Abnormalities of Temporomandibular Joint
The glenoid fossa of the temporal bone and the mandibular condyle constitute a compound, diarthrodial joint, separated by an articular cartilage and enclosed by a synovial membrane and capsular ligament. The upper compartment (meniscotemporal articulation) permits gliding motions, and the lower compartment (meniscocondylar articulation) functions as a hinge.
Muscle action and the occlusal relations of the teeth are the major determining factors in movements of the joint, with the anatomy of the bony surfaces and the articular ligaments less important than in other joints. Facial height and form depend largely on the proper growth of the mandible. Whereas the maxilla grows by apposition against the anterior base of the cranium, the mandible develops vertically by an epiphyseal type of growth from the head of the condyle. Interference with the chondrogenic zone of the condyle has a disastrous effect on the facial profile. Growth arrest, partial or complete, may be the result of otitis media, radiation, arthritis, condylar fracture, or trauma by obstetric forceps. Complete arrest of mandibular growth or ankylosis occurring during childhood gives rise to micrognathia, a deformity in which the condyloid process is shortened, with an obtuse mandibular angle, a stunted mental protuberance, and a concave lower border of the bone resulting from the powerful action of the depressor muscles. Dislocation of the condyle may ensue from a blow on the ramus or chin when the mouth is open, from yawning, or from excessive manipulation of the jaw under general anesthesia. The dislocation is nearly always forward, the condyle resting anterior to the articular eminence. Backward displacement occurs rarely from a forceful blow, at the expense of the posterior attachment of the meniscus. The condyle then rests on the bony surface of the fossa, with a slight tilting of the mandible and an open position of the anterior teeth. Other dislocations are seen only with fractures of the condyle or base of the skull. Chronic injury to the joint ligaments as, for example, by malocclusion of the teeth may lead to subluxation. A hypermobility of the condyle is accompanied by a clicking or snapping sound at the termination of opening. This sound is due to the condyle’s slipping anteriorly past the meniscus and then striking the articular eminence. The same action occurs when the attachment of the external pterygoid muscle to the capsule has been lost through injury.
Fracture of the condyle is seen frequently as a result of a frontal blow on the chin. Displacement of the condylar head is sometimes caused by the trauma itself; more commonly, it is caused by the pull of the external pterygoid muscle inward and forward. Following bilateral fracture of the condylar necks, the molar teeth close prematurely but the incisors are still separated.
Ankylosis may result from injury or inflammation of the joint. Occasionally, extraarticular causes, such as fibrosis and cicatrization of the muscles attached to the mandible, cause a false ankylosis. This may happen in healing of extensive wounds of the face and in postradiation cases of head and neck cancer. In ankylosis proper, the cause can be a comminuted fracture of the condyle, suppurative arthritis or osteomyelitis, intracapsular hemorrhage, or rheumatoid arthritis. The ankylosis may be fibrous, with perceptible but slight movement, or bony. Unilateral ankylosis is characterized by a marked limitation of movement and a deviation toward the affected side on opening of the mandible. The uninjured condyle describes an arc about the injured condyle, which acts as a fulcrum. Muscles on the normal side hypertrophy; on the injured side, they atrophy.