Chapter 4

 CH 4) GROWTH OF THE MODERN HUMAN MASTICATORY SYSTEM

Within the last couple of centuries, in a development which was extremely sudden by evolutionary standards, the industrialization of our food made it so soft that the masticatory system stopped receiving adequate functional forces to stimulate intramatrix growth and to regulate the growth of the face.  The human diet had been softening ever since the use of fire for cooking and softened further in the transition from hunting to farming.  Then, with the rapid spread of industrialization in the nineteenth and twentieth centuries, food became so soft that it no longer provides a coherent resistant food bolus on which the mandible can pivot during power-crushing.  The resulting change in functional mandibular range of motion and in jaw muscle development has had surprisingly important impacts on facial growth. 

LOSS OF JAW MUSCLE STRENGTH

Softening of our diet has weakened our jaw muscles by half.  Without tough resistant foods to stimulate the ideal type of exercise the jaw muscles get from healthy functional chewing, bite forces no longer increase rapidly with age after the primary dentition.  Similar losses of jaw muscle strength can be produced by raising various species of animals on soft food diets.

LOSS OF OCCLUSAL STABILITY

Accompanying the loss of jaw muscle strength has been a loss of occlusal stability.  Pre-industrial humans always had stable occlusions.  Their jaw muscle strength was proportional to their overall muscle strength, because it was a function of how much muscle could grow where it was needed.  In contrast, modern humans often have unstable occlusions that prevent the jaw muscles from developing to their full potential.  An irregular occlusal interface is a difficult template for the jaw elevator muscles to exercise against, because it continually activates protective reflexes that limit jaw muscle forces.  The masticatory system functions more carefully, somewhat like the way your leg muscles would function when walking barefoot on gravel.  Studies have shown that restrictive occlusal interferences inhibit jaw closing forces and cause narrowing of the range of movement of the mandible, while elimination of occlusal interferences leads to increased bite forces.  Our jaw muscles cannot develop to their full potential if the template against which they exercise does not support strong masticatory activity.  As a result, jaw muscle strength is now better correlated with occlusal stability than with overall muscle strength.  Even within individual masticatory systems, the jaw muscles are stronger on the side of the more stable posterior occlusion.

The occlusal tables in modern humans are no longer smoothly contoured surfaces that perfectly fit the mandibular range of motion.  Instead, many modern dental occlusions have an occlusal surface comprised of small facets that only fit simultaneously when the mandible is braced within a small central area.   The occlusal interface has become a surface with irregular jagged peaks and valleys instead of a smoothly sloping bowl-like  shape.  An illustration of a typical modern occlusal interface can be seen below:

 

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Conversely, jaw muscle weakness can cause occlusal instability by limiting the forces that maintain the alignment of the occlusal surfaces of the teeth.  The bracing and masticatory forces of the jaw muscles produce and maintain occlusal stability.  Diminishing those forces diminishes occlusal stability.  Studies have shown that baboons raised on soft food have decreased occlusal stability.

Additional contributers to modern occlusal instability include the increased prevalence of caries and periodontal disease that have closely followed the spread of industrialized foods around the world.  Caries can cause drifting of teeth by removing tooth structure that was maintaining interproximal contacts or occlusal contacts.  Subsequent fillings can restore lost tooth structure, but they are rarely able to reverse any misalignment which may have occurred.  Periodontal disease can cause drifting of teeth by changing the bony support around their roots.   

NARROWING OF THE MANDIBULAR RANGE OF MOVEMENT  

Softening of our diet has also narrowed our mandibular range of motion.  Chewing pathways automatically widen in response to tough foods, while they stay close to the midline for simple mashing of softer foods. 93  Pre-industrial humans ripped, tore, and crushed food in long gliding strokes that pass over the intercuspal area and follow through to the non-working side; while modern mastication involves primarily mashing the bolus by squeezing it between the teeth, while the mandible stays close to the midline and often stops for about 100 msec before the next cycle begins.  The average change in masticatory pattern can be seen in the illustration below comparing an Australian aborigine chewing pattern (on top) with a modern european chewing pattern (below). 

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The dramatic change that has occurred in chewing pathways during the last couple of centuries can also be seen by comparing frontal tracings of the mandible during chewing in modern adults (left below) with those of young and middle aged Australian aborigines (center and right below).  The chewing pathways of young aborigines are already flatter than those of modern adults, and adult aborigine chewing pathways are much flatter still.   While the contact glide in Australian aborigines is about 3 to 4 mm long, the contact glide in modern Europeans is only about 1 mm long.94 

 

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VERTICALIZATION OF MASTICATORY SYSTEM COMPONENTS

The narrowing of the functional mandibular range of motion has caused a narrowing of the articular components of the masticatory system.  Growth is redirected vertically - a good example of function dictating form.    The resulting verticalization of the TMJs can be seen in deeper glenoid fossae and steeper articular eminentia.  Studies of rabbits raised on soft food show  similar changes in the contours of the TMJs. 

The verticalization of the occlusal table can be seen in the deep interdigitation and steep curves of Wilson and Spee which are now maintained throughout life instead of flattening with age as they did in our pre-industrial ancestors.  Masticatory forces have so little influence on teeth positions that the contours of the occlusal table are now more strongly determined by the angles at which teeth erupt than by the functional range of motion of the mandible; even though those eruption paths were designed to continually supply tooth structure at the occlusal table – not to determine the functional range of motion of the mandible.  When the functional forces are too weak to overcome the resistance provided by a form designed for aligning the newly erupting teeth, form dictates function.

MAXILLO-MANDIBULAR SYNOSTOSIS

The verticalization of the occlusal table has caused a relative locking together (partial synostosis) of the maxillo-mandibular suture.  The translation of the mandibular corpus is restricted by connecting it to a maxilla that expands rather than translates, and the expansion of the maxilla is restricted by connecting it to a mandibular corpus that cannot expand but can only translate.  Experimental synostosis of craniofacial sutures in animals restricts growth of the most directly involved bones and disturbs the growth pattern in the whole region. 95   A partial synostosis of the maxillo-mandibular suture may have similar effects. 

RESTRICTED INTRAMATRIX GROWTH

The partial synostosis of the maxillo-mandibular suture and the loss of jaw muscle functional forces have acted synergistically to produce many of the same changes in craniofacial form.  At times it may be difficult to determine which is cause and which is effect.  Even in otherwise well controlled animal experiments, the effects of weaker chewing forces and a narrowed mandibular range of motion cannot be separated, because the animals automatically narrow the mandibular range of motion in response to the softening of the food that also evokes less forceful jaw muscle use and less jaw muscle development. 

Together, the partial synostosis of the maxillo-mandibular suture and the loss of jaw muscle functional forces have caused an average change in primarily the lateral and lower portions of the face.  Comparing pre-industrial human skulls with modern human skulls makes them look like two different species.  However the change in growth of the the lateral and lower portions of the face has to be seen against the background of other changes, such as a rounding of the cranium and increased elongation of the cranial base, that have also occurred during the same time period.  For that reason, all these changes will be discussed below.

ROUNDING OF THE CRANIUM

The expansion of the neurocranium occurs about 90% prenatally, therefore most of its growth is unaffected by its functional environment.  However, the average weakening of skeletal muscles which has gradually accompanied our change to a less physically demanding life style has had some effect on postnatal neurocranial expansion and thereby has slightly affected the shape of the cranial vault.  On average, the cranial vault has become slightly rounder as a result of a growth pattern that is influenced less by muscle forces and more by the circumferential expansion of the brain.  Long skulls have become shorter, and wide skulls have become narrower, with both dolichocephalics and brachycephalics normalizing to become more mesocephalic, much like infant skulls which have not yet been influenced by the pulls of the musculoskeletal system.96 97 98 99 100

INCREASED CARTILAGINOUS ELONGATION 

At the same time, the rate and extent of elongation at cartilaginous growth centers has increased.  The cause is thought to be the increased sugar and carbohydrate consumption on growth hormone production. 101 102 103 104 105 106 107 108 109 110 111 112 113    One result of the increased cartilage growth is that people have become significantly taller within a generation or two.  A second result result of the increased cartilage growth is that vertical facial height has increased by pulling the mandible further downward relative to the rest of the cranium.  A third result is that protrusion of the center of the face has increased by pushing the nose and surrounding structures further anteriorly relative to the rest of the cranium.  

REDUCED LATERAL AND LOWER FACIAL GROWTH

Alongside the slightly increased protrusive and vertical growth in the center of the face and beneath the slightly rounder cranium, the downward and forward growth of the lateral portions of the midface and the entire lower face has changed more radically and in a very different manner.  This intramatrix growth has changed its average direction.  It grows more downward and less forward, especially at the front of the face.  The intramatrix growth processes involving expansion of the maxilla, anterior translation of the mandibular corpus, and forward rotation of the mandibular corpus have been inhibited.  Much of the inhibited facial growth has been redirected vertically.  Similar changes in facial growth have been produced experimentally in rats, monkeys, rabbits, hyraxes, and minipigs by raising them on soft diets.  Similar changes have also been recorded in humans with muscle disease.

DECREASED MAXILLARY EXPANSION 

One of the most obvious effects of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been a decrease in the rate and extent of maxillary expansion.  On average, modern palates are remarkably narrower than those of our ancestors.123 Similar maxillary narrowing has also been produced in animals raised on soft diets124 125 126  , and in humans without masseter and pterygoid muscles.127  Monkeys raised on soft diets often develop crowding of the upper teeth much like that frequently seen in modern children.128  

Decades ago Sir Arthur Keith observed, "Misplacements of the teeth, long narrow dental arches, high vaulted palates, and carious teeth, which are so common among Englishmen of today, were almost unknown amongst the British people of the Neolithic and Early Bronze periods; these conditions make a sporadic appearance as the Roman period is approached, becoming more frequent in this period.  They are conditions which are rarely seen amongst the remains from Saxon graveyards.  Indeed they do not assume anything approaching their present frequency until the eighteenth century is reached and England entered upon her life of industrialism." 

The diminished maxillary expansion has affected the zygomatic processes that buttress the lateral portions of the maxilla against the cranium.  They rotate more down than out, causing the appearance of “sunken” cheekbones that many people have observed when comparing modern humans to pre-industrial humans.

DECREASED PROTRUSION OF THE CORPUS

A second effect of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been a decrease in the rate and extent of anterior translation of the mandibular corpus.   A comparison of late medieval and recent Finns (minimizing genetic mixing) showed a 6% decrase in mandibular length despite overall skull size increases.(Varrela J. Dimensional variation of craniofacial structures in relation to changing masticatory functional demands. Eur J Orthod 1992;14:31-36.)  Similar decreases in size of the corpus and retrusion of the posture of the corpus also occur in patients with injury or disease that weakens the craniofacial muscles.  

While prognathism was positively associated with jaw muscle strength in pre-industrial humans, it is only positively correlated with jaw muscle strength in modern humans when jaw muscle strength is high enough to exert a significant effect on growth.   In many people there is so little anterior facial translation produced by intramatrix facial growth that total facial prognathism is determined more by cranial base shape than by functional forces or jaw muscle strength.  When jaw muscle strength is so low that it doesn't stimulate anterior translation of the corpus, the position and posture of the corpus is determined more by the resting tensions in the surrounding myofascial curtains than by stimulation of intramatrix growth processes.  

One result of the retrusion of the mandibular corpus is that class two malocclusions have become much more common today.  In pre-industrial humans, class two malocclusions comprised only about the same small percentage of the population that they did in other primates; but in modern humans they have come to comprise about a quarter of the population. 

A second result of the retrusion of the mandibular corpus is retrusion of other facial features.   The corpus usually leads the rest of the face in its growth pattern, but the midface follows.  Studies have shown that many modern children who have a class 1 malocclusion nonetheless have both upper and lower jawbones in a retrusive position relative to the cranium.  Studies comparing modern and ancestral populations of Japanese 114 , Egyptians, and Americans 115 have shown that the modern skulls have more retrusive midfaces than those of the recent past.116

A third result of the relative retrusion of the mandibular corpus, that does not occur when it is combined with backward facial rotation, is a pattern of tissue damage and regressive remodeling at the posterior aspects of the condyles and progressive remodeling on the anterior aspects of the condyles.117 118  Over time such remodeling may produce a forward bend in the condylar neck.  A similar pattern of remodeling has been produced in the TMJs of animals by using inclined planes or chin cups to retrude the corpus.  Autopsy studies have shown that the lesions of osteoarthritis often appear first on the posterior surface of the condyle, and a scanning electron microscope study found osteoclastic resorption on the posterior slope of the condyle in 19 of 20 joints examined.  The remodeling of the TMJs may be influenced in an opposite direction by backward rotation of the corpus, as described below.

A MORE BACKWARD FACIAL ROTATION                     

A third effect of the weakening of the jaw muscles and the verticalization of the mandibular range of motion has been an average change in the direction of rotation of the mandibular corpus. Generally faces no longer rotate so strongly forward, and some rotate backward, a situation never seen in pre-industrial humans.

The facial shelves, in proportion to their distance from the cranial base, have followed the rotation of the corpus and fanned out anteriorly more than they used to.  Even the angle of the cranial base has been affected, becoming slightly more acute in modern humans as the front portion of the cranial base has rotated backward along with the rest of the face.121 122

The role of masticatory forces in the changed pattern of facial rotation has been demonstrated experimentally.  In one study of children with backward rotating faces, using an exercise gum temporarily reversed their direction of facial rotation. 119 

 One result of this more backwardly rotating direction of facial growth is that the gonial angle, where the backwardly rotating corpus meets the very stable ramus, has become more obtuse. 120   At the border between these two divergently growing regions, just anterior to the gonial angle, it's become common to find an antegonial notch.  

A second result of this more backwardly rotating direction of facial growth is to change the orientation of the mandibular anterior teeth.  With the roots of the mandibular incisors carried down and back by their basal bone, their crowns don't upright as readily as they did previously. The long axes of upper and lower incisors form a smaller angle than in they did in pre-industrial humans.  

A third result of this more backwardly rotating direction of facial growth is regressive remodeling on the anterior aspect of the condyle.  Some children with this condition have been diagnosed as having "idiopathic condylar resorption".  Actually the resorption is caused by the mandible rotating backward around a pivot point somewhere in the molar area rapidly enough to drive the anterior aspects of the condyles into the articular eminentia with enough force to trigger rapid regressive remodeling.

VERTICAL INCREASES AT THE FRONT OF THE FACE

Much of the facial growth which is restricted horizontally has been redirected vertically, especially at the front of the face.  Loss of mandibular elevator muscle forces has been shown to lead to dramatic increases in the vertical dimensions of the anterior face - whether the loss is caused by disease which impairs muscle development129, cutting or removing muscles or motor nerves, or natural trauma.  When experimental impairment of the mandibular elevator muscles is carried out unilaterally in animals, increased dental height occurs on the side of impairment. In population studies, the height of the front of the face is inversely proportional to jaw muscle strength.130 131  In experimental studies, adolescents with a facial growth pattern charactertized by excessive vertical growth experienced improved their facial growth patterns when using an exercise gum to increase jaw muscle strength.( Ingervall B, Bitsanis E. A pilot study on the effect of masticatory muscle training on facial growth in long-face children. Eur J Orthodont 1987;9:15-23)  Orthodontists have long recognized that many of the problems in their patients today are due to excess height at the front of the face.132  

The front of the midface follows the verticalized growth of the corpus.  One result is an increase in the distance from the incisal edges of the upper central incisors to the nasal floor.   Gummy smiles, never seen in pictures of tribal peoples, have become common.  In some modern faces, the framework of bones and teeth has become so long that it impinges on facial muscle resting lengths.  The lips and other perioral muscles may show visible strain when trying to maintain a seal during swallowing or even at rest, the freeway space may be obliterated, and the passive tension of stretched masseters may narrow the maxillary arch.  

The vertical increases at the front of the face now continue significantly during adulthood.  While our pre-industrial ancestors had facial heights which remained steady to maintain the optimal resting length of the mandibular elevator muscles during adulthood, many modern faces keep getting longer at a rate which has been measured at .37 mm per year in the third and fourth decades of life. 133    

One contributor to the increase in height at the front of the face may be the eruption force of the teeth and the surrounding alveolar bone.   If their eruptive force is greater than the axial pressure on teeth produced by the jaw closing muscle forces that naturally limit tooth eruption, the teeth can supererupt – elongating the whole framework of bones and teeth at the front of the face.  It's interesting that the rate at which teeth continually wore down and erupted in some pre-industrial cultures is very similar to the rate at which faces now continuously lengthen in modern post-industrial cultures where teeth no longer wear down significantly.      

AVERAGE OVERALL CHANGE IN FACIAL MORPHOLOGY

The changed pattern of intramatrix growth is only partially absorbed by remodeling at the interfaces between the intramatrix growth processes and the rest of the craniofacial skeleton.  As a result of the intramatrix facial growth that is not masked by remodeling, modern human faces grow, on average, longer vertically, narrower, and more retrusively than they did just a couple of centuries ago.  This average change in facial shape has not affected everyone the same way, and it is not easy to quantify against a background of genetic mixing and normal variation which sometimes dwarf it.   However, when large samples are compared and genetic mixing is minimized; the same tendency toward longer, narrower, and more retrusive facial features has occurred in all racial groups in all parts of the industrialized world.   

The average change in profile can be seen in a superposition of the facial shape of modern Swedes (dotted lines) with those of Australian Aborigines (straight lines) in a sagittal plane as shown below.

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The average change can also be seen in the increased convexity of the modern midface, as shown on the left below. 

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With the face becoming longer, narrower and more retrognathic - even in those with strong skeletal muscles and short wide cranial vaults, mismatches may be produced between the shapes of the vault and the face.  Pre-industrial humans with strong overall musculature had shorter and wider vaults (brachycephalic) with shorter and wider faces (euryprosopic), and pre-industrial humans with weaker overall musculature had longer vaults (dolichocephalic) with longer faces (leptoprosopic).  The correlation was so consistent that some researchers even refer to short wide faces as brachyfacial and to long narrow faces as dolichofacial.  However, today some people with strong musculature may have relatively long narrow faces, because they failed to develop the jaw muscles as much as their other muscles.  Thus, unlike in skeletal remains, leptoprosopic faces can be found on brachycephalic crania.

AVERAGE OVERALL CHANGE IN HEAD POSTURE

Accompanying the average change in facial shape has been a change in the average posture of the head in a sagittal plane.   As the mandibular corpus has shifted posteriorly relative to the long axis of the spine, the center of mass of the head in normal standing posture has shifted anteriorly relative to the long axis of the spine.  Forward posture of the head and backward posture of the mandible are highly correlated in population studies. 

Forward head posture and backward mandibular posture are synergistic - either one can cause the other.  Forward head posture can cause backward mandibular posture by stretching the muscles and fascia that attach the mandible to the clavicles and sternum and thereby preventing the mandible from shifting as far forward as the head. 134 Backward mandibular posture can cause forward head posture by evoking adaptations to protect the airway. The muscles of the head and neck are controlled by a hierarchy of neuromuscular reflexes, and airway protection is at the top of that hierarchy.   All the muscles of the area acquire whatever resting postures are needed to hold the bones in whatever positions they need to maintain an adequate airway. Because the mandible surrounds the airway on three sides and the cervical spine borders its fourth side, backward mandibular posture can constrict the airway between the mandible and the cervical spine (middle illustration below) and thereby trigger these airway protective reflexes. The muscles respond by extending the head in order to rotate the mandible forward away from the cervical spine and out of the airway space.

 However the head cannot just tip backward, because the visual orientation reflex keeps it level with the horizon.  This reflex was able to bend the entire cranium in mice forced to live standing on their hind legs, and it causes extension of the head to maintain a useful visual field in spite of the drooping eyelids in people with palpebral ptosis.  In people with mandibular retrusion, it causes extension of the head to be accompanied with forward shifting of the head in order to pull upward and forward on the mandible while maintaining its line of sight, as seen in the illustration on the right below.

 

 

 

 While the causal relationship between backward mandibular posture and forward head posture can go both ways, there is good evidence that backward mandibular posture usually comes first. Growth studies have found much stronger associations between mandibular growth and subsequent body posture than between body posture and subsequent mandibular growth.140 141 142  The ability of backward mandibular posture to cause forward head posture can be seen in natural experiments when children who undergo TMJ ankylosis or other injuries or defects that prevent the mandible from translating forward with the rest of the face during growth acquire extreme forward head posture simply as a result of the extreme backward mandibular posture. The ability of pharyngeal airway blockage to cause extension of head posture has been demonstrated by studies which showed that many children with swollen tonsils have extended head posture135, which reverses quickly after surgery to remove the swollen tonsils.136 The ability of extension of the head to move the hyoid bone anteriorly137 and thereby increase pharyngeal airway space has been demonstrated by imaging.138 139

 

 

Backward mandibular posture can be caused by a retrognathic or distalizing dental occlusion because, within the boundaries needed to maintain an adequate airway, one of the most important determinants of mandibular posture is the location of the habitual mandibular bracing position. This position provides a home base for the masticatory system. The jaw muscles are programmed to brace the mandible there by clamping it up against the underside of the cranium immediately whenever danger is detected, when the postural system needs stabilizing for applying external forces, and at the beginning of each swallow. Because bracing of the mandible is so central to the function of the masticatory system, the jaw muscles are programmed to hold the mandible in a postural position located just below its habitual bracing position in order to maintain fast easy access to that position.

 

The accomodation of mandibular posture to the location of the mandibular bracing position has been demonstrated in different planes.  Vertically, an immediate increase in freeway space follows the first occlusal contact after placement of a bite raising appliance, and an immediate return to the pre-treatment freeway space follows the first occlusal contact after removal of the bite raising appliance.13 Laterally, children who develop unilateral cross-bite undergo a shifting of the mandible in both bracing and postural positions to the side of the cross-bite motivated by an increased resting tension in the posterior temporalis on the side of the cross-bite14 15 ; and their jaw muscle resting tensions return to symmetry after correction of the cross-bite.16    Antero-posteriorly, monkeys who experimentally receive a protrusive occlusal interference exhibit an immediate increase in the tonus of the ipsilateral superior lateral pterygoid muscle.17

 

THE INFLUENCE OF PERSONALITY

The way an individual craniofacial structure is affected by restricted intramatrix facial growth  depends partly on the way that individual’s neuromuscular system responds to stresses and growth strains, and the response of each individual’s neuromuscular system depends partly on personality.  Because the end plates of the motor nerves are anatomically and physiologically  extensions of the brain, the state of tension in the skeletal muscles directly reflects the electrical state of the brain.  Therefore personality affects muscle resting postures which affect growth.  Studies have shown that individuals have relatively consistent, unique, physiological response patterns to a variety of stressors.  For example, a "muscle responder" will respond repeatedly with tension in the same set of muscles to a wide range of emotional stimuli.  Similarly provocation studies found that different types of people have different responses to a the same bite change.  Some people seem to compulsively focus on an experimentally placed bite interference and develop a habit of grinding against it, thereby increasing their mandibular elevator muscle activity.  Other people given the same bite interference will avoid it by decreasing their mandibular elevator muscle activity – even learning to swallow without touching teeth by inserting the tongue between the teeth to stabilize the mandible at the beginning of each swallow. 

Because of the influences of personality, a different modification of the growth process occurs in each different personality type.  The two most distinct personality responses, the aggressive responders and the passive responders, are illustrated below:

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AGGRESSIVE RESPONDERS  

Aggressive responders seem to react to a growth restriction by fighting against it.  They may develop unusually strong parafunctional habits (clenching or grinding) which can significantly alter the pattern of late craniofacial growth.  Often they are able to limit verticalization of the anterior facial skeleton and maintain a forwardly rotating facial growth pattern, as shown on the left in the illustration above.  

However these people often develop a deep overbite, because their anterior teeth do not provide a stable incisal platform against which the mandibular elevator muscles can brace and on which the mandibular corpus can rotate during growth.  Instead, the mandibular corpus rotates around a center closer to the molar area.  The posterior teeth may become intruded while the anterior teeth become extruded, leading to a deep overbite and a bilevel occlusal table in which the maxillary and mandibular anterior teeth are noticeably taller than the posterior teeth.   

In some cases aggressive responders establish a dual bite, characterized by two distinct bracing positions for the mandible.  In one bracing position, the mandible is locked back at the posterior end of its normal range of motion.  In the other bracing position, the mandible rests further down and forward against an alternative occlusal platform.   The presence of this second bracing position seems to protect the system, because it is not well associated with the presence of symptoms.  

Bruxing is not isometric like clenching, and may even be rhythmic like chewing, therefore it is generally a healthier form of exercise than clenching.  In addition, bruxing may occur with sufficient force to eliminate at least some aspects of the maxillo-mandibular synostosis.  However, in many people with relatively deep overbites, bruxing only delivers significant axially directed bite forces to the posterior teeth, and it intrudes or reduces only the posterior segments of the dentition. Even when the bruxism does include the anterior and posterior teeth in a healthy proportion, the pattern of mandibular movements that gets worn in generally lacks the variability produced by healthy chewing function in which the mandible pivots around a resistant bolus. 

PASSIVE RESPONDERS

Passive responders seem to react to a facial growth restriction by avoiding it as much as possible.  A strained bite is usually avoided by lowering the posture of the mandible sufficiently to avoid frequent tooth contacts.  The retrusive effect of the maxillo-mandibular synostosis may be minimized in the short term, but in the long term compensatory vertical growth is evoked by the loss of elevator forces and maxillary width is limited by the medially directed pressure of passively stretched masseter muscles.  As a result, the face rotates backward, as shown in the right side of the illustration above.  The face may grow extremely long anteriorly, with lips that are open at rest and visibly strained when trying to create a lip seal during swallowing. 

 

ADAPTIVE TONGUE POSTURES

Some of the responses to inhibited facial growth involve aberrant tongue postures.  The tongue may respond to airway demands by acquiring a posture very low in the mouth, as can be seen in some of the monkey experiments with blocked airways.  It may also acquire a resting posture interpposed between the teeth in order to provide a cushioned platform against which the mandible can rest.  This results in the tongue scalloping that is frequently seen in people with narrow palates or TMJ disorders.

 

LOSS OF GROWTH COORDINATION

The loss of jaw muscle strength and the narrowing of mandibular range of motion have also disrupted some of the regulatory mechanisms that are needed to ensure coordination of facial growth processes.  These regulatory mechanisms relied on the jaw muscles to provide stable symmetrical restings forces which shape bones and jaw muscle functional forces to symmetrically stimulate the diverse growth processes in upper and lower jaws. Without sufficient functional development, mandibular movement pathways are more irregular. In Aborigines chewing, opening and closing movements rarely cross, while in modern humans they often cross.143  

A result of this increased irregularity and decreased muscle symmetry is that faces grow, on average, significantly more asymmetrically and irregularly. Animals raised on soft diets develop less symmetrical craniofacial structures than normals, and anthropologists have noted that symmetry of the craniofacial skeleton and the range of variation of several facial angles and dimensions have risen markedly in recent centuries.  Weston Price observed that, in tribes still eating traditional diets, the people all looked like brothers and sisters; but when they switched to modern diets, they lost their resemblance. 

Increased irregularity and asymmetry can be seen in the pattern of adaptive remodeling which occurs in the TMJs.   While condylar remodeling in pre-industrial humans was generally consistent in direction and proportional to age, condylar remodeling in modern humans is more dependent on mechanical factors than on age.   As a result, it has become much more variable than it was in our ancestors.   

Increased irregularity and asymmetry can also be seen in the prevalence of malocclusion, which has recently risen  far beyond the 10 percent level found in earlier humans and in primates.144   Corruccini collected a great deal of cross-cultural bite data using a variety of native populations before and after adopting a modern diet and concluded that the increases in malocclusion occur directly in proportion to the change of diet.  He noted, "Cross cultural data dispel the notion that considerable occlusal variation is inevitable or normal.  Rather it is an aberrancy of modern urbanized populations.  Furthermore, the transition from predominantly good to predominantly bad occlusion repeatedly occurs within one or two generations' time in these (and other) populations, weakening arguments that explain high malocclusion prevalence genetically.  Cumulatively, over these study samples, there is no chance for consistent inbreeding, racial mixing, or genetic change accounting for the transition."145

As a result of the combination of lost growth coordination and average directional changes in craniofacial growth, there are now extreme variants in craniofacial structures, even among populations relatively sheltered from genetic mixing. Certain craniofacial skeletal structures that have become fairly common today are rarely or never found in skeletal material from pre-industrial societies. 

 

93   Proschel P, Hoffman M. Frontal chewing patterns of the incisor point and their dependence on resistance of food and type of occlusion. J Pros Dent 1988;59:617-624.

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