The effect of gamma irradiation on the anisotropy of bovine cortical bone

Bone is an anisotropic non-homogenous composite material composed of inorganic bone mineral embedded in an organic matrix. The mechanical behaviour of bone is governed by the volume fraction of these constituents, their mechanical properties, the degree of crystallite-collagen orientation and the bonding between phases. This study aims to evaluate the mechanical role of these constituents in the expression of anisotropy by using gamma irradiation to alter the mineralised collagen fibrils. Bovine cortical cubes were prepared, treated and mechanically tested in uniaxial compression in the axial, radial and tangential orientations. Ultimate stress, ultimate strain, energy to failure and stiffness were evaluated. This study confirmed deleterious effect of gamma irradiation on the axial compressive properties of cortical bone with a dose dependent decrease in ultimate stress of 6% (P = 0.231) and 16% (P = 0.001) at 15 and 25 kGy respectively. This corresponded to a 39% (P = 0.058) and 30% (P = 0.167) reduction in energy to failure. In the radial orientation there was also a dose dependant decrease in ultimate stress which was consistent with a statistically significant decline in ultimate strain (31% (P = 0.003) and 36% (P = 0.000)) and energy to failure (36% (P = 0.053) and 45% (P = 0.008)) at both doses. In the tangential orientation there was a significant 22% (P = 0.01) and 23% (P = 0.02) decrease in stiffness; though these changes did not alter ultimate stress considerably. This study provides valuable insights into the role of collagen in the radial and tangential orientation when loaded in compression; while also building on the body of work related to the use of gamma irradiation for load bearing bone allografts.