Pelvis-Based Three-Dimensional Hip Morphology in Japanese Women: Reference Distributions and Spatial Relationships of Femoral Head Position, Acetabular Orientation, and Coverage

Abstract

Background: Conventional radiographic assessment of the hip primarily describes coronal acetabular coverage and does not fully characterize the three-dimensional spatial relationship among the pelvis, femoral head, and acetabulum.This study aimed to establish a pelvis-based three-dimensional morphometric reference framework for femoral head position, acetabular orientation, and regional coverage in Japanese women without clinically recognized hip disease.Methods: This retrospective cross-sectional study included 172 Japanese women (344 hips; mean age, 45.8 ± 10.7 years; range, 19-59 years) who underwent chest-abdominal computed tomography for nonorthopedic indications.Three-dimensional reconstructions were standardized using the anatomic pelvic plane.Femoral head position was quantified using the anterior index (AI) and lateralization index (LI), and acetabular morphology was assessed using acetabular anteversion (AV), anterior, posterior, and horizontal acetabular sector angles, conventional coverage parameters, and coverage areas.Statistical analyses accounted for clustering of bilateral hips.Sensitivity analyses were performed in hips with a center-edge angle of 25°-40° and in right hips only.Results: The mean AI and LI were 62.5 ± 3.3 and 68.4 ± 5.7, respectively.After adjustment for age, side, and bilateral clustering, greater AI was associated with greater AV (standardized β = 0.62), smaller anterior acetabular sector angle (β = -0.40),and larger posterior acetabular sector angle (β = 0.32; all p < 0.001).LI showed weaker associations with acetabular orientation and regional coverage.No statistically significant side-to-side or age-related differences were observed in the principal three-dimensional positional and angular parameters.The principal associations were preserved in both sensitivity analyses.Conclusions: Pelvis-relative femoral head position was consistently associated with acetabular orientation and the anterior-posterior distribution of regional coverage.These findings provide a population-specific three-dimensional anatomical reference framework that complements conventional coronal hip measurements and may serve as a basis for future comparisons with morphologically abnormal hips.