Interpretation of calcaneus dual-energy X-ray absorptiometry measurements in the assessment of osteopenia and fracture risk

Dual-energy X-ray absorptiometry (DXA) of the calcaneus is useful in assess ing bone mass and fracture risk at other skeletal sites. However, DXA yield s an areal bone mineral density (BMD) that depends con both bone apparent d ensity and bone size, potentially complicating interpretation of the DXA re sults. Information that is more complete may be obtained from DXA exams by using a volumetric density in addition to BMD in clinical applications. In this paper, we develop a simple methodology for determining a volumetric bo ne mineral apparent density (BMAD) of the calcaneus. For the whole calcaneu s, BMAD = (BMC)/A(DXA)(3/2), where BMC and A(DXA), are, respectively, the b one mineral content and projected area measured by DXA. We found that A(DXA )(3/2) was proportional to the calcaneus volume with a proportionality cons tant of 1.82 +/- 0.02 (mean +/- SE). Consequently, consistent with theoreti cal predictions, BMAD was proportional to the true volumetric apparent dens ity (rho) of the bone according to the relationship rho = 1.82 BMAD. Also c onsistent with theoretical predictions, we found that BMD varied in proport ion to rho V-1/3, where Vis the bone volume. We propose that the volumetric apparent density, estimated at the calcaneus, provides additional informat ion that may aid in the diagnosis of osteopenia. Areal BMD or BMD2 may allo w estimation of the load required to fracture a bone. Fracture risk depends on the loading applied to a bone in relation to the bone's failure load. W hen DXA is used to assess osteopenia and fracture risk in patients, it may be useful to recognize the separate and combined effects of applied loading , bone apparent density, and bone size.