A rare cause of cervical spinal stenosis: posterior arch hypoplasia in a bipartite atlas.

We describe CT and MRI of a previously unreported combination of atlantoaxial anomalies consisting of posterior arch hypoplasia in a bipartite atlas with an os odontoideum, in a 30-year-old woman presenting with neck and left arm pain. MRI showed the os odontoideum, marked stenosis of the spinal canal at the level of the atlas, with cord compression and evidence of myelopathy. CT revealed a bipartite atlas with midline clefts in anterior and posterior arches, thickening in the anterior arch and hypoplasia of the posterior arch with incurving of both hemiarches. Flexion and extension radiographs demonstrated atlantoaxial instability.