A biomechanical study of odontoid fractures and fracture fixation.

The purpose of this study was to measure the stability of the odontoid process after fracture and subsequent screw fixation. To accomplish this, we mechanically reproduced Type II and Type III odontoid fractures on isolated C2 vertebrae by varying the direction of load. These fractures were subsequently stabilized with a single 3.5 mm screw and retested for multidirectional stability and load to failure. Reduced and instrumented specimens were found to have a stiffness equivalent to one half of that of the unfractured odontoid. Load to failure was also slightly less than one half of the original fracture force (average 160 lb). Screw failure was by a cut-out mechanism in all Type III fractures and by bending of the screw in all Type III fractures. Our findings, in conjunction with the existing literature, strongly suggest that Type III fractures result from extension forces, whereas Type II fractures result from lateral or oblique loading forces. Single screw fixation of an odontoid fracture will provide stability equal to approximately one half that of the unfractured bone.