Estimation of trochanteric soft tissue thickness from dual-energy X-ray absorptiometry

Low body mass index is considered to be an important risk factor for fractures in postmenopausal women, in part reflecting its association with lower bone mineral density (BMD). In contrast, obesity is thought to be protective against fracture because of higher BMD and reduced transmitted force of falls as a result of soft tissue padding. Dual-energy X-ray absorptiometry (DXA) is most widely used for the assessment of osteoporosis. We hypothesized that trochanteric soft tissue thickness, as measured directly on whole-body DXA scans, could be estimated using the regional measures of anteroposterior tissue thickness from hip and spine DXA. We identified 376 adult patients who underwent DXA evaluation of the lumbar spine, hip, and whole body at the same visit. The population was randomly divided into 2 equal-sized subgroups, one used to derive prediction equations for trochanteric soft tissue thickness and the other for independent validation. Compared with the actual measurement from the whole-body scans in the validation cohort, the DXA-based estimate gave an unbiased prediction of trochanteric soft tissue thickness (adjusted R² was 0.60 with a standard error of the estimate of 1.35cm), which was significantly better than estimation obtained without DXA information (p < 0.001). Area under the curve discrimination for trochanteric soft tissue thickness in the lowest and highest tertiles was 0.901 (95% confidence interval: 0.849-0.953) and 0.859 (95% confidence interval: 0.805-0.915), respectively. Model-based prediction of trochanteric soft tissue thickness in the lowest and highest tertiles had sensitivities of 78.5% and 91.9% and specificities of 64.4% and 86.8%, respectively. We conclude that regional DXA scans of the spine and hip can be used to estimate the trochanteric soft tissue thickness.