Assessment of primary and salvage lateral mass screw insertion torque in a cadaveric model.

This study was performed to determine the insertion torques of three types of lateral mass screws and to explore their relationship to bone mineral density (BMD). The peak insertion torque of primary articular screws was measured, and the holes were then stripped by overtightening. Larger diameter "salvage" screws were placed in the stripped holes, and peak insertion torque was measured. Peak primary screw insertion torque and BMD were positively correlated (r = 0.48, p = 0.00001). The ratio of salvage to primary screw insertion torque varied significantly among the systems (p = 0.0003). The positive correlation between BMD and primary screw peak insertion torque confirms the importance of bone density for satisfactory screw purchase. The large variation in the ratio of salvage to primary screw insertion torque among systems may be due to differences in primary and salvage screw design.