Os odontoideum: report of three cases.

Os odontoideum is a condition in which a smoothly corticated ossicle exists dorsal to the anterior arch of C1, taking the place of the rostral dens, but with no bony connection to the body of the axis. Three patients presented with this condition: the first with Lhermitte's phenomenon 10 years after significant trauma, the second as an incidental finding during routine cervical spine imaging following a road traffic accident, and the third with recurrent transient quadriparesis precipitated by falls from a surfboard. Patients had at least 10 mm of sagittal instability on dynamic imaging and the second patient had a minimum sagittal canal diameter of only 11.5 mm. Posterior atlanto-axial fixation was successfully achieved in all cases using polyaxial screws and rods with the assistance of computed tomography-based image guidance. Image guidance provided an invaluable aid to preoperative planning and intraoperative placement of the posterior spinal instrumentation.