Predictive value of proximal femoral bone densitometry in determining local orthogonal material properties

Models which are based on non-invasive bone measurements may in the future be able to successfully identify individual subjects at an increased risk for hip fracture; thus, we designed a study to determine the usefulness of dual-energy X-ray absorptiometry (DXA) and quantitative computed tomography (QCT) in predicting the local material properties of human proximal femoral cancellous bone. There has been some disagreement in the scientific literature regarding appropriate predictive models for local material properties of cancellous bone. We sought to confirm that density-mechanical property relationships were consistent from subject to subject, and that three-dimensional QCT measurements were stronger predictors of mechanical properties than two-dimensional DXA results. Linear and power fit relationships between these densitometric measures and material properties were also examined to determine which were more appropriate. Bone cubes from specific regions of highly oriented trabeculae were analyzed separately to determine if cube orientation had an effect on mechanical properties independent of bone density. Ten pairs of ex vivo femurs (five male, five female; age 30-93, mean age 62) were prepared such that specific anatomic planes were visible radiographically. Both QCT and DXA measurements were made on all 20 femurs. Cancellous bone cubes were obtained proceeding along two distinct directions from the proximal end of each femur pair. Unexpectedly, the density-modulus relationships among these ten donors were found to be significantly different at p <0.01 (83 percent of the tests were different at p <0.0001). Density-strength regressions were also significantly different at p <0.01, but this effect was not as consistent nor as statistically significant. In general, the QCT method did not produce predictions of local cancellous bone material properties superior to the DXA method. The linear and power fit models appeared to produce consistent results, with neither being obviously more advantageous. These density measurements explained at best 30-40 percent of the variance in modulus and 50-60 percent of the variance in ultimate stress. The orientation of cancellous cubes in the principal compressive trabeculae region was a significant contributor to mechanical properties (p= 0.0001) independent of bone density. This finding was not as dramatic in the femoral neck cancellous bone region.