LAUSR

STUDY DESIGN Retrospective review. OBJECTIVE To assess the change in pelvic incidence (PI) after lumbo-pelvic fixation and the differential impact of the type of pelvic fixation: S2-alar-iliac screws(S2AI) versus Iliac screws(IS) on post-operative PI. SUMMARY OF BACKGROUND DATA Recent studies suggest that changes occur to the previously assumed fixed PI after spino-pelvic fixation. METHODS Adult spine deformity(ASD) patients who underwent spino-pelvic fixation with≥4 levels fusion were included. Pre-operative and post-operative PI, lumbar lordosis(LL), thoracic kyphosis(TK), pelvic tilt(PT), sacral slope(SS), PI-LL mismatch and the Sagittal Vertical Axis(SVA) were analyzed on EOS imaging. A significant PI change was established at≥6⁰. Patients were categorized based on the type of pelvic fixation (S2AI vs. IS). RESULTS One-hundred-forty-nine patients were included. Of these, 77(52%) had a>6⁰ change in their PI post-operatively. In patients with high pre-operative PI (>60⁰), 62% had a significant PI change compared to 33% of patients with normal PI (40⁰-60⁰) and 53% in patients with low PI(<40⁰; P=0.01). PI was likely to decrease in patients with high baseline PI (>60⁰) and to increase in patients with low baseline PI (<40⁰). Patients with a significant PI change had a higher PI-LL. Patients in the S2AI group (n=99) and those in the IS group (n=50) were comparable at baseline. In the S2AI group, 50 (51%) patients had>6⁰ change in their PI compared to 27(54%) patients in the IS group(P=0.65). In both groups, patients with high pre-operative PI were more prone to significant post-operative changes (P=0.02 in IS, P=0.01 in S2AI). CONCLUSION PI changed significantly in 50% of patients post-operatively, especially in those with high/low pre-operative PI and those with severe baseline sagittal imbalance. This occurs similarly in patients with S2AI and those with IS screws. Surgeons should keep in mind these anticipated changes while planning ideal LL, as this impacts post-operative PI-LL mismatch. LEVEL OF EVIDENCE IV.