Biomedical response of femurs in male Wistar rat in chronic hypergravity environments

Abstract

Bone is sensitive to mechanical stimulation and plays a loading-bearing role in the human body. However, regulation of bone biomechanical properties in chronic hypergravity environments is still unclear. In this study, male Wistar rats exposed to chronic hypergravity environments (4×g, 8×g, 10×g, and 20×g) for 4 weeks were set as the hypergravity groups, and rats exposed to the normal gravity as the control group. Morphology parameters and bone remodeling factors were obtained by means of micro-CT, Western blot, and q-PCR. Mechanical properties of femurs were measured utilizing three points bending test and creep test and were fitted into a viscoelastic-viscoplastic constitutive equation. The results indicate osteoporosis occurred in femurs of hypergravity groups. Accordingly, the protein and gene expressions of bone remodeling factors (OPG, RANKL, runx2) in hypergravity groups were significantly different from that in the control group, demonstrating that bone formation level increased and bone resorption level decreased. Meanwhile, mechanical properties of femurs in hypergravity groups showed that Young's modulus of femurs in the 20×g group was significantly higher than that in the control group. The viscoelastic-viscoplastic properties of bone tissue were changed in hypergravity environments. Among them, the 8×g group was closest to the control group in morphology and mechanical properties. To sum up, the biomechanical response regulation of rat femur under 4-20×g chronic hypergravity environments was presented. Hypergravity environments could lead to osteoporosis. The balance between bone formation and bone resorption would be disrupted in hypergravity groups due to bone adaptation. 20×g environment has a significant effect on elastic modulus on femurs. Due to the difference in biomechanical response of femurs, the viscoelastic-viscoplastic characteristics of femurs have a nonlinear relationship with hypergravity values. Bone tissue was least affected by 8×g hypergravity in morphology and mechanical properties.