The Role of The Bite in TMJ Disorders

 
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.