The TMJ, or temporomandibular joint, is the jaw joint, and there are two, one on either side, working together to allow the motion of the upper temporal bone and lower mandible, or the jawbones. The defining feature of the TMJ is the articular disk, formed of fibrous tissue similar to cartilage in the ear and nose that is found between the two bones, forming the joint. It divides this joint into two parts, allowing for rotational movement (opening and closing) and translational motion (forward, backward, and side to side).

The TMJ is susceptible to all the same afflictions as other joints in the body, including arthritis, dislocation, trauma, ankylosis (joint stiffness), neoplasia (abnormal growth of tissue), and other developmental difficulties. Sometimes abbreviated incorrectly as TMJ, which is actually the name of the joint, temporomandibular joint disorder (TMD) or TMJ syndrome is the mot common disorder associated with this particular part of the body. It can involve a wide variety of conditions and symptoms, including both acute and chronic inflammation of the joint, as well as significant pain and impaired range of motion.

Signs and symptoms of TMJ disorder can be difficult to interpret because their presentation can vary and be quite complex, mimicking other ailments and involving a number of components. Muscles, ligaments, tendons, nerves, bones, teeth, and connective tissue can all be affected by temporomandibular joint syndrome. Muscles are the source of the majority of patient complaints and can be affected by overuse (nail biting or excessive gum chewing), as well as clenching teeth. These are the simple causes, though muscle pain from TMD can also be caused by infection, bruising, and scarring.

In any case where a dysfunction of the muscles persists, the teeth may line up incorrectly with each other, leading to trauma from the over- or under-bite and causing sensitivity to the teeth, compounding other pain in the mouth. Due to the fact that this joint, easily one of the most complex in the human body, is so close to the ear, many TMJ patients complain of ear pain and are unaware of their disorder at first. In fact, TMJ disorder is attributed as a major possible cause of secondary otalgia (chronic earache).

In severe cases of TMD, destruction of the supporting bone can cause movement in teeth, as can heavy forces placed on the teeth by the disorder. As teeth move, it affects the overall interaction between teeth, muscles, and joints, especially when the mouth is closed and can lead to other pains of the mouth, including inflammation of the gums, dental pulp, and pulpitis.

While there is not really a cure for temporomandibular joint disorder, there are treatments. If the surfaces of teeth have been damaged and are highly sensitive, the proper occlusion must be restored through the modification of the occlusal surfaces of the teeth. For pain relief, standard analgesic pain killers, as well as NSAIDs can provide some assistance, though most of the pain is neuralgic and unresponsive to such therapies. Long term solutions include utilizing certain prescription medications, surgery, and therapy, as well as eliminating habits that may aggravate symptoms.