MECHANICAL LOADING THRESHOLDS FOR LAMELLAR AND WOVEN BONE-FORMATION

Bone formation was measured in rat tibiae after 12 days of applied loa ding. Bending forces were applied using a four-point loading apparatus . Sham loads were applied at the same magnitudes as bending forces but the loading pads were arranged so that bending was minimized. Bending and sham loading were applied to the right tibiae of rats and the lef t tibiae served as contralateral controls. Loading was applied as a si ne wave with a frequency of 2 Hz for 18 s (36 cycles) per day. The pea k magnitude of applied load was 27, 33, 40, 52, and 64 N. Woven bone w as observed on the periosteal surface in all animals subjected to load s of 40 N or greater. Periosteal woven bone formation occurred in both bending and sham loading groups. Woven bone formation on the perioste al surface was either absent or responded at a maximal Fate if the sti mulus threshold was surpassed. The amount of new woven bone and the wo ven bone-forming surface were independent of the magnitude of applied strain. Bone formation on the endocortical surface was exclusively lam ellar. Lamellar bone formation was stimulated by applied bending of th e tibia but not by sham loading. Bending strains above a loading thres hold of 40 N or about 1050 mu strain increased both bone-forming surfa ce and the mineral apposition rate and subsequently increased the bone formation rate as much as sixfold. No evidence of increased bone form ation was seen for applied strains below 1050 mu strain. Examination o f bulk stained sections from animals exposed to the highest applied lo ads showed no evidence of microcracks. Thus, microcracks have not been implicated as a causal factor for the observed increases in bone form ation rate.