Objective：To assess the effects of immobilization on the three-dimensional microstructure of cortical and cancellous bone.

Methods：Eight-week-old specific-pathogen-free Wistar rats were divided into two groups：the control (n＝12) and immobilized group (n＝12). The hind limbs of the rats in the immobilized group were fixed using orthopedic casts for 4 weeks. The cortical bone at the mid-shaft and the cancellous bone at the distal metaphysis of the femur were analyzed using micro-computed tomography.

Results：The values of total cross-sectional area, cortical bone area, and cortical thickness in the immobilized group were significantly lower than those in the control group. Meanwhile, the relative bone volume (bone volume/total volume) and mean trabecular thickness in the immobilized group decreased and the structure model index significantly increased compared with the values in the control group (P＜0.05).

Conclusion：Our rat model can evaluate the effects of load-permitting immobilization. The results of this study indicate that structural changes in immobilized osteopenia-affected bone arise mainly from thinning of the cortex and reduction of cancellous bone volume caused by a reduction of trabecular width.

Objective：This study aimed to evaluate the mechanical strength of immobilized osteopenia using a fixed limb rat model.Methods：Eight-week-old specific-pathogen-free male Wistar rats were divided into two groups, a control group (n＝32) and an immobilized group (n＝32). The hind limbs of the immobilized group were fixed using an orthopedic cast, and the fixation periods were set for 1, 4, 8, and 12 weeks. Feeding and weight-bearing were permitted. After each fixation period, the length of the right femoral bone was measured, and three-point bending at the midshaft and uniaxial compression test at the distal metaphysis were performed with a universal material testing apparatus. The maximum force and breaking force in the bending test and ultimate load in the compression test were statistically analyzed.Results：The values of the maximum force and breaking force in the immobilized group were significantly lower than those in the control group at 4, 8, and 12 weeks (p＜0.05). The ultimate load of the distal metaphysis in the immobilized group decreased from 1 week after fixation, and the gap with the control group widened as the fixation period extended.Conclusion：The results of this study indicate that bone weakness caused by immobilized osteopenia arises after 1 week in the metaphysis and after 4 weeks in the midshaft.