Mechanical strain threshold for lamellar bone formation

Bone formation was measured in rats´ tibiae after 12 days of applied loading. Bending forces were applied to the right tibia through 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. Loading was applied as a sine wave with a frequency of 2 Hz for 18 seconds (36 cycles) per day. The peak magnitude of applied load varied from 27 to 64 N. Lamellar bone formation was stimulated by applied bending of the tibia, but not by sham loading. Bending strains above a strain threshold of 1050 mu strain increased both double-labeled surface and the mineral apposition rate and subsequently increased the bone formation rate as much as 6-fold. No evidence of increased bone formation was seen for applied strains below 1050 mu strain.