The impact of immobilization and strength exercise strategy to bone change: An animal study

Abstract

Objective: The aim of this study is to evaluate whether an immobilization model can be used in studies of the mechanism of structural adaptation to mechanical usage. Methods: Six-week-oldmale rats were subjected to right hindlimb immobilization or served as control for 0, 4, 6, 8, 12 or 24 weeks. They were double-labeled with bone markers prior to sacrifice. The right hindlimb was immobilized by sciatic nerve denervation and considered unloaded, while the left hindlimb was overloaded during ambulation, such as strengthening exercise. Static and dynamic histormorphometric studies were performed on 20-μm-thick Villanueva stained, undercalcified sections of the proximal tibia metaphysis and the tibia shaft. Results: In the unloaded limb, immobilization induced cancellous and cortical bone loss. The bone loss was accompanied by poorer trabecular architecture. Unloading shut off nearly both peri-osteal bone formation and accelerated bone marrow expansion. In the overloaded limb, significant changes occurred in proximal tibial metaphyses: trabecular area increased, trabecular number increased, and trabecular separation decreased. A significant increase in mineral apposition rate was found only at 4 weeks of overloading. Significant decrease in both eroded and labeled bone surfaces occurred at all time periods. These histomorphometric changes indicated that increased cancellous bone mass was caused by an increase in bone formation activity (i.e., increase in mineral apposition rate) and a decrease in remodeling space (i.e., decrease in bone eroded surface). Although cortical bone mass was unchanged, increased peri-osteal bone formation activity (i.e., increases in mineral apposition and bone formation rates) was detectable. Conclusions: These findings indicate that the rat skeleton can quickly adapt to mechanical usage. Therefore, the one-legged immobilization model can be used in studies of the mechanism of structural adaptation to mechanical usage.