Regional femoral head Hounsfield units mirror acetabular undercoverage in acetabular dysplasia

Acetabular dysplasia is a three-dimensional (3D) disorder characterized by regional femoral head undercoverage and altered joint loading. Percentage femoral head coverage (%FHC) enables regional assessment of acetabular coverage, but its relationship with femoral head computed tomography (CT) attenuation, expressed in Hounsfield units (HU), remains unclear. This study evaluated associations between 3D acetabular coverage and femoral head HU values in symptomatic dysplastic hips.

This retrospective radiologic study included 40 patients with bilateral acetabular dysplastic morphology and complete bilateral CT imaging scheduled for periacetabular osteotomy between 2023 and 2026, resulting in 80 hips. Total and regional %FHC were calculated in the anterolateral (AL), anteromedial (AM), posteromedial (PM) and posterolateral (PL) regions. Lateral %FHC was defined as the mean of AL and PL coverage. Femoral head HU values were measured in 12 regions of interest on three axial slices across the same four quadrants, and lateral HU was defined as the mean of AL and PL values. Associations were assessed using age-adjusted linear mixed-effects models with a random intercept for participant.

Intraobserver and interobserver reliability were good to excellent. Age was independently associated with lower HU values across models. Lateral %FHC was positively associated with lateral HU (β = 10.68 HU per 1% increase; 95% confidence interval [CI], 3.76-17.60; p = 0.002). Regionally, %FHC_AL was associated with anterolateral femoral head Hounsfield unit value (UH_AL; β = 10.59; 95% CI, 2.73-18.44; p = 0.009), and %FHC_PL was associated with posterolateral femoral head Hounsfield unit value (UH_PL; β = 7.36; 95% CI, 1.58-13.13; p = 0.013). Total, AM and PM coverage were not associated with corresponding HU values.

In this exploratory imaging study, lower lateral, AL and PL 3D acetabular coverage were associated with lower HU values in corresponding femoral head regions. These findings suggest that regional femoral head HU mapping may provide an imaging correlate of spatial acetabular undercoverage in dysplastic hips, although its clinical, diagnostic and prognostic utility requires further validation.

Level III, diagnostic study.