TMJ anatomy and animal models.

The temporomandibular joint (TMJ) is a cardinal feature that defines the class Mammalia and separates mammals from other vertebrates. As befits its late evolutionary origin, the TMJ also makes a tardy developmental appearance 1. The TMJ is interesting because its constituent bones, the mandible and the squamous temporal, are intramembranous in origin. Thus, the tissue that covers each articulating surface is a secondary cartilage with a fibrous skin, derived from the periosteum. Another nearly constant feature is the intra-articular disc. The disc, even when incomplete, is associated with the lateral pterygoid muscle 2 , which has led some authors to speculate that it arose as a tendon which became pinched by the new joint 3. An additional interesting feature of the TMJ is its role in growth. Intramembranous bones do not have epiphyseal plates, but their growth is effected (or at least affected) by nearby cartilaginous structures. In the cranium, the cranial base cartilages and the nasal septum are thought to be the major drivers of growth at cranial sutures 4. It seems likely that in most vertebrates, Meckel's cartilage performs a similar role for the multiple bones of the lower jaw. The evolution of the TMJ accompanied the almost complete removal of Meckel's cartilage from the sutureless lower jaw of mammals , thus eliminating this mechanism of growth. The substitute is the secondary condylar cartilage, which is a major growth site in addition to being an articular covering. Despite its status as a mammalian identifier, the TMJ shows remarkable morphological and functional variation in different species, reflecting not only the great mammalian adaptive radiation in feeding mechanisms, but also a freedom from constraints such as bearing body weight. The most extreme evolutionary variants include: (1) loss of the synovial cavity in some baleen whales; (2) loss (or possibly primitive absence) of the disc in monotremes, some marsupials, and some edentates (anteaters and sloths) 2,5 ; (3) variations in the orientation of the joint cavity from parasagittal (many rodents) to transverse (many carnivorans); and (4) reversal of the usual convex/con-cave relationship so that the mandibular condyle becomes the female element (many artiodactyl ungulates such as sheep and cattle 6). In addition, the relative size of the joint is exceedingly variable. Soft tissue details such as capsular ligaments 7 and col-lagen orientation in the disc 8 are also highly species-specific. The striking anatomical differences in TMJs are clearly tied to biomechanics. …