A Physical Model for Dual-Energy X-Ray Absorptiometry–Derived Bone Mineral Density

Abstract

Dual-energy X-ray absorptiometry (DXA)–derived areal bone mineral density (BMD) is an established predictor of osteoporotic fractures and reflects bone strength as well. The goal of this study was to develop and validate a physical model for appropriate interpretation of BMD. DXA and peripheral quantitative computed tomography investigations of the distal tibia (n = 45), proximal tibia (n = 12), distal femur (n = 26), and distal radius (n = 34) were carried out. The DXA-derived BMD was analytically modeled as a nonlinear function of volumetric bone mineral apparent density and the cross-sectional area (eCSA) of given bone; ie, BMDmod = apparent BMD × √eCSA. At every measured skeletal site, the relationship between BMD and BMDmod was systematically stronger than that observed separately between BMD and apparent BMD or cross-sectional area. The models (r2) explained 85%, 94%, 87%, and 74% of the variability in BMD at the distal tibia, proximal tibia, distal femur, and distal radius, respectively. The mutual contributions of bone density and size to BMD can vary to some extent in a site-dependent fashion. This dual nature of BMD on one hand provides a reasonable mechanical explanation for why BMD is a good surrogate of bone strength and a predictor of osteoporotic fractures but on the other hand, complicates its detailed interpretation.