Effects of decreases in mechanical stress on disuse bone atrophy

Introduction

Osteocytes, osteoblasts, and osteoclasts are bone cells that recognize mechanical stress, but effects of decreases in mechanical stress on reduction of the bone mineral density are yet to be clarified. In this study, we examined the bone mineral density and morphology in several regions, and evaluated the effects of decreases in mechanical stress on disuse bone atrophy.

Methods

The experimental animals were 21 male 9-week-old Sprague-Dawley rats. The animals were divided into the control group (n=8), exercise group (n=6), and plaster fixation group (n=7). After one-week preliminary breeding, plaster fixation was performed during the period of 10-17 weeks of age. The animals in the exercise group were allowed to perform free running in a rotatory cage.Plaster fixation of the left hind leg was performed in the position of knee joint flexion and foot joint dorsiflexion. Under anesthesia with nembutal, fixation was performed by perfusion with Karnovsky’s solution, and the femur was excised. The bone mineral density was measured in the metaphysis and diaphysis of the proximal femur and the metaphysis of the distal femur (DXA method, QDR-2000), and osteoblasts were observed by light microscopy and scanning electron microscopy (SEM).

Results

The bone mineral density in the femur was significantly lower in the plaster fixation group than in the exercise and control groups (p<0.01). The bone mineral density in the metaphysis of the distal femur was highest in the exercise
group, followed by the control and plaster fixation groups, with the differences being significant (p<0.01). In the plaster fixation group, light microscopy revealed decreases in osteoblasts, and SEM demonstrated the irregularity of osteoblasts covering the bone surface and disarrangement of collagen fibers.

Discussion/Conclusions

Development of disuse bone atrophy is considered to be strongly affected by osteoblastic function in the ossification system and local stress responses. We found that reduction of mechanical stress decreased the activity of osteoblasts and the bone mineral density in the metaphysis of the distal femur.