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

A radiographic study of computed tomography scans of the pelvis converted to 3-dimensional imaging.

The objective of this study was to determine the optimal length and trajectory of S2 alar iliac (S2AI) screws.

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.

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.

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.

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.