INCREASED BONE-FORMATION IN RAT TIBIAE AFTER A SINGLE SHORT-PERIOD OFDYNAMIC LOADING IN-VIVO

Based on our quantum concept for mechanically adaptive bone formation, we hypothesized that a single bout of loading would increase bone for mation at the endosteal surface in rat tibiae, with a maximal response 4-8 days after loading and a stimulus-response relationship for load magnitude. Bending loads were applied to right tibiae of rats at 31, 4 3, 53, or 65 N for a single bout of 36 or 360 cycles; bone formation w as assessed 1-4, 5-8, or 9-12 days after loading. A single loading epi sode increased lamellar bone formation rate (BFR) in all groups (P < 0 .05) and was maximal 5-8 days after loading. A distinct dose-response relationship was not evident among all load magnitudes or for duration , but 65 N was significantly more osteogenic than loads of 31-53 N (P < 0.05), consistent with a threshold response to loading. There was al so evidence for a significant increase in BFR (P < 0.05) and double-la beled surface (P < 0.01) within 4 days of loading, suggesting that bon e-lining cells were activated directly by the stimulus. Thus subtle ch anges in BFR may occur by modulating the activity of surface cells, bu t large modeling drifts and anabolic responses require recruitment and differentiation of osteoprogenitor cells near the bone surface.