Bony Hip Congruency and Coverage Varies by Acetabular Region and Activity in Asymptomatic Young Adults

Understanding how hip coverage and congruency change during dynamic loading may provide insight into the etiology of long-term degenerative pathology and susceptibility to hip instability. The goal of this study was to determine how hip congruency and femoral head coverage change during activities of daily living and their relationship with bone morphology in an asymptomatic cohort.

Twenty-four healthy young adults (13 women, 21.9 ± 2.2 years) stood, walked, squatted, and ascended a step while synchronized biplane radiographs of the hip were collected. In-vivo bone motion was determined with sub-millimeter accuracy using a validated tracking technique that matched subject-specific bone models to the biplane radiographs. Dynamic congruency index (CI) and femoral head coverage were calculated across each activity. Mixed effects analyses determined effects of activity, region, and their interaction on CI or coverage, separately, and multiple linear regression determined the associations between alpha angle, lateral center edge angle (LCEA), and CI or coverage in each region and activity.

During lower-flexion activities, Anterior-Inferior, Anterior-Superior, and Posterior-Inferior congruency decreased up to 30% (all p < 0.02), Superior and Posterior-Superior congruency increased up to 40% (all p < 0.01), and Anterior and Inferior coverage decreased (up to 40.1% and 9.4%, all p < 0.01) compared to higher-flexion activities. LCEA was a better morphologic predictor of coverage and congruency than alpha angle.

These observed regional variations in hip coverage and congruency in asymptomatic individuals provide a potential mechanistic explanation for common patterns of cartilage degeneration in the anterior acetabular region and a reference for evaluating pathologic hips.