EP9.15 Coronal Pelvic Drop as a Compensatory Mechanism in Acetabular Over-Coverage: Associations with Dynamic Lateral Center-Edge Angle Normalization, Patient-Reported Function, and Reoperation Risk

Abstract Background Coronal pelvic drop during walking alters dynamic lateral center-edge angle (LCEA) relative to static radiographic values. In over-covered hips, pelvic drop reduces coverage toward the normal range, potentially serving a protective role. We hypothesized that the magnitude of pelvic drop translates into measurable differences in patient-reported outcomes and revision risk in femoroacetabular impingement (FAI) patients with acetabular over-coverage. Methods We included 101 FAI patients (age 33.9 ± 9.7 years, BMI 28.4 ± 5.2 kg/m2, 5 male / 10 female, LCEA 43.9 ± 3.3°) in this prospective cohort. Pelvic drop magnitude was correlated with (a) percentage of walking frames in the normal LCEA range (25.1–39.9°), (b) seven patient-reported outcome scores (PROS)(iHOT-12, HOS-SS, PROMIS Pain Interference, Physical Function, Depression, Anxiety, EQ-5D VAS), and (c) revision surgery status. Results Among 11 over-covered patients with walking data, larger pelvic drop correlated positively with time in the normal LCEA range (r = +0.63, p = 0.04). This correlation was negative in borderline patients (r = −0.49, p = 0.04) and null in normal patients (r = −0.18, p = 0.15), confirming direction-specificity. In over-covered patients, drop magnitude correlated with PROMIS Physical Function (r = +0.77, p = 0.009; ρ = +0.94, p = 0.0001), HOS-SS (r = +0.63, p = 0.04), and iHOT-12 (ρ = +0.64, p = 0.03). No PROS reached significance in the normal or borderline subgroups. Over-covered patients with above-median pelvic drop (>4.9°) had a revision rate of 1/7 (14.3%) versus 5/8 (62.5%) in the below-median group (Fisher p = 0.12). Conclusion In over-covered FAI, coronal pelvic drop may function as a compensatory mechanism that reduces dynamic LCEA toward normal during loading. This compensation correlates with better patient-reported function and trends toward lower revision rates. The effect is direction-specific: the same drop is neutral or harmful in normal and borderline hips. These findings reframe pelvic drop in over-covered FAI — not necessarily as abductor weakness but as a biomechanical compensation with implications for symptom burden and reoperation risk.