Investigation of ultrastructural and mechanical properties of rat cortical bone using micro-CT, three-point bending testing, and the reference point indentation technique

Characterization of bone is important as it aids in the understanding of bone pathologies and their corresponding treatments. Assessment of cortical bone morphology has been identified as an important aspect of overall bone quality as it contributes significantly to the mechanical strength of bone. In the first part of this study, rat cortical bone was assessed at different hierarchical levels using micro-CT imaging, and changes within the cortical network morphology as a function of age were investigated. Decomposition of the intracortical porosity of the rat into the canal network and the osteocyte lacunar system allowed for the quantification of morphometric indices describing the size, shape, and spatial orientation of these systems. The porosity due to canals decreases with age while the porosity due to lacunae decreases slightly with age. Overall, it was found that the lacunae element-based parameters were independent of age. The results of this study serve as valuable inputs into the computational modeling of bone at different hierarchical scales and will help provide new insights into the assessment of bone quality in the future. In addition to micro-CT imaging, the mechanical properties of rat cortical bone as a function of age were assessed with three-point bending testing. Additionally, a novel microindentation technique, termed reference point indentation (RPI),that is capable of in vivo testing was used to assess the mechanical properties of rat bone. These results were compared to traditional bending test results for validation of this new RPI technique. Parameters including apparent modulus, stiffness, mean energy dissipation, and ultimate bending strength were reported. Furthermore, a parameter unique to the RPI technique, the indentation distance increase (IDI), was introduced and used to investigate the fracture properties of bone as a function of age. It was concluded that the RPI technique is capable of providing information regarding bone fracture toughness as well as other parameters, and these parameters were similar to those obtained using traditional three-point bending tests. Although these results are promising, a more thorough comparison of the RPI technique with traditional mechanical testing is warranted to provide a better understanding of the relationship between these testing methods