Congenital anomalies of the odontoid process.

Anomalous development of the odontoid is uncommon, and its clinical significance lies in its potential for producing serious neurologic sequelae due to atlantoaxial instability. Although there are several recognized variations (aplasia, hypoplasia, and os odontoideum), clinically they share the same signs and symptoms, and the treatment is identical. Symptoms are usually due to instability of the atlantoaxial joint, with compression of the spinal cord and anteriorly against the axis or posteriorly from the ring of the atlas. Patients may present with no symptoms, with persistent neck complaints, with transient or permanent neurologic deficits, or with sudden death. Symptoms from cranial nerve irritation seldom occur, but occasionally symptoms of cerebral and brain stem ischemia are noted as a result of compression of the vertebral arteries in the area of the atlas. If the condition is suspected, the diagnosis usually can be confirmed on lateral flexion-extension roentgenograms. Special techniques are often required, particularly lateral laminagrams, and flexion-extension stress roentgenograms are necessary to determine the presence and degree of atlantoaxial instability. The role of prophylactic surgical stabilization is not yet established. If instability greater than 5 mm. is demonstrated or the patient has clinical findings of neurologic compromise, surgical fusion should be performed.