OBJECTIVETo investigate the influence of bone remodeling in extraction sites and orthodontic forces on tooth movement with the aim of providing a basis for selecting optimal orthodontic forces, time of tooth movement and reducing the time for tooth moving into extraction sites.

METHODSExtraction of upper first molars were performed on 36 male Sprague-Dawley rats which were divided equally into 3 groups. A method for quantification of orthodontic tooth movement in the rats was presented. Orthodontic appliance was placed at different time after tooth extraction. Different forces were used to move the maxillary second molars mesially into the extraction spaces. X-ray was taken before appliance activation and after 1, 3, 5, 7, 10, 14 days since appliance activation. Tooth movement was measured cephalometrically by Imagine Analysis Technique, adjusting for magnification by using the known digitized length of the broach.

RESULTS1. The tooth on the recent extraction side moved faster than that on the healed side. 2. Tooth movement at all time points on the 0.30 N curve differed from those on the higher force curves (P < 0.01), either moving into recent extraction sites or healing sites. Comparison between 0.60 N and 1.36 N indicated that mesial molar movement did not differ from each other after day 5. 3. The classical tooth movement curve had three parts that represent distinctly different processes: early movement; delay; later movement.

CONCLUSION1. The tooth on the recent extraction side moved faster than that on the healed side. 2. Moderate force maybe was the optimal orthodontic force. It could be overloaded, but resulted in no further enhancement of tooth movement.