A function of occlusal scheme and Muscular Intensity
(click here to see animations)
Posterior, bilateral and equivalent occlusal contacts provide for maximal temporalis (t) contraction intensity (clenching), and no strain on the TM joints (1) (F).
A unilateral posterior occlusal contact allows for maximal temporalis contraction on the same side (G). Since there is no stabilizing occlusal stop on the opposite side, that lateral pterygoid can contract and pull the condyle medially, toward the point of resistance (force vector: fv), which is the unilateral posterior occlusal contact (2) (H). The more medial the strain, the more damaging to the joint complex.
As the occlusal contact moves anteriorly to the canine, the potential maximum temporalis contraction intensity decreases, but is still near maximal (I) (in mammals, the stronger the temporalis, the larger the canine teeth (Slide 1)). The direction of the force generated on the joint complex by the opposite side lateral pterygoid's contraction is now less medial and more anterior, thereby decreasing the strain on the joint complex. This is the reason for "canine rise", or "cuspid guidance", so that in an excursive movement, the canines occlude, and cause the posterior teeth to disclude, preventing the scenario in (2).
As the occlusal stop reaches the anterior midline, maximal temporalis contraction intensity has been decreased by over 2/3s (J) (4). The direction of the force generated bilaterally by the lateral pterygoids is anterior,
seating the condyles into their most musculoskeletally stable position (K).
The TMJ is not a load bearing joint and when the terms "take the load off the joint", or "support the joint" are used, they are referring to bilaterally equilibrating the occlusal scheme (usually with a splint), thereby preventing the unilateral lateral pterygoid contraction intensity. Ultimately, any strain on the TMJs is a function of the occlusal scheme and the degree of intensity of the muscular contractions (without an occlusal scheme (rest position), there is no joint strain). During clenching parafunction, for example, when occlusal contacts are bilaterally equivalent, there is no strain on the TMJs or facial structures (this explains how some tension-type headache patients can have no signs of a TMD). Some TMD patients with ideal occlusal schemes may habitually "excursive clench", whether or not they are wearing their TMD splint.