Bone-loading response varies with strain magnitude and cycle number

Mechanical loading stimulates bone formation and regulates bone size, shape , and strength. It is recognized that strain magnitude, strain rate, and fr equency are variables that explain bone stimulation. Early loading studies have shown that a low number (36) of cycles/day (cyc) induced maximal bone formation when strains were high (2,000 mu epsilon) (Rubin CT and Lanyon LE . J Bone Joint Surg Am 66: 397-402, 1984). This study examines whether cycl e number directly affects the bone response to loading and whether cycle nu mber for activation of formation varies with load magnitude at low frequenc y. The adult rat tibiae were loaded in four-point bending at 25 (-800 mu ep silon) or 30 N (-1,000 mu epsilon) for 0, 40, 120, or 400 cyc at 2 Hz for 3 wk. Differences in periosteal and endocortical formation were examined by histomorphometry. Loading did not stimulate bone formation at 40 cyc. Compa red with control tibiae, tibiae loaded at -800 mu epsilon showed 2.8-fold g reater periosteal bone formation rate at 400 cyc but no differences in endo cortical formation. Tibiae loaded at -1,000 mu epsilon and 120 or 400 cyc h ad 8- to 10-fold greater periosteal formation rate, 2- to 3-fold greater fo rmation surface, and 1-fold greater endocortical formation surface than con trol. As applied load or strain magnitude decreased, the number of cyc requ ired for activation of formation increased. We conclude that, at constant f requency, the number of cyc required to activate formation is dependent on strain and that, as number of cyc increases, the bone response increases.