Association of S2 Alar–Iliac Versus S1-Only Fixation with Spinopelvic Mobility and Acetabular Component Orientation in Total Hip Arthroplasty: A Standing–Sitting EOS Study

Background: S2 alar–iliac (S2AI) fixation crosses the sacroiliac joint, unlike fixation terminating at S1, and may be associated with reduced spinopelvic mobility in patients undergoing total hip arthroplasty (THA). We compared spinopelvic mobility, femoropelvic reorientation, acetabular component orientation, and hip-specific function between S2AI and S1-only fixation. Methods: This retrospective comparative cohort study included 60 patients undergoing THA after adult spinal fusion with S2AI (n = 30) or S1-only (n = 30) fixation. Standing and sitting EOS imaging at 1 year assessed changes in sacral slope (ΔSS) and pelvic–femoral angle (ΔPFA). Immediate postoperative CT measured static acetabular component orientation. Hip-specific function was assessed using the Hip Disability and Osteoarthritis Outcome Score (HOOS). Multivariable regression evaluated associations with ΔSS and HOOS. Results: Spinopelvic stiffness (ΔSS < 10°) was more frequent with S2AI fixation (66.7% vs. 13.3%; p < 0.001). Compared with S1-only fixation, S2AI was associated with lower ΔSS (7.7 ± 4.8° vs. 18.1 ± 7.0°; p < 0.001) and greater ΔPFA (68.8 ± 11.7° vs. 58.7 ± 10.8°; p = 0.001). Static cup orientation was similar. One-year HOOS was lower with S2AI fixation (79 [68–86] vs. 86 [72–92]; p = 0.01), although ΔHOOS did not differ significantly (p = 0.07). After adjustment, S2AI fixation remained associated with lower ΔSS (β = −7.67; p < 0.001), while greater ΔSS was associated with higher HOOS (β = 3.42 per 5°; p = 0.003). Conclusions: S2AI fixation was associated with reduced spinopelvic mobility and greater femoropelvic reorientation despite similar static cup orientation. Residual confounding by spinal construct complexity precludes causal inference.