Os odontoideum: diagnosis and role of imaging

Objective To elucidate the imaging manifestations of os odontoideum, establish the diagnosis and guide surgical therapy. Methods Clinical and imaging data, including X-ray, CT and MR of 24 patients with os odontoideum, were retrieved and reviewed retrospectively. Results Os odontoideum with intact cortex was divided into round, conical and blunt tooth types. Four cases of orthotopic and 20 cases of dystopic os odontoideum were included. There was anterior displacement of the base of the dens in six cases, posterior displacement in nine cases and no displacement in nine cases. A widening of anterior atlanto-axial space was shown in 14 patients with varying degrees. Thickening of the soft tissue posterior to the dens was observed in 19 patients, spinal canal stenosis in 21 patients, cervical myelopathy in 10 patients and craniocervical junction malformation in 9 patients. Posterior C1-C2 pedicle screw fixation and fusion was performed in 12 patients and 4 patients underwent posterior occipito-cervical fixation and fusion. Conclusion Radiographically, os odontoideum is defined as an independent ossicle of variable size with smooth circumferential cortical margins separated from the axis. Imaging can be used to assess atlanto-axial instability, associated normal or abnormal anatomical structures and guide surgical therapy.