| The microscopic changes in cortical bone may be partially responsible for the increased prevalence of hip fractures in the elderly, which are serious public concern. The influence of these microstructural changes of cortical bone on its mechanical properties was examined by a combination of experimental and theoretical methods. First, mechanical tests and histology analysis were performed on specimens from middle diaphysis of human femurs to determine elastic properties of cortical bone as a transversely isotropic material and their relationship with Haversian porosity and resorptive porosity. Significant correlations were found between longitudinal Young's modulus or longitudinal shear modulus and porosity of cortical bone. In addition, the elastic moduli in the longitudinal direction were more sensitive to the changes of porosity than those in the transverse direction. Next, a two-level composite model of cortical bone was developed to predict effective elastic constants of cortical bone using a generalized self-consistent method. In the first level, a single osteon was modeled by considering Haversian canal as a single inclusion and osteon tissue as matrix. In the second level, osteons and resorptive cavities were considered as multiple inclusions while interstitial bone tissue was regarded as matrix. The elastic properties of microstructural components were modeled as transversely isotropy and were taken from the existing literature on nano-indentation measurements of bone tissue. The predictions of Young's modulus and shear modulus were in good agreement with results from mechanical tests of bone specimens.; For the first time, the debonding strength of cement lines in human cortical bone was determined by performing osteon pushout tests. Twenty specimens were tested under the condition of a small hole in the supporting plate, in which the cement line debonding occurred. The cement line debonding strength had an average of 7.96 ± 1.99 MPa. In addition, ten specimens were tested under the condition of a large hole in the supporting plate, in which the shear failure inside osteons was observed. The average shear strength of cortical bone was 73.71 ± 15.06 MPa, which was significantly higher than the debonding strength of cement lines. Therefore, our results suggest that the cement line is indeed a weak interface. |
