Chapter 3
CH 3 – NATURAL GROWTH OF THE HUMAN JAW SYSTEM
FUNCTIONALLY STIMULATED GROWTH - The other reason for the success of the human jaw system was that a delayed growth process enabled each individual jaw system to grow in response to the bite forces it receives in order to produce a structure that almost perfectly fit its functional needs, so it could acquire and maintain such a functional harmony. Genetics ensures the proliferation of tissues according to a pre-determined sequence, but the final form is also very much determined by the tissue responses to the functional environment in which the genetics are expressed.
In many ways, humans evolved from an ape born while still in an embryonic stage.49 This was especially true of the jaw system. In humans at birth, the TMJs have not yet formed, the squamous portion of the temporal bone is essentially flat, and the mandible is just a bulbous tube enclosing a collection of tooth crowns with a midline that is still unfused, making it not yet even capable of receiving or transmitting significant forces. Its growth requires functional forces. The alveolar processes (the bones supporting the teeth) do not form if there are no teeth to receive forces. The TMJs don't even acquire their contours until they start receiving biting forces,76-77 they continue deepening well into adulthood,16-17 and they can change shape at any time in response to a change in bite forces.18, 78-80. When animals are raised on a liquid diet to reduce mechanical loading; their TMJs fail to enlarge, their TMJ cartilage does not thicken properly,19-21 and their TMJ mechano-receptors fail to mature normally.22 Experimental studies in mice, rats, rabbits, and non-human primates have shown that mechanical loads are vital for maintaining normal growth, morphology, and function of the secondary cartilage of the temporomandibular joint. In vitro studies confirm that normal mechanical loading stimulates cell division, matrix synthesis, and enzyme activity in the tissues of the TMJ.
The bite forces that stimulate craniofacial bone growth are distributed around the whole front half of the cranium, as shown below by a researcher who applied bite forces to a cranium coated with pressure sensitive paint.
Those forces are highest near the bite table in the lower face, moderate in the middle face, and very low in the upper face.93 The tooth bearing portions of the jawbones and their extensive frameworks of supporting bones experience compressive forces, regions of muscle attachment and insertion (such as the zygomatic arch and the coronoid process) experience tensile forces, and areas between these two portions experienced twisting, bending, and shearing forces.83 In front, the nasal processes carry incisal biting forces to the medial (inner) aspects of the supraorbital ridge. At the canine and premolar areas, the walls of the maxillary sinuses and nasal cavity transfer biting forces up to the sides of the supraorbital ridge and the front portions of the zygomae (cheekbones). At the molars, the bony prominences over the buccal roots transfer biting forces to the rear portions of the cheekbones, which in turn transfer these forces to the front portions of the temporal bones. Medially, biting forces are transferred to the cranial base via the walls of the maxillary sinuses and the wings of the sphenoid bones.
Bones respond to mechanical stimulation in a manner that depends on force and time. Absence of force leads to bone loss (atrophy). Excessive force can lead to osteoblast apoptosis and destruction of bone tissue. Where muscles pull on bones, those bones develop protuberances for the attachments of the muscles. Where muscles bend bones, those bones develop an internal architecture aligned to withstand bending forces. Muscle activity causes the bones at their origins and insertions to strengthen just enough to be able to withstand whatever forces they can apply, and weakness or paralysis of muscles produces bones that are extremely thin and mechanically deficient.50-51
Bite forces that alternate rhythmically, like in chewing, probably stimulate growth by increasing oxygen supply due to a pumping effect. Stimuli that produce osteogenic activity are frequency-specific. Repetitive loading is osteogenic, while constant loading is not.54-55 In this process, oxygen seems to be a "master controlling switch" – affecting osteoblast metabolism, osteoclast metabolism, and osteoid calcification.52-53.
The ability of bite forces to stimulate growth is well documented. In animals, softening the diet produces thin craniofacial bones, hardening the diet produces thick craniofacial bones 67, and unilaterally damaging jaw muscles or extracting teeth warps the cranium.56-66 In monkeys, a new layer of bone forms on the supraorbital ridge just after the arrival of each new molar68, and forceful biting opens the sagittal suture that runs along the top of the head. In pre-industrial eskimos, powerful chewing produced a distinct thickening along the sagittal suture.69
In modern humans, bite forces are smaller, but bones are thinner. In the lower face, the mandible thickens as much as needed to resist the functional stress it encounters. The size of the chin is proportional to maximal bite force, the thickness of the condyles is determined by the functional loads they receive 70 71 72 73, the size of the gonial angle varies as a direct function of the size of the masseter and internal pterygoid muscles74-75, and the growth of the coronoid process depends on the presence of the temporal muscle. In the midface, the distribution of bone becomes virtually identical to the stress distribution experienced by that area during loading, and the midface becomes a lightweight and efficient honeycomb of thin membrane bones ideally constructed to withstand and distribute the strong functional forces they experience.
In fact, much of postnatal human craniofacial growth and development may be a process of adaptation to bite forces. The density of all the bones of the cranial vault varies according to jaw muscle strength.81 Moss pointed out, "The external form of the human skull ... is related directly to imposed loadings, a point verified experimentally in miniature swine skulls. It is known that unit strains are greatest in infant skulls, less in adolescent skulls and least in adult skulls, suggesting that the growing skull increasingly adapts its structure to masticatory loadings."82
BITE FORCES
The first bite forces to shape the craniofacial area are produced by squeezing the mandible against the tongue or nipple resting between the gum pads. In response, the TMJs begin to form between the condyles and the temporal bones where opposing periosteal envelopes rubbing against each other produce secondary cartilage on the surfaces of the bones and a fibrocartilaginous pressure-bearing disc from the tendon of the lateral pterygoid muscle, the pressure-bearing articular surfaces of the TMJs become avascular, the TMJ disks acquire a thin central zone surrounded by peripheral wedges, and the upper portions of the TMJs develop bony slopes (articular eminences).
When the teeth arrive, a bite table replaces the tongue as the platform against which the mandible is squeezed, and the jaw closing muscles replace the facial muscles as the source of stabilization for the mandible during swallowing. Freeing the facial muscles from their crude infantile suckling and swallowing functions allows them to take on the more delicate and complicated functions of speech and facial expression. The consistency of the food is at least partly responsible for the change in neuromuscular control. Older children given a bottle revert to infantile suckling, swallowing, and respiratory rhythms.
Bite forces shape the TMJs. The glenoid fossae of the TMJs deepen, the bony slopes at the front of the TMJs begin to develop their characteristic S-shaped profiles, and the cheekbones (zygomatic processes) thicken and begin to bow outward. By the time the primary dentition is complete, the TMJs are well established and the bony slopes of the TMJs have gained more than half of their adult form.
Bite forces also shape the bite table. The consistency of mandibular bracing stops the eruption of all the teeth along the same plane, and the consistency of the functional mandibular range of motion prevents any teeth from supererupting into a position that limits the mandibular range of motion. At the same time, the steady soft tissue pressures directed inward from the lips and cheeks and outward from the tongue position the teeth bucco-lingually by creating a trough, called a neutral zone, into which the teeth drift at the end of their eruption pathways, which need that guidance, because they cannot be controlled very precisely by genetics. Because of the importance of these functional forces in aligning the teeth, dental arch dimensions show very low heretability compared with most other craniofacial dimensions.86-88
The bite table quickly achieves stability. The front teeth meet with an overbite that serves to prevent the faster growing lower jawbone from growing past the upper jawbone. The back teeth meet with an interdigitation that refines their alignment by means of the so-called cone and funnel mechanism, illustrated below. Since each upper back tooth interdigitates with two lower back teeth and each lower back tooth interdigitates with two upper back teeth, the interdigitating arrangement of opposing teeth due to this cone and funnel mechanism spreads along the dental arches.
To ensure that the teeth line up in a manner that does not restrict the mandibular range of motion before it is wide and strong enough prevent excessive eruption from impinging on a healthy mandibular range of motion, initial chewing patterns in very young children include wide lateral thrusts on opening, (as seen below right). The lateral opening thrusts ensures that, even if a young child is not yet chewing foods tough enough to produce lateral thrusts in power-crushing, the teeth do not super-erupt. After early childhood, the wide lateral opening thrusts disappear, and the chewing pattern normalizes, with the mandible opening near the midline and closing medially from a more laterally displaced position for power-crushing (as seen below left).
The stable bite table established on the baby teeth is then preserved during the transition to adult teeth, because the adult teeth first erupt in front and in back of the primary bite table to form a structural tripod that supports and extends the bite table before replacing its members (the baby teeth) one or two at a time.
Subsequently, bite forces have important roles in craniofacial growth. Three different growth processes are affected - neurocranial expansion, cartilaginous elongation of the cranial base, and antero-inferior translation of the sides of the face.
NEUROCRANIAL EXPANSION
The cranial vault and orbits grow early in life in response to expansion of the enclosed neural mass. The cranial vault is a tabular structure of membrane bones that forms a continuous shell enclosing the expanding brain, and the tabs drift away from the center of the cranium as they get pushed apart by expansion of the brain. Serial X-ray tracings of a growing head look like a slow motion picture of an explosion, as seen below. Between the tabs, "fill in" growth at the fibrous sutures occurs in whatever amounts are needed to accommodate the expansion. Even when the cranial contents expand much more quickly than normal in hydrocephaly, the sutures are still able to produce enough bone to maintain a continuous shell around the grossly enlarged cranium.
Similarly, the orbits also grow to perfectly enclose the expanding mass inside them. Removal of the eyeball during growth results in deficiencies in the anterior and lateral growth of the midface.
While the expansion of the cranium and the orbits is motivated by enlargement of the enclosed neural contents, the final shape they acquire is also slightly affected by externally imposed forces.84 85 Some pre-industrial human tribes altered head shapes in infancy, apparently without impairing brain development. Limiting cranial growth in one direction causes it to expand in other directions, like pushing on a lump of clay. Compression of the cranium vertically, as from bite forces, favors growth horizontally. In our pre-industrial ancestors, people with stronger muscles had crania that were relatively short and wide (brachycephalic), and people with weaker muscles had crania that were relatively tall and narrow (dolichocephalic).
CRANIAL BASE ELONGATION
The cranial base is also only slightly affected by bite forces. It grows interstitially due to endochondral ossification like the long bones of the limbs or the vertebrae, on a time schedule slightly later than the cranium and orbits but much earlier than the jawbones. Structurally, the cranial base forms a thick spline on which the brain rests, and it does not burgeon out in response to neurocranial expansion as readily as the thin plates of membrane bone bordering the rest of the cranial vault. Instead, the cranial base holds its shape while the rest of the cranium expands, like the reinforced bottom of a box which contains an expanding mass, as seen in the illustration below left. The relative independence of the cranial base from neurocranial expansion can be seen in the way it is only slightly affected in microcephaly and hydrocephaly, while the rest of the cranial vault becomes grossly distorted.
As seen above right, the cranial base is comprised of two sections, an anterior portion and a posterior portion, (the basi-occiput and the baso-sphenoid), which are actually phylogenetically cephalized vertebrae that function like the top end of the spinal column. These two cartilaginous growth plates are arranged back to back and connected at an angle of about 130 to 135 degrees. That angle determines the direction at which the middle of the face grows down and out relative to the spinal column. Strong bite forces flatten it only slightly.
Elongation of the more vertically oriented posterior portion of the cranial base increases facial height by pushing the cranium up and away from the shoulder girdle and chest, which causes facial height to increase in proportion to body height and leads to dramatic changes in facial proportions between childhood and adulthood, as seen below.
Elongation of the more horizontally oriented anterior portion of the cranial base pushes the center of the face forward. The nasal cartilage functions like the anterior end of the anterior cranial base.
FACIAL TRANSLATION
From the underside of the elongating cranial base in the midline and the underside of the anterior cranial vault and orbits on the sides, the face grows down and forward. However, that growth process is complex, because the upper and lower jawbones grow by completely different mechanisms and in slightly different directions. Harmonious growth requires coordinating these diverse growth processes, and the bite table between them plays a central role in that coordination.
MANDIBULAR ADVANCEMENT - The mandibular corpus leads the way. This curved piece of thick cortical bone, which holds all the lower teeth, maintains its shape and also shifts forward due to the condyles and the rami pushing it forward from behind like elongating handles. The amount of growth at those handles depends on the amount of bite forces received.
MANDIBULAR ROTATION - The mandibular corpus is positioned vertically by the balance between the eruption force of the dentitions pushing it inferiorly and bite forces pushing it superiorly. Strong bite forces cause the mandibular corpus to rotate upward in front, (forward rotation), which keeps the front of the face short. Weak bite forces allow the mandibular corpus to rotate down and back, which lengthens the front of the face. Generally, the rotation of the mandibular corpus can be seen most clearly relative to the two rami from which it is suspended. The rami are stable architectural landmarks, because their positions are controlled by the postural tensions of the temporalis muscles. The rami meet the corpus at the gonial angles, which are acute in people with strong jaw muscles and obtuse in people with weak jaw muscles.
The mandibular growth that produces both the advancement and the rotation of the mandibular corpus is designed to continually carry the lower teeth further up into the bite table in order to compensate for the loss of tooth structure caused by continual tooth wear. Because this growth was stimulated by bite forces and tooth wear was usually proportional to bite forces, people with relatively strong bite forces experienced and also needed more mandibular advancement and forward rotation to compensate for that wear.
CONDYLAR ADAPTATION
To accommodate growth of the mandibular corpus in any direction needed to adapt to whatever bite forces were produced, the growth at the mandibular condyles stays highly adaptive throughout life due to periosteum-like properties from a proliferative layer of undifferentiated mesenchymal cells that grow as much as needed to maintain the necessary length of the handle; even when the corpus become displaced due to disease, injury, extreme functional habits, loss of teeth, or surgery. Surgical removal of a condyle is routinely followed by spontaneous regeneration of a new condyle with a functional articular head and condylar neck contained within a normal synovium, and in some cases with a fibrocartilaginous cap.
Many researchers have commented on the important role that condylar adaptability plays in human growth. Enlow says, "The variable capacity of condylar growth provides adaptation to different facial types, different articular patterns between the individually variable configuration and dimensions of the cranial floor and maxilla, different occlusal patterns, and normal structural changes occurring in conjunction with progressive growth. As the whole mandible becomes displaced in whatever vectors are involved at different ages and in whatever variations occur among different individuals, the condylar cartilage and the contiguous membranes forming the intramembranous bone of the condylar cortex and the condylar neck grow in whatever directions and in whatever amounts are required to sustain constant functional position and articulation with the cranial floor.”
MIDFACE EXPANSION
Between the bottom of the relatively steady orbital floors and the top of the relatively variable mandibular corpus, the midface grows to fill-in the spaces left between them at a series of circum-maxillary sutures, which are oriented perpendicular to the direction of growth, as shown below. These cirum-maxillary sutures function like shock absorbers. Continuous slight interosseous movements, like from biting,91.2 keeps sutures open, while immobilization of sutures leads to ossification.90 91 91.1 Softening the diet, which diminishes movement at the sutures by exposing them to smaller bite forces, produces premature obliteration of facial sutures in rats. In humans, the circum-maxillary sutures are the only cranial sutures that do not ossify.
The bite table formed by the upper jawbone (the maxilla) moves down and forward in front of these sutures, expanding while shifting down and forward to fill in the space left between the downward and forward shifting mandibular corpus and the orbits. The expansion of the maxilla is due to separation of the two (right and left) maxillary bones at their midline connection and unfolding. The separation at the midline widens the suture there. The unfolding enlarges the maxilla, like spreading wings, until it becomes a platform almost perfectly designed for receiving those bite forces.
One way the maxillary bones unfold is by rotating around an axis through the midpalatal suture, as can be seen from left to right in the illustration below. As chewing forces drive the bones upward and outward around their midline suture, they lower the suture and flatten the roof of the palate. In our ancestors, strong chewers had flat wide palates.
The other way the maxillary bones unfold is around their front end, as seen from left to right in the illustration below. In our ancestors, strong chewers also had flatter faces.
Expanding the maxilla enlarges the nasal airway by widening the floor of the anterior nasal airway as well as the buttresses that carry chewing forces around the nasal airway. Pre-industrial humans who were strong chewers developed wide flat midfacial structures, including small zygomaxillary angles, large nasomalar and zygomaxillary angles, and flared zygomas. As the structural components of the midface shift laterally, their inner aspects resorb, leaving room for expansion of the sinuses and the nasal airway. The role of bite forces in this maxillary expansion can be seen in the dramatic expansion and outward splaying of all the upper teeth created by wearing a spinal realignment tool called a Milwaukee brace, which pushes up forcefully on the mandible to tip the head back and thereby applies strong continuous bite forces to the upper teeth.
JAWBONE GROWTH COORDINATION
With upper and lower jawbones growing adaptively in response to the same functional forces, they also grow to fit each other, which causes their composite facial growth pattern over time to exhibit remarkable stability. The coincidence of advancing the mandibular corpus and expanding the maxilla keeps the lines of upper and lower teeth in close proximity, because a wider portion of the lower dental arch comes to lie under a directly widening portion of the upper dental arch; but it also requires some shifting of teeth in the bite, because it brings the mandibular buccal cusp tips up the posteriorly and medially facing inclines of the buccal cusps of the maxillary teeth, even as expansion of the maxillary bite table moves the maxillary teeth further laterally. The mandibular molars shift anteriorly relative to the maxillary molars while the maxillary molars move laterally relative to the mandibular molars.
With growth coordinated by functional forces, and with exterior boundaries maintained by muscle resting postures; the overall growth of the face has a steadiness much like a matrix. Although longitudinal studies of children have shown that annual increments of individual facial bones are not evenly distributed among the various bony elements, (some grow faster one year and others grow faster the next year); diminished growth of any one bone is always compensated for by increased growth in neighboring bones, and the steadiness of the overall matrix is preserved. Even after an experimental bite interference has caused a growth deformity, removal of the experimental bite interference is followed by compensatory growth which re-establishes symmetry. Similarly, after functional orthodontic appliances are used to alter growth, the treatment period is often followed by "catch up" growth that at least partially restores the previous facial growth pattern.
In this craniofacial matrix, the resting postures of the craniofacial and craniocervical muscles create a field of relatively consistent uniform tonus that envelopes the bones in a "neutral zone" that controls their external shapes. Any shift of a bone away from the neutral zone creates an imbalance between opposing myofascial pulls and thereby causes a constant light tension pulling the bone back toward its neutral zone or returning it to its original shape. Surgeons find that rapid resorption occurs in any part of a bone graft which is located beyond the tension zone controlled by the musculature, and any change in the passive tension of the myofascial curtain can reshape the underlying bones. The passive tension of the myofascial curtains draped from the front of the cranium down onto the shoulder girdle and sternum in resting posture maintains the resting position of the mandible. The stability of a typical craniofacial growth pattern can be seen below.
At the front of this craniofacial matrix, the particular arrangement of facial muscles of each individual produces a relatively consistent mask which stays unique and recognizable while it gradually swings out from the underside of the front of the cranium, as if it were hinged at the forehead, and the profile flattens between infancy and adulthood as shown below.
INTRAMATRIX FACIAL GROWTH
Hidden within that steady craniofacial growth matrix, the upper and lower jawbones have surprisingly rapid growth patterns that were designed to keep supplying tooth structure at the bite surface, especially the mandibular corpus advancing while rotating forward (upward in front) to continually carry the lower teeth up and forward into the upper teeth surrounding them. This intramatrix growth was not even recognized until researchers began using implants to track longitudinal growth in youth and adolescents. It is so hidden, because it is surrounded by a neutral zone produced by the light steady forces of postural tonus. Wherever the jawbones run into the neutral zone maintaining the craniofacial matrix, surfaces get resorbed; and wherever jawbone growth carries a bone away from the craniofacial matrix, enough new bone gets deposited to fill in the space.
THE MAXILLO-MANDIBULAR SUTURE
Connecting and coordinating these diverse growth processes in the upper and lower jawbones, the bite functions like one long horseshoe shaped suture. Alterations in the growth of one area of the upper jawbone affects growth in the opposing area of the lower jawbone, and vice-versa.92 This adaptive growth maintains the stability of the bite table despite damage to a tooth or even an area of teeth. It also ensures that the bite table remains stable, whether wear of the teeth is slow or fast.
AGING AND AIRWAY
During adulthood, the continuation of this intramatrix facial growth is important not only for compensating for tooth wear but also for reducing airway flow resistance in order to compensate for the natural weakening of muscles at the rate of about 5% per decade, so the respiratory muscles do not have to keep increasing their effort with age. Mandibular advancement reduces resistance in the pharyngeal airway by shifting the mandibular corpus farther from the cervical spine, while maxillary expansion reduces resistance in the nasal airway.
EVOLUTIONARY SUCCESS
This sophisticated longlasting craniofacial growth system helped enable humans to spread out all over the surface of the earth. It could grow a jaw system to fit almost any type and degree of chewing task and still achieve a functional harmony. However, it could not grow a jaw system to achieve functional harmony without sufficient bite forces to regulate it.
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