Relationship of posterior superior iliac spine morphology and ideal S1 pedicle screw trajectory: A cross-sectional study of closed versus open posterior superior iliac spine morphology.

Introduction: S1 pedicle screws are commonly used in spine surgery. As these screws are often the most caudal part of the construct, it is important to achieve solid fixation via the ideal pedicle screw placement with the use of a medialized screw into the sacral promontory, the "tricortical S1 screw." However, this trajectory can be precluded by the presence of a closed posterior superior iliac spine (PSIS) morphology, leading to suboptimal fixation in the sacrum. The aim of this study was to define the prevalence of closed PSIS morphology. Methods: Computed tomography (CT) imaging of the pelvis of a consecutive cohort of patients at a single institution was reviewed to identify the ideal S1 screw trajectory. PSIS morphology was identified as either open or closed, depending on whether a line from the ideal S1 pedicle screw trajectory intersected the PSIS. Statistical analyses included descriptive statistics (95% confidence intervals), group comparisons (one-sample proportion tests, Chi-square, t-tests, and Mann-Whitney U-test), correlations, and logistic regression for morphology predictors. Effect sizes (Cohen's d and Cramer's V) assessed clinical significance alongside statistical significance. Results: Of the 132 patients included in this study, 91 (68.9%) patients demonstrated closed PSIS morphology. Gender was the only significant predictor of closed PSIS morphology, with females being 33% less likely to have closed PSIS morphology. Conclusion: Preoperative CT imaging should be carefully reviewed and scrutinized to assess for closed PSIS morphology, and if closed PSIS morphology is present, plans for alternative fixation methods should be considered and available at the time of surgery.