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Bites: Current Concepts

OCCLUSAL CONFUSION

Dentists in school are taught that occlusion is an act that must be carefully controlled, but they are not taught why. They learn that each tooth should provide a centric bite stop, but they don't learn how the teeth work together orthopedically to provide a platform for the mandible to rest on and exercise against, and how this platform affects craniofacial muscle tonus, facial growth, and body posture. The bite table is among the most stable of craniofacial growth landmarks in mammals, including our recent ancestors, while other craniofacial components fluctuate more widely around it. However, when we industrialized our food supply, human bite tables became much less stable structurally, more postero-inferiorly located, and more asymmetrical. Those changes have had profound effects on our health, but dentists have been unable to connect them with the bite changes that cause them, because the parameters by which dentists measure bites have no relevancer to their orthopedic function or health.  

The inability of traditional dentistry to understand how bites work is revealed by the way bites are managed clinically. Altering a bite has been described as "sailing into uncharted waters". Most dental authorities consider any bite treatment, even stabilizing a bite, to be invasive and therefore only to be used as a last resort. Clinical dentistry treats the bite table as a wall with a number of critical support areas (centric stops), each of which must be carefully preserved; because reducing it could cause the existing bite to become lost and difficult or impossible to find again. Therefore, bite changes are considered dangerous, because they are irreversible. The reluctance to change a bite is so widespread in clinical dentistry today that successful phase one disk recapturing procedures using oral orthopedic appliances are usually followed by an attempt to return to the pre-treatment bite, even though that process causes more than half of the patients to undergo recurrence of the disk displacement and at least some recurrence of symptoms. 

If a bite is corrected according to conventional standards and patients complain of discomfort, their discomfort is assumed to be caused by factors other than bite strain. Patients who have become uncomfortable with the feel of their bite following proper dental reconstruction have been diagnosed as having occlusal disease, occlusal neurosis, occlusal dysesthesia,1 occlusal hyperawareness, positive occlusal sense, body dysmorphic disorder, somatoform disorder, monosymptomatic hypochondriacal psychosis,2 or phantom bite syndrome3. Studies of this condition have found that it is not associated with any recognized anatomical features or proprioceptive changes, such as altered interdental thickness discrimination.4-5 Treatments reportedly used for it have included cognitive behavioral therapy, counseling, psychotherapy, and medications including pimozide (a neuroleptic drug),6-7 dothiepin (antidepressant),8 tricyclic antidepressants,9-10 serotonin-selective reuptake inhibitors, seratonin-norepinephrine reuptake inhibitors,9,11-12 monozide (anti-psychotic),13 mirtazapine (noradrenergic and specific serotoninergic antidepressant), and aripripazole (a dopamine partial agonist).9   

Most of the clinical research on bites has attempted to precisely locate the best mandibular position for simultaneous contact of all the teeth, or at least all the back teeth, because the conceptual frameworks by which dentists evaluate bites grew out of laboratory techniques that employ a hinge articulator to mount the models of the upper and lower dentitions. However, human mandibles, as all mammalian mandibles, do not work like hinges, even multi-adjustable hinges. Instead, they always allow the mandible some horizontal freedom of movement within the central bite area. In fact, that horizontal freedom of movement due to the TMJs was the breakthrough that distinguished mammals from reptiles, because it enabled mammals to rub their teeth together in order to chew food, rather than just tear and swallow it like reptiles. Thus the mammalian bite table functions as a platform that occupies an area. That area is certainly smaller today than it was in our pre-industrial ancestors, but there is no reason to expect it to be zero.

The bite table is not a wall, but a dynamic platform in which each structual member is constantly adapting its position to keep its articular surfaces aligned with functional forces. Normal healthy human teeth have a mobility of 50 - 100 microns,14-15 and they can adaptively shift position by hundreds of microns overnight to accommodate bite forces. Therefore, reversing symptoms produced by a bite change does not require precisely reproducing all the critical support areas that existed before the change - it requires restoring the ability of the bite to provide a stable central bracing platform and a range of motion that is acceptable to the rest of the postural system. The barrier to using bite change therapeutically has been our inability to predict the effects of altering a bite in any particular manner, because we have not understood how bites work orthopedically.  

BITES AND TMJ DISORDERS

Bite strain can certainly cause TMJ disorders. Symptoms have been produced experimentally by adding high fillings (bite interferences) to centric stops,16-20 working side excursions,21-23  and balancing side contacts.24-31 In one subject, an experimental temporary bite interference only .25 mm tall caused symptoms that persisted for nine months until they were treated with a bite plate.32  It's reasonable to expect the bite to affect TMJ health, because it determines the braced location and functional range of motion of the condyles; and the location of the braced (close packed) position and the functional range of motion of the articulating bones in other joints affects their health. It's also reasonable to expect the bite to affect the health of the jaw muscles, because it provides their primary exercise template. In other parts of the body, the physical features of the template against which muscles exercise affects the health of those muscles, and the jaw muscles have been shown to react almost immediately to even minute changes in the contours of their exercise template.33-38 

However, researchers have been unable to find a connection between TMJ disorders and bites.39-40 A few bite parameters (deep overbite, anterior open bite, loss of posterior support, and unilateral cross-bite) show weak correlations with TMJ disorders at extreme values; but normal variants of most bite parameters show no correlation with any functional condition.41-42 As a result of this failure to correlate, some researchers have concluded that the bite plays no significant role in TMJ disorders and therefore that evidence based treatment provides no justification for bite treatment.

However, lack of evidence of a correlation is not evidence of lack of a correlation. It may be that we just don't know enough about bites or TMJ disorders to demonstrate the correlation between them. Indeed there is good evidence to support that explanation. The parameters that we use to compare and contrast different bites include static measurements of spatial relationships between upper and lower teeth and some measurements of sliding and incidental tooth contacts, however none of those parameters has ever been well correlated with jaw system health or TMJ disorder symptoms. For decades we have known that some people with chronic TMJ disorder symptoms and difficulty chewing have textbook perfect bites, while other people with very irregular bites have excellent jaw system health and function. In research, the bite is an uncontrolled variable. Studies that attempt to evaluate the effect of altering bite parameters yield inconsistent results, because they are probably helping some people and hurting others. Even balancing side interferences are correlated with TMJ disorder symptoms in some studies,43-44 and not in others.45-47 

BITE STABILITY

The only bite parameter that we at least try to measure which has any functional relevance is stability.48 On average, people with TMJ disorders have less stable bites than  normals.49 However even that relationship has been difficult to demonstrate, because our techniques for measuring bite stability are so crude compared with the sensitivity of the system. There are three problems with our current techniques for measuring bite stability.  

One problem is the thickness of the measuring device. Bite stability is measured as the ability to evenly distribute biting pressure on a 100 micron thick piezoelectric film sensor (Tek Scan or Accura) or a slightly thinner and less sensitive plastic sheet (Prescale Occluzer System), to mark bite surfaces with 40 to 60 micron thick carbon paper or inked cloth, or to penetrate a sheet of thin wax. Yet all these materials that must be interposed between the teeth are much too thick to give clinically relevant information. The neuromusculature of the masticatory system reacts to interferences less than 8 microns tall,50-52 yet even the thinnest marking device is at least twice that thick, and most are several times that thick.

A second problem is tooth mobility. Teeth at rest are so delicately suspended in the middle of their sockets that they move easily over small distances.53 When a patient bites, the first teeth to contact shift to allow other teeth to contact, and our marking devices cannot distinguish between the first teeth to hit and the subsequent contacts.

A third problem is the variability of mandibular closing trajectories. The order of bite contacts at the termination of any closure of steeply intercuspated teeth depends on the precise closing trajectory, which depends on variables such as posture and mental state. It only becomes consistent after a series of consecutive mandibular closures has allowed the jaw muscles to hone in on whatever bracing position provides the most stable bite platform after the teeth have shifted in any way they need to provide maximal stability in that bracing position.  

BITE PHILOSOPHIES

Even if dentists can't measure the functional aspects of bites or understand how they work with the rest of the body, they need techniques for managing bite problems and reconstructing bites in clinical practice. The techniques that are currently used and the conceptual models that have been built to rationalize them are described below:  

CENTRIC RELATION 

Centric relation (CR), which is still the most commonly held bite philosophy and a deeply held belief system among many dentists, is a conceptual model that grew out of the use of hinge articulators to set denture teeth a century ago. Dentists learned that, if the mandible is pushed back as far as possible, it can be made to consistently rotate open and closed like a hinge around an axis drawn between the TMJs; and, if the denture teeth are set to all contact at the same height on that hinge-axis closing trajectory, the dentures remain stable during chewing and swallowing. Later, when dentists needed to construct effective chewing surfaces for making crowns and bridges, they found that making the teeth fit together on that same hinge axis trajectory seemed to enable effective chewing and also prevent the rapid periodontal breakdown that sometimes occurred when other positions were used to build the bite. CR seemed to work clinically, and it provided a consistent reference point for comparing different bites and changes to bites.

CR was embraced by dentistry. A bite in which all the teeth contact simultaneously in CR was considered to be the only proper bite. People who sometimes use a more forward bite position were assumed to be "posturing" for psychological reasons or suffering from some other pathology, such as spasm of the superior lateral pterygoid muscles. People who did not have a stable bite in CR were assumed to have a malocclusion characterized by CR interferences that triggered chronic firing of the superior lateral pterygoid muscles to avoid striking the interferences by pulling the mandible anteriorly away from CR (the CR slide). 

As dentistry became more precise, CR became more narrowly defined. Dawson described CR as a point -"a definite apex from which no forward or backward movements of the condyle-disk assemblies can occur unless they move down on the bony slope of the fossa".54  The ideal mandibular opening and closing trajectory was thought to be a pure hinge axis rotation, like a hinge articulator, which terminates at 138 simultaneous tripodized bite contacts. Researchers measured CR centric slides in .1 mm increments and tried to correlate them with TMJ disorders. 

CR philosophy spread to all areas of dentistry. Periodontists removed centric interferences to diminish tooth mobility. TMJ specialists designed nightguards and splints to provide stable contacts in CR. Orthodontists pushed back the upper teeth to fit closely around the lower teeth when the mandible is in CR.  

Equilibration techniques, in which dentists perfected techniques for locating and removing all interferences to CR closure, became the popular way to treat TMJ and bite disorders. Some dentists performed full mouth rehabilitation on otherwise healthy patients just to create CR bites. Grinding CR closure into a normal bite changes the normal envelope of mandibular movement (Posselt), seen on the left side below, to fit CR theory, as seen on the right side below. Superimposing the two illustrations would see the apex at CRO created by grinding in CR.

FIGURE 1   -  REDRAWING THE ENVELOPE OF MANDIBULAR MOVEMENT     

                          POSSELT                                DAWSON

CCdawson posselt lowres.jpeg

Elaborate conceptual frameworks were concocted to explain why CR bites are healthy. Dawson claimed that CR is the only mandibular position which is stable, because it is braced by bone; and therefore it is the only mandibular position that allows full relaxation of the superior lateral pterygoid muscles. Ramfjord claimed that the jaw muscles function with harmonious low level activity when the teeth contact evenly in CR.55 Other researchers hypothesized that centric interferences cause spasm or hyperactivity of the superior lateral pterygoid muscles by denying access to CR, and that chronic hyperactivity of the superior lateral pterygoid muscles displaces the TMJ articular disks by pulling them antero-medially off the condyle. Medical illustrations were drawn with the entire superior lateral pterygoid muscle attached directly to the front edge of the disk in a manner that would enable that muscle to pull the disk off the condyle. 

These hypotheses were combined to produce an explanatory model of TMJ disorders that is still widely accepted today. A recent article summarized, “Any sort of hit-and-slide from CR into MI will cause the condyles to translate down and forward out of the fossae. Once the condyles are positioned down and forward on the slippery slope of the eminentiae, the inferior belly of the lateral pterygoid muscle must contract to hold the condyles in this down and forward position, while the superior belly of the lateral pterygoid must also contract to keep the disc properly positioned between the condyle and eminence. Periods of prolonged contraction of the lateral pterygoid result in fatigue or spasm of the muscle, which can be experienced as pain and discomfort to the patient. These symptoms can be exacerbated if the patient has a clenching or bruxing habit because the temporalis, medial pterygoid, and masseter elevator muscles will be highly active and will be in direct contrast to the already contracted lateral pterygoid muscles. This dysfunction and constant opposition between the elevator muscles and condyle positioning muscles will further increase the fatigue and strain on all of the muscles of mastication. Also, constant tension within the superior belly of the lateral pterygoid muscle (the portion of the lateral pterygoid with attachments to the articular disc) will result in continuous stretching of the ligaments that attach the disc to the posterior surface of the condyle. This constant stretching can eventually create an unstable condyle-disk assembly, resulting in a disc that can click or pop off of and onto the lateral pole of the condyle during function.”56

Research has undermined all these assumptions. Provocation experiments found that centric interferences are as likely to decrease jaw muscle activity as to increase it.67-68 MRI studies showed that disk displacements occur in many different directions and not usually in the path of the superior lateral pterygoid muscles.69 Anatomical studies showed that 80% to 85% of the superior lateral pterygoid fibers attach to the condyle rather than the disk, making it unlikely that they could pull the disk off the condyle anyway.70-71 Radio telemetry showed that, even after removing all CR interferences, CR is rarely used.60-63  Intrajoint catheters showed that CR is the only mandibular position that produces increased intra-articular fluid pressure.64 EMG studies showed that retruding the mandible causes increased elevator muscle tension65 and hyoid instability.66 Kinematic studies showed that the concept of a pure hinge axis closure was a mechanical abstraction; because, even in CR, the condyles do not operate like hinges.  All natural jaw movements combine rotation and translation. 

Supporters of CR had to explain why most people with perfectly healthy masticatory systems lack the characteristics of an ideal or even a good bite according to CR theory. Centric slides can be found in all pre-industrial human dentitions,72-73 and they are still found in 90% of modern dentitions.74-76  When patients have full mouth reconstruction to eliminate their centric slide, it usually returns.77  Dawson claimed that these centric slides in apparently perfectly healthy jaw systems were due to an "adaptive centric posture", which occurs when "deformed TMJs have adapted to a degree that they can comfortably accept firm loading". Okeson blamed the anterior shifting of the condyles in CR on pathological elongation of the temporomandibular ligaments. 57 Many researchers blamed lateral shifting of the condyles in CR on a pathology they named an "immediate side shift", and studies found that it varies in length from 0 to 3 mm, yet it appeared to have no clinical significance.58-59 Even the plausible sounding warning that condyles should not be held down on the slopes of the articular eminences for more than very short intervals turned out to be baseless when dentists treating sleep apnea learned that mandibles can be held in extreme protrusion all night without causing problems in most people.

Recently, as a result of the problems sometimes associated with the clinical application of CR, supporters of CR have softened their positions. Most have stopped pushing the mandible backward so forcefully, some have redefined CR as a superior or superior-anterior condylar position instead of a posterior or supero-posterior one, and some advocate freedom in centric - either a long centric or a wide centric. The glossary of prosthodontic terms has 7 definitions for CR, with the most recent moving the focus away from the interdigitation of the teeth by defining CR as a disk-condyle relationship, even though about thirty percent of modern adults have a dislocated disk in at least one TMJ. Most authorities no longer recommend changing a functional and asymptomatic bite to fit CR theory. Okeson simply advocates for a musculoskeletally stable mandibular position.

The repeatability of CR makes it convenient, but that does not make it a healthy treatment position. The repeatability is due to the fact that CR is a border position. Border positions in joints are not functional positions – they provide movement limitations that protect the joint structures from injury. The ligaments that become taut when the mandible reaches its posterior border position are designed to function passively as restraining devices, not to enter actively into joint function. They can be used to limit jaw closing to one posteriormost opening and closing trajectory, but joints need a range of motion that ensures adequate circulation to all areas of their articular surfaces to stay healthy, and it's difficult to envision how confining the mandibular range of motion to its posterior borders could benefit the TMJs. The optimal location for stable mandibular bracing is probably, on average, about 1 mm to 1.5 mm forward from CR, but even that varies too much to provide a guide for choosing a bite position.  

Restoring dentitions in CR usually works clinically, because CR is located close to the posterior border of the functional mandibular range of motion, where the jaw muscles automatically bring the mandible for power crushing and therefore where teeth are most vulnerable to damage by extreme chewing forces. A tooth that contacts prematurely near CR is more likely to be injured than a tooth that contacts prematurely in a more forward mandibular position, where jaw muscle forces are lower.  If a facial pain condition is due to frequent activation of neuromuscular reflexes protecting a hypersensitive molar from bite trauma, eliminating a CR interference on that tooth can relieve TMJ disorder symptoms. However, the success that CR dentistry has occasionally had in those types of patients is certainly not an indication that CR is the ideal or even a desireable location for the central mandibular bracing platform in all people.

CANINE GUIDANCE

For clinical work, dentists also need to choose mandibular pathways in and out of that central bracing platform. Cusps that are too steep can collide. Cusps that are too flat can prevent the cutting actions that are needed for effective chewing.   

In the 1960's, a researcher named D'Amico studied the skeletal remains of a tribe of American Indians and saw that they, like all tribal people, lose their overbite and overjet to acquire end to end occlusion as their teeth wear down with age. To D'Amico, their faces seemed too short, and he mistakenly concluded that this change in their bites was pathological. His reasoning was, "If the edge to edge relation of the anterior teeth were a hereditary functional relation, it would be seen in man today, with unabraded normal tooth structure".78 He went on to hypothesize that the tooth wear in these tribes could be prevented by more overlap of their canines with their long roots, because "nature intended the canines to protect the posterior teeth by guiding the mandible into CR".  

Actually the changes in facial form that D'Amico saw as pathological were the natural results of mechanisms designed to compensate for wear, including a continual dento-alveolar eruption force of at least a few grams and an adult facial growth pattern that continually moves the lower dentition up and forward into the upper dentition just far enough to supply as much tooth structure as needed for the bite table to remain steady despite wear.79-84 In tribal people, face height generally maintained its proportion to body height throughout life. 

D'Amico correctly observed that canine guidance decreases functional jaw muscle forces. He said, "contact of the upper cuspids by the opposing mandibular teeth during eccentric excursions causes transmission of periodontal proprioceptive impulses to the mesencephalic root of the fifth cranial nerve, which in turn alters the motor impulses transmitted to the musculature."  He advocated employing that reflex protective mechanism to reduce jaw muscle forces by increasing the steepness of the canine guidance to diminish tooth wear, because he had perceived excessive jaw muscle forces to be the problem that had caused the loss of vertical dimension in the Indian tribe he studied. 

Later other researchers using EMG affirmed his finding that canine contact reduces biting forces. They found that group function contacts caused both the ipsilateral temporalis and the masseter to fire; but a canine contact only triggered firing in the ipsilateral temporalis.85-86  Thus, canine guidance reduced functional jaw muscle forces, which the researchers took to be a justification for increasing canine guidance, because they believed that TMJ disorders were due to excessive jaw muscle forces.

Dentists came to believe in canine guidance, much like they believed in CR. These were articles of faith in the dental community, and they still are. Clinically, dentists steepen canine guidance to treat jaw muscle pain and bite problems.

However, the researchers were looking at the wrong muscle forces. The problem in TMJ disorders is excessive jaw muscle resting forces (tonus), not excessive functional jaw muscle forces; and canine guidance only reduces jaw muscle functional forces.87 Canine guidance shuts down functional masseter activity by triggering neuromuscular reflexes that are designed to protect the TMJs. In apes, lateral movements of the mandible during chewing can thrust the molars laterally to produce a powerful grinding action, while the contacts between the backward facing surfaces of the lower canines and the forward facing surfaces of the upper canines protect the mandible from posteriorly directed forces. In modern humans, lateral thrusts produce contacts between the forward facing surfaces of the lower canines and the backward facing surfaces of the upper canines, which can drive the ipsilateral mandible posteriorly and thereby activate the posterior temporalis muscles earlier in the chewing cycle to pull it away from the canine contact. In addition, while steepening canine guidance narrows the mandibular range of motion, there is good evidence that most people would be better served by a wider range of motion. When jaw muscles are rehabilitated and bites are stabilized, the functional mandibular range of motion naturally widens.100-102  

The canines provide important contributions to the anchorage of natural dentitions, but they do not play a unique role in the bites of hominids103, and they are not specially designed to remove all horizontal forces from the other teeth. They are designed to work together with the other teeth to provide stable support for the mandible throughout its normal range of motion, and they are the primary support for the mandible when it moves antero-laterally.

ANTERIOR GUIDANCE

The canine guidance concept was extended to include the anterior (front) teeth. In healthy natural dentitions, when the mandible moves anteriorly, it rides up onto the incisors, onto a stable incisal bite platform. Because of the dominance of CR, a stable incisal bite platform is rarely considered important when restoring the anterior teeth. Insufficient anterior guidance is ICD 10 code M26.54.

MUTUAL PROTECTION

Combining CR with steep anterior and canine guidance led to an occlusal philosophy known as mutual protection, based on the belief that teeth should only receive forces axially (straight downward into their sockets). The steep anterior and canine guidance protect the back teeth by preventing them from all horizontal vectors of force, and stable CR contacts on the back teeth protect the front teeth from the powerful jaw closing forces produced during bracing. Labwork is facilitated; because the back teeth just need to provide stable centric stops in one mandibular position, and they do not require simultaneous sliding contacts; and the front teeth can be designed for esthetics without also requiring enough stability to provide an incisal bracing platform. 

BIOESTHETICS 

Some dentists took these concepts of mutual protection, centric relation, and canine and anterior guidance to their extremes by rebuilding all their teeth to their original unworn shapes, including CR contacts on the front teeth. The proponents of bioesthetics claim that wear and nocturnal bruxism are pathologies that result from the bite being "out of balance", and these pathologies can be prevented by restoring balance to the bite, usually. by increasing the steepness of the anterior and canine guidance. 

However, the foundational beliefs of bioesthetics also have no scientific basis. Nocturnal bruxism is a normal sleep behavior, usually associated with microarousals following increased sympathetic activity, and it reflects no particular bite condition.106 It cannot be caused by bite problems or eliminated by bite treatment,107-108 and it is not correlated with TMJ disorders.109-113 In fact, some tooth wear is probably normal and desireable.  Most species of mammals do not even achieve effective chewing function until wear has reduced the complex arrangement of cusps and fossae that cover the biting surfaces of newly erupted teeth into a series of closely fitting facets that crush food between the facets and cut food at the facet edges.114 Our unworn enamel covered bite surfaces were designed to align the dental arches and then provide a constant supply of ridges that helped to grate food and edges that cut food - not to maintain a continuous protective layer on teeth or limit the range of motion of the mandible.115 The teeth protect each other by sharing the loads they encounter in whatever direction the mandible moves, like the walls of a socket in which a ball moves around.

Also, in all natural dentitions, teeth do not just receive forces axially - they have a healthy natural range of movement that helps keep them healthy.  The concept that teeth should only move straight up and down defies biologic common sense. All joints, including the dento-alveolar (periodontal) joints between the tooth roots and their sockets, are designed to benefit from hydrostatic forces generated by a normal functional range of movement. The extensive network of small vessels and anastomoses that fill the tooth sockets  communicate directly with surrounding bone marrow spaces. During healthy mastication, these vessels act like hydraulic lines to absorb shocks and circulate fluids.116-118 Compression of a tooth pumps fluids out of the socket and into venous circulation, then release of the compression allows new blood to flow back into the socket with an articular pulse that gradually returns the tooth to its rest position.119 The circulatory benefit from alternating compression and release probably explains why reducing or eliminating masticatory forces causes atrophic periodontal changes,120-121 much like immobilizing synovial joints causes degenerative arthritic changes in those joints. Also like in synovial joints, alternating compression and release of a tooth affects one area of its surrounding periodontium at a time; therefore a healthy functional range of motion for a tooth requires sufficient variability, including a vertical (axial) component, a transverse (bucco-liongual) component, and even a mesio-distal component between adjacent teeth, complete with interproximal contact areas shaped to function as interproximal articular surfaces.

GROUP FUNCTION

Group function is the natural state of the bite in all mammals, including humans.122 Until the industrialization of our diet in the last couple of centuries, canine guidance was only present temporarily in some newly erupted dentitions, where its role was to provide enough early coupling between the diverse growth patterns in the maxilla and the mandible to maintain the proximity of the dental arches in a sagittal plane despite their diverse growth patterns. Once the bite table was formed, the front teeth did not restrict or "guide" mandibular movements but cooperated with the neighboring teeth to provide a stable bracing platform for the mandible in whatever directon it moved anteriorly or antero-laterally. Omnidirectional group function gave the dentition longevity by evenly distributing wear. The teeth wore in together, and they wore out together. 

Today omnidirectional group function has become less common and less uniform, however most natural bites still operate with some degree of group function.123 The trouble that dentistry has had with group function is that it has been difficult to create prosthodontically with the hinge articulators that are still used by dental labs. In the near future, a computerized mandibular movement simulator will recreate the complex functional movements of the mandible and take into account the independent movements of teeth within their sockets and even the bending of the mandible and the compression and release of the circum-maxillary sutures in order to reproduce mandibular movements accurately enough to enable us to build group function bites in the dental laboratory. 

NEUROMUSCULAR DENTISTRY

The "neuromuscular" bite philosophy is generally been posited as the alternative to CR, although it also has no scientific basis. Their effects on mandibular posture are completely opposite.While CR and anterior and canine guidance techniques almost always shift the mandibular bracing position backward, "neuromuscular" techniques almost always shift the mandibular bracing position forward.  

Neuromuscular dentistry began in 1969 when a well know and respected researcher, Dr. Barney Jankelson, thought he had discovered a better way to locate and record CR which was technologically advanced and measureable. His method involved placing a pulsing TENS (transcutaneous electrical nerve stimulation) source directly over the motor root of the trigeminal nerve to fire all the jaw closing muscles evenly, and then increasing the TENS to electronically close the mandible directly into its ideal bite - the so-called myocentric position. He claimed that the myocentric position is so important that the pathways into and out of it do not matter, and he used articulators that had only straight vertical opening and closing movements. He advocated using the system to treat TMJ disorders. When Dr. Jankelson was near the end of this life, the ADA awarded his equipment its seal of acceptance.  

Soon afterwards, scientific research undermined all the assumptions on which neuromuscular dentistry was based. Researchers showed that small changes in the location of the EMG electrodes caused significant differences in the results – making any longitudinal monitoring useless.125-126 Anatomical studies using needle electrodes showed that the pulsing TENS does not cause the jaw muscles to fire evenly but simply stimulates the muscle fibers that are closest to the source.127-128 One study found that the myotronics diagnostic package was unable to even distinguish between TMJ disorder patients and "normals".129 TENS is used in medicine to provide pain relief, not muscle relaxation,130 and there is no good evidence that it actually relaxes jaw muscles, except as a secondary effect of diminishing pain which had been triggering muscle tightening.131-133 Applying TENS over the cheeks usually causes the mandible to shift anteriorly, because the muscles closest to the source are the superficial masseters, which are oriented in a more forward direction than the other elevator muscles. Treatment that brings the mandible anteriorly often provides relief of symptoms, because most TMJ disorders are caused by longstanding retrusion of the mandible, not because TENS has some special ability to relax the jaw muscles.  

CONDYLAR POSITION ON X-RAY

There have also been attempts to determine the optimal mandibular bracing location by the radiographic positions of the condyles in the glenoid fossae in transcranial X-rays, but that parameter turns out to be much too variable to be useful clinically.142-143  Condyles are often displaced in unexpected directions and distances.144. In addition, the positions of the condyles relative to the glenoid fossae are poor reference points for evaluating condyle position, because the glenoid fossae are not inert frameworks. They are attached to the condyles in many ways, and they relocate with the condyles in response to functional forces.145 Attempts to change the positions of the condyles relative to the glenoid fossae by full mouth reconstruction, equilibration, or orthodontics have generally been unsuccessful.146-148

OCCLUSAL SOLUTION

The most consistent and physiologically healthy mandibular closing trajectory occurs during swallowing. Studies have indicated that swallowing reflexively recruits all of the jaw closing muscles in a way that appears less adaptive to bite interferences than the normal voluntary closure trajectory.  However, there is still natural variation in the mandibular closing trajectories used during swallowing, and that variation defines an area rather than a point. Therefore to reproduce the contours of an ideal mandibular bracing area would require recording a large number of swallowing events after the jaw muscles have been deprogrammed while tipping the head into different postural positions. The data could be integrated digitally to mill or mold a platform that provides orthopedic support for the working side of the mandible throughout its functional range of motion like one large ball-and-socket joint. When the ball moves in any direction within the socket, it rides up on the socket walls in that direction, anteriorly, laterally, and even posteriorly.  

OCCLUSAL CONCLUSION

For almost a century, dentists have reconstructed bite tables by choosing one mandibular bracing position, stabilizing it with as many simultaneous tooth contacts as possible, and then surrounding it with sloping walls of tooth structure that "guide" the mandible's range of motion by confining it anteriorly and laterally. Such a simple mechanical approach was useful when dental lab work was dependent on simple hinge devices to reproduce jaw movements; however, building a bite table around a single central bracing position does not even resemble natural jaw movement pathways, and it does not recognize the important role that jaw muscle exercise and range of motion play in the physiology and natural orthopedic function of the mandible.  

FOOTNOTES

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