Importance of canine guidance/What happens in Bruxism?

Group function versus canine guidance

When a person bruxes his or her teeth side to side, keeping the teeth in constant contact, the cusps of the upper and lower teeth slide over each other forcing the lower jaw to drop slightly as they approach a cusp-tip to cusp-tip relationship.  The number of cusps that remain in contact during lateral excursions of the lower jaw vary from person to person.

• Canine Guidance--In young persons with ideal occlusal relationships, the upper and lower teeth contact evenly throughout the entire dental arch when the teeth are fully together in that person's centric occlusion. However, as soon as he or she begins a lateral excursion, all the teeth (anterior and posterior alike) lose contact, except for the upper and lower canines on that side.  In other words, the canines are situated and inclined in such a way that, while they allow full contact of all teeth in centric occlusion, they force the jaw to open as the upper and lower canines slide over each other.  This disengages the cusps of all other teeth as the person begins to grind side to the side.  (This phenomenon is called "cuspid rise" in deference to the fact that most articulators are hinged in such a way that the upper teeth move instead of the lower.  This artificial way of mounting the models makes the upper canines appear to rise instead of the lower canines drop, which is what happens in a real mouth.)  In fact, canine guidance is considered the most physiologic of all  occlusal relationships because it protects the teeth from wear and tends to prevent bruxing in most persons who are likely to brux only occasionally.  In the absence of chronic bruxing habits, this relationship often persists throughout life. 
• Group Function--On the other hand, if a person is a habitual bruxer, the combination of tooth movement and cuspal wear over a period of years reduces, and eventually eliminates the prominence of the canine prematurely.  This causes more and more posterior tooth cusps to remain in contact over more and more of the excursive movements.  The process continues until, eventually, all the cusps of the back teeth remain in contact throughout the entire lateral excursion.  This "group functioning" of all the posterior teeth now replaces the original canine rise in causing the lower jaw to drop during excursions. 
  An occlusion in group function is more prone to perpetuate the bruxing habit leading to greater and greater wear on all teeth.  Eventually, the occlusion is worn flat, eliminating any tendency of the lower jaw to drop at all during lateral excursions.  In other words, all, or most of the teeth remain in contact throughout the entire lateral excursion, and fail to disclude, as they do in canine guided occlusions.  This may cause extreme wearing of the anterior teeth as well as the posteriors.  Tooth wear from bruxing is called attrition.  Continual bruxing leading to continual wear of the teeth also changes the relationship of the patient's centric occlusion to their centric relation, causing a slow, continuing protrusion of the lower jaw bringing about more and more wear on the anterior teeth.  Many dentists believe that by recreating a canine guided relationship they can stop a severe bruxing habit and save a dentition otherwise doomed to "death by attrition".