TP1.4 Frame-by-Frame Analysis of Dynamic Acetabular Coverage During Operative-Stance in Femoroacetabular Impingement

Abstract Background The temporal behavior of acetabular coverage during the load-bearing phase of the operative hip has not been well characterized. We hypothesized that during operative-limb stance — when body weight is transmitted through the hip — dynamic acetabular coverage as reflected by dynamic lateral center-edge angle (LCEA) classification differs from coverage measured by static radiographic LCEA. Methods Motion capture data were collected from 101 femoroacetabular impingement (FAI) patients (38 male / 63 female; mean age 31.1 ± 11.7 years) during level walking with normal speed. A dynamic LCEA was computed for each frame as the sum of static radiographic LCEA and instantaneous coronal pelvic tilt. Operative-stance frames were classified as dysplastic (<20°), borderline (20–25°), normal (25–40°), or over-covered (≥40°). The percentage of operative-stance time in each classification group was computed for each patient. Results Of 94 patients with complete operative-stance data, 61 (64.9%) spent at least one frame in a classification different from their static baseline. Cohort-mean operative-stance time was 69.5% normal, 13.3% borderline, 2.4% dysplastic, and 14.8% over-covered. Among 66 statically normal patients, 18 (27.3%) entered the borderline range and 21 (31.8%) crossed into over-coverage. Among 17 statically borderline patients, 10 (58.8%) reached the dysplastic range, spending a mean 22.6% of operative-stance time below 20°. Among 11 statically over-covered patients, 4 (36.4%) crossed into the normal range. In stride-level testing, 56 of 94 patients (59.6%) showed a significant static-to-dynamic LCEA difference after False Discovery Rate correction (Benjamini-Hochberg correction method)(p < 0.001). Conclusion During operative-limb stance, nearly two-thirds of FAI patients experience coverage classification different from their static baseline. Borderline patients show the most severe pattern, with more than half reaching the dysplastic range and spending about one-fifth of loaded stance-time in dysplastic hip range. Over-covered patients rarely cross more than one group downward. Further research should relate these dynamic exposures to cartilage health and patient-reported outcomes.