Sacral Endplate Penetrating Screw for Lumbosacral Fixation: A Cadaveric Biomechanical Study

BACKGROUND: Cortical bone trajectory is a relatively new alternative for instrumentation of the lumbar spine. When performing lumbosacral instrumentation, a novel S1 endplate penetrating screw (EPS) has been recently shown to have higher insertional torque than the traditional trajectory screw, but the biomechanical properties of this new trajectory are yet to be verified with the cadaveric studies. OBJECTIVE: To evaluate 2 screw trajectories in sacra using cyclic loading and pullout tests, and to determine whether bone quality had different effects on the 2 trajectories. METHODS: Nine cadaveric sacra were used, 5 of which had normal bone mineral density (BMD) and 4 were osteoporotic. Each side of the sacra was randomly assigned to either EPS trajectory or S1-alar screw (S1AS) trajectory. Each screw then underwent cyclic loading followed by pullout force measurement. A mixed-design 2 way ANOVA test was used to detect differences between the groups. RESULTS: The EPS group had less relative rotation at the bone-screw interface during cyclic loading than the S1AS group (P = .016) regardless of bone quality. The pullout force following the cyclic loading was significantly higher in the EPS group (2349 +/- 838 N) than the S1AS group (917 +/- 909 N) in normal bone (P < .0001). The difference was more pronounced in osteoporotic bone with the EPS (1075 +/- 216 N) compared to the S1AS (365 +/- 422 N; P < .0001). CONCLUSION: The S1 EPS trajectory is significantly more stable against loosening and has a higher pullout force compared to the S1AS trajectory. The difference between the 2 trajectories is more pronounced in osteoporotic bone.