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.
