Microscopic alterations in the physis of long bones in response to hind limb immobilization in the rat

The growth plate increases its activity in response to exercise. Likewise, decreased physical activity exerts a negative effect on bone growth and development, leading to rarefaction of the subepiphyseal bone. Limb immobilization inhibits the growth plate's activity, indirectly shown by a recorded arrest in longitudinal growth of the long bones. However, there is no direct evidence concerning the growth plate itself.

To determine whether the growth plate exhibits measurable microstructural changes in response to decreased levels of physical activity.

Histomorphometric analysis was used to qualitatively and quantitatively assess the changes in the epiphyseal plate in response to single hind limb immobilization in the rat. In 16 of 25 Sprague-Dawley male rats the left hind limb was immobilized for 3 weeks; the remaining 9 rats served as controls. The left proximal tibia of each animal was examined by computerized image analysis.

There was a decrease in epiphyseal height, cell column density and subepiphyseal trabecular area--all indices of growth plate activity. Metaphyseal cortical thickness was also depressed, thereby confirming the efficacy of the immobilization method applied.

Limb immobilization in the rat induces inhibitory histological changes in the epiphyseal growth plate, which are in contrast to the excitatory microscopic changes seen with exercise. These changes can be assessed quantitatively. Their potential for reversibility remains to be determined by future experiments.