Occipitoatlantal instability associated with articular tropism.

The authors report the case of a patient with rotatory posterior subluxation of the occiput on the atlas associated with tropism of the O-C1 articulations. Lateral flexion-extension plain films demonstrated 5 mm of posterior translation of the occiput on the atlas. Tomographs revealed tropism of the O-C1 articulations, and CT scans on extension demonstrated posterior rotatory subluxation of the occiput on the atlas. The patient had no osseous abnormality caudally below the atlantoaxial joint, and underwent posterior occipitoatlantal fusion by wiring. At the 4-year follow-up, the grafted bone had been incorporated, and the patient was completely free from preoperative symptoms. In the present patient, occipitoatlantal instability is presumed to have derived from articular tropism.