BMI-stratified performance of native CT hounsfield units in the proximal femur for osteoporosis detection and fracture association

BACKGROUND: Opportunistic computed tomography (CT) bone density assessment using Hounsfield units (HU) is increasingly used. Body mass index (BMI) may influence dual-energy X-ray absorptiometry (DXA), computed tomography X-ray absorptiometry of the hip (CTXA), and CT attenuation values, potentially affecting HU thresholds and fracture discrimination. We quantified associations between BMI, femoral HU, hip T-scores, and prevalent fractures. METHODS: We retrospectively analyzed 600 consecutive patients (65.3 ± 14.1 years; 81% female; BMI 26.7 ± 6.5 kg/m²) who underwent CTXA hip assessment. Trabecular HU were measured on coronal CT images using a circular ROI in the femoral head and an irregular ROI in the proximal femur. BMI subgroups followed WHO categories. Spearman correlations (ρ) and linear regression analyses (R²) were performed. Multivariable linear and logistic regression models were adjusted for age and sex. RESULTS: BMI correlated positively with femoral HU and hip T-score (ρ = 0.46; p < 0.001). In multivariable models, each 1 kg/m² increase in BMI was associated with higher HU values (+ 3.32 HU; 95% CI 2.61–4.02) and higher hip T-score (+ 0.081; 95% CI 0.064–0.098; p < 0.001). Fracture prevalence decreased across BMI categories. HU values discriminated patients with and without hip fracture (AUC up to 0.99). Given the cross-sectional design, these values reflect discrimination within the study cohort rather than prospective predictive validity. No proximal femur fractures occurred above the osteoporosis threshold of 94.77 HU. CONCLUSIONS: BMI was independently associated with CT attenuation and CTXA-derived bone density parameters and inversely associated with prevalent fractures in this cohort. BMI should be considered when interpreting opportunistic HU measurements and applying fixed thresholds.