Proposal of a new indicator of hip compensation for spinopelvic-hip mismatch: a retrospective study in Japan.

Study design: retrospective study.

Purpose: This study aimed to develop an indicator of the compensatory capacity of hip joints in response to spinopelvic mismatch using standard radiographs.

Overview of literature: EOS imaging has enabled detailed analysis of hip and lower extremity compensation in sagittal malalignment. However, its high cost and limited availability hinder widespread clinical use. Currently, there are no established indicators to assess hip compensation for spinopelvic mismatch using standard radiographs.

Methods: A total of 209 patients with osteoporosis and 54 with adult spinal deformities were included. Patients were divided into two groups based on pelvic incidence-lumbar lordosis (PI-LL): <20° and ≥20° groups. The sagittal vertical axis (SVA), thoracic kyphosis, PI, pelvic tilt (PT), LL, sacral slope, and pelvic femoral angle (PFA) were measured. Health-related quality of life (HRQOL) was assessed in 86 patients using the Japanese Orthopedic Association Back Pain Evaluation Questionnaire (JOABPEQ). A new index, PFA-(PI-LL), was defined as spinopelvic-hip mismatch. Correlation coefficients were calculated for each radiographic parameter, and the coefficient of determination (R2) for the relationship of each parameter with SVA was evaluated in both groups. The correlations between SVA, PT, PI-LL, PFA-(PI-LL), and JOABPEQ domain scores were also analyzed.

Results: PI-LL correlated with PFA in the PI-LL <20° group (r=0.56, p<0.001) but not in the PI-LL ≥20° group. Among all parameters, PFA-(PI-LL) demonstrated the strongest association with SVA, indicating its superior ability to explain variations in sagittal alignment in both groups and across all patients (all patients, R2=0.77). Significant correlations were observed between the radiographic parameters and JOABPEQ scores across all domains.

Conclusions: PFA-(PI-LL), which represented spinopelvic-hip mismatch, was a reliable indicator of hip compensatory function in terms of anterior trunk inclination and HRQOL.