Occlusion: Orthopedic Perspectives
Understanding how modern dental occlusions function requires first understanding how dental occlusions were designed to function in our ancestors and then how that function has changed due to the recent softening of our diets. Pre-industrial occlusal dynamics resembled those of a large ball-and-socket joint; with the mandibular dentition as ball and the maxillary dentition as socket. Its close packed position was a centrally indented (intercuspal) area in the fit between the articulating surfaces, its range of motion was determined by group function of the teeth in the direction of mandibular movement, and a remarkable congruence between the articulating surfaces in all mandibular positions was produced by dento-alveolar remodeling and refined by occlusal wear. Now that soft diets have weakened our jaw muscles and narrowed our jaw movements, these orthopedic features can be incorporated into modern dental occlusions with proportions modified to fit our more delicate and verticalized masticatory systems.
THE MAXILLO-MANDIBULAR JOINT
The dental occlusion functions in many ways like a synovial joint that has been re-engineered to house teeth on its articular surfaces. Resiliency is provided by the fluid dynamics of the periodontal ligament spaces rather than layers of soft tissues, functionally stimulated circulation is created by the alternating hydraulic pressures in the periodontal ligament spaces rather than by expressing and reabsorbing fluids at the cartilage surfaces (weeping lubrication), protective neuromuscular reflexes respond to a vast network of mechanoreceptors surrounding all of the tooth roots instead of just those lining the joint periphery and ligament attachments, and congruence (goodness of fit) in the close packed position and throughout the normal functional range of motion is created and maintained by adaptive shifting of teeth and then refined by occlusal wear rather than by bone remodeling.
The microstructure of the maxillo-mandibular joint absorbs forces with progressive resistance like the microstructure of a synovial joint, with morphology that was designed to protect the specialized articular surfaces – whether cartilage or teeth. In synovial joints, protection is provided by a series of increasingly stiff layers of tissue - the surface-active phospholipids in the plastic lubricating film and fibrous articular covering, the calcified cartilage, the thin subchondral bone, the trabeculae aligned to resist functional forces, and the cortical bone. In maxillo-mandibular joints, protection is provided by the viscoelastic and hydrostatic properties of the periodontal ligament spaces. Intruding a tooth into one of those spaces compresses the collagenous ground substance, tugs on the principle fibers, drives fluids into nearby vessels, and bends out the socket walls.1-7
Furthermore, the macrostructure of the maxillo-mandibular joint absorbs forces with progressive resistance like the macrostructure of a synovial joint. The articular surface deforms in the area of compression in order to distribute the load onto a larger portion (more surface area) of the joint. In synovial joints the articular surfaces deform by expressing water molecules from proteoglycans in the area of compression. In the maxillo-mandibular joint, the articular surfaces deform by the rapid shifting of teeth. Teeth at rest are so delicately suspended in their sockets that each demonstrates a vascular pulse,8-9 and they move easily from their rest positions over short distances,10 enabling the rock-like occlusal surfaces to absorb impacts like a cushion. For example, during light closure in the center of the maxillo-mandibular joint, the load falls on the teeth in the middle of the dentition – primarily the first molars and the premolars. More forceful closure depresses those teeth until the load is spread out to include the canines and the second molars.11
The articular surfaces of the maxillo-mandibular joint also resist forceful compression by interproximal bracing. Teeth at rest are separated by interproximal gaps of three to twenty-one microns.12 Occlusal force not only depresses teeth, but also tips them mesially and lingually until their interproximal gaps close and the adjacent teeth make physical contact. The extreme friction produced by this forceful interproximal contact locks together groups of neighboring teeth and thereby enables them to function as a single structural unit, like one long tooth supported by many roots.
The most important source of these compressive forces is bracing. This stabilizing of the mandible occurs hundreds and often thousands of times a day when the elevator muscles clamp the mandible forcefully up against the maxilla through the medium of the occlusal table. During swallowing, the braced mandible provides a stable framework against which the suprahyoid, pharyngeal, and tongue muscles can pull on the nearby tissues to create a wave of contraction. During trauma, bracing of the mandible provides a critical role by preventing a blow to the protruding chin from driving the condyles like a hammer into the vital structures of the inner ear and the temporal lobes. For postural stability, bracing the mandible fixes it to the cranium and thereby enables the anterior chain of postural muscles to pull down on the front of the head by pulling down on the mandible.
THE HABITUAL MANDIBULAR BRACING AREA (HMBA)
Mandibular bracing is so frequent and forceful that it shapes a central portion of the occlusal table to serve as the close packed position of the maxillo-mandibular joint. Teeth that receive large compressive forces intrude. Teeth that receive less compressive forces than their inherent eruptive force extrude. By this process, functional forces shape the occlusal table like tamping down bricks in sand until they create an almost perfect fit between the inferior component (the mandibular dentition) and the superior component (the maxillary dentition).
The resultant mandibular bracing platform is commonly referred to by its dental characteristics, - intercuspal position (ICP) or maximal intercuspation (MI); but functionally it is most accurately described as a habitual mandibular bracing area (HMBA). It functions as a platform for receiving mandibular bracing. The word habitual is included, because some people also use other platforms (dual bite). The word area is used, rather than the word position, because the HMBA allows the mandible to move around horizontally at least a small distance before encountering an increase in vertical dimension.
Such transverse (horizontal) freedom of mandibular movement is a hallmark feature of mammalian mastication. In reptiles, any transverse mandibular movement threatens to break off teeth; therefore the mandible is moved straight up and down, and food is just torn off and swallowed – it is not really chewed. Even the teeth that do not break off are continually dulled from abrasion and thus require continuous replacement. The evolutionary breakthrough that gave rise to the age of mammals was the development of temporomandibular joints which allow transverse mandibular movements, which create gliding tooth contacts, which create attrition, which can create and maintain a pattern of wear facets bordered by edges that stay sharp in spite of abrasion. The closely fitting facets form effective crushing surfaces, and the edges of the facets retain their cutting ability. The presence of wear facets on the teeth is considered the best indicator of the transition to mammals in the archeological record.
This formula for lifelong effective mastication based on transverse mandibular movement works for all different types of mammalian masticatory systems. In carnivores, the facets are vertically oriented, and a small transverse component of mandibular movement (about 5 mm in tigers) keeps the teeth sharp by working them together like shears. In herbivores, the facets are horizontally oriented, and wide lateral mandibular movements grind food like a mortar and pestle. In rodents, long antero-posterior mandibular movements allow gnawing at the incisors alternating with power-crushing at the molars.
In hominids, all these different chewing components were combined to produce a highly adaptable hybrid masticatory system with transverse freedom of movement provided by the articular disks and with the jaw muscles centered over the bolus for power and oriented in all directions for control. Generally the masticatory system transitioned from more carnivorous features in youth to more herbivorous features in late adulthood.
The HMBA had to be large enough to accommodate variables in mandibular position produced by gravity, body posture, and temporalis tonus.13-14 When the head is extended, the mandible lands on the posterior aspects of the HMBA close to CR. When the head is flexed, the mandible lands on the anterior aspect of the HMBA, with the anterior teeth providing a stable incisal bracing platform. When the head is turned to one side, the mandible lands on the opposite (working side) of the HMBA.
The HMBA also had to be wide enough to accommodate a twisting of the mandible in power-crushing. During this final phase of the masticatory stroke, as the mandible moves from a lateral position into and through the HMBA, it also rotates around a vertical axis passing generally through the bolus powered by a force couple consisting of the working side superior lateral pterygoid and balancing side posterior temporalis muscles. This twisting of the mandible on top of the bolus helps crush it like crushing out a cigarette butt with the ball of the foot.
MANDIBULAR RANGE OF MOTION
Surrounding the HMBA in pre-industrial human masticatory systems, the mandibular dentition continued to create pathways for guiding the mandible into and out of it like tamping down bricks in sand until these pathways formed broad continuous slopes like curved walls that extended the HMBA in all directions. When the mandible moved laterally, the working side mandibular buccal cusps rode up onto the working side maxillary buccal cusps in the direction of the excursion while all the teeth on the balancing side cleared. When the mandible moved anteriorly, the mandibular anterior teeth rode up onto the posteriorly facing palatal slopes of the maxillary anterior teeth while all the other teeth cleared. When the mandible moved posteriorly, the distal facing slopes of the terminal mandibular molars rode up onto the mesially facing slopes of the terminal maxillary molars while all the other teeth cleared - posterior guidance.
This omnidirectional group function was apparently even more important in our ancestors than bilateral centric stops. In many apparently perfectly healthy Aboriginal dentitions, strong jaw muscle forces make the maxilla grow so much wider than the mandible that the teeth are unable to achieve bilateral contact. Group function provides adequate support for bracing the mandible unilaterally on either side, as shown in figure 2 below.15
FIGURE 1: ALTERNATELY UNILATERAL MANDIBULAR BRACING
The same functional forces that shaped the maxillo-mandibular joints in pre-industrial masticatory systems also shaped the temporomandibular joints. These joints are very much products of functional forces. The temporal component is flat at birth, it is not fully formed until at least age 12, it continues deepening well into adulthood,16-17 and it changes shape in response to a change in occlusal forces at any age.18 In the temporomandibular joints of animals raised on a liquid diet to experimentally reduce mechanical loading; the components fail to enlarge, the cartilage does not thicken properly,19-21 and the mechano-receptors fail to mature normally.22
With maxillo-mandibular and temporomandibular joints acquiring similar shapes by adapting their contours to the same functional forces, these joints also ended up perfectly fitting each other. In our ancestors, temporomandibular joint and occlusal contours were more highly correlated with each other than with age or sex.23-24
The group function that supported the mandible in its direction of movement also included the basal bones that supported the articular components. Incising loaded the premaxilla and anterior nasal spine. Anterolateral excursions loaded the canine prominence, the lateral border of the nasal cavity, and the infraorbital shelf on the working side. Lateral excursions loaded the anterior aspect of the zygomatic process and the lateral portion of the infraorbital shelf on the working side. Posterolateral excursions loaded the posterior aspect of the zygomatic process and the maxillary tuberosity on the working side.
During mastication, the point of compression moved across all these structural components like a wave. It usually started at the distal end of the dentition on the working side and spread mesially. At each point of compression, elevator muscle forces drove the mandibular buccal cusps superiorly, medially, and anteriorly through the maxillary central fossae and marginal ridge areas while the mandibular condyle was driven superiorly, medially, and anteriorly through the middle of the glenoid fossa and the facial bones were loaded sequentially. Frequently the wave crossed the midline to the non-working side in a follow-through stroke.
During the long periods of mastication in our pre-industrial ancestors, these rhythmic waves of compression moving across the craniofacial area provided circulation that stimulated growth in the most directly affected structures. Each wave drove teeth in and out of their sockets, bent the membrane bones back and forth, and compressed and released the circum-maxillary sutures. The rhythmically repetitive nature of the waves worked the mandible like a pump handle against the underside of the cranium. People with more powerful mastication underwent more pumping and therefore more growth. Since abrasives were present in almost all food, these people also experienced more occlusal wear and thereby needed more growth. In this manner, functionally stimulated masticatory system growth effectively compensated for occlusal wear.
In this ball-and-socket analogy used to approximate the dynamics of the maxillo-mandibular joint, the diameter of the ball is remarkably close to eight inches. After carefully studying several thousand extremely healthy dentitions in both living people and skeletal remains in order to determine how to set teeth for dentures, early dental researchers all concluded that the mandibular teeth should be set on the circumference of an eight inch sphere.25-27 This so-called spherical occlusion is illustrated in figures 2-6.
FIGURE 2 - MONSON'S SPHERE
Bonwill believed that the radii of this sphere formed such a perfect triangle that it proved the existence of God.
FIGURE 3 - BONWILLS' TRIANGLE
The curves of Wilson and Spee facilitate setting prosthetic teeth using an articulator.
FIGURE 4 - CURVE OF WILSON
FIGURE 5 - CURVE OF SPEE
These early dental researchers also concluded that the long axes of the teeth should be aligned along the radii of the same sphere, as can be seen in Monson's cone with its apex at the glabella in figure 6.
FIGURE 6 - MONSON'S CONE
A generally similar alignment can be seen in the angles at which the teeth erupt, as illustrated in figure 7.
FIGURE 7 - ANGLES OF ERUPTION OF THE TEETH
Natural occlusal contours are certainly much more complex than the surface of an eight inch sphere, even after the cusps are all worn off. In extremely worn natural dentitions from all parts of the world, the occlusal tables form longitudinal twists like a propeller blade.27-28 These so-called helicoidal curves, illustrated in figure 7, have been characteristic of hominid dentitions for over 2 million years. They comprise a registration in tooth structure of the natural functional range of motion of the mandible after overcoming all the lateral obstructions produced by the interdigitation of the teeth.
FIGURE 8 - THE HELICOIDAL CURVE
Even such a featureless occlusion would be difficult to restore accurately with a mechanical articulator, because the occlusal plane changes depending on the sagittal location in the arch. For example, the curve of Wilson is flat at the second molars, it slopes in one direction at the first molars, and it slopes in the opposite direction at the third molars, as illustrated in figure 9.
FIGURE 9 - A CHANGING CURVE OF WILSON
Modern human occlusal interfaces are far more complex. Our jagged irregular occlusal tables reflect our weaker and more vertical masticatory pattern. A typical modern occlusal interface, seen in figure 11, looks more like a mountain range than the smooth curves of a ball in a socket.29
FIGURE 10 - THE MODERN OCCLUSAL INTERFACE
Jagged occlusal features may not be problematic, because even point-to-point contacts can still provide adequate occlusal stability for bracing, and the functional pathways of the mandible during mastication are smoothed by the bolus. However, there are three occlusal problems that have become common in modern human dentitions – loss of occlusal stability, restrictions to the functional mandibular range of motion, and displacement of the HMBA.
LOSS OF OCCLUSAL STABILITY
The dentition is designed to achieve occlusal stability early in life and then maintain it throughout life. Occlusal stability was preserved during the transition from primary to permanent dentition by an eruption sequence in which permanent teeth erupt in front of and behind the primary occlusal table to extend and stabilize it before the primary teeth are replaced one or two at a time. Occlusal stability was then maintained into old age despite occlusal wear and injury by dento-alveolar eruption and functionally stimulated facial growth.
Our ancestors had very strong jaw muscles and very stable occlusions. Of the thousands of pre-industrial dentitions that are still intact in museums, the only ones that lack extreme occlusal stability are those that had apparently undergone serious injury, and even many of them re-established occlusal stability by incorporating unusual morphologic features such as notched interproximal contacts or wear facets on the buccal or lingual surfaces. Pre-industrial dentitions display such a remarkable congruence between occlusal surfaces almost any way they are put together that most researchers have been unable to locate a single clear centric or intercuspal position.
Modern occlusions are much less stable, and most of their stability is confined to a small area so that minor changes of the mandibular closing trajectory can completely alter the location of the occlusal contacts. For example, when the mandible moves horizontally, occlusal contacts may occur in different directions than the path of the movement. The relatively sudden loss of occlusal stability that occurs when people transition from a traditional rural to a modern urban life style has been well documented in several different cultures.32-33
An occlusion can be destabilized at any age by small changes in the TMJs or the teeth. Because the mandible is such a long bone, even a slight loss of condyle height due to regressive remodeling or an effective increase in condylar height due to TMJ inflammation can significantly disrupt occlusal stability. Subsequently the occlusion may restabilize naturally due to the same balance of eruptive and intrusive forces that established occlusal stability in the first place. However, natural occlusal restabilization may be prevented by features such as tight interproximal contacts, irregular tooth alignment, and pressure from soft tissues.
Intervention to restabilize an occlusion may involve reducing high areas and/or building up low areas. The choice depends on facial height and the state of the dentition. Composite resin is an easy way to build up low areas; and it wears in before it wears out, providing a more accurate occlusal fit than we can produce in a dental laboratory. To retain that accuracy, during a window of time after the resin wears in and before it wears down excessively, the refined contours can be duplicated in more permanent materials using transfer techniques.
Maintaining a stable occlusion also requires enough bracing force and consistency to keep the teeth aligned. Occlusal stability is highly correlated with jaw muscle strength.34 The jaw muscles can be strengthened by chewing on a resistant wafer, tough foods, or an unsoftened gum base (exercise gum). The jaw muscles also need to exert their forces within an area that is narrow enough to define an HMBA or the mandible seems to look around for a better bracing platform, biting more frequently than normal over a wide range of mandibular positions.
RESTRICTIONS TO THE FUNCTIONAL MANDIBULAR RANGE OF MOTION
The occlusal table is also designed to develop contours that guide the mandible smoothly into and out of the HMBA along contours that define a normal mandibular range of motion. That range of motion is established during childhood by masticating with a distinct wide lateral opening thrust that served to prevent erupting teeth from impinging on it before normal mastication achieves enough strength and consistency to maintain it.35 After the normal range of motion is well established, the pattern of mastication reverses, and the childhood lateral thrust on opening is replaced by a normal adult masticatory pattern in which the mandible opens near the midline and then shifts laterally while open in preparation for a powerful mesial thrust during the final stages of closing.36
When teeth fail to receive enough functional forces to align their occlusal surfaces in harmony with a healthy functional mandibular range of motion, the orientation of their occlusal surfaces depends partly on conditions like soft tissue forces, the positions of neighboring teeth, eruption pathways, and unworn cuspal anatomy. As a result, one or more of their occlusal surfaces can limit the mandibular range of motion with orthopedic obstacles such as balancing side interferences and steep asymmetrical guidance. The range of motion limits that result from such interferences can then affect subsequent positioning and wear of teeth in a way that further reinforces those limits.
Restrictions to the mandibular range of motion can impair mastication by triggering neuromuscular reflexes that protect teeth by rapidly shutting down elevator forces. The neuromuscular system avoids continually triggering these protective reflexes by establishing an engram that produces a more acceptable pattern of functional mandibular movements. As a result of engrams that restrict the mandibular range of motion to avoid occlusal interferences, mastication can become a process of mashing the bolus in the middle of the HMBA rather than crushing it while dragging it through the HMBA. Mashing can break down most refined foods adequately for digestion, but it does not provide healthy exercise for the jaw muscles, because penetrating the soft bolus produces tooth contact which triggers an EMG “silent period” - a sudden stoppage of jaw elevator muscle activity that prevents them from completing a normal chewing stroke.
Restrictions to the mandibular range of motion are well tolerated during childhood when a steep, vertically oriented, and often irregular HMBA surface can be negotiated by a fast and agile neuromuscular system. Young tissues are full of water, enzymes, and elastic fibers that enable them to withstand diverse articular stresses and unanticipated impacts while hyperactive neuromuscular reflexes and strong versatile jaw muscles can cope with frequent sudden changes in firing patterns or forces applied. Chewing strokes can deftly work the mandible across a jagged occlusal table while performing intricate dances to avoid traumatic collisions between irregular overlapping cusps.
As adults age, changes in the character of the tissues favor a steadier and less ballistic masticatory pattern. Adaptability diminishes as metabolic activity slows and the body becomes less able to rapidly increase local circulation. In muscles, the availability of ATP dwindles and the number of contractile fibers declines. Nerve tissue undergoes age related declines. Neuromuscular reflexes become slower due to a delay in processing rate and conduction velocity. In the performance of tasks, there is a loss of coordination and precision of movement.
In our ancestors, these progressive losses of strength and adaptability with age were accompanied by changes in the form of the occlusal table that made mastication require less strength and adaptability. The size of the HMBA decreased due to interproximal wear, mesial drift, and abrasion of the buccal and lingual surfaces. Occlusal stability increased due to the enlargement of the interproximal and occlusal facets. Functional mandibular pathways became smoother and steadier and therefore more easily negotiated by the jaw muscles without triggering protective reflexes.
In modern adults, progressive losses of strength and adaptability of the masticatory system with age continue, but the occlusal table no longer matures so consistently in a manner that makes it progressively more compatible with these losses. At some point, the masticatory neuromuscular system may become unable to negotiate jagged, irregular, or restrictive occlusal contours without straining the jaw muscles or allowing damage to the dentition or temporomandibular joints.
DISPLACEMENT OF THE MANDIBLE
The most clinically significant occlusal problem today is displacement of the mandible. During normal growth, the orientations at which the various mandibular elevator muscles converge onto the mandible from origins distributed widely around the cranium make the occlusal table one of the most stable landmarks of the craniofacial area. However, when a craniofacial growth pattern lacks regulation by strong healthy jaw muscles, the occlusal table can acquire contours that displace the mandible in various directions and in various planes.
Vertical displacement inferiorly has become relatively common in the last couple of centuries, in part because the anterior aspect of the average modern face grows longer vertically due to the inability of our weaker elevator muscles to oppose the forces of dento-alveolar eruption. While our ancestors maintained a relatively steady face height in proportion to body height, the modern corpus descends inferiorly faster than it used to and keeps descending during adulthood at a rate that is similar to the rate of occlusal wear in our ancestors.37-38 A similar excessively vertical facial growth pattern can be produced experimentally by anything that weakens or impairs the mandibular elevator muscles. Excessive vertical dimension is ICD 10 code M26.37.
In contrast, vertical displacement superiorly can be caused by extreme bruxism, loss of tooth structure, or worn out dentures. The loss of vertical dimension produced by superior mandibular displacement among edentulous patients led to the discovery of TMJ disorders in the 1930s. Insufficient vertical dimension is ICD 10 code M26.36.
Vertical displacement is only an occasional problem, because the elevator muscles are usually able to keep the height of the framework of bones and teeth that support the HMBA within an acceptable range; and that height can be easily altered therapeutically if necessary. Excessive vertical dimension can be treated by equilibration, jaw muscle strengthening, or using passive myofascial stretch to apply intrusive forces to the dentition during sleep by wearing an oral appliance with a tall stable occlusal platform. Insufficient vertical dimension can be treated by building up or extruding the dentition and stretching the elevator muscles.
Horizontal displacement is a more persistent problem, because it can maintain itself by altering the pattern of subsequent facial growth in a manner that accomodates and thereby perpetuates the displacement. For example, a misaligned permanent canine can erupt into a position that alters the location at which all subsequent mandibular bracing occurs rather than having its position corrected by the forces of mandibular bracing. Subsequently, the rest of the dentition may realign to acquire a stable HMBA in the displaced location while the temporomandibular joints remodel to acquire a close packed position there and the jaw muscles acquire adaptive firing patterns to brace the mandible there. The adaptive muscle resting postures that accompany the altered mandibular bracing position change the pattern of light steady forces that control facial growth.39-40 The effect on facial morphology is most easily seen in unilateral cross-bite, but it probably occurs in a posterior direction unilaterally or bilaterally in many TMJ disorder patients.
To detect horizontal displacement of the HMBA requires temporarily interrupting the flow of afferent periodontal signals that has been continually programming the jaw muscles to direct all mandibular bracing onto the HMBA. The afferent flow can be interrupted with simple mechanical devices such as cotton rolls or a Lucia jig, or even just by anesthetizing the teeth, but the most reliable method is to wear an anterior flat plate appliance during sleep. Even flat stabilization appliances are ineffective deprogrammers, because they provide posterior occlusal contacts, and the jaw muscles will reflexively find and brace the mandible in whatever position provides posterior occlusal contacts bilaterally.41-42
The time required for deprogramming depends on muscle health. It often occurs overnight in children and adults with strong healthy jaw muscles. It may require weeks or even months of nightly appliance wear along with jaw muscle rehabilitation in some chronic TMJ disorder patients.
After deprogramming, a range of acceptable locations for the HMBA can be best approximated by observing mandibular bracing behavior during swallowing. Mandibular bracing is among the cascade of neuromuscular reflexes that accompany each swallow. At the onset of the swallow, the mandible braces to provide a stable base of operation from which the tongue can thrust its tip anteriorly into the front of the palate to gather the food and form a bolus. Then later in the swallow the mandible stays braced to provide a stable base of operation toward which the suprahyoid muscles can pull the hyoid bone superiorly and anteriorly to let the bolus pass behind it. To power this mandibular bracing during swallowing, the elevator muscles exhibit a relatively consistent and uniform pattern of firing activity. Studies of children with laterally forced bites show that the elevator forces are least unbalanced during swallowing.43
Even in the short closures from rest position to the HMBA, the consistent elevator activity in swallowing creates a range of mandibular closing trajectories that defines a bracing area rather than a position.44-45 On the level of precision that we use for prosthodontics, each closing trajectory varies slightly depending on posture, emotion, and probably a wealth of cognitive influences that we do not yet fully understand. The HMBA needs to be able to accommodate those variables.
The guidance surrounding the HMBA should distribute the forces of bruxism widely to protect the teeth. Since occlusal contours cannot prevent bruxism, they should be designed to withstand it. In most dentitions, the best protection is group function. Its precise contours can be refined during a provisional restoration phase in which the new occlusal surface is fabricated in resin and allowed to wear in slightly. Occlusal wear occurs rapidly enough on resin to recreate group function similar to that produced by years of occlusal wear on natural teeth in our ancestors. Subsequently the same contours can be duplicated in gold or porcelain using transfer techniques.
In some cases, the guidance surrounding the HMBA and even the contour of the HMBA itself may be customized in order to protect a vulnerable joint or even a tooth. The mandibular range of motion can be limited in specific directions by steep contours or facilitated in other directions, such as toward the midline, by shallowing contours in that direction.
In the near future, occlusal surfaces will be digitally engineered by integrating data from mandibular movements recorded under the influence of various degrees of loading with data from tiny vibration sensors that display the actual order and timing of each occlusal contact while mandibular movements in the occlusal plane are tracked with micron level precision. Storing and combining the occlusal contact and jaw tracking data from multiple occlusal acts will enable us to produce highly pixellated topographical maps of entire occlusal interfaces, alter them with the click of a mouse, and reproduce them prosthodontically by computerized milling or molding. Contours may be customized to protect articular structures or alter facial growth. Chewing efficiency may be maximized by large numbers of small perfectly fitting facets. However, we also need a better conceptual model for applying these technologies. Studying the evolution of dental occlusion can help us understand how to construct such a model.
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