Initial cell kinetics (15 min-20 h) of mechanically induced osteogenesis was studied with 3H-thymidine (3H-Tdr) autoradiography. Continuous orthodontic force elicited a three-stage cell proliferative reaction within rat molar periodontal ligament (PDL): (a) brief, generalized response, characterized by a burst of mitotic activity from 75 min to 2 h and a cyclic change (decrease, increase and return to control levels) in percent labeled cells from 15 to 2 h; (b) transient, generalized response, involving an increased mitotic index (MI) from 2 to 13 h, associated with elevated labeling index (LI) from 6 to 12 h; and (c) sustained, localized osteogenic response (12 h to several days) in which cell proliferation was confined primarily to the immediate area of new bone formation. These results suggest that the initial effect of mechanical perturbation on relatively quiescent adult bone progenitor cells is a generalized, transient release of G1 and G2 blocked cells. This response may correlate clinically with the regional acceleration phenomenon (RAP), which follows bone trauma, fracture, or surgical injury.

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