HSA: Beyond BMD with DXA

The measurement of bone mineral density is a surrogate for the measurement of bone strength. Bone strength is comprised of many components including, but not limited to bone architecture, geometry, cortical porosity and tissue mineralization density. A new application for dual energy X-ray absorptiometry (DXA), called hip structural or hip strength analysis (HSA), allows the measurement of geometric contributions to bone strength in the proximal femur. With this approach, the cross-sectional area, section modulus and buckling ratio can be quantified. These parameters are measures of strength in axial compression or bending. The limitations of HSA with DXA are primarily those associated with the two-dimensional nature of DXA. Because of the two-dimensional nature of DXA, assumptions must be made regarding the symmetry of the bone cross-sections used in the HSA regions of interest. In one proprietary approach to HSA, an index, called the Femur Strength Index, has been created in an attempt to relate the force of a fall on the greater trochanter to the strength of the proximal femur. Studies using HSA with DXA have demonstrated discordant behaviors between the bone mineral density (BMD) and the section modulus. The geometric parameters are predictive of fracture risk although they do not seem to be better predictors of risk than a conventional measurement of BMD. Various bone active agents have been shown to have desirable effects on these geometric parameters. Direct measurement of these components of bone strength may result in improved fracture risk prediction or therapeutic monitoring. Minimally, a better understanding of the changes in these components of bone strength in disease and during therapy may result from HSA.