Extending DXA beyond bone mineral density: understanding hip structure analysis.

The hip structure analysis (HSA) method was introduced to extract geometric strength information from archived hip dual-energy x-ray absorptiometry (DXA) scans acquired in large research studies. Research has shown that strength effects are not easily inferred from conventional DXA measures, so there is growing interest in the clinical community in more direct evaluation of bone strength in patients. This article reviews the factors that govern the strength of an object, how they are used in engineering simulations, and how those properties can be extracted from DXA data. It is important to recognize that that although DXA scanners can be used to measure geometric strength, they were not designed to do so. The current HSA method is fundamentally limited to evaluating bending strength in the plane of the image, so precision is sensitive to consistent femur positioning. The positioning issue and other limitations of the HSA method are discussed, as well as the critical importance of body-size scaling when interpreting bone geometry. Also discussed is how current HSA limitations could be ameliorated in a "next-generation" DXA scanner that is optimized for the purpose.