Spinopelvic–Hip Harmony [PFA−(PI−LL)] as a Radiographic Indicator of Hip Extension Compensatory Reserve in Kyphotic Adult Spinal Deformity: A Multicenter Study

Study Design Retrospective multicenter observational study. Objectives In kyphotic adult spinal deformity (ASD), hip extension, assessed by pelvic femoral angle (PFA), compensates for sagittal malalignment, but no radiographic measure quantifies residual “hip extension reserve.” When hip compensation fails, knee flexion, assessed by femoral inclination (FI), is recruited and forward trunk inclination, assessed by sagittal vertical axis (SVA), worsens. We aimed to identify which sagittal radiographic parameter best explains FI, SVA, and health-related quality of life (HRQOL), and to evaluate whether spinopelvic–hip harmony, defined as PFA minus spinopelvic mismatch [PFA−(PI−LL)], reflects hip extension reserve. Methods We included 115 kyphotic ASD patients from three spine centers. Spinopelvic parameters, PFA, and FI were measured; PI−LL and PFA−(PI−LL) were calculated. HRQOL was assessed using the Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ). We performed Spearman correlations and hierarchical linear regression (baseline: age, sex, and body mass index), adding each radiographic parameter individually and comparing ΔR 2 and the corrected Akaike information criterion (AICc) (complexity-penalized fit). Results PFA−(PI−LL) correlated with FI (ρ = −0.71) and SVA (ρ = −0.73). After adjustment, PFA−(PI−LL) yielded the greatest incremental explanatory power for FI (ΔR 2 = 0.43) and SVA (ΔR 2 = 0.56), with ΔAICc >10 vs the others. PFA−(PI−LL) was also associated with JOABPEQ lumbar function and walking ability. ROC analyses showed PFA−(PI−LL) cutoffs for FI >10° and SVA >95 mm were around 160°. Conclusions In kyphotic ASD, PFA−(PI−LL) best explained both knee flexion recruitment and forward trunk inclination and may indicate hip extension reserve.