MD Candidate The George Washington University School of Medicine and Health Sciences Washington, DC, US
Disclosure(s):
Aadit Mehta, B.S.: No financial relationships to disclose
Introduction: Pelvic fixation has become a sine qua non for stabilization of long-segment spinal fusions crossing the lumbosacral junction, shielding relatively weaker sacral pedicle screws from strong cantilever forces that lead to pseudoarthrosis or hardware failure. The S1-alar-iliac (S1AI) screw trajectory remains an underreported and perhaps underutilized option for primary spinopelvic fixation despite its conception a decade ago. This study reviews the paucity of data on S1AI screws compared to the accepted fixation methods of S2-alar-iliac (S2AI) and iliac screws.
Methods: A Boolean literature search was conducted on the PubMed and Scopus databases using the terms "S1AI screw," "S1 alar-iliac screw," "S1-AI screw," and "S1 alar iliac screw," which yielded 111 abstracts. After review, just eleven studies on S1AI screws were identified as germane and included. The bibliographies of these studies were also searched for additional relevant papers, but this failed to identify further studies for inclusion. The studies were then qualitatively reviewed.
Results: S1AI screws are technically straightforward to place. They share a start point with the familiar S1 pedicle screw; this both aligns them with pedicle instrumentation above (simplifying rod placement) and obviates dissection onto the sacrum and iliac crest. One study reported a lower risk of sacral breach compared to S2AI screws despite similar trajectories. S1AI placement is made more facile with enabling technologies like neuronavigation and robotics. Biomechanically, S1AI screws enjoy many of the advantages of other pelvic fixation methods. The trajectory is long, engages corticalized regions of the sacrum and ilium, and can accommodate very large diameter screws, all of which confer strong anchorage and pull-out strength. S1AI screws extend far beyond McCord’s lumbosacral pivot point and traverse the third sacropelvic zone of O’Brien, both of which increase construct stability. One biomechanical study found S1AI screws to be comparable to S1 pedicle plus S2AI screws, but more rigid in lateral bending.
Conclusion : The S1AI screw is considered a salvage option despite technical and biomechanical features comparable to iliac bolts and S2AI screws. Incumbent upon further noninferiority testing and comparative studies, the S1AI may be a trajectory less traveled to primary spinopelvic fixation.