Risk Factors for S2 Alar-Iliac Screw Misplacement in the Sacroiliac Joint Using O-Arm Navigation

Study Design: Retrospective cohort study. Objective: In this study, we investigate factors associated with screw deviation, focusing on sacroiliac joint (SIJ) morphology. Material and Methods: This retrospective observational study included patients with S2 alar-iliac (SAI) screws inserted under O-arm guidance. The clinical characteristics and SIJ morphology of each patient were assessed using preoperative computed tomography (CT). Our morphologic evaluations included the presence or absence of SIJ fusion and the vacuum phenomenon (VP), and the degree of sclerosis. Results: A total of 177 patients (354 screws) were analyzed. The median age was 76 years, and 55.4% of the patients were female. The most common indication for surgery was degenerative spinal deformity (45.2%), followed by spinal canal stenosis (26.6%). Screw deviation occurred with 10 of the 354 screws (2.82%). Significant differences in preoperative severe SIJ sclerosis and the presence of VP were found using post hoc analysis between the deviation and nondeviation groups. Logistic regression analysis confirmed both severe sclerosis and VP as significant risk factors. All deviated screws were correctly reinserted intraoperatively, and no screw-related complications occurred during follow-up. Conclusion: This study demonstrates that, even with O-arm navigation, SAI screw deviation can occur. Severe SIJ sclerosis and the presence of VP were identified as significant risk factors. Therefore, careful preoperative evaluation of SIJ morphology is essential. Particular caution should be exercised in patients with these risk factors to minimize the risk of screw deviation. Evidence Level: Level III.