Optimal Trajectory and Length of S2 Alar Iliac Screws: A 3-Dimensional Computed-aided Design Study.

STUDY DESIGN: A radiographic study of computed tomography scans of the pelvis converted to 3-dimensional imaging. OBJECTIVE: The objective of this study was to determine the optimal length and trajectory of S2 alar iliac (S2AI) screws. SUMMARY OF BACKGROUND DATA: Solid pelvic fixation is a necessary component in thoracolumbar deformity surgery to protect sacral fixation and promote arthrodesis. The S2AI technique has been promoted as a way to reduce hardware prominence and instrumentation issues associated with traditional iliac fixation. MATERIALS AND METHODS: In total, 64 of 100 patients randomly selected from our institution's spine registry were able to be converted to 3-dimensional imaging. Virtual screws were then placed in the optimal position for an S2AI screw on each side of the pelvis. The lateral and inferior angles were measured off-axial and sagittal planes, respectively. The distances from the notch and the remaining available screw length were also recorded. RESULTS: The average patient age was 38±16 years. The average lateral angle was 42.5±2.0 degrees and the inferior angle was 18.2±1.8 degrees. The screws fit bilaterally in all 64 patients without cortical breach. The remaining available screw distance was measured in all patients and found to be 40.5±8.7 mm. The average distance from the sciatic notch was 13.8±4.1 mm. Men had significantly more potential screw length when compared with women. CONCLUSIONS: We found an average inferior angle of 18.2 degrees, which is less than the previously described angle range of 20-40 degrees. These findings suggest that an ideal trajectory may be significantly different than the previously described trajectory. We found that with the optimal trajectory, a 100 mm screw can fit in all patients without concern for cortical breach of the pelvis or violation of the hip joint.