Chapter 3
CH 3 – NATURAL GROWTH OF THE HUMAN JAW SYSTEM
FUNCTIONALLY STIMULATED GROWTH - The other reason the human jaw system was so adaptable is that a delayed growth process enabled each individual jaw system to grow to fit its own functional needs. In many ways, humans evolved from an ape born 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. 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 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 Even after the TMJs are fully formed, they lose their contours if the condyles are removed or fractured.78-80 Delaying the growth of the jaw system until it was exposed to functional forces enabled each individual to grow a jaw system that best fit the particular forces it would have to endure. Thus, genetics provides the motive forces, the raw materials that are bound to cause proliferation of tissues according to a pre-determined sequence, but the final form taken by the tissues is also very much determined by the system’s response to the functional environment in which the genetic tendencies are expressed.
The bite forces that stimulate 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. In pre-industrial eskimos, powerful chewing produced a distinct thickening along the sagittal suture.69
The strains produced by biting 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 from the upper jawbone 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 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. More centrally, biting forces are transferred to the cranial base via the walls of the maxillary sinuses and the wings of the sphenoid bones.
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. As a result, 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 are rhythmic, like in chewing, probably stimulate growth by increasing oxygen supply due to the pumping effect of rhythmically alternating forces. In this process, oxygen seems to be a "master controlling switch" – affecting osteoblast metabolism, osteoclast metabolism, and osteoid calcification.52-53 Stimuli that produce osteogenic activity are frequency-specific. Repetitive loading is osteogenic while constant loading is not.54-55 In monkeys, a new layer of bone forms on the supraorbital ridge just after the arrival of each new molar68, and forceful biting bends the whole cranium, opening the sagittal suture that runs along the top of the head. The mandible thickens as much as needed to resist the functional stress it encounters; therefore 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 animals, softening the diet produces thin craniofacial bones, hardening the diet produces thick craniofacial bones 67, and unilaterally damaging jaw muscles or extracting teeth affects the shape of the cranium.56-66
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
Three different growth processes are involved - neural expansion of the cranium and orbits, cartilaginous elongation of the cranial base, and antero-inferior translation of the face. These growth processes each respond differently to bite forces.
NEUROCRANIAL EXPANSION
The cranial vault and orbits grow in respond 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 get pushed apart by expansion of the brain inside them. As a result, they drift away from the center of the cranium. Lateral and frontal serial X-ray tracings of a growing head look like a slow motion picture of an explosion, as seen on the right side of the illustration below. Between the individual cranial bones, "fill in" growth at the fibrous sutures occurs in sufficient amount 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 at least partly determined by externally imposed forces.84 85 Some pre-industrial human tribes successfully altered head shapes by binding their infants' heads, 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. For example, compression of the cranium vertically by strong jaw closing muscles favors growth horizontally. As a result, pre-industrial humans with relatively strong overall musculature had crania that were relatively short and wide (brachycephalic), while pre-industrial humans with relatively weak overall musculature developed crania and faces that were relatively tall and narrow (dolichocephalic).
CRANIAL BASE ELONGATION
The cranial base, which functions as the top end of the spinal column, grows interstitially due to endochondral ossification, which causes it to elongate like the long bones of the limbs or the vertebrae. Its growth peaks at puberty but then continues at least through the second decade of life after other cartilaginous growth has ended. Structurally it 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, much like the reinforced bottom of a box which contains an expanding mass, as seen in the illustration below left. The relative independence of cranial base growth 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 cartilaginous growth plates arranged back to back and connected at an angle of about 130 to 135 degrees.
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.
The angle formed by the cranial base is one determinant of the way the jaws fit together. Sharp cranial base angles produce faces with more rearward growing upper jawbones, while obtuse cranial base angles produce faces with more forward growing upper jawbones, at least in the midline. The angle of the cranial base is mostly determined genetically. There is some evidence that strong bite forces flatten it slightly.
FACIAL TRANSLATION
From the underside of the cranial base, the face translates down and forward, perpendicular to the orientation of the circum-maxillary sutures, which respond adaptively. As the face translates down and forward, bone fills-in behind it at the circum-maxillary sutures, shown below. Unlike the rest of the cranial sutures, these cirum-maxillary sutures stay open throughout life, because they function like shock absorbers for the maxilla. Continuous slight interosseous movements keeps sutures open, while immobilization of sutures leads to ossification.90 91 91.1 The jaw muscles produce continuous slight movement of the maxillae, which prevents closure of the circum-maxillary sutures.91.2 Softening the diet, which diminishes movement at the sutures by exposing them to smaller bite forces, produces premature obliteration of facial sutures in rats.
MAXILLARY EXPANSION
The upper jawbone, which is buttressed and cushioned by these sutures, grows by expanding. Its two halves, the shell-like maxillary bones (maxillae) separate and unfolding away from their midline connection, like unfolding a box or spreading wings, to create a platform that can absorb the power-crushing forces that the mandible applies upward and forward against it.
One way the upper jawbone unfolds is by its right and left maxillary bones 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 upper jawbone unfolds is around its front end, as seen from left to right in the illustration below. In our ancestors, strong chewers also had flatter faces.
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.
The expansion of the upper jawbone enlarges the nasal airway by widening the floor of the nasal airway as well as the buttresses that carried 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. Over time, the distribution of bone in the midface 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.
ADVANCEMENT OF THE MANDIBULAR CORPUS
On the other side of the bite table, the horseshoe shaped portion of the mandible that houses the lower dental arch (the mandibular corpus), shown below right in black, advances due to growth of bone at its back end (the posterior surface of the ramus) and especially at the mandibular condyles, as if it were suspended by continuously elongating handles.
ROTATION OF THE MANDIBULAR CORPUS
While its growth displacement moves the corpus anteriorly, it also moves the corpus superiorly, rotating it upward in front and shortening the front of the face. For that reason, strong jaw muscles are correlated with short anterior faces.
This intramatrix growth involving both the advancement and the rotation of the mandibular corpus is designed to continually carry the roots of the lower teeth further up into the bite table and against the upper teeth in order to compensate for the loss of biting surface caused by continual tooth wear. People with relatively strong chewing activity and strong jaw muscles needed and also experienced more advancement and rotation of the mandibular corpus, because they were the people who experienced the most tooth wear and thereby needed the increased facial growth to compensate for that wear.
GROWTH ADAPTATION
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 that holds the front half of the lower jawbone under the upper jawbone; even when it become displaced due to disease, injury, extreme functional habits, functional orthodontic appliances, loss of teeth, or surgery. In rats, experimentally holding the jaw partly open redirects condylar growth until it creates a new handle with a curved shape that reconnects the back ends of the lower jawbone in it new position with the glenoid fossae in the temporal bones to maintain the TMJs. In rabbits, experimentally shifting the rear portion of the lower jawbone further backward triggers an increase in condylar growth until the integrity of the handle is re-established. In humans, 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. During normal growth, as the corpus of the mandible advances, osteoblasts lay down as much bone as necessary behind the advancement so the ramus can push the corpus forward and upward while maintaining its position as a member of the postural system and controlled by the tendon of the temporalis muscle.
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.” Petrovic says, “The condylar cartilage growth is integrated into an organized functional whole having the form of a servosystem, which is able to modulate the lengthening of the condyle in such a way that, through postnatal growth, the lengthening of the lower jaw adapts to the lengthening of the upper jaw.”
THE CRANIOFACIAL GROWTH MATRIX
With upper and lower jawbones growing in response to the same functional forces, they also grow to fit each other, which is one feature that causes their composite facial growth pattern over time to exhibit remarkable stability. The other source of stability in the overall facial growth pattern is that it is regulated by the resting postures of the craniofacial and craniocervical muscles, which are part of a field of relatively consistent uniform tonus that envelopes the body and creates a "neutral zone" that controls the external shapes of the bones. 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 growth, the front of the face forms a relatively consistent mask due to the particular arrangement of facial muscles which keeps each face unique and recognizable throughout life. This facial mask 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.
In this manner, 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.
INTRAMATRIX CRANIOFACIAL GROWTH
However, hidden within that steady craniofacial growth matrix, the upper and lower jawbones have surprisingly rapid independent growth patterns. This intramatrix growth in youth and adolescents was not even recognized until researchers began using implants to track longitudinal growth. It was apparently designed to contribute to the stability of the matrix by compensating for whatever rate of tooth wear occurred to maintain a steady bite table. The growth was stimulated by functional forces, so it was effective at compensating for wear. People who chewed more underwent more compensatory growth and also usually needed more growth due to more tooth wear. However, in this intramatrix growth, the upper and lower jawbones grow by completely different mechanisms, in slightly different directions, and at different rates. Between them, the bite table functions like a suture to compensate for differences between the maxillary and mandibular growth patterns. This intramatrix growth was hidden because, all around it, the light steady forces of postural tonus limit its effect on the overall matrix. Wherever jawbone growth runs into the steady structural framework maintained by the muscle postures, bone gets resorbed; and wherever jawbone growth carries a bone away from the steady matrix maintained by the muscle postures, just enough new bone gets deposited to fill in the space.
The dynamics of intramatrix craniofacial growth are a function of bite forces, therefore understanding them requires understanding the role of bite forces on growth.
EARLY 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, the bite table replaces the tongue as the platform against which the mandible is squeezed for bracing 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.
As the jaw muscles develop, biting 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. Subsequently, as long as there are still enough teeth to form a bite platform, the structural components of the jawbones keep growing in response to the complex gradient of craniofacial strains produced by biting forces.
Bite forces also align the teeth. The eruption pathways that the teeth follow into the mouth cannot be controlled very precisely by genetics, so their final pathways as they enter the mouth need to be refined by functional forces, including the light steady soft tissue pressures directed inward from the lips and cheeks and outward from the tongue. Those forces define a trough, called a neutral zone, into which the teeth tend to drift. 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
BITE STABILITY
The bite table quickly achieves stability, because the eruption of each tooth stops when it receives enough bite forces, and the consistency of the biting forces stops the eruption of all the teeth along the same plane. The front teeth meet with an overbite that serves to couple the growth of the upper and lower jawbones and thereby 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, initial chewing patterns in children include wide lateral thrusts on opening, (as seen below right). After the bite table is established, the wide lateral opening thrusts disappear, and the chewing pattern reverses to become the normal adult chewing pattern, with the lower jawbone opening near the midline and closing from a more lateral position for power-crushing (as seen below left).
As soon as the bite table is established, it becomes a central architectural landmark and a key structural component in the facial growth process. The stable bite table established on the baby teeth is preserved during the transition to adult teeth by a process in which 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, the face grows largely in response to bite forces.
THE EFFECT OF BITE FORCES
The stable bite table then affects subsequent jawbone growth by providing a healthy exercise template for functional operations of the jaw muscles and a pathway for bite forces to load the midface. Bite forces continue to stimulate advancement of the lower jawbone and more gradual expansion of the upper jawbone. A stable symmetrical bite table coordinates the growth in the upper and lower jawbones by molding them to fit the same bite forces.
In the upper jawbone, bite forces cause a flattening by rotating the two maxillary bones away from the midline suture. The anterior component of power-crushing causes anterior growth of these lateral areas of the upper jawbone, in contrast to the medial area of the upper jawbone, which is controlled more by cartilaginous elongation of the cranial base and nasal septum.
In the lower jawbone, bite forces cause the front portion, the corpus which contains the dental arch, to advance and rotate upward in front. Generally, the upward rotation of the mandibular corpus can be seen most clearly relative to the two rami behind it, which function as the handles from which the corpus is suspended. The rami are stable architectural landmarks, because their positions are controlled by the steady postural tensions of the temporalis muscles, so they function somewhat as a reference point from which the rotation of the corpus can be measured at their intersection - the gonial angles, which are acute in people with strong jaw muscles and obtuse in people with weak jaw muscles.
THE FREEWAY SPACE, THE MAXILLO-MANDIBULAR JOINT SPACE
Bite forces are necessary for maintaining the space at rest between the upper and lower jawbones (AKA the freeway space), because growth adaptation is unable to limit vertical growth with the normal controls provided by postural tonus. Other joint spaces in the body are maintained by light passive forces due to the resting tonus of the muscles which cross the joint; but the teeth are wired with too many neuromuscular reflexes to have them maintain light steady contact. Therefore to maintain the vertical height of the face in concert with the rest of the postural system, the eruption force in the teeth and their surrounding tissues during sleep must be counterbalanced by bite forces during the day. As a result, the length of the front of the face is inversely proportional to jaw muscle strength. When disease or injury damages the jaw muscles, the mandibular corpus rotates sharply down and back. 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.
COORDINATION OF GROWTH IN THE JAWBONES
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. Thus, growth 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. In such a manner, the mandibular molars shift anteriorly relative to the maxillary molars while the maxillary molars move laterally relative to the mandibular molars. The net effect is to keep the upper and lower teeth in close proximity for efficient chewing while some slippage between them allows the upper and lower jawbones to shift in slightly different directions as a result of their diverse growth processes.
THE MAXILLO-MANDIBULAR SUTURE
Connecting and coordinating these diverse growth processes in the upper and lower jawbones, the bite functions like a cranial suture in that it adaptively connects the growth processes on its two sides. 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 Adaptive growth at the dental joint maintains the stability of the bite table despite damage to a tooth or even an area of teeth. Adaptive growth at the dental joint also accommodates both the advancement of the mandibular corpus and the expansion of the maxilla while maintaining a stable bite table, whether wear of the teeth is slow or fast.
AGING
During adulthood, the continued slow growth of the jaw system is important for reducing resistance in airway flow, just as the natural weakening of muscles at the rate of about 5% per decade makes them progressively less able to strongly pull in air. Mandibular advancement reduces resistance in the pharyngeal airway, while slower maxillary expansion reduces resistance in the nasal airway. However, these growth process were both stimulated by functional (chewing) forces. The human jaw system was not designed to deal with a situation that was almost never encountered in evolution - insufficient functional forces.
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