The TMJ is an encapsulated articulation of the mandibular condyle with the glenoid fossa of the skull base. An intervening fibrocartilaginous disk sits above the superior surface of the condyle and divides the joint into superior and inferior joint spaces, which are lined by synovium. There are two components to normal TMJ movement. Rotation is the initial motion whereby the mandibular condyle rotates within the fossa. This is initiated by the lateral pterygoid and suprahyoid muscles and accounts for the first 20 mm of jaw opening and is accompanied by little movement of the disk. Rotation is followed by translation whereby the condyle and disk glide forward and inferiorly until they exit the glenoid fossa and articulate on the lip of the anterior wall of the fossa (the articular eminence). This accounts for the last 20 to 55 mm of jaw opening.

Disorders of both the TMJ and surrounding masticatory and neck muscles usually occur as a result of a precipitating factor such as trauma or muscle pain in a person with a predisposition to develop a TMD. Symptoms then persist in the presence of perpetuating factors such as genetic predisposition and abnormal pain processing by the central nervous system.

Trauma (from protracted, wide mouth opening or facial injury) can lead to intraarticular inflammatory changes that alter the functioning of normal tissues and result in degenerative changes. Subconscious grinding of the teeth (bruxism) and jaw clenching as well as other nonfunctional jaw movements such as gum chewing and fingernail biting (parafunctional habits) cause muscle hyperactivity and fatigue leading to pain.

Psychobiologic influences are believed to include a genetic predisposition to hyperalgesia and decreased thresholds to noxious stimuli as well as hormonal influences (such as the low-estrogen phase of menstruation). Like other chronic pain disorders, abnormal pain processing within the central nervous system has been observed (see Chapter 236). Psychological problems such as depression, anxiety, heightened somatic awareness, and maladaptive pain coping as well as history of abuse and posttraumatic stress disorder are heavily associated with TMD and likely play a role in both predisposing and perpetuating TMD.

Internal derangement of normal joint anatomy occurs when the articular disk is malpositioned in relation to the mandibular condyle. Most commonly, the disk is displaced anteriorly, sitting in front of the condyle when the jaw is closed. Infection, previous TMJ surgery, and systemic inflammatory connective tissue diseases can play a role in the etiology of TMD.

Dull, aching, unilateral facial pain localized to the mandible, TMJ, or muscles of mastication is the most common presenting symptom. The pain may radiate to, or present in, the temporal and periorbital regions, ears, posterior neck, or shoulders. While chronic, the pain does usually have a waxing and waning pattern with pain-free periods and known triggers such as mandibular movement, chewing, or yawning. Limited or asymmetrical jaw opening and complaints of the jaw locking open or closed are also frequent. Sounds such as clicking and popping within the TMJ during movement are common in TMD. While asymmetrical opening and TMJ sounds can occur in up to 50% of the general population, less than 5 % of the general population experiences limitation in mandibular range of motion.

TMDs are most common in the young and middle age (20 to 50) with a female-to-male ratio as high as 9:1. Muscle and TMJ tenderness to palpation are common with tinnitus and dizziness sometimes reported at time of presentation.