S2AI vs. iliac screws in spinopelvic fixation for adult spinal deformity: a propensity score-matched analysis.

Purpose: The purpose of this study was to compare the S2-alar-iliac (S2AI) technique with the iliac screw (IS) technique in adult spinal deformity (ASD) patients in terms of clinical and radiographical outcomes, focusing on reoperations, complications, and change in radiographic parameters.

Methods: This is a retrospective review of a prospective, multicenter database. ASD patients who underwent long fusion to the pelvis with 2-year postoperative follow-up were included. To compare outcomes (radiographic, clinical, and complications), matching was performed based on the type of pelvic fixation (IS vs. S2AI) using propensity score matching (PSM), 1:1 ratio, caliper 0.1, tolerance ≤ 0.001, with a 95% confidence interval. Kaplan-Meier survival curves were generated for each group and compared between the two groups by the log-rank test. Hazard ratio (HR) was calculated using the Cox proportional hazards model.

Results: Out of 1442 patients undergoing intervention with a 2-year follow-up, 555 were identified as having pelvic instrumentation. Among them, 52 patients fixed with S2AI screws were matched with 52 patients fixed with IS using PSM for age, body mass index (BMI), number of fused levels, and global tilt. No significant differences were found in radiographic correction, reoperation rates, or infection rates. The percentage of mechanical complications (MC) was higher in the IS screw group, with a statistically significant increase in MC-free survival in the S2AI screw group (80.6 vs. 61.2 months; p = 0.022), with a HR of 0.43 (p = 0.027). Patients with S2AI screws reported higher immediate postoperative pain at 6 weeks, with this difference leveling off in subsequent assessments. At 2 years, a higher percentage of radiolucent halos were observed in the S2AI screw group (59.6% vs. 34%; p = 0.017), but there were no differences in pain assessments in the quality-of-life tests.

Conclusion: After a thorough comparison, both pelvic fixation methods showed similar deformity correction and reintervention rates. However, iliac screws had more mechanical complications, while S2AI screws, crossing the sacroiliac joint, led to higher short-term postoperative pain and increased radiological loosening at 2 years.