The study on the mechanical characteristics of articular cartilage in simulated microgravity

The microgravity environment of a long-term space flight may induce acute changes in an astronaut＇s musculo-skeletal systems. This study explores the effects of simulated microgravity on the mechanical characteristics of articular cartilage. Six rats underwent tail suspension for 14 days and six additional rats were kept under normal earth gravity as controls. Swelling strains were measured using high-frequency ultrasound in all cartilage samples subject to osmotic loading. Site-specific swelling strain data were used in a triphasic theoretical model of cartilage swelling to determine the uniaxial modulus of the cartilage solid matrix. No severe surface irregularities were found in the cartilage samples obtained from the control or tail-suspended groups. For the tail-suspended group, the thickness of the cartilage at a specified site, as determined by ultrasound echo, showed a minor decrease. The uniaxial modulus of articular cartilage at the specified site decreased significantly, from （6.31 ± 3.37） MPa to （5.05 ± 2.98）MPa （p 〈 0.05）. The histology- stained image of a cartilage sample also showed a reduced number of chondrocytes and decreased degree of matrix staining. These results demonstrated that the 14 d simulated microgravity induced significant effects on the mechanical characteristics of articular cartilage. This study is the first attempt to explore the effects of simulated microgravity on the mechanical characteristics of articular cartilage using an osmotic loading method and a triphasic model. The conclusions may provide reference information for manned space flights and a better understanding of the effects of microgravity on the skeletal system.

Changes of cortical bone pores structure and their effects on mechanical properties in tail-suspended rats

Cortical bone is the main mechanical bearing structure of bone,and the mechanical properties of materials are not only related to bone mineral density,but also largely depend on its pores microstructure which affected by blood vessels.However,the change of pores structure in cortical bone under microgravity was still unclear.In this study,in order to clear the changes of pore structure with cortical vascular pores and its effect on bone mechanical properties,rat tail-suspension was used to simulate microgravity and the changes of the microstructure in rat tibia cortices were investigated by high-resolution micro-CT(3μm)while the bone mechanical properties were measured via three point bending test.The results showed the bone mineral density of cortical bone didn't change in tail-suspended rats.However,the pore structure of cortical bone in tail-suspended rats changed significantly,the proportion of pores greater than 15μm(cortical vascular pores)increased while that less than 15μm decreased.The mechanical properties of bone(such as maximum load and maximum stress)in tail-suspended rats deteriorated.And the volume ratio of pore vessels(vessel volume/tissue volume)was negatively correlated with the mechanical properties.In conclusion,the increase of cortical vascular pores in rats caused by the simulated microgravity contributes to the decrease of mechanical properties.