Introducing functional dysplasia: Dynamic pelvic mechanics during running reduce femoral head coverage

Purpose: To investigate the influence of peak contralateral pelvic drop and ipsilateral femoral adduction during running on traditional radiographic parameters of femoral head coverage. Methods: This cross-sectional retrospective study included 19 patients (38 hips), 9 males and 10 females, with a mean age of 40 ± 10 years, all presenting with symptomatic unilateral femoroacetabular impingement. Participants underwent a three-dimensional running analysis and anteroposterior pelvic radiographs. Using specialised software, the femur and pelvis were rotated in the coronal plane based on the peak angles of contralateral pelvic drop and femoral adduction obtained from the biokinetic running analysis. After prior validation, traditional femoral head coverage parameters were assessed on both standard and adjusted radiographs, and the variation of each radiographic measurement was obtained. Results: < 0.001), respectively. After image adjustment, the percentage of hips classified as dysplastic increased based on the lateral centre-edge angle (0%-18%), acetabular index (11%-45%), extrusion index (5%-21%) and sharp angle (45%-76%). The femoro-epiphyseal acetabular roof index classified five hips (13%) as unstable. Linear regression demonstrated that each degree of pelvic drop resulted in a decrease of 0.92° of the lateral centre-edge angle and increases of 0.94°-1.13° in the other radiographic measurements. Conclusion: This study demonstrated that femoral head coverage significantly decreases due to contralateral pelvic drop during running. This results in an increased number of dysplastic findings on adjusted radiographs. Patients with significant contralateral pelvic drop and ipsilateral femoral adduction during running may be at risk for functional dysplasia. Level of Evidence: Level III, retrospective cross-sectional.