Changes induced in growing rat bone by immobilization and remobilization

Abstract

We studied changes in bone mass and histology in growing rats after different relatively short periods of immobilization and during subsequent remobilization. Immobilization-induced loss of bone weight is mainly due to mineral losses as indicated by changes in wet weight, ash weight, and calcium content. 45Ca 2+ incorporation was found to be decreased in immobilized bones and showed strong dependence upon the age of the rats. Histological examination showed rapid and extensive trabecular bone loss, and external measurements of bone length and diameter confirmed that a substantial part of the decrease in bone mass was due to actual trabecular bone loss and not the reduction of external bone volume. Two of the methods studied, cast immobilization and reversible neurectomy, allow subsequent remobilization and thus enable recovery of the bone to be studied. Bone ash weights were 12.3 ± 1.12% and 13.1 ± 1.82% below the control values in the tibia and the femur, respectively, after three weeks of cast immobilization and 12.0 ± 1.10% and 9.2 ± 0.90% below after three weeks of immobilization by reversible neurectomy. The bone mineral mass recovered by 40% ( p < 0.053) in the femur and 67% ( p < 0.027) in the tibia during the three weeks' remobilization following one week of cast immobilization, and 62% ( p < 0.001) in the tibia but only 38% ( p < 0.073) in the femur after three weeks of cast immobilization. Mobility of the extremity was restored after three weeks of immobilization by reversible neurectomy, whereupon about half of the lost bone mass was recovered in both the tibia and the femur during six weeks of reinnervation. The results indicate that the loss of the bone mass is reversible to a certain extent, and that the recovery depends on the duration of the immobilization period.