Bone mineral density averaged over a region of interest on femur is affected by age-related change of bone geometry.

Femur expansion occurs during normal aging in both men and women. Average bone mineral density (BMD) over a region of interest (ROI) on the femur may considerably decrease with age even in healthy people, and therefore, it is inaccurate if used to monitor treatment-induced bone change. INTRODUCTION: Areal bone mineral density (BMD), averaged over a region of interest (ROI) on the femur, is widely used in the diagnosis of osteoporosis, assessment of fracture risk, and monitoring of treatment effectiveness. We studied the effect of age-related change in femur geometry on average BMD. METHODS: The effect of age-related bone geometric change on averaged BMD was investigated by a cross-sectional study. Total 83 healthy subjects were selected for this study. For each subject, QCT of left femur was scanned using clinical scanner. For each standard volume of interest (VOI), integral/cortical/trabecular bone volume, volumetric BMD (vBMD), and bone mass were measured using QCT Pro; the corresponding areal BMD (aBMD) was projected using CTXA-Hip. Both QCT Pro and CTXA-Hip are commercial software. Correlations between bone volume/density/mass and age were studied. RESULTS: In the studied population, there was no association between body weight/BMI (body mass index) and age, correlation between normalized femoral neck width and age was 0.24 (p < 0.05). Both aBMD and integral vBMD decreased with age (after adjusted by BMI, for aBMD, r = - 0.21 to - 0.24, p ≤ 0.05 except at trochanter; for vBMD, r = - 0.20 to - 0.31, p < 0.05); cortical vBMD had no significant change; trabecular vBMD decreased at all VOIs except at trochanter (after adjusted by BMI, r = - 0.22 to 0.32, p ≤ 0.05). Integral volume showed slight increase but only significant at the trochanter after adjusted by body size, cortical volume showed insignificant decrease, and trabecular volume considerably increased with age in all VOIs (after adjusted by body size, r = 0.27-0.40, p < 0.05). Integral, cortical, and trabecular mass had no significant change in all VOIs, except that at the trochanter trabecular mass slightly increased with age (r = 0.31, p < 0.05). CONCLUSIONS: Even though there is no change in bone mass, average BMD may considerably decrease with age due to bone expansion. Comparatively, aBMD is less affected than vBMD.