The effects of immobilization on bone histomorphometry in rats

Abstract

To determine whether immobilization acts directly on bone by alteration of mechanical loading or systemically, studies of the effects of immobilization were carried out on histomorphometry of diaphyses of tibiae and on subcutaneous implants of demineralized allogenic bone matrix of rats. The right hind leg of growing rats was denervated by severing the tibial nerve. A sham operation on the right hind leg was performed in control animals. Bone formation at the endosteal and periosteal surfaces was significantly lower in tibiae from limbs with severed nerves as compared to tibiae from the intact limbs of nerve-sectioned rats and from both limbs of sham-operated control rats. Bone formation was decreased at both 3 and 7 weeks after immobilization. The decreased formation resulted in significant reductions in cross-sectional area. At 3 weeks post denervation, the periosteal bone formation rate was lower in tibiae of intact limbs from denervated rats as compared to tibiae from intact limbs of sham-operated animals. This finding was attributed to reduced physical activity of the denervated rats. In the implants, nerve section did not alter the amount of implant matrix resorbed, the amount of bone matrix synthesized, or the amount of calcium in the implant. These findings support the hypothesis that inhibition of bone formation at the tibial diaphysis in response to immobilization resulted from altered mechanical loading and not from the production of substances acting systemically. Whereas the mean medullary area of tibiae was not altered by nerve section, it was decreased in tibiae of all groups compared to the values of basal controls, indicating that bone formation was greater than bone resorption. The findings are interpreted to mean that, in the rat, reducing the mechanical loading of an extremity inhibits bone formation at the tibial diaphysis of the affected limb and does not influence bone resorption at that site.