Modified brooks fusion for an os odontoideum associated with an incomplete posterior arch of the atlas

The etiology of os odontoideum still remains speculative. A recent review concluded that it is an acquired abnormality 'sencondary to trauma or perhaps occasionally to infection'. Only four of the 35 cases had an associated congenital anomaly which the authors thought was too small a percentage if os odontoideum were a fundamental congenital anomaly as others have postulated. We report a case in which os odontoideum was associated with a failure of synostosis of the posterior neural arch of the atlas.