The Role of The Bite in TMJ Disorders

SUMMARY

In almost everybody, there is a single lower jawbone location at which all or almost all of the back teeth fit together fully.   You can find this lower jawbone location by tapping your back teeth together a few times until your bite feels solid on the back teeth of both sides. The way the teeth fit together here acts like a home base for the chewing system.  All jaw movements begin and end there, and the back ends of the lower jawbone (the condyles) are fully seated in the depths of their sockets there, cushioned by the disks.  The ease with which the teeth come together at this location and the comfort of the jaw muscles and the fit of the jaw joints (TMJs) at this location influence the resting tensions in your jaw muscles, which in turn influences your facial growth pattern as well as the health of your postural muscles.  If the bite is unstable, it can be stabilized by shaving down high spots or building up low spots.  If the bite is displaced, there are various ways to shift it to a better position depending on your dental condition.


BACKGROUND

The way the teeth fit together in the bite has long been recognized as a central factor in TMJ disorders.  TMJ disorders were first discovered when people who had no teeth experienced dramatic relief of their symptoms after getting dentures.  Since then there have been hundreds of clinical reports published by reputable dentists detailing how TMJ disorder symptoms in their patients have been altered by permanently or temporarily changing the patient’s bite.  Even minute changes to the contour of the bite platform can cause TMJ disorder symptoms experimentally.  In one provocation study, fillings that were intentionally left only 1/4 mm high caused typical TMJ disorder symptoms, some of which persisted for months after the high filling was removed. 
 
The way the teeth fit together in the bite affects the TMJs, because it determines the location of the lower jawbone and therefore also the location of the balls (condyles) in the sockets of the TMJs during bracing and chewing.  It also affects the jaw muscles, because it determines the height and contours of the template against which the jaw muscles exercise. 
If the lower jawbone needs to twist or torque to fit the teeth together, both jaw muscles and TMJs can be put under strain.
   
 
BITE STABILITY 
A bite that is stable over time is a fundamental requirement for long term health of the jaw system.  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 in and around a central bracing position gives the lower jawbone a consistent "home base" and easy pathways into and out of that home base.  The jaw closing muscles need easy access to that home base, because they brace the lower jawbone there hundreds or thousands of times each day.   Studies have shown that people with TMJ disorders and difficulty chewing or other functional disorders of the jaw system have less stable bites than people without those disorders.
  
THE BITE ACHIEVES STABILITY early in life by a process in which each tooth keeps erupting out of its basal bone and toward the opposing teeth until its eruption is stopped by the bite force pushing back down on the teeth.  As long as the bite force is consistent, the opposing teeth end up meeting at the same time (stable bite).  
 
THE BITE CAN BE DESTABILIZED at any age.  Loss of bite stability can occur rapidly due to cavities or gum disease, or it can occur gradually when caused by a strained facial growth pattern.  
 
DISLOCATION OF A TMJ DISK is one event that commonly destabilizes bites.  Dislocating the disk removes a cap from the top of the condye and thereby effectively shortens that condyle much like removing a shim from under a table leg.  The resulting slight change in the cant of the lower jawbone usually causes the back teeth of that affected side to hit before the back teeth of the other side.  
 
ARTHRITIC DEGENERATION of a TMJ can cause a similar loss of condylar height due to regressive remodeling of the bone. In diseases like rheumatoid arthritis, regressive remodeling of the condylar bone can occur so rapidly that the whole lower jawbone rotates around its contacts on the rearmost teeth and creates an open bite (no contact) between all the other teeth.
 
INFLAMMATION in a TMJ can destabilize a bite by effectively lengthening the affected condyle like inflating a bag of water under a table leg, because the swelling associated with the inflammation pushes the affected condyle down and away from the skull.  The back teeth on that side may not contact at all, and any attempt to touch them produces pain, because it drives the condyle into the swollen joint.
 
CONTINUOUSLY CHANGING BITE often occurs in chronic TMJ disorders, because the bite shifts back and forth over time in response to small fluctuations in the state of inflammation and the arthritic degeneration of the joints.  The patients often report feeling like they have many different bites or no bite at all.  The continuously changing bite forms a poor target for the remodeling of bone that constantly attempts to improve the fit between bones at joints.
 
SHIFTING OF TEETH due to cavities, gum disease, excessive nocturnal clenching and grinding, or normal nocturnal clenching and grinding against a partial coverage appliance like a lower nightguard can also destabilize a bite.   Teeth tend to tip into spaces opened up by cavities between the teeth, and they shift around easily in response to localized gum disease.  Nocturnal clenching or grinding, especially when using a lower nightguard, can confine those powerful compressive forces to the back teeth and cause them to shorten by sinking (intruding) into the jawbones while the front teeth lengthen by elongating (extruding) out of the jawbones.  The resulting increase in overbite can destabilize the bite.  The bite becomes unstable when you cannot bite the back teeth forcefully together without the front teeth also hitting forcefully.  The bite becomes very unstable when you can’t even touch the back teeth.  In such cases, because the front teeth contact on a steep and slippery incline, their contact can wedge the lower jawbone backward toward the ears.  It is this type of backward shifting of the lower jawbone that causes many TMJ disorders.
  
BITES CAN RESTABILIZE on their own.  The same adaptability that brought the teeth into a good fit originally can also recreate a good fit after the bite has been disrupted.  Teeth were designed to keep erupting throughout life in order to maintain a stable bite by compensating for the constant wear that also continued throughout life during evolution, and eruption of teeth that have become too low can bring them back up into a good bite at any age.
 
However, some modern bites are unable to restabilize naturally because of dental features such as tight contacts between adjacent teeth, irregular alignment of the teeth, or frequent shifting of the lower jawbone.  In such cases, the bite must be restabilized by a dentist who will shave down high spots and/or build up low spots.  
  

BITE STRAIN 
Even if your bite is stable, it may still be strained.   In other words, even if your bite forms a stable platform on which all of your back teeth hit at the same time, that platform may still be located in a lower jawbone position that is too far back or off to one side and is not easily accommodated by your muscles or TMJs.  
 
The jaw muscles are arranged in bilaterally symmetrical slings that converge down onto the lower jawbone at various angles from their widespread attachments on the skull.  When the lower jawbone is at rest, it should hang from these slings in a central position. 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.  At the top of that trajectory should be a stable platform formed by the simultaneous fitting together of most or all the back teeth, the central bracing or intercuspal position of the lower jawbone.
 
In many TMJ disorder patients, the central bracing position of the lower jawbone is not located at the top of an unstrained lower jawbone closing trajectory.  Instead, for the lower jawbone to access its central bracing position, it must shift away from its preferred natural closing trajectory and torque or twist into a different location.  That process can displace the condyles within the sockets of the TMJs and force the jaw muscles to do all of their exercise in a strained position - like riding a bicycle with the seat too low or lifting weights with your feet planted slightly off to the side.  As a result, those jaw muscles may spend hours every day clenching or grinding and still never become strong and healthy like muscles in other parts of the body that are exercised for hours every day.
 
Even if your bite forces your lower jawbone into an extremely strained position each time your mouth is fully closed, you are unlikely to become aware of the strain, because the teeth are protected by neuromuscular reflexes that continuously program the jaw muscles to only close the jawbone wherever the teeth fit together.  During evolution, the jaw muscles were strong, and the teeth were delicate.  Neuromuscular reflexes protected them so well that even a very strained bite can feel perfectly natural.
 
 

DIAGNOSING BITE STRAIN
To diagnose bite strain requires deprogramming the jaw muscles so they temporarily stop bringing the lower jawbone to the one place where the teeth fit.  The jaw muscles can be deprogrammed by nightly wear of a front flat plate appliance as described in a separate paper on this website.  When you wake up every morning, your jaw muscles will be deprogrammed and your jaw will open and close along a trajectory that is determined by the jaw muscles without the influence of the teeth.  Then, when you first remove the appliance in the morning and bite down lightly, and you can see where your back teeth should fit together.  If you have some teeth that hit first and slide your jawbone from its deprogrammed position into your natural bite, those prematurely contacting teeth are in the way of your bite and likely the primary cause of your bite strain.
 

ADJUSTING BITES
There are many ways to eliminate bite strain.  Teeth can be drilled down, built up, and/or moved orthodontically.  The choice depends on your dental needs, your finances, and your face height.  People with excessively long faces can most benefit from reducing high spots to make them shorter, and people with excessively short faces can most benefit from building up teeth to make them taller.
 
REDUCING high spots involves accurately locating them with little blue carbon paper and then reducing them with a high speed drill in a process known as equilibration.  Equilibration does not harm teeth or make them more susceptible to developing cavities.  In fact, until MRI became popular and made us realize that the vast majority of TMJ disorder patients have a dislocated TMJ disk, equilibration was the primary means used by dentists to treat TMJ problems, and the largest TMJ professional organization in the world is still the American Equilibration Society.  Equilibration is especially useful in people with steeply interdigitating tooth inclines which restrict the range of motion of the lower jawbone.   It enables longer smoother chewing strokes.
 
BUILDING UP low spots usually just involves bonding tooth colored filling material directly to the tops of the teeth.  The process can significantly reinforce teeth that already have fillings, because material bonds to enamel with incredible strength.  Also the material is versatile, because it can be easily ground down or added to itself later. The primary disadvantage of using composite resin for building up teeth is that the material tends to wear down.  Teeth were designed to wear down slowly and steadily over time, so such wear of composite resin may not be a problem. However, in people who have strong jaws and vigorous tooth grinding habits, wear of the composite resin may occur rapidly enough to destabilize the bite, so more longlasting materials like gold or porcelain is required – especially on the molars which receive most of the bite force.  
 
WEAKENED TEETH due to the presence of cavities or previous fillings will require gold or porcelain onlays or crowns eventually anyway, and that permanent dental work can be very helpful in recreating a healthy bite.   Filling materials prevent food from collecting in a defect in a tooth, but they do little to reinforce the tooth structurally.  As teeth become more brittle with age, they tend to crack or chip from around the filling.  Thus, a tooth with a filling will eventually need to be covered by a solid piece of gold or porcelain such as a crown or onlay so that no part of the natural tooth structure surrounding the filling can fracture. 
 
GOLD is the ideal material for reconstructing bite surfaces.  It is extremely accurate and biocompatible.  It has a hardness very similar to natural tooth structure – which minimizes wear.   In addition gold is strong even when thin, and it can be bonded to enamel with impressive strength.   Gold tooth build-ups usually don't require novocaine.
 
PORCELAIN has excellent esthetics, however it also has disadvantages.  Porcelain needs bulk for strength, so the tooth preparation process requires significant reduction of tooth structure, which increases the chance of the tooth later needing a root canal.  It’s also difficult to create an accurate bite with porcelain, because porcelain cannot be simply cast to fit the bite but instead must be initially built too high and then ground down in the dental lab until it fits.  Finally, porcelain is so much harder than natural teeth that, insead of wearing down itself, it tends to wear away the opposing natural teeth.