Experimental Research on the Nanoindentation of Bovine Compact Bone under Fatigue Loading

Abstract Background The role of bone fatigue damage at the nanostructural level, and its effect on fatigue properties is an understudied and important subject. An understanding of the subtler aspects of bone’s properties is crucial to understanding bone fragility caused by aging, disease, and fatigue damage. The primary strength of this study was the demonstration of the degradation of bone’s microscopic mechanical properties by nanoindentation following fatigue loading. Methods 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 30×120 µm region before fatigue loading and after loading to fracture. We analysed the effects of residual indentations on bone’s fatigue resistance using an optical microscope and scanning electron microscope. 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. Results The morphology of the initial residual indentation before fatigue loading appeared the same as that after loading to fracture. 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%. The p-value was significantly lower (p < 0.001) for reduced modulus compared to all other mechanical properties. Conclusion Residual indents different from fatigue damage created no crack initiators that would weaken bone’s resistance to fatigue loading. Fatigue loading has the most heavy effect on bone’s reduced modulus compared to all other microscopic mechanical properties.