusal table and the functional harmony in the pre-industrial human masticatory system transformed gradually in a manner that also fit the changes which naturally occur in aging tissues.  The fast adaptable jaw muscle firing patterns and hyperactive protective reflexes that can deftly work the mandible in an intricate dance against a steeply interdigitating and often irregular occlusal table in youth became replaced by smoother steadier jaw muscle firing patterns that operated in long contact glides on well established facets in dental arches which continually shortened and narrowed due to occlusal wear .

The growth and development of this harmoniously functioning and gradually transforming human masticatory system was a sophisticated postnatal process which integrated several diverse growth processes by making them each grow to fit the same functional demands and thereby also made them grow to perfectly fit each other.   From the underside of an expanding cranial vault and an elongating cranial base, the upper and lower jawbones translated down and forward in the same general direction but motivated by very different mechanisms.  The mandible grew by elongation of the condyles and rami pushing its corpus anteriorly. The maxilla grew by expansion of a structure composed of membrane bones that were connected in the middle and widely buttressed in all directions against the skull above it.  With upper and lower jawbones both continually adapting their growth to fit the same set of normal functional stimuli, these structures also continually grew to fit each other.  

Jawbone growth was designed to compensate for all different rates of occlusal wear.  Because both mandibular and maxillary growth processes were stimulated by forceful mastication and the force of mastication was generally proportional to the occlusal wear, the upper and lower jawbones grew roughly in the amounts that were needed to compensate for occlusal wear. As a result, the face maintained a steady height, and the resting tensions of the muscles maintained a steady growth pattern throughout life regardless of the rate of occlusal wear.  In addition, because occlusal wear continued throughout life, these growth processes also continued throughout life, unlike growth in other parts of the body where adaptation continues but there is no persistent pattern of growth after the second decade of life.  Slow continual adult growth in response to functional stimuli kept the dentitions aligned and maintained an effective bracing platform and exercise template for the jaw muscles.

 

However the human masticatory system was never designed to adapt to a condition of insufficient functional stimuli.   In modern humans, without a consistent set of functional forces to which all the parts can mutually adapt, the parts never develop the remarkable goodness of fit that is seen in the masticatory systems of even our recent ancestors.  They used to wear in together and then slowly wear out together.  Now they often don't even wear in properly, and they fail not because they wear out but because their dysharmonious function causes damage of the one or more components.

 The cause of the dysfunction in the modern human masticatory system is the recent softening of our diet to the industrialization of our food.  This change has caused a significant weakening of our jaw muscles and narrowing of our chewing movements, which has redirected horizontal facial growth vertically.  The weakening of the jaw muscles has shifted the anterior and superior growth of the mandibular corpus downward and backward.  The rest of the structures of the craniofacial area have been affected generally in proportion to their distance from the corpus.  As a result, the average modern human face is significantly longer, narrower, more convex, and more retrognathic (compared to the rest of the skull) than the average human face from only a couple of centuries ago.  Anthropologists have noted the same changes in a wide variety of diverse populations in various parts of the world, and very similar changes have been experimentally induced in several species of animals by simply softening their food.  

While facial growth has altered its overall direction, it has also become more irregular and asymmetric due to a combination of genetic mixing and the weakening of the craniofacial muscles, which has diminished their ability to regulate facial growth.  As a result of the increase in irregularity and the average directional change in facial growth, there are skulls today that don't even resemble any of those found in the skeletal remains of our recent ancestors.

Symptoms only occur when adaptation fails, but the loss of functional jaw muscle forces has also diminished adaptive capacities.  Less vigorous rhythmic pumping of the mandible against the underside of the skull diminishes the potential for adaptive growth in areas such as the craniofacial sutures and the periodontium.   Diminished interproximal wear has reduced the ability of the teeth to form a continuous line of stable interproximal contacts which act like a line of interspinal joints connecting all the members of each dental arch in a way that maintains the range of motion, spacing, and alignment of the teeth.   Diminished occlusal wear has reduced the ability of the dentition to accomodate diverse facial growth patterns.

One system that has been affected is the airway.  The change in the direction of growth of the mandibular corpus to a more downward and backward rotation impinges on the pharyngeal airway. The simultaneous narrowing of the maxilla due to redirection of its growth can impinge on the nasal airway.  One response has been the extension and protrusion of head posture to restore the airway, a feature which has also become prevalent during the same general time period as mandibular retrusion.  

To eliminate these disorders in society does not require returning to hard diets like those of our ancestors, it just requires using an understanding of how the  masticatory system was designed to work in order to learn how we can establish a new functional harmony that suits our modern human masticatory systems. Chapter 1 describes how our masticatory system evolved.  Chapter 2 describes the functional harmony it attained.  Chapter 3 describes how it grew and developed in our ancestors.  Chapter 4 describes how it grows and develops now.  Chapter 5 describes some of the effects of this change.