Time course for bone formation with long-term external mechanical loading

Increased mechanical loading of bone with the rat tibia four-point bending device stimulates bone formation on periosteal and endocortical surfaces. W ith long-term loading cell activity diminishes, and it has been reported th at early gains in bone size may reverse. This study examined the time cours e for bone cellular and structural response after 6, 12, and 18 wk of loadi ng at 1,200-1,700 microstrain (mu epsilon). Bone formation rates, measured by histomorphometry, were compared within groups, between loaded and contra lateral nonloaded tibiae, and between weeks. Formation surface, mineral app osition rate, and bone formation rate on periosteal and endocortical surfac es were elevated after 6 wk of loading. By 12 wk of loading, periosteal and endocortical formation surface and endocortical mineral apposition rates w ere elevated. By 18 wk of loading, periosteal adaptation appeared complete, whereas endocortical mineral apposition rate remained elevated. No periost eal resorption was observed. Average thickness of new bone formed, from bas eline to collection, was greater in loaded than nonloaded tibiae by week 6 and was maintained through week 18. Early increases in bone formation resul t in periosteal apposition of new bone that persists after formation ceases .