Chapter 4
CH 4) STRAINED CRANIOFACIAL GROWTH
REDUCTION OF BITE FORCES
Within the last couple of centuries, in a development which was extremely sudden by evolutionary standards, our food became so soft that our jaw systems stopped receiving the bite forces they need to stimulate and regulate healthy facial growth. 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. However, these changes did not result in pathology or asymmetry. 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 properly regulate and coordinate the diverse growth processes in the upper and lower jawbones, leading to a widespread TMJ disorders as well as associated breathing and postural symptoms. Without tough resistant foods to stimulate the ideal type of exercise the jaw muscles get from healthy functional chewing, our bite forces only achieve about half the strength they did in our ancestors. Similar losses of jaw muscle strength have been produced experimentally by raising various species of animals on soft food diets, along with smaller body mass, less dense bone, narrower maxillae, smaller mandibles and condyles, and thinner tooth sockets.
REDUCTION OF BITE STABILITY
Bite stability is a reflection of jaw muscle strength, and our bites have become more unstable as our jaw muscles have become weaker. Baboons raised on soft food have less stable bites, and humans with a more stable bite on one side usually have stronger jaw muscles on that side. A study of modern humans chewing gum showed that strengthening their jaw muscles caused an increase in bite stability.1
Unstable bites sustain themselves, because they cause collisions between teeth, which trigger protective reflexes that limit jaw muscle forces, which prevent bites from receiving enough consistent closing forces to align the teeth properly. When chewing soft food, the teeth penetrate the bolus and collide, triggering a jaw opening reflex that immediately shuts down the activity of the jaw closing muscles. When a bite is unstable, the jaw muscles function in a manner that is guarded and hypervigilant, with increased resting tensions and decreased functional forces, somewhat like the way your leg muscles would function when walking barefoot on gravel. Studies have shown that bite interferences which cause collisions between teeth inhibit jaw muscle forces, while elimination of bite interferences increases jaw muscle forces.
The instability in modern human bite tables can be seen in its "occlusal interface", composed of jagged peaks and valleys, as seen below. In pre-industrial bites, the teeth fit well over such a large area that investigators cannot identify a single centric bite position. This phenomenon is not just due to wear, because it can also be seen in tribes that ate largely tree fruit, like bananas, and encountered little tooth wear. In contrast, in post-industrial bites like ours, if the teeth fit well, they only fit well within a small central area.
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 the interproximal contact areas are smaller than it used to be, they still help stabilize the arches. It's hard to understand why dentists try to make restorations with convex interproximal contact areas. They are not biomimetic in functioning teeth.
NARROWED MANDIBULAR RANGE OF MOVEMENT
Soft foods also narrow the mandibular range of movement. Chewing pathways automatically widen in response to tough foods, while they stay close to the midline for simple mashing of softer foods.2 Experiments have shown that introducing experimental bite interferences cause narrowing of the mandibular range of movement, while eliminating bite interferences causes widening of the mandibular range of movement. 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 human chewing mashes food while keeping the mandible close to the midline; and the mandible often stops for about 100 msec at the end of the closing stroke. The illustration below is from a study comparing an Australian aborigine chewing pattern (on top) with a modern european chewing pattern (below).
The same change has also been observed and recorded in a frontal view contrasting chewing pathways in modern adults (left below) with chewing pathways in 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.3
The forces applied by our modern mashing of food remain close to the midline, which prevents the delivery of transverse bite forces to the structures of the jaw system.
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 in both the bite table and the TMJs. The 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. Studies of rats show that a soft food diet leads to greater eruption of the teeth.4 The verticalization of the bite table in modern humans can be seen in its deep interdigitation and steep curves (Spee and Wilson), which were designed to align the teeth and then provide a constant source of tooth structure at the biting surfaces for as long as possible - not to dictate the range of motion of the mandible. The human bite table was designed to adapt its form to the range of motion of the mandible, not to limit or "guide" the mandibular range of motion. However, when the functional forces are too weak to overcome the resistance to mandibular movement posed by malpositioned teeth, form can inhibit function; and the contours of the bite table end up dictating the range of motion of the mandible. For example, the thick enamel on the palatal cusps of the upper molars and the buccal cusps of the lower molars was designed to provide a longlasting supply of working surface where it was needed, not to create balancing side contacts which shut down jaw muscle activity during chewing.
EFFECTS ON FACIAL GROWTH
Because the face grows to fit bite forces, the change in the bite forces created by our change in diet and the verticalization of its structural components has changed the way our faces grow. For example, on average, the mandibular corpus used to rotate slightly forward with age (upward in front); but it now rotates slightly backward with age. However, many of these changes in form due to altered facial growth are difficult to quantify, because craniofacial growth is a complex blend of different growth processes. Therefore, to quantify the effect of reductions and displacements of bite forces on facial growth, they need to be seen against other concurrent changes including rounding of the cranium and increased elongation of the cranial base.
ROUNDING OF NEUROCRANIAL EXPANSION
Since the expansion of the neurocranium occurs about 90% prenatally, most of its growth has been unaffected by our soft diets, but the weakening of our postural muscles has diminished the downward traction they exert on the cranium - making the cranial vault more affected by the circumferential expansion of the brain and less affected by the muscle tonus vertically. As a result, the cranium has become rounder. 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 Even the shapes of the orbits are probably affected. They have become less rectangular as their lateral borders have shifted inferiorly.
INCREASED CRANIAL BASE ELONGATION
At the same time, the increased rate of elongation at cartilaginous growth centers that is found in modern humans has increased body height as well as the elongation of the cranial base. The increased growth in the posterior cranial base has lengthened the face by pushing the cranium further up and away from the shoulder girdle. The increased growth in the anterior cranial base has elongated the face sagitally by pushing the center of the face further anteriorly. However, the lateral borders of the face are less influenced by cranial base elongation and more influenced by bite forces, as described below.
DECREASED MAXILLARY EXPANSION
Beneath the slightly rounder cranium and alongside the slightly increased anterior and vertical growth in the center of the face due to slightly increased cranial base elongation, our upper jawbones expand much less than they used to, making palates remarkably narrower than they used to be.6 Similar maxillary narrowing has also been produced in animals raised on soft diets, and in humans without masseter and pterygoid muscles.7 Monkeys raised on soft diets often develop crowding of the upper teeth much like that frequently seen in modern children.
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.
The decreased maxillary expansion is a result of the upper jawbone failing to receive enough bite forces to make the two paired maxillary bones unfold far enough to fit around the tongue. Much of the growth that is inhibited horizontally is redirected vertically, and the average midface now increases more in vertical length than in horizontal width with age.
CHANGE IN MAXILLARY SHAPE
Because the loss of bite forces primarily affects the sides of the face, causing it to grow more down and back, while the midline of the face is less affected by the loss of bite forces and more affected by the cartilaginous elongation that advances it; our faces have also become more pointed and less flat in a coronal view, as seen below.
REDIRECTED TRANSLATION AND UPWARD ROTATION OF THE MANDIBULAR CORPUS
Beneath the narrowed upper jawbone, the rotation of the mandibular corpus (the front half of the mandible) has been redirected. On average, it now grows downward and backward (backward growth rotation), producing a more obtuse gonial angle, where the backwardly rotating corpus meets the very stable ramus (controlled by temporalis posture). At the border between these two divergently growing regions, just in front of the gonial angle, it's become common to find an antegonial notch.
The vertical component of backward growth rotation now causes the average face to grow longer vertically with age after overall body height stops increasing. The vertically lengthening face further restricts maxillary expansion by tightening the jaw closing muscles and thereby increasing the force by which they push in on the outer edges of the dentition. 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 cheeks out away from the teeth.
The backward component of backward growth rotation 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. The backward component of backward growth rotation has also caused forward head posture, as described in the next chapter.
The role of decreased bite forces in causing this change in the direction of mandibular growth is demonstrated dramatically when the decreased bite forces are caused by disease which impairs muscle development, cutting or removing muscles or motor nerves, or natural trauma. When biting forces during the day are not strong enough to counterbalance the eruption forces that occur during sleep, the mandible lowers and the front of the face lengthens vertically. When experimental impairment of the jaw closing 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 backward rotating facial growth pattern changed to forward rotation while using an exercise gum to increase jaw muscle strength.147
The critical role of bite forces in this growth can be assessed by the effect of their absence. 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, because there are no bite forces to oppose the influence of gravity and eruption of the teeth and the alveolar processes. The tongue in its resting posture to protect the airway has prevented the upper jawbone from following the lower jawbone as far down and back.
FACIAL GROWTH IN A PATIENT WITH MUSCULAR DYSTROPHY
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, as described later). 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 where the teeth are located. The orbits have become less rectangular as their lateral borders have shifted down and back following the mandible. Even the angle of the cranial base may be affected, becoming slightly more acute in modern humans as the front portion of the cranial base has rotated downward and backward following the mandible.121-122
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 pictures of tribal peoples, 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 surrounding it, the lips and other perioral muscles show visible strain when trying to maintain a seal during swallowing. The passive tension of stretched masseter muscles narrows the upper jawbone, making vertical increases and midfacial narrowing synergistic changes. Frequently the redirection of mandibular growth direction down and back tips the upper central incisors palatally due to the palatally (inwardly) directed force from the lower lip. The activity of the lower lip, especially during swallowing, controls the positions of the incisal edges of the maxillary central incisors, and carrying the lower lip down and back with the mandible can bring those incisal edges down and back. As a result, while our pre-industrial ancestors had facial heights which maintained a steady proportion to their overall height, the average modern face keeps getting longer at a rate which is about as fast as our teeth used to wear down. The average vertical increase in facial height has been measured at .37 mm per year in the third and fourth decades of life. 133
MAXILLO-MANDIBULAR SYNOSTOSIS (LOCKING TOGETHER OF THE JAWS)
The steeply interdigitated teeth and overbite at the dental suture connecting the upper and lower jawbones further restrict the expansion of the upper jawbone as well as the advancement and upward rotation of the mandibular corpus by locking them together like a partial synostosis of the maxillo-mandibular suture. Canine contacts drive the mandible posteriorly, instead of the simian canine contacts which brace the mandible anteriorly. The mandibular corpus cannot translate forward easily, because it is locked by the bite to an upper jawbone that can only expand, and the upper jawbone cannot expand easily, because it is locked by the bite to a mandible that can only translate. 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 average change in profile due to modern diets and life styles has been recorded in Nubians before (solid lines) and after (dotted lines) switching from hunting-gathering to an agricultural diet, shown below left; and in comparing modern Swedes (dotted lines) with Australian Aborigines (solid lines), as shown below middle. Very similar changes have been recorded 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, who always had short and wide (brachycephalic) craniofacial regions, no longer always have shorter and wider faces (euryprosopic) to match. Instead, there are now brachycephalic crania with long (leptoprosopic) faces, - what Enlow called the Dinaric head shape.
The loss of bite forces may also have affected the shapes acquired by the bony orbits, which are comprised on their lower and inner (medial) aspects by membrane bones of the midface and on their roofs by a plate of membrane bones that gets pushed forward by growth of the anterior part of the cranial base. The orbits grow longer and narrower. Long orbits may be the reason for the epidemic of myopia (nearsightedness), because the shape of the lens is a reflection of the shape of the inside of the orbit.
LOSS OF SYMMETRY
At the same time, craniofacial growth has also become remarkably more asymmetrical, especially in people with relatively weak jaw muscles. The asymmetry usually appears most extreme in the lower face, with the rest of the facial features following in proportion to their distance from the mandible. The 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 the influence of the bite. 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
One cause of the increased asymmetry is the inhibition of normal growth. In animals, restrictions to growth cause increased variation of growth patterns. For example, when Harvold experimentally blocked nasal respiration in a group of monkeys, they each developed a different type of malocclusion. In modern humans, mandibular movement pathways have become much more irregular. In Aborigines, opening and closing movements rarely cross, while in modern humans they often cross.143
VARIABILITY AND PERSONALITY
The response of each individual’s neuromuscular system to strained craniofacial growth 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. Many are able to limit verticalization of the anterior facial skeleton and the downward component of downward and backward rotation of the mandibular corpus because of strong jaw closing muscle activity. In contrast, passive responders seem to react to a facial growth restriction by avoiding it, which further reduces bite forces. A strained bite may be avoided by lowering the posture of the mandible sufficiently to avoid frequent tooth contacts, resulting in a face that grows extremely long anteriorly.
FOOTNOTES:
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