Bone mass homeostasis and bisphosphonate action.

The evidence supporting the concept of bone mass homeostasis controlled by mechanical loads is summarized. The well-known adaptation of bone structure to mechanical loads can only be achieved if an increase in load stimulates bone formation and a decrease stimulates bone resorption. This defines the feedback system that can play a role in the coupling of bone formation to bone resorption. The two processes are not determining bone mass, but serve as means to maintain it at the homeostatic level. Imbalance produced by excess resorption, which cannot be effectively matched by increased formation, a slower process, causes bone loss. Slowing of bone resorption can facilitate the restoration of bone mass to homeostatic levels and, since bone formation is mechanically driven, the newly evolving structure would best be suited for mechanical usage and should reduce the risk of fractures.