Chapter 4
CH 4) ALTERED GROWTH OF THE HUMAN JAW SYSTEM
The human diet had been softening ever since the use of fire for cooking, and then further in the transition from hunting to farming, accompanied by slight rounding of the cranium and lengthening of the face; but it was only after the rapid spread of industrialization in the nineteenth and twentieth centuries, when we started consuming most of our calories in liquids, oils, and soft food; that our jaw muscles became too weak to stimulate the horizontal growth needed to maintain adequate space for airway passage and to coordinate the diverse growth processes in the upper and lower jawbones.
REDUCTION OF JAW MUSCLE STRENGTH
The dramatic reduction in our chewing activity has caused an average loss of almost half of our jaw muscle strength. Similar losses of jaw muscle strength have been produced experimentally by raising various species of animals on soft food diets.
REDUCTION OF BITE STABILITY
Bite stability is a reflection of jaw muscle strength, and our bites have become less stable as our jaw muscles have become weaker. Baboons raised on soft food also have less stable bites.1 Our bites have become unstable, not only when measured as the number of simultaneous tooth contacts that occur in the most stable bite position, which is how we measure bite stability, but also as a dramatic reduction in the area within which the stability occurs. Even when our teeth fit perfectly in a textbook CR or MI bite, they don't fit so well when the mandible is moved to a different location. In contrast, pre-industrial bites fit well over such a large area that investigators cannot even identify a single centric bite position.
In modern humans, unstable bites sustain themselves, because they cause collisions between teeth, which trigger a reflex (the jaw opening reflex) that immediately shuts down the firing activity of the jaw closing muscles. Studies have shown that bite interferences reduce functional jaw muscle forces, while eliminating bite interferences increases functional jaw muscle forces. When a bite is unstable, the jaw muscles try to minimize collisions between interferences, which leaves them functioning in a manner that is guarded and hypervigilant, with increased resting tonus and decreased functional forces, somewhat like the way your leg muscles would function when walking barefoot on gravel. The guarded functional behavior then prevents those bites from receiving enough forces to align the teeth.
UNSTABLE INTERPROXIMAL JOINTS
One contributer to modern bite instability is the smaller interproximal contact areas that allow teeth to shift more easily buccally or lingually. The interproximal joints from a modern teenager are shown below. They can be contrasted with those from rural India in the last century, seen in chapter 2.
Although our interproximal contact areas are smaller than they used to be, they still help stabilize the arches. Matrix products should produce small flat contact areas bordered by convex curves, rather than convex contact areas, which are inherently unstable.
NARROWED MANDIBULAR RANGE OF MOVEMENT
Soft foods also narrow the mandibular range of movement. Chewing pathways automatically widen in response to tough foods.2 Experimental bite interferences cause narrowing of the mandibular range of movement, while eliminating bite interferences causes it to widen. Pre-industrial human chewing ripped, tore, and crushed food in long gliding strokes that followed through to the opposite (non-working) side. In contrast, modern humans mash most of our food, keeping the mandible opening and closing close to the midline, with the mandible often stopping completely for about 100 msec at the end of the closing stroke. In Aborigines, opening and closing movements rarely cross, while in modern humans they often cross.143 The illustration below is from a study comparing an Australian aborigine chewing pattern (on top) with a modern european chewing pattern (below).
The frontal view below contrasts chewing pathways in modern adults (left below) with chewing pathways in young and middle aged Australian aborigines (center and right below). 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.3
VERTICALIZATION OF STRUCTURAL COMPONENTS
As form follows function, the structural components of the human jaw system have also narrowed and lengthened. Generally the jawbone growth that is inhibited horizontally has been redirected vertically.
The verticalization can be seen at the TMJs in deeper glenoid fossae and steeper articular eminentia. Studies of rabbits raised on a soft food diet show similar changes in the contours of the TMJs.
The verticalization can also be seen in the dentitions. Teeth achieve balance in a location between the eruption forces and bite forces. In rats, a soft food diet leads to greater eruption of the teeth.4 In modern humans, the average facial height keeps increasing at an average rate of .37 mm per year in the third and fourth decades of life, which is about as fast as our teeth used to wear down.133
The verticalization of the bite table in modern humans can also be seen in the steep curves (Spee and Wilson), along which the teeth first erupt. These curves were designed to align the teeth and then continue supplying tooth structure at the bite table for as long as possible - not to limit or "guide" the mandibular range of motion. When functional bite forces are too weak to overcome the resistance to mandibular movement posed by these curves or other irregularities in the bite table, the bite table can dictate the range of motion of the mandible instead of being shaped by it. In modern humans, mandibular movement pathways have become much more irregular, because they are more frequently determined by the bite table than the jaw muscles.
The verticalization of the bite table and TMJs, the loss of bite stability, and the narrowing of the mandibular range of motion have significantly altered the way our faces grow; but quantifying these effects requires seeing them against the other changes that have occurred at the same time, including rounding of the cranium and increased elongation of the cranial base.
ROUNDING OF THE CRANIUM
Since the expansion of the neurocranium occurs about 90% prenatally, it is only slightly affected by the loss of bite forces. The pull of the jaw muscles downward on the sides of the cranium has decreased, - making the cranium rounder, because its growth is more affected by the circumferential expansion of the brain and less affected by vertical muscle tonus. Long crania have become shorter, and wide crania 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.5
INCREASED CRANIAL BASE ELONGATION
Cranial base elongation occurs later than neurocranial expansion, and it is also only slightly affected by the loss of bite forces. The general rate and extent of cartilaginous elongation has increased in modern humans, making us taller, making the face longer vertically by pushing the whole cranium further up and away from the shoulder girdle, and making the face more protrusive by pushing its center further anteriorly.
DECREASED ADVANCEMENT OF THE MANDIBULAR CORPUS
Beneath the front of the slightly rounder cranium and alongside the elongated cranial base, the growth at the sides of the face has changed direction, usually following the mandibular corpus. The average mandibular corpus used to advance while rotating slightly forward with age (upward in front); but it now advances less while rotating slightly backward with age. The inhibition of advancement has shortened the mandible. A comparison of late medieval and recent Finns (minimizing genetic mixing) showed a 6% decrease in mandibular length, despite overall skull size increases.
Instead of advancing while shifting upward in front (forward rotation), the mandibular corpus on average now shifts down and back (backward rotation). The backward rotation produces a more obtuse gonial angle, often just behind an antegonial notch where it meets the steady orientation of the mandibular rami.
The role of decreased bite forces in causing this change in the direction of mandibular growth can be implied from population studies, in which the height of the front of the face is inversely proportional to jaw muscle strength.130-131 It can also be implied from observations of facial growth when jaw muscles are diseased or damaged. Experimental impairment of the jaw closing muscles unilaterally in animals causes increased dental height on the side of impairment. Impairment of the jaw closing muscles by disease in humans results in extreme downward and backward rotation of the mandible. A longitudinal growth study of a patient with muscular dystrophy below shows extreme downward and backward mandibular rotation, compared with the white line showing normal growth in the X-ray on the right side below.
FACIAL GROWTH IN A PATIENT WITH MUSCULAR DYSTROPHY
In contrast, increasing bite forces in adolescents who used an exercise gum to increase jaw muscle strength changed backward rotating facial growth patterns to forward rotating facial growth patterns for the year of gum chewing, and then reverted back after the gum chewing stopped.147
One result of the verticalized growth of modern jawbones is an increase in the distance from the incisal edges of the upper central incisors to the nasal floor. Gummy smiles, never seen in tribal people, have become common. In many modern faces, the framework of bones and teeth has become too long to be covered comfortably by the curtain of soft tissues hanging down from the front of the cranium, so the perioral muscles show visible strain when trying to maintain a lip seal during swallowing. Frequently the redirection of mandibular growth direction down and back tips the upper central incisors back palatally, following the lower lip, which controls its position by impacting it during each swallow.
The rest of the craniofacial growth components generally follow the mandibular corpus in proportion to their distance from it, except when the tongue intervenes to protect the airway by altering muscle resting postures in ways that prevent the bones attached to them from following the corpus. Studies comparing modern and ancestral populations of Japanese 114, Egyptians, and Americans115 have shown that modern midfaces are more retrusive than those of the recent past.116 The cheekbones (zygomatic processes) that buttress the lateral portions of the midface seem to "sink" as they follow the mandible down and back. The facial shelves (orbits, upper jawbone, and lower jawbone) diverge and fan out anteriorly more than they used to, especially at the sides of the face. The orbits have become less rectangular as their lateral borders have shifted down and back following the mandibular corpus. Even the angle of the cranial base may be affected, becoming slightly more acute in modern humans its anterior portion rotates downward and backward following the mandible.121-122
NARROWING OF THE MIDFACE
With the sides of the face growing more down and back than the midline of the face, the palate has become narrower and deeper.6 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." James Sim Wallace found that average palate widths had shrunk from an average of 2.37 inches before the Industrial Revolution to an average 2.16 inches by the late nineteenth century. Monkeys raised on soft diets often develop crowding of the upper teeth, much like that frequently seen in modern children.7
The vertically lengthening of the face restricts maxillary expansion by tightening the lips and cheeks. The role of stretched jaw closing muscles in restricting maxillary expansion can be seen dramatically in the ability of Frankel appliances to expand the maxilla by simply holding the lips and cheeks out away from the teeth.
MAXILLO-MANDIBULAR SYNOSTOSIS (LOCKING TOGETHER OF THE JAWS)
The steeply interdigitated teeth and anterior overbite can also restrict both the expansion of the maxilla and the advancement of the mandibular corpus by locking them together, like a partial synostosis of the maxillo-mandibular suture. The mandibular corpus cannot translate forward easily, because it is locked by the bite to a maxilla that grows by expanding rather than translating, and the maxilla cannot expand easily, because it is locked by the bite to a mandibular corpus that cannot expand. Experimental synostosis of craniofacial sutures in animals disturbs the growth pattern in the whole region.95 The partial synostosis of the maxillo-mandibular suture disturbs growth throughout the face.
NET CHANGE IN FACIAL MORPHOLOGY
The net effect of these changes can be seen in studies by researchers who measured skeletal remains to compare the profiles of Nubians before (solid lines) and after (dotted lines) switching from hunting-gathering to an agricultural diet, shown below left. It can also be seen in a comparison between the average profiles of modern Swedes (dotted lines) with Australian Aborigines (solid lines), as shown below middle. Very similar changes can also be seen in the illustration on the right comparing patients with myotonic dystrophy (dotted lines) with normals.
NUBIANS BEFORE AND AFTER MODERN SWEDES AND ABORIGINES MYOTONIC DYSTROPHY AND NORMALS
With the growth of the face now influenced more by the bite contours than by the ability to build muscle, mismatches may be produced between the shapes of the vault and the face. For example, pre-industrial humans with strong overall musculature always had short and wide (brachycephalic) craniofacial regions, but there are now brachycephalic crania with long (leptoprosopic) faces, - which Enlow called the Dinaric head shape.
With the sides of the face growing more downward and backward than the midline of the face; our faces have become more pointed and less flat in a coronal view, as seen below.
LOSS OF SYMMETRY
The weakening of our bite forces has made craniofacial growth more asymmetrical, usually led by the position of the mandibular corpus, with the rest of the facial features following, in proportion to their distance from it.
One cause of the increased asymmetry is the loss of muscle strength. The muscles regulate growth. Muscle strength and symmetry are well correlated.
Another cause of the increased asymmetry is the inhibition of normal growth. In experimental studies, when normal growth is impaired symmetrically by a barrier such as nasal airway blockage, it results in more irregular growth. Monkeys with experimentally blocked airways show divergent responses.
The typical change that has resulted from changing to softer foods can be seen in the photo below of a mother and daughter from rural India. The mother's face is short and symmetrical. Her daughter's face is longer in front and twisted slightly to the right.
VARIABILITY AND PERSONALITY
The response of each individual’s neuromuscular system to the growth restriction depends on personality; therefore, a different modification of the growth process occurs in each different personality type. 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.
Aggressive responders seem to react to a growth restriction by fighting against it. In provocation studies, they 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 until they can wear it down or intrude the tooth. Many aggressive responders are able to limit verticalization of the anterior facial skeleton due to strong jaw closing muscle activity, although the mandibular corpus is usually locked back posteriorly behind a deep anterior overbite, because strong bite forces intrude the posterior teeth but cannot apply axial forces to limit the eruption of the anterior teeth.
In contrast, passive responders react to a facial growth restriction by avoiding it, which further reduces bite forces. They lower mandibular posture sufficiently to avoid frequent tooth contacts, resulting in a face that grows extremely long anteriorly.
FOOTNOTES:
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