Understanding the os odontoideum.

Five cases of os odontoideum and associated instability of C1-2 involving patients who underwent a posterior spinal fusion are reviewed, along with the literature. Four of the patients had Down's syndrome, and two of them were symptomatic at the time of presentation. In the two asymptomatic patients, the os odontoideum was an incidental finding on routine roentgenographic survey. The fifth patient presented with trauma and neck pain. The operation was successful in all of the patients with Down's syndrome but one, who had preoperative signs of myelopathy. Patients known to have an os odontoideum must be followed closely; if there is instability, posterior spinal fusion is recommended to avoid the risk of neurologic compromise. An understanding of the pathoanatomy of this condition is essential to recognize and properly treat these patients.