| Summary: | Timeline factors such as the rest interval between load cycles were, as well as the many factors that characterize dynamic mechanical loading to bone, reported to have significant effects on bone formation. Noticing the possible importance of timeline-dependent phenomena in mechanotransduction, we examined the effects of both the duration of constant load and the duration of rest interval in a 3-point bending model for rat tibiae. A square-wave loading was used, in which loading waveform was composed of constant load and rest periods. Newly formed bones were labeled with fluorescent dyes, and the amounts of new bone were measured using confocal laser microscopy. Loading to the medial surface of tibiae with a waveform composed of 0.5 s of constant load and 39.5 s of rest periods induced bone formation on the medial surface, whereas that composed of 39.5 s of constant load and 0.5 sec of rest periods induced bone formation on the lateral surface. Furthermore, the length of the rest period and the number of loading cycles were found to have significant effects on the amount of bone formation. Based on these results, a hypothetical mechanism on the mechanotransduction system in bone was discussed.
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