Contribution of bone micromechanical behavior beyond lamellar length scale to the macroscopic bone quality of hind limb unloading rats

Long duration space flight would cause serious bone loss and bone quality deterioration. For better understanding the underlying mechanism and countermeasures, it is necessary to know the structural-mechanical relationship of the tissue. Bone is a hierarchically structured composite material with up to seven hierarchical levels of organization. At different length scales, it has distinct mechanical behaviors. In a previous study, we studied the effects of microgravity on the nano-indentation properties of bone, showing that microgravity has some effects on the nano-indentation properties of bone. However, this result obtained at nanoscopic length scale is not consistent with the previous observations that microgravity would cause deterioration in the bulk mechanical property of bone at higher structural hierarchies. To address this issue, we used micro-indentation technique in this study to examine the effects of microgravity on the mechanical property of bone at microscopic length scale. After 21 days tail suspension, the elastic modulus (E), hardness (H), and E/H ratio of rat femur were significantly decreased and such changes were different between cortical and trabecular bone. This result was similar with those of three-point bending test, but different from those of nano-indentation test. The results suggest that the decrease of macro-mechanical property under unloading-simulated microgravity (SMG) condition is most likely due to changes at microstructural level (across lamellae) instead of at sub-micron or nanoscopic length scales (within lamellae).