Mechanical characterization in shear of human femoral cancellous bone: torsion and shear tests

In order to investigate and compare the mechanical behaviour of human cance llous bone during different shear loading modes, two tests were performed t o characterise human femoral cancellous bone in sheer: a torsion test until failure and a shear test using a sharpened stainless steel tube. Paired cy lindrical samples were core drilled from 12 human femoral heads, symmetrica lly with respect to the coronal plane and along the primary trabecular dire ction. The distal part of the sample was assigned to a torsion test and the shear test was performed on the proximal part along two perpendicular anat omical directions. Apparent densities and tissue densities were measured on both torsion and shear specimens. The mean torsion properties were shear m odulus G, 289 (183) MPa, ultimate stress tau(torsion), 6.1 (2.7) MPa, ultim ate strain gamma(ultimate), 4.6 (1.3)%, yield stress tau(yield), 4.3 (1.9) MPa and yield strain gamma(yield), 1.8 (0.3)%. Strong correlation was obtai ned between G and tau(torsion) (r'=0.853, p<0.001). These torsion propertie s were correlated with apparent density of torsion specimens showing, respe ctively: r'=0.713, p=0.005 and r'=0.671, p=0.008. Properties from the shear test were invariable with regard to the two tested directions then isotrop ic ultimate shear stress and isotropic elementary shear stress, which repre sent the mean values of the two tested directions were, respectively, tau(s hear), 10.0 (4.5) MPa and tau(elem), 18.8 (6.1) MPa. Both shear stresses we re correlated with apparent density of shear specimens: tau(shear), r'=0.56 4, p=0.045 and tau(elem), r'=0.636, p=0.024. Apparent densities for shear s pecimens were superior than for torsion specimens (p=0.06) and the comparis on was the opposite for tissue densities (p=0.028), showing strong density gradients of cancellous bone in the femoral head. These torsion and shear t ests which permit the evaluation of cancellous bone behavior under two diff erent types of shear loading, may be performed on different human sites and the measured shear properties may be compared to structural properties of cancellous bone. (C) 2000 IPEM. Published by Elsevier Science Ltd. All righ ts reserved.