Torsional testing and peripheral quantitative computed tomography in rat humerus

Peripheral quantitative computed tomography (pQCT) is a noninvasive method mainly used to evaluate the densitometric and geometric properties of bone. In the present study, we evaluate the different variables provided by pQCT examination and their ability to predict the mechanical strength properties of the rat humerus. Humeri from 68 female rats were utilized. These humeri represented bone with a wide range of mechanical and densitometric properties as well as geometric dimensions. Various characteristics, such as volumetric cortical density, total mineral content, cortical thickness, total cross-sectional area, cortical area, and polar strength strain index (SSI), were measured by pQCT. The reproducibility of these measurements was good, with a coefficient of variation (CV) ranging from 0.8% to 4.9%. Bone composition (e.g., ash weight, water content, and inorganic content) and bone dimensions (e.g., length, waist, and volume) were also determined. The mechanical properties (maximum torque, torsion at failure, and stiffness) were measured by torsional testing. Stepwise multiple linear regression was performed to identify the best explanatory variables for each mechanical parameter. Total cross-sectional area and polar SSI were equally well correlated to stiffness (r = 0.57, p < 0.001), whereas ash weight was superior to the pQCT variables to explain maximum torque (r = 0.42, p < 0.001). No other independent pQCT variable entered the two models in the stepwise regression analysis. It was found to be feasible to measure properties of the rat humerus with pQCT. Cross-sectional area and the polar SSI were shown to be the best explanatory variables for stiffness, whereas ash weight was the best predictor for maximum torque.