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The bite: orthopedic perspectives

THE MAXILLO-MANDIBULAR JOINT

To understand the dynamics of the bite in a mammalian jaw system, it must be seen as one large tooth-lined ball-and-socket type of joint that has been re-engineered to house teeth on its articular surfaces, with the lower dentition acting as the ball and the upper dentition acting as the socket. Its close packed position is a centrally located slightly indented flat area where the convex buccal (outer) cusp tips of the lower dentition fit up into the larger concave contours formed by the central grooves and marginal ridges of the upper dentition. This indented flat area, in which the mandible can move freely in a horizontal plane, is very small in carnivores and very large in herbivores. In a healthy natural human bite, it occupies at least 4 square mm, within which the articular surfaces (the bite tables and the TMJs) can withstand maximal compressive loading. The central flat bracing area is surrounded by sloping walls that provide continuous support for the mandible like the walls of a socket in its direction of movement throughout its functional range of motion. 

To understand the dynamics of the bite in humans, it also has to be seen as an integral component of the postural system. That relationship is not described below, but it is described extensively in the file entitled BITES AND BODY POSTURE just beneath this file.

The bite, as a maxillo-mandibular joint, has four characteristics of synovial joints and three unique characteristics: 

SHARED JOINT CHARACTERISTICS OF SYNOVIAL AND MAXILLO-MANDIBULAR JOINTS

1) The area receiving maximal compressive force responds to loads by deforming to distribute the compressive force onto a larger portion of the joint.  In synovial joints, the progressively increasing resistance to loads is produced by a series of increasingly stiff layers of tissue - plastic lubricating film, fibrous articular covering, calcified cartilage, thin subchondral bone, trabeculae aligned to resist functional forces, and cortical bone. In the maxillo-mandibular joint, progressively increasing resistance to loads is produced by embedding each tooth in a sophisticated hydraulic shock absorbing system that allows it to move easily from its rest position over small distances in response to even light imposed loads.10 As a result, each tooth that comes under load shifts out of the way in order to distribute the load onto more teeth, and the collection of teeth gives the bite table resiliency like a big cushion. For example, during light closure in the center of the maxillo-mandibular joint, the load falls on 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  

2) Functional forces enhance circulation at the articular surfaces.  Synovial joint articular surfaces are flushed of waste products by weeping circulation due to rubbing of the cartilage that forces fluids out when an area is compressed and then the articular pressure which replenishes that area  when the compression has moved on to a different area. In the maxillo-mandibular joint, each tooth socket is flushed of its waste products by moving around within its socket in response to bite forces, which compresses the collagenous ground substance, tugs on the principle fibers, drives fluids into nearby vessels, and bends out the socket walls in the direction of movement of the root.1-7  Then, as the force in that direction is released, that area of the socket which was compressed gets replenished by arterial pressure that causes the tooth to rebound towards its resting position. 

3) Neuromuscular reflexes protect the articular surfaces of inflamed joints by increased resting tonus and decreased functional forces. In synovial joints, the muscles surrounding the joint react to noxious afferent signals from the joint capsule and ligament attachments. In the maxillo-mandibular joint, the muscles surrounding the joint (the jaw muscles) react to noxious afferent signals from the vast network of mechanoreceptors surrounding the tooth roots.

4) Remodeling of the joint surfaces continuously maintains the goodness of fit between them.  In synovial joints, the bony articular surfaces are remodeled by osteoblasts and osteoclasts. In the maxillo-mandibular joint, the bite tables are shaped by the adaptive shifting of teeth and their sockets, and the fit is refined by wear on the biting surfaces. 

UNIQUE MAXILLO-MANDIBULAR JOINT CHARACTERISTICS

This maxillo-mandibular joint also has unique features. 

1) Each tooth functions like its own joint with its socket, complete with range of motion, source of functional circulation, continuous remodeling, and reflex neuromuscular protections.

2) Each tooth forms an interproximal joint with its neighboring teeth, including a resting joint space of three to twenty-one microns12.  On the sides of the interproximal joint space are two central flat contact areas surrounded by a periphery that defines a range of motion in all directions. At rest, the adjacent teeth are not in contact. Biting depresses them while it also tips them lingually and mesially until their interproximal gaps close and the adjacent teeth make physical contact. The 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 with many roots. Pre-industrial and modern interproximal joints are photographed in chapters 2 and 4 of ETIOLOGY.

3) The resting maxillo-mandibular joint space is not maintained by resting forces in the surrounding tissues.  While synovial joint spaces are maintained by light passive forces due to the resting tonus of the muscles which cross the joint, the freeway space cannot be maintained by passive intrajoint forces, because the teeth are wired with too many neuromuscular reflexes to have them maintain light steady contact during sleep. The freeway space is protected by neuromuscular reflexes which respond immediately to changes in tooth contact patterns, but those reflexes respond to any increased tooth height, such as a partial or full denture, by lowering the mandible to restore freeway space. By altering mandibular resting posture, those reflexes can redirect facial growth vertically. They can also prevent people from applying enough biting forces during the day to counterbalance and thereby reverse the eruption of the teeth and their supporting alveolar bone that occurs during sleep.

THE CENTRAL BRACING AREA

Much like the central bracing areas of synovial joints are shaped by the remodeling of their bony articular surfaces in response to postural forces, the central bracing areas of maxillo-mandibular joints are shaped by remodeling of the tooth sockets in response to biting forces. Teeth that receive large compressive forces intrude. Teeth that receive less compressive forces than their inherent eruptive force (probably a few grams) extrude. As a result, bite forces shape the maxillo-mandibular joint like tamping down bricks in sand until they create a flat central area with an almost perfect fit between the upper and lower dental arches surrounded by slopes that support the mandible in its direction of movement whenever the mandible moves away from its flat central area. 

The flat central area of the central bracing platform of a mammalian maxillo-mandibular joint may be small, but it is never a point. In all mammalian dentitions, the mandible can move around horizontally at least a small distance before encountering an increase in vertical dimension. In fact, the feature that made temporomandibular joints (TMJs) so valuable in evolution was their ability to support transverse mandibular movements, which allowed forceful rubbing of teeth, which enabled bite surfaces to undergo attrition that maintains sharp cutting edges and closely fitting facets that form effective crushing surfaces despite constant abrasion. The presence of such wear facets is considered the best indicator of the transition to mammals in the archeological record.

The flat area occupied by the central bracing platform varies greatly in size. In carnivores, it is small (about 5 mm in tigers) to enable shearing for cutting. In herbivores, it is large enough to enable wide lateral mandibular movements to grind food like a mortar and pestle. In rodents, it has long antero-posteriorly extended channels to enable gnawing at the incisors alternating with power-crushing at the molars. In hominids, these features of previous mammalian dentitions were combined to produce a new hybrid chewing system with increased adaptability and with the jaw muscles centered over the bolus for power and oriented in all directions for control.

THE JOINT PERIPHERY

Surrounding the central bracing platform, the bite continues to provide support (not guidance), and proprioception continues to occur in the direction of movement of the mandible, like a ball riding up the walls of a larger socket. When the mandible shifts to one side, the lower dentition rides up on the upper dentition of that side (the working side) like riding up the sides of a bowl, which provides sensory feedback from that side, while the rest of the teeth separate and stop providing sensory feedback. When the mandible moves forward, the lower front teeth ride up onto the backwardly facing slopes of the upper front teeth (anterior guidance). When the mandible moves backward, the terminal lower molars ride up onto the forward facing slopes of the terminal upper molars (posterior guidance). 

In our recent ancestors, the ability of the central bite platform to provide orthopedic support in whatever direction the mandible moved was apparently even more important than the bilateral centric stops that are now considered the cornerstone of a healthy bite. In many healthy Aboriginal dentitions, strong jaw muscle forces made the upper jawbone grow so much wider than the mandible that the upper and lower buccal segments (molars and premolars) are unable to achieve bilateral contact simultaneously, but adequate bracing is achieved on one side at a time, as shown below.15.  

ALTERNATELY UNILATERAL MANDIBULAR BRACING 

 

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With the mandible supported simultaneously at maxillo-mandibular and temporomandibular joints that have adapted their contours to the same functional forces, these three joints also end up perfectly fitting each other during bracing and throughout the normal range of motion of the mandible. For example, lateral excursions of the mandible are supported in perfect harmony by the outer (buccal) cusps of the lower teeth on the chewing side riding up onto the inward facing slopes of the upper buccal cusps just as the lateral borders of the condyle on the chewing side ride up onto the inward facing slopes of the disk and lateral walls of the glenoid fossa. As a result, their TMJ and bite contours were more highly correlated with each other than with age or sex,23-24 unlike modern craniofacial structures. 

This support for the mandible in its direction of movement even extends to the basal bones. Incising loads the premaxilla and anterior nasal spine and the articular eminence. Anterolateral excursions load the canine prominence, the lateral border of the nasal cavity, and the infraorbital shelf on the working side. Lateral excursions load the anterior aspect of the zygomatic process and the lateral portion of the infraorbital shelf on the working side. Posterolateral excursions load the posterior aspect of the zygomatic process and the maxillary tuberosity on the working side. During powerful chewing, the point of compression moved through these supporting structural components of the facial skeleton like rhythmic waves of compression. Power-crushing forces started at the back end of the dentition on the working (chewing) side and spread forward. At each point of compression, the movement of the mandible carried the outer cusps of the lower teeth upward, forward, and medially (toward the midline).  After reaching the midline, the wave of compressive force crossed to the non-working side in a follow-through stroke.  At each location of the mandible, compression drove the teeth into their sockets, bent the membrane bones, and compressed the circum-maxillary sutures.  Then, as the location of the mandible shifted, those teeth and their supporting bones rebounded, while other areas were compressed.  

THE SPHERICAL BITE TABLE

In the ball-and-socket dynamics of the maxillo-mandibular joint, the ball closely resembles the surface of a sphere with a 4 inch radius,25-27 which is about the length of the temporalis muscles, the postural muscles from which the mandible hangs and which is how all the early dental researchers described excellent natural bites, as shown below.

MONSON'S SPHERE

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BONWILLS' TRIANGLE

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CURVE OF WILSON

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CURVE OF SPEE

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Monson also described a cone made of 4" radii as a model along which the teeth are aligned

MONSON'S CONE

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Teeth even erupt at angles that resemble the radii of the same sphere.  

ANGLES OF ERUPTION OF THE TEETH

Of course, natural bite table contours are much more complex than the surface of a 4" sphere. The mandible hangs from three slings of muscles; and, even if it just swung passively from these slings, its movement would trace a pattern that has compound curves.  In fact, extremely worn natural dentitions  form longitudinal twists that resemble a propeller blade.27-28  These so-called helicoidal curves, illustrated below, comprise a registration in tooth structure of the natural functional range of motion of the mandible after overcoming the resistance of the teeth.   

THE HELICOIDAL CURVE

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Even such a featureless bite table would be difficult to restore accurately with a mechanical articulator, because its surface angle changes continuously. 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 below.  

A CHANGING CURVE OF WILSON

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MODERN OCCLUSION

Modern human bites have much more complex surfaces.  A typical modern bite table, seen below, looks more like a mountain range than smooth compound curves.29 

THE MODERN OCCLUSAL INTERFACE

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Jagged bite features themselves are not necessarily problemmatic, because numerous point contacts can still create a stable bracing platform, and  chewing pathways are smoothed by the bolus.  The problemmatic features of modern bites include loss of stability, restrictions to the mandibular range of motion, and displacement of the location of the central bracing platform.

LOSS OF BITE STABILITY

A sudden loss of bite stability occurs whenever people transition from a traditional rural to a modern urban life style.32-33  This loss of stability can be seen in the number of contacts and the surface areas contacting and by the smooth group function occurring all around it.

RANGE OF MOTION RESTRICTIONS

When the teeth fail to receive enough functional forces to align them in harmony with a healthy functional mandibular range of motion, they can restrict the mandibular range of motion. The range of motion restrictions then get embedded in the neuromuscular system by an adaptive firing pattern (engram) that results in a narrowed mandibular range of motion. In steeply interdigitated teeth, chewing movements may be confined to mashing the bolus in the middle of the central bracing area. 

Restrictions to the mandibular range of motion are well tolerated during childhood. 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 can cope with frequent sudden changes in firing patterns and strong versatile jaw muscles can deftly work the mandible across a jagged bite table while performing intricate dances to avoid traumatic collisions between irregular overlapping cusps.

With advancing 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. Neuromuscular reflexes become slower due to a delay in processing rate and conduction velocity.  Task performance diminishes due to 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 bite table that made chewing require less strength and adaptability. The size of the bite table diminished due to wear on the tops and sides of the teeth, while bite stability increased due to wear interproximally (between the teeth). Functional mandibular pathways became smoother and steadier. 

In modern adults, progressive losses of strength and adaptability of the jaw system with age continue, but the bite table no longer matures in a manner that makes it progressively more compatible with these losses. As a result, range of motion restritions often become symptomatic, especially in the jaw muscles.

DISPLACEMENT OF THE CENTRAL BITE PLATFORM 

The most clinically significant result of the recent change in bite table contours is displacement of the location of the central bite platform, which produces a parallel displacement of the mandible's resting posture, which in turn affects body posture and the growth pattern of the face.

VERTICAL DISPACEMENT

The central bite platform can be displaced inferiorly or superiorly, depending on the balance between bite forces and the nightly pressure of dento-alveolar eruption. 

Superior displacement can occur due to extreme bruxism, loss of teeth, or worn out dentures. Superior mandibular displacement due to inadequate dentures led to the discovery of TMJ disorders in the 1930s. Insufficient vertical dimension is ICD 10 code M26.36.  

Inferior displacement of the mandible is a much more common problem today, because our weak jaw muscles no longer provide enough bite forces to limit vertical growth produced by eruption forces in the teeth and surrounding bones. While our ancestors maintained a relatively steady face height during adulthood in proportion to body height, our faces on average now grow longer during adulthood due to inferior mandibular displacement.  Our mandibles shift downward on average at about the same rate as the teeth used to wear down.37-38 An excessively vertical facial growth pattern can be produced experimentally by anything that weakens or impairs the jaw closing muscles. Excessive vertical dimension is ICD 10 code M26.37.  

HORIZONTAL DISPLACMENT

Horizontal displacement is a more persistent problem than vertical displacement of the central bite platform, because horizontal displacement maintains itself by altering the pattern of subsequent facial growth in a manner that accomodates and thereby perpetuates the displacement. For example, if a permanent canine erupts into a displaced position, it can alter the location at which all subsequent mandibular bracing occurs rather than having its final erupted position corrected by the forces of mandibular bracing. Subsequently, the rest of the dentition realigns to acquire stability only in the displaced location, and the jaw muscles acquire adaptive firing patterns (engrams) to brace the mandible there, which in turn reflexively alters jaw muscle forces to hold the mandible in a posture just beneath that position.  Posture provides the light steady forces that control bone growth.39-40  As a result, the displaced mandibular posture maintains a displaced mandibular growth pattern.  

DEPROGRAMMING

To detect horizontal displacement of the mandible requires temporarily interrupting the flow of afferent periodontal signals (engrams) that has been continually programming the jaw muscles to direct all mandibular bracing into the current central bite platform. The afferent flow can be interrupted with simple mechanical devices such as cotton rolls, a Kois deprogrammer, a Lucia jig, or even just by anesthetizing the teeth; but the most reliable method is to wear a front flat bite plate appliance during sleep. The time required for deprogramming depends on muscle health.  It often occurs overnight in young people 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.

SWALLOWING

After the jaw muscles are deprogrammed, they can best demonstrate the ideal location for the central bracing area during swallowing, which is accompanied by a relatively consistent and uniform pattern of firing activity in the jaw closing muscles. Studies of children with laterally forced bites show that the jaw closing forces are least unbalanced during swallowing.43-45  To make a record of that central bracing area would require having the patient swallow multiple times in varied head postures with the jaw muscles fully relaxed so that myofascial forces alone determine closing trajectories.

RESTORING BITE STABILITY

To restabilize bites orthopedically, the concave upper bite surfaces (central fossae and marginal ridges) are constructed to house the lower buccal cusp tips.  The cuspal inclines which form the walls of each housing provide group function buccally and clearance palatally.  Initial contacts occur at the premolars and first molars in neutral head posture, at the second molars when the head is extended, and at the anterior teeth  when the head is flexed, such as in the alert feeding position. After the ideal bite is constructed, maintaining it requires biting forces that are strong enough and distributed widely enough to keep the teeth in their desired positions. 

The contour of the bite table surrounding the central bracing platform should distribute the forces of bruxism in a manner that protects the teeth (group function). Perfect group function is difficult to construct in the laboratory, but its accuracy can be improved during a provisional restoration phase in which the new bite surface is fabricated in resin and allowed to wear in slightly before duplicating the same contours in gold or porcelain using transfer techniques.

In some cases, bite contours may be customized in order to protect vulnerable articular structures in the TMJs or the teeth. For example, the mandibular range of motion can be limited in specific directions, by steepening cuspal contours in that direction; or it can be facilitated in other directions, such as toward the midline, by shallowing cuspal contours in that direction.

OCCLUSAL SOLUTIONS 

In the near future, bite tables will be digitally engineered by integrating data from mandibular movements recorded under the influence of various degrees of loading with data from tiny piezoelectronic vibration sensors that display the actual order and timing of each bite contact while mandibular movements are tracked with micron level precision by position sensing detectors (PSDs). Storing and combining the bite contact and jaw tracking data from multiple biting acts will enable us to produce highly pixellated topographical maps of entire bite interfaces, alter them with the click of a mouse, and reproduce them prosthodontically by computerized milling or 3D printing to maximize chewing efficiency by creating well defined central bracing areas supported by large numbers of small perfectly fitting facets surrounded by slopes constructed to fit mandibular range of motion suited for each individual jaw muscle pattern. 

FOOTNOTES

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