The way the teeth fit or don't fit together in the bite has long been recognized as a central factor in TMJ disorders.  TMJ disorders were first discovered during the 1930's in people who had lost all their teeth and experienced dramatic relief of their symptoms after getting new dentures.  Since then thousands of dentists have treated TMJ disorders by changing the bite or by making various bite appliances, and there have been hundreds of clinical reports published by reputable dentists detailing how TMJ disorder symptoms in their patients have been eliminated or otherwise altered by permanently or temporarily changing the patient’s bite according to some bite philosophy.

 

However dentists and dental researchers have been unable to determine the role of bites in TMJ disorders, because they have never had a functional way of evaluating bites.   Dentists measure bites by means of static parameters such as overbite, overjet, and various patterns of tooth contact; but it’s been known for decades that these static bite parameters are unrelated to the health of the jaw system.  For example, it's frequently been observed by researchers and dentists that some people with obvious bite strain, difficulty chewing, and severe TMJ problems have textbook perfect static bite parameters and patterns of tooth contact; while other people with very irregular bites have obviously strong healthy jaws, the ability to chew very tough foods easily, and no signs of TMJ disorders.

 

Because researchers can find no correlation between traditional bite parameters and TMJ disorders, the use of bite treatment for TMJ disorders has recently become controversial.  Some dental researchers advise against treating bites on the basis of the conclusion that, because they cannot find evidence of a connection between bites and TMJ disorders, the role of the bite in causing or treating TMJ disorders must be minor.  However, other researchers have pointed out that lack of evidence of an association is not evidence of lack of an association.  In other words, just because we can’t prove that bites and TMJ disorders are connected doesn’t mean that they are not connected.  It may be simply that we don’t know enough about bites to be able to show how they are involved in TMJ disorders and other jaw problems.  Indeed there is strong evidence for such a conclusion.

 

TMJ disorders can certainly be caused by even minute changes in the bite.  In provocation studies, TMJ disorder symptoms can be produced within hours by a filling which is only 1/2 mm high, and the symptoms can persist for weeks until the high filling is removed.  It's reasonable to extrapolate that a bite irregularity which is present for years could cause TMJ disorder symptoms to become chronic.

 

It’s not surprising that a structure as unusual as the bite of the teeth has been difficult to measure. The bite platform functions as a unique type of joint between the upper and lower jawbones.  No other joint has articulating surfaces even remotely similar to a line of jagged interdigitating rocks which meet at a hard end point.

 

It’s also not surprising that the bite would affect the health of the TMJs and the jaw muscles. The precise shape of the interdigitating rocks and the way they fit together can determine the range of motion for the lower jawbone and the quality of the exercise template for the jaw muscles.  The range of motion of bones which articulate at a joint are well known to affect the health of that joint, and the quality of the template against which muscles exercise is well known to affect the health of those muscles.

 

The only quality of a bite which we know is related to TMJ disorders and other functional problems is bite stability.  A stable bite is characterized by the ability to close the lower jawbone with a sharp tapping sound produced by multiple nearly simultaneous tooth contacts or the ability to leave marks on most of the teeth when biting on a piece of carbon paper or inked cloth placed between the teeth.  By any way it’s measured, people with TMJ disorders and other functional problems involving the chewing system have less stable bites than people without those disorders.

 

A bite that is stable over time is necessary for the orthopedic stability of the lower jawbone.  In its bracing position, the lower jawbone is supported at its front end by the bite and at its two back ends by the two TMJs.  Stable simultaneous contact on this three legged table gives the lower jawbone a consistent “home base”.  The jaw closing muscles can then use this home base to brace the lower jawbone securely against the underside of the skull for stability needed during swallowing and postural activities, to provide a good template for the jaw muscles to exercise against during chewing, and to enable the TMJs to acquire the same goodness of fit between bones which is seen in all healthy joints.  If there is not  a single lower jawbone position which is used consistently for chewing and bracing, the three legs have to constantly reshape their contours to try and hit a constantly moving target,.  

  

Teeth are designed to acquire a stable bite early in life by a process in which each tooth keeps erupting out of the gums and toward the opposing teeth until its eruption is stopped by the biting force between the opposing teeth.  As long as the bite force which stops the teeth from erupting is produced in the same consistent lower jawbone position, the opposing teeth all end up meeting at the same time in that lower jawbone position, and the lower jawbone achieves a stable bracing position resting on all the teeth simultaneously in that lower jawbone position.  That stable bite then protects the individual teeth by providing a platform which spreads out biting forces among all the teeth.

 

However the stability of the bite can be disrupted at any stage of life by any of a number of events.  These events can occur rapidly when caused by injury or gradually when caused by discrepancies in facial growth.

 

One event that can destabilize a bite is dislocation of the disk in one of the TMJs.  The TMJ disk forms a cap on the top of the condyle.  When it is in place, it effectively lengthens the underlying condyle.  Dislocating the disk removes this cap.  The effect is much like removing a shim from under a table leg.  As a result, that side of the lower jawbone becomes about ½ mm shorter, the cant of the whole lower jawbone changes slightly toward the side with the dislocated disk, and usually the back teeth of that side will usually become the first teeth to hit.  

 

A second event that can destabilize a bite is inflammation in a TMJ.  The swelling that accompanies inflammation expands the TMJ, effectively lengthening the condyle of that side by pushing it a little further down and away from the skull.  As a result, the cant of the whole lower jawbone changes slightly away from the damaged side.  This change usually causes the back teeth of the other side to hit first.  The back teeth on the affected side may not contact at all.   In these cases, the bite often shifts day to day in response to small fluctuations in the state of inflammation of the affected TMJ.

 

A third event that can destabilize a bite is shifting of teeth due to cavities, gum disease, or a period of excessive clenching or grinding during sleep, especially when using a lower nightguard.  Nighttime clenching or grinding usually causes loss of height only on the back teeth, leaving the front teeth as the primary biting contacts.  A lower nightguard can make the problem worse, because it often prevents any bite forces from being distributed to the front teeth.  As a result of confining bite forces to the back teeth, the back teeth get shorter by intruding into the bone, and the front teeth get taller by extruding out of the bone.  In this way the overbite increases and bite stability decreases.  The bite becomes unstable when you can’t bite the back teeth forcefully together without the front teeth hitting.  The bite becomes very unstable when you can’t get the back teeth to bite forcefully at all because the front teeth are in the way.  The front teeth then act like a wedge and drive the lower jawbone backward toward the ears.  It is this type of backward shifting of the lower jawbone which causes most TMJ disorders.

 

After a destabilizing event, bites may restabilize on their own.  The same adaptability which brought the teeth into a good fit during childhood can also recreate a good fit after it has been disrupted in adulthood.  Teeth were designed to keep erupting throughout life in order to compensate for the constant wear which continued throughout life in evolution.  Continual eruption provided a constant supply of tooth structure at the working bite platform much like the spring-loaded flint on a cigarette lighter.  Today the eruption force of the teeth still continues throughout life even though most of the wear has stopped, and it can cause enough eruption of  teeth which are low to bring them back into a good biting relationship with the opposing teeth at any age.

 

However, many modern bites lack the adaptability to restabilize naturally because of dental features such as tight contacts between adjacent teeth, irregular alignment of the teeth, or frequent shifting of the bracing location of the lower jawbone.  In such cases, the bite must be restabilized by a dentist.  Usually the process involves shaving down high spots and/or building up low spots until simultaneous contact is achieved on multiple teeth in the habitual bracing position for the lower jawbone and stable support is achieved for smooth gliding movements of the lower jawbone into and out of that habitual bracing position.  Although this process sounds simple, there are many different techniques and different philosophies regarding how to restabilize bites.   Dentists with different bite philosophies will often shave down or build up different areas of teeth in an attempt to restabilize the same bite.

 

Even if the bite is stable in the habitual bracing position for the lower jawbone, it may still be strained in that position.  In other words, even if the bite forms a stable platform on which all the teeth hit at almost the same time. that platform may still have jagged inclines which force the lower jawbone off center and into a position which is not easily accomodated by the muscles or joints.  

 

The jaw muscles are arranged in bilaterally symmetrical slings which converge at various angles from their widespread attachments on the skull down onto the lower jawbone.  When the lower jawbone is at rest, it should hang in a central position determined by the anatomy of these supporting slings, much like a playground swing hangs naturally in a central location.  Then, when the jaw muscles contract evenly, they should close the lower jawbone along a trajectory that is also determined by the anatomy of the slings, much like there’s a natural trajectory along which your arm swings a weight up and down.  At the top of that trajectory should be a stable platform formed by the simultaneous fitting together of all the back teeth, full dental interdigitation.

 

In many TMJ disorder patients, the position of full dental interdigitation is not located at the top of an unstrained jaw closing trajectory.  Instead, for the lower jawbone to go into the position of full dental interdigitation, it must shift sideways or backward during closing.  The jaw muscles, forced to do all of their exercise with the lower jawbone in this strained position, cannot grow healthy and strong.  The effect is much like riding a bicycle with the seat too low or lifting weights with your feet planted slightly off to the side.  You can spend many hours exercising your jaw muscles in clenching or grinding and still have jaw muscles which get tired easily chewing.  

 

Even if the bite forces the lower jawbone into an extremely strained position each time the mouth is fully closed, you are unlikely to become aware of the strain, because the teeth are so well protected by neuromuscular reflexes.  During evolution, loss of teeth was fatal and the jaw muscles were strong, so it was the teeth rather than the muscles which were protected.  The teeth were surrounded by a vast network of sensory nerves wired to the jaw muscles in a way that causes your lower jawbone to automatically close wherever your teeth fit, even if it requires that the jaw muscles perform an intricate dance to avoid hitting teeth that would be in the way of a more natural unstrained closing pathway for the lower jawbone.  Thus even when you simply close your mouth in a way that feels natural, your jaw muscles may still be forcing your lower jawbone into a strained position. 

 

Over the years, dentists have developed a number of ways to detect bite strain.  All use some technique for locating the optimal bite position for the lower jawbone and then comparing it with the natural bite position for the lower jawbone.  However there are big controversies associated with the techniques for locating the optimal bite position for the lower jawbone.  Each technique has an associated philosophy and a band of dedicated adherents who claim it gives them consistent clinical success.  However the three most popular techniques have been shown to have no scientific basis.  The fourth, which is much simpler and based on a common sense understanding of muscle and joint physiology, is explained in the paper entitled, THE FRONT FLAT BITE PLATE APPLIANCE. 

 

 Manipulating the lower jawbone to a position as far backward as possible (known as centric relation) was the first technique used to find the optimal bite.  Originally this technique was discovered as a useful tool for making dentures, because it provided a consistently reproducible jaw closing trajectory which facilitated the setting of artificial teeth.  Later centric relation became widely accepted for use in making crowns and bridges, and then also for use in adjusting bites.  The consistency of the centric relation position of the lower jawbone enables dentists to create a precise fit of the teeth, even if the fit occurs in a strained lower jawbone position.

Many dentists who deeply believed in centric relation built elaborate conceptual frameworks for evaluating bites relative to centric relation.   The length of the slide of the lower jawbone between centric relation and the natural position of full dental interdigitation was carefully measured in thousands of patients.  Many had their bites reconstructed to eliminate any slide between the two positions.  It was theorized and widely accepted that most people with TMJ disorders have bites which are pathological because they push the closing lower jawbone forward and thereby prevented it from accessing a stable bite in centric relation.  Researchers searched for scientific reasons to justify the apparent clinical successes that dentists were seeing from using centric relation.

 

However, in spite of numerous attempts, science failed to find any reason to justify the use of centric relation.  Some studies showed that, in natural bites, the centric relation position of the lower jawbone was rarely used – thus it was a border position rather than a functional position.  Clinical studies found that 90% of "normals" have a slide between centric relation and the position of full dental interdigitation.  Anthropological studies showed that virtually all of the thousands of human skulls found in museums demonstrate a slide between centric relation and the position of full dental interdigitation.  Experimental studies using a catheter inserted in the TMJ showed that centric relation was the only position of the lower jawbone which produced unnaturally high fluid pressure in the TMJs.  Still other studies showed that bites reconstructed with the lower jawbone in the centric relation position are unstable and spontaneously shift to accommodate a slightly more forward bracing lower jawbone position.

 

Recently, as a result of the problems sometimes associated with clinical application of centric relation theory, supporters of centric relation have modified their positions.  Some have stopped pushing the lower jawbone so far backward, others advocate a lower jawbone position which allows some freedom to move forward slightly (called long centric), and still others have redefined centric relation as an upmost and rearmost position of the lower jawbone instead of just a rearmost position.  

In retrospect, centric relation dentistry often works clinically not because it is an ideal or correct position for the lower jawbone, but because it is located relatively close to the rearmost bracing position of the lower jawbone where the jaw muscles automatically bring the lower jawbone for power-crushing and therefore where the teeth are most vulnerable to bite forces.  If one tooth hits first at that rearmost bracing position of the lower jawbone, that tooth is likely to get injured because of the heavy forces it will have to withstand.   Manipulating the lower jawbone back into that position and equilibrating the bite in that position so many teeth hit at the same time when the lower jawbone is in that position can prevent damage to a tooth which hits before others when the lower jawbone goes back for heavy chewing.  This ability to relieve a symptomatic tooth has convinced many dentists of the validity of centric relation dentistry.  In addition, the repeatability of centric relation helps dentists adjust bites, because it creates a reference point which helps eliminate the difficulty posed by having to deal with many different jaw closing trajectories, each causing a different set of teeth to hit first.  However the convenience of being able to locate a repeatable jaw closing trajectory cannot be considered evidence that the only jaw closing trajectory which is consistently repeatable is somehow the healthiest bracing position for the lower jawbone.  Centric relation is clearly a border position for the TMJs, and border positions in joints serve as range of motion limitations which protect the joint structures from movements which have the potential to cause injury.

 

A few decades ago Dr. Jankelson pointed out that, because centric relation was a system for finding the bite based solely on the mechanical fit between the jawbone and skull, it ignored any role for nerves or muscles.  He introduced a theory that placing a pulsing TENS (transcutaneous electrical nerve stimulation) source directly over the root of the jaw muscles can fire all the jaw muscles evenly and thereby cause the lower jawbone to close into the ideal bite position.  He called his technique neuromuscular dentistry and started a Seattle company called Myotronics to market the pulsing TENS machine along with an EMG detector and a jaw tracking device.  When Dr. Jankelson was near the end of this life, the ADA gave the equipment their seal of approval.  The equipment was expensive, but its high tech appearance and the label "computerized diagnostic testing" made it an easy sell to patients.

 

After Dr. Jankelson’s death, scientific research undermined every aspect of the basis for using the Myotronics equipment.  Some researchers showed that it could not separate TMJ disorder patients from normals.  Other researchers showed that the pulsing TENS machine cannot cause the jaw muscles to fire evenly but only stimulates the muscle fibers in proportion to their distance from the source of the TENS.  It usually causes the lower jawbone to shift forward, because the muscles closest to the pulsing TENS source are the superficial masseters, which are oriented in a more forward direction than the other jaw muscles.  Treatment which brings the lower jawbone forward often provides relief of symptoms, because many people with TMJ disorders have problems caused by longstanding retrusion of the lower jawbone, not because TENS has some special ability to relax the jaw muscles and certainly not because a pulsing TENS unit has some unique ability to determine the ideal bite position for the lower jawbone.  

 

During the same time period, Dr. Robert Lee popularized a technique for finding the bite based on the idea of reconstructing all the teeth to the shapes they had when they first erupted.  His philosophy was that wear of the teeth is abnormal because it results from individual components of the chewing system being out of balance, and that putting the system back into balance will prevent tooth wear.  His three principles were that the lower jawbone should fit in centric relation, the front teeth should have an overbite which is steep enough to separate the back teeth during any movement of the lower jawbone, and the anatomy of unworn teeth enhances chewing efficiency.  In practice his technique usually involved increasing the steepness of the overbite of the front teeth and sometimes involved performing crowns on all the teeth to restore their original unworn anatomy.

To explain why steep overbite is needed to separate the back teeth during chewing, Lee pointed to the experiments which show that the jaw muscles are unable to bite forcefully when the back teeth are separated.  There were many theories that TMJ disorders result from the jaw closing muscles overloading the structures of the chewing system.  Therefore Lee concluded that restoring the portions of the front teeth which had been worn away would cause a steeper overbite and thereby relieve TMJ symptoms by decreasing the jaw muscle forces used. 

However, researchers showed that it was the resting jaw muscle forces rather than the functional jaw muscle forces which are too high in TMJ disorder patients.  In fact, people with TMJ disorders generally have weaker jaw muscles than "normals".  One researcher commented that we can treat TMJ disorders by either increasing their functional jaw muscle forces or decreasing their resting jaw muscle forces.  Steeper overbite decreases only the functional jaw muscle forces (not resting jaw muscle forces), and it probably does so by activating an arthrokinetic reflex which protects the TMJs from the lateral forces which are applied to the working side condyle when the laterally moving lower jawbone runs into a stop or obstacle at the front teeth or canines.  Such pivoting around a stop or obstacle at the front of the mouth forces the lower jawbone to pivot around that stop or obstacle and thereby forces the condyle of that side laterally where it can damage the collateral ligaments that hold the disk in place.  

Researchers also showed that tooth wear is a normal and often essential part of the development of the chewing system in nearly all mammals - including humans.  The shapes of the teeth when they first erupt is designed to align the newly erupting teeth so they meet correctly and form a good bite, not to function efficiently in chewing.  In almost every species, some tooth wear is necessary before the chewing system even becomes efficient.  This process is explained in much more detail in chapter 1 or the paper entitled SOCIETAL ETIOLOGY OF TMJ DISORDERS.   Therefore restoring teeth to their original "unworn" shape does not increase chewing efficiency.

In addition, hundreds of studies have failed to turn up any evidence that nocturnal tooth grinding or clenching results from an improper bite.  Changing the bite does not change the frequency or force of nocturnal tooth grinding or clenching.   Sleep studies have shown that nocturnal tooth grinding or clenching is a neurological response to changes in sleep stages and reflects no particular bite condition.

 

At this time the only reliable means for determining the ideal position for the lower jawbone in the bite is using a front flat plate appliance which eliminates the neuromuscular signals from the teeth so they no long program the lower jawbone to go wherever the teeth fit.  As a result, the lower jawbone is able to locate itself wherever the jaw muscles feel comfortable holding it.  The use of a front flat plate appliance to accomplish this goal is explained in the paper entitled, THE FRONT FLAT BITE PLATE APPLIANCE.