Model of hindlimb unloading in adult female rats: Characterizing bone physicochemical, microstructural, and biomechanical properties

Prolonged bedrest and microgravity induce alterations to bone, leading to bone fragility and compromising the quality of life. In this study, we characterized the physicochemical changes, microstructure, and biomechanics of the femurs of female adult rats in response to hindlimb unloading for 21 days. Twenty 6-month-old Wistar female rats were distributed into control (CON) and hindlimb unloading (HLU) groups. Analysis the in vivo bone mineral density (BMD) by dual energy x-ray absorptiometry (DXA) from the femurs was performed at the beginning and end of the experiment; plasma levels of calcium, phosphorus, and alkaline phosphatase, tartrate-resistant acid phosphatase activity, assessed by spectrophotometry, and estradiol, measured by enzyme-linked immunosorbent assay, was performed after the experiment. We evaluated changes in the trabecular and cortical structure of the femur, after disuse, by micro-computed tomography with high resolution, for analysis of cortical porosity, Raman spectroscopy to measure the amount of physicochemical properties, and the biomechanical test to estimate the changes in biomechanical properties. Our results demonstrated that, after 21 days, HLU animals had decreased femoral BMD, deteriorated bone microarchitecture, particularly in the cortical compartment, with changes in the physicochemical properties and porosity, and reduced deformation capacity of the bone and resistance to the bone stresses. Nevertheless, this study showed the critical role of mechanical stimulation in maintaining the structure of the skeleton in female adults and that disuse, even for a few days, leads to microscopic changes in the structure of the bone matrix, which increases the risk of fracture.