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 failu re strains, are not well understood despite their potential importance in age-related fractures and prosthesis loosening. We hypothesized th at shear failure strains (yield and ultimate) are independent of appar ent density and trabecular orientation, i.e. are homogeneous and isotr opic. We measured the shear failure properties of bovine tibial trabec ular bone, where specimens were loaded to failure in torsion longitudi nally (n = 25) or transversely (n = 23) relative to the primary trabec ular orientation. We found that although failure stresses depended str ongly on apparent density (r(2) = 0.61-0.80), failure strains were ind ependent of apparent density for both trabecular orientations. Althoug h 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 (longit udinal group 4.60 +/- 0.77% vs transverse group 4.24 +/- 1.25%, p = 0. 20). These findings indicate that shear failure strains are homogeneou s and largely isotropic. By combining our shear data with compressive data from a previous experiment, we also predicted that trabecular bon e can fail in shear when subjected to compressive loads that are nor a ligned with the principal trabecular orientation. If this prediction h olds for human bone, shear may be a dominant failure mode during off-a xis loading of trabecular bone in vivo, such as during falls on the hi p. Copyright (C) 1996 Elsevier Science Ltd.