The dependence of shear failure properties of trabecular bone on apparent density and trabecular orientation.

The shear properties of trabecular bone, in particular the shear failure strains, are not well understood despite their potential importance in age-related fractures and prosthesis loosening. We hypothesized that shear failure strains (yield and ultimate) are independent of apparent density and trabecular orientation, i.e. are homogeneous and isotropic. We measured the shear failure properties of bovine tibial trabecular bone, where specimens were loaded to failure in torsion longitudinally (n = 25) or transversely (n = 23) relative to the primary trabecular orientation. We found that although failure stresses depended strongly on apparent density (r2 = 0.61 - 0.80), failure strains were independent of apparent density for both trabecular orientations. Although the mean (+/-S.D.) yield strain in the longitudinal group (1.46 +/- 0.19%) was 10% higher (p = 0.01) than in the transverse group (1.33 +/- 0.15%), indicating a slight anisotropy of shear yield strains, the mean ultimate strains did not depend on trabecular orientation (longitudinal group 4.60 +/- 0.77% vs transverse group 4.24 +/- 1.25%, p = 0.20). These findings indicate that shear failure strains are homogeneous and largely isotropic. By combining our shear data with compressive data from a previous experiment, we also predicted that trabecular bone can fail in shear when subjected to compressive loads that are not aligned with the principal trabecular orientation. If this prediction holds for human bone, shear may be a dominant failure mode during off-axis loading of trabecular bone in vivo, such as during falls on the hip.