Occipital screw pullout strength: A biomechanical investigation of occipital morphology

Study Design. A three-group design with consistent pullout strength measures. Objectives. To determine pullout strength of three fixation types (unicortical screws, bicortical screws, wires) and to investigate their correlation with respect to occipital morphology. Summary of Background Data. A secured, multidirectional occipitocervical fusion requires internal fixation. Devices secured at occipital protuberance were suggested to offer the greatest pullout strength because of this region's thickness. Methods. Twelve fresh human cadaveric occiputs were sketched with a grid delineating 21 fixation sites. Each site was drilled and hand-tapped. Four specimens were instrumented with unicortical screws on one side of the midline and bicortical screws on the other. Another four were instrumented with bicortical screws and wires, and the remaining four were instrumented with unicortical screws and wires. Two points on each specimen were secured with identical fixation to examine side-to-side symmetry. An MTS materials testing apparatus (MTS Systems Corporation, Eden Prairie, MN) was used to displace the fixators. Pullout strengths at different anatomic locations were recorded. Results. The greatest pullout strength was at the occipital protuberance for all fixation types. The bicortical pullout strength was 50% greater than unicortical. The wire pullout strength was not significantly different from that of the unicortical screw (P > 0.05). Seventy-eight percent of wires broke at 1100 N. Unicortical pullout strength at occipital protuberance was comparable with that of the bicortical screw at other locations. Conclusions. Unicortical screw fixation at occipital protuberance offers acceptable pullout strength without the potential complications of bicortical screws or wire fixation.