Musculoskeletal recovery following hindlimb immobilization in adult female rats

Bone loss during immobilization is well known. Common medical treatments of aged individuals that involve temporary immobilization may create an iatrogenic medical problem, a permanently increased risk of fracture. The goal of this article is to understand recovery of musculoskeletal tissues from temporary immobilization. Six-month-old female Sprague-Dawley retired breeder rats were used. A baseline group was killed; half the rest were immobilized (unilateral right hindlimb taping). After six weeks, groups of immobilized and control rats were killed and the immobilized rats were released from taping. After six more weeks, all remaining rats were killed. All rats received in vivo dual fluorochrome labeling. Lower leg muscles and tibiae and femurs were collected. Histomorphometric analysis of the proximal tibial metaphysis and cortical bone near the tibio fibula junction was completed. During immobilization, cancellous bone mass and trabecular number declined. Cortical bone mass did not change, but formation was lower and resorption was higher. Muscle weight fell. During the recovery period, bone mass rose, but did not exhibit complete recovery. The same was true for muscle weight. This model seems adequate for studying musculoskeletal recovery following immobilization. Both muscle and bone require a time period longer than the period of immobilization in order to make a complete recovery from the temporary deterioration. Bone structure deteriorates markedly during immobilization and recovers poorly. Longer studies are necessary to evaluate the ability of the adult skeleton to make a full recovery without intervention.