Deformation behaviour of bovine cancellous bone.

Repetitive cyclic loading from daily activities is reported to induce fatigue damage and microcracking in bone structures. In terms of osteoporotic structures or in cases of serious damage of skeleton segments and the replacement by metallic implants the degree of damage due to cyclic loading will be even more pronounced. It is generally assumed that fatigue induced cracking and crack propagation essentially act as driving forces for complex physiological phenomena such as remodelling processes of bones and the adaptation to applied loads. In cases where the crack propagation rate exceeds the remodelling velocity, sudden and unexpected fracture of the bone is observed. Especially for implant reinforced structures the deviation in stiffness to the bone material can induce high peak stresses and accelerate crack propagation. Whereas, for cortical bone the mechanical behaviour under cyclic loading is sufficiently described, only rough data are available for trabaecular structures. In this study the deformation behaviour of bovine vertebra trabecular bone specimens is investigated under cyclic compressive loading. A powerlaw relationship was found between the applied load ratio and cycles to failure. A linear decrease of maximum, integral strains at failure with increasing applied load ratio was observed. Optical deformation measurement of the surface strains revealed that low strains (0-1 increasing applied load ratio whereby the higher strains behave directly opposite. This indicates that different failure mechanisms are acting at low cycle and high cycle fatigue, respectively.