Dislocation of the temporomandibular joint (TMJ; see the image below) is a painful condition that occurs when the mandibular condyle becomes fixed in the anterosuperior aspect of the articular eminence.
TMJ dislocation is due to either trauma (force against a partially opened mandible) or, more commonly, excessive opening of the mandible (such as may occur during yawning and dental procedures). Other mechanisms include passionate kissing, eating, yelling, singing, endoscopy, and intubation. Spasm of the masseter, temporalis, and internal pterygoid muscles results in trismus, preventing return of the condyle to the temporal fossa. 
More commonly, dislocation occurs bilaterally, resulting in a mandible that is fixed in a symmetrically open position so that only the most posterior teeth may be contacting. Infrequently, unilateral dislocation occurs with resultant deviation of the jaw to the unaffected side. 
With dislocation, the mandibular condyles may be palpated anterior to the articular eminence. The diagnosis should be obvious through history and physical examination, and radiographs should not be necessary for confirmation. However, in the setting of trauma, radiographs should be obtained to exclude concomitant fracture.
Patients who have experienced previous dislocation are more prone to recurrence. Additionally, patients with weakness of the joint capsule, anatomic aberration of the joint, or injury to the associated ligaments are at greater risk of dislocation.
Indications for reduction of a TMJ dislocation include the following:
Acute dislocation of the TMJ, either unilaterally or bilaterally; acute episodes are most easily managed with manual reduction
Contraindications for reduction of a TMJ dislocation include the following:
Extensive facial trauma
Multiple prior unsuccessful attempts