Atlantoaxial instability associated with an orthotopic os odontoideum: a multimodality imaging assessment.

Radiography, computed tomography (CT), and magnetic resonance imaging exams of the cervical spine were performed in a 29-year-old man who was ultimately diagnosed with an orthotopic os odontoideum during admission for injuries sustained in a motor vehicle collision. Initial radiography suggested either os odontoideum or an acute fracture of the dens. Further imaging with CT and flexion and extension radiographs confirmed os odontoideum and excluded a dens fracture. Although rare, os odontoideum is an important cervical spine anomaly to consider and to distinguish from an acute fracture of the dens.