Causes
TMD is a symptom complex (i.e. a group of symptoms occurring together and characterizing a particular disease), which is thought to be caused by multiple, poorly understood factors, but the exact etiology is unknown. There are factors which appear to predispose to TMD (genetic, hormonal, anatomical), factors which may precipitate it (trauma, occlusal changes, parafunction), and also factors which may prolong it (stress and again parafunction). Overall, two hypotheses have dominated research into the causes of TMD, namely a psychosocial model and a theory of occlusal dysharmony. Interest in occlusal factors as a causative factor in TMD was especially widespread in the past, and the theory has since fallen out of favor and become controversial due to lack of evidence.
Disc displacement
In people with TMD, it has been shown that the lower head of lateral pterygoid contracts during mouth closing (when it should relax), and is often tender to palpation. To theorize upon this observation, some have suggested that due to a tear in the back of the joint capsule, the articular disc may be displaced forwards (anterior disc displacement), stopping the upper head of lateral pterygoid from acting to stabilize the disc as it would do normally. As a biologic compensatory mechanism, the lower head tries to fill this role, hence the abnormal muscle activity during mouth closure. There is some evidence that anterior disc displacement is present in proportion of TMD cases. Anterior disc displacement with reduction refers to abnormal forward movement of the disc during opening which reduces upon closing. Anterior disc displacement without reduction refers to an abnormal forward, bunched-up position of the articular disc which does not reduce. In this latter scenario, the disc is not intermediary between the condyle and the articular fossa as it should be, and hence the articular surfaces of the bones themselves are exposed to a greater degree of wear (which may predispose to osteoarthritis in later life).