Os odontoideum. A cause of atlanto-axial instability.

Os odontoideum is a rare condition in which instability may damage the upper cervical cord. A delay in diagnosis is not uncommon. This paper describes a series of 11 patients with os odontoideum. The presenting symptoms were divided into three groups: posttraumatic neck-pain, gradually appearing signs of medullary compression and an asymptomatic group. Eight patients had atlanto-axial instability. Six had interlaminar fusion between atlas and axis. No serious complications were seen. Stability was obtained in all patients postoperatively. In case of significant instability, fusion is indicated. In patients with cerebral palsy a closer examination of the upper cervical cord is recommended.