The disc of the human temporomandibular joint: design, function and failure.

The directions and limits of mandibular movements are controlled by muscles and nerves, and by biomechanical constraints in the dentition and the temporomandibular joints with their associated ligaments. This paper analyses biomechanical constraints in relation to shape, function, stability and dysfunction of the disc in the joint. It is argued that the dense part of the disc is modelled in response to condylar forces which compress it into the articular eminence thereby thinning its centre and squeezing out a thickened anulus around its rim. Because of a lack of congruence between the two articulating surfaces of the joint and a very low coefficient of friction, the disc is potentially squeezed off the condyle by compression forces during opening, closing and mastication. It is stabilized mainly by its anulus whose thick rim is unable to prolapse between the condyle and articular eminence. An important function of the elastic tissue behind the thick part of the disc may be, by contracting, to prevent the soft tissues behind the joint being nipped between the condyle/disc/temporal bone when the jaw is closing. The author suggests that during mastication the function of the disc is not only to spread the joint forces but also to limit the depth to which the condyle is compressed into the soft tissues covering the temporal bone and thereby allow the condyle (and disc) sufficient freedom to spin and slide over the temporal bone without distorting and damaging the surfaces. In other words, the function of the disc is to destabilize the condyle. In terms of biomechanical constraints the disc seems more liable to prolapse anteriorly than posteriorly. It prolapses when the compression forces which tend to wedge it off the condyle cannot be resisted by its thickened anulus. If the energy of compression is stored within the distorted disc and suddenly released there is an audible click.