Functional dependence of cancellous bone shear properties on trabecular microstructure evaluated using time-lapsed micro-computed tomographic imaging and torsion testing.

When compressed axially, cancellous bone often fails at an oblique angle along well-defined bands, highlighting the importance of cancellous bone shear properties. Torsion testing to determine shear properties of cancellous bone has often been conducted under conditions appropriate only for axisymmetric specimens comprised of homogeneous and isotropic materials. However, most cancellous bone specimens do not meet these stringent test conditions. Therefore, we studied the application of the stepwise torsion testing system in biologic specimens with viscoelastic behavior. We explore the functional dependence of cancellous bone shear properties on trabecular microstructure and its spatial distribution, specifically the contribution of the subregion with the minimum polar moment of inertia to the overall failure properties. Torsional properties of whale trabecular specimens obtained by the incremental application of stepwise torque were not different from those obtained via continuous testing. Average polar moment of inertia accounted for 82 and 67% of the variation in shear modulus and shear stress, respectively. However, torsional properties were better predicted by the subregion with minimum polar moment of inertia, describing 87 and 74% of the variation in shear modulus and shear stress. The use of a novel torsion testing system for nonhomogeneous, orthotropic cancellous bone using stepwise application of torsion and simultaneous micro-computed tomographic imaging was further studied. Most importantly, a heterogeneous cancellous bone microstructural environment, the subregion with the minimum polar moment of inertia, hence the weakest spatial distribution of bone, predicted the shear properties for the entire bone volume.