Immobilisation induced bone loss in the sheep is not modulated by calcitonin treatment

The effect of calcitonin on immobilisation-induced loss of cortical and trabecular bone was studied in adult female sheep. The left calcaneus of 24 adult female Welsh mountain sheep was protected from normal loading by placing an external fixator across the hock joint, from the tibia to the metatarsus. In vivo strain gauge recordings from similar animals showed that this procedure resulted in a 50% reduction in principal strain magnitude in the shaft of the calcaneus during walking. All animals received intramuscular injections three times weekly. Half of the sheep received 100 i.u. of salmon calcitonin while the remainder received vehicle alone. The experiment was conducted blind. Over the 12 week period of the experiment, dual photon absorptiometry was performed at monthly intervals to measure the bone mineral content (BMC) of the calcanei. In all the animals, the loss of bone associated with the functional protection afforded by the fixator was highly significant (p < 0.0001). There was however, no significant difference in either the amount or rate of bone loss between animals which had received calcitonin and those which had not. Post mortem measurement of the cortical thickness of the shaft of the calcaneus revealed a specific pattern of loss which was also not different in the two groups. In this model, calcitonin treatment was ineffective in preventing or retarding loss of bone associated with reduced functional load-bearing over a 12 week period.