ARCHITECTURAL MODIFICATIONS AND CELLULAR-RESPONSE DURING DISUSE-RELATED BONE LOSS IN CALCANEUS OF THE SHEEP

The results of simple biomechanical unloading in models of acute-disus e osteoporosis are influenced by systemic and regional effects of the method used to generate the bone loss. A model in which strain-gauge m easurements confirmed that the os calcis was unloaded in healthy ewes during ambulation was assessed by histomorphometry. Twelve nonovariect omized adult female Welsh mountain sheep were submitted to hock joint immobilization by an external fixation procedure from the tibia to the metatarsus for a period of 12 wk. Histomorphometric analysis showed t hat this model was able to produce pure local bone loss, as transiliac bone biopsies failed to reveal any difference between the initial and final results. Immobilized and nonimmobilized calcanei were both remo ved postmortem. After the 12 wk of the study, osteoclastic activity wa s increased in accordance with the usual disuse process. An unexpected increase of osteoblastic activity was also observed, possibly related to recovery after the initial dramatic bone loss, but an artifact of the surgical procedure such as a regional acceleration phenomenon cann ot be definitively excluded. However, the increased osteoblastic activ ity was not sufficient to prevent accentuation of the negative bone ba lance, resulting in a 29% decrease of trabecular bone volume in immobi lized calcanei compared with nonimmobilized calcanei. This reduction w as due to thinning of trabeculae (72.4 +/- 12.1 vs. 98.9 +/- 15.9 mu m ; P < 0.05) without any change in trabecular number (2.74 +/- 0.72 vs. 2.79 +/- 0.40/mm(2); not significant). In conclusion, this model only locally increased both osteoclastic and osteoblastic activities leadi ng to bone loss and architectural modifications. The decreased bone fo rmation usually observed in other models of disuse osteoporosis may th erefore not constitute a local phenomenon generated by unloading.