Changes in bone’s micromechanical properties caused by fatigue fracture

The role of bone fatigue damage at the nanostructural level, and its effect on fatigue properties is an understudied and important subject. In this study, nanoindentation was used to probe the micro-mechanical properties of bovine tibiae subjected to fatigue loading in four-point bending. Indentation tests were conducted in the same 30 μm × 120 μm region before fatigue loading and after loading to fracture. Using an optical microscope and scanning electron microscope, the morphology of the initial residual indentation before fatigue loading appeared the same as that after loading to fracture. The mechanical properties calculated using nanoindentation were reduced modulus and hardness, the time constant based on creep, long-term creep viscosity and the dissipated work. Differences of each parameter before loading and post fracture were examined using paired t-tests. The results show that the reduced modulus decreased significantly (p = 9.47 × 10^–3) by 7.62–15.16% after fracture whereas the time constant of creep increased slightly (p = 0.049) by 2.81–5.41%. There was no clear change in hardness or dissipated work with fatigue loading. Fatigue loading has the largest effect on bone’s reduced modulus compared to all other mechanical properties.