 At the trabecular sites of the radius and tibia, total volumetric bone mineral density vBMD, mgcm3 and total cross-sectional area CSA, mm2 will be measured to calculate bone strength index BSI; mg2mm4, Equation 1, an estimate of bones ability to withstand compressional strains. At the cortical bone sites of the radius and tibia, cortical vBMD mgcm3, cortical CSA mm2, and cortical thickness mm will be measured to calculate the strength-strain index SSI, mm3, which reflects torsional bone strength. The SSI is calculated as the integrated product of the section modulus and the density of cortical bone Equation 2. Section modulus is calculated as a x d2dmax, where a is the CSA of a voxel, d is the distance of the voxel from the center of gravity, and dmax is the maximum distance of one voxel to the center of gravity. The ratio of cortical vBMD and normal physiological density ND 1200 mgcm3 provides an estimate of the modulus of elasticity. 