Structural changes in rat bone subjected to long-term, in vivo mechanical loading.

Woven bone formation is commonly observed when grossly altered loading conditions are imposed upon living bone tissue. The fate of this woven bone with time has not been fully characterized. In this study, rats underwent daily bending of the right tibia for a period of 3 to 14 weeks. New bone was formed in the region of maximum bending stresses on the right tibiae of all rats that underwent daily loading. The new bone was at first poorly mineralized with disorganized collagen structure. With time, the new bone consolidated into a well mineralized primary bone structure similar in appearance to pre-existing nonlamellar bone within the tibial cortex. Using the data from this study and previous studies, we were able to outline the sequence of events that occur during bone adaptation in the rat tibia loading model. Explosive new woven bone formation began to occur five days after the initial four-point bending session, and the amount of woven bone reached a peak after about 15 days. After the third week the new bone began to consolidate. Rapid mineralization occurred during the third and fourth weeks, with less rapid mineralization occurring for several weeks thereafter. After the 14 weeks, the new bone was fully mineralized, and new bone formation had stopped.