Skeletal adaptations to mechanical usage: results from tibial loading studies in rats.

Wolff's law defines a static relationship between stress trajectories and trabecular architecture. More recent theories have attempted to describe the dynamic relationship between the form of bone and its mechanical environment. Frost's mechanostat theory is unique among these in its distinction between modeling and remodeling processes, lamellar and woven bone formation, mechanical usage windows for activation and its application to disorders of bone and mineral metabolism. Our studies suggest that lamellar and woven bone formation are very different not only in histological appearance, but in the temporal characteristics of their formation. Thus, it is important to distinguish these two histological types when interpreting studies of adaptive bone formation. Studies using the in vivo 4-point bending model in rat tibiae show that static loads do not play a role in mechanotransduction and that bone formation is threshold-driven and dependent on strain rate, amplitude and duration of loading. They have also provided strong indirect evidence that mechanical strains cause interstitial fluid flow that, in turn, activates the bone cell response. Based on these observations, we hypothesize that strain rate determines the vigor of osteoblastic activity and the regularity of loading bouts determines osteoblast recruitment in a "quantum" fashion.