OBJECTIVEThe purpose of this study was to investigate the physiological process of healing after tooth extraction and the biological reaction of tooth movement into extraction sites with the aim of selecting optimal time of tooth movement into extraction sites clinically.

METHODSExtraction of upper first molars were performed on 30 male Sprague-Dawley rats which were divided equally into 5 groups. Orthodontic appliance was placed at different time after tooth extraction in order to move the maxillary second molars mesially into the extraction spaces. The animals were injected continuously with tetracycline and calcein for two days before appliance activation and animal sacrifice. Undecalcified mesio-distal specimans 65-100 microns of thickness were prepared. The quantification of bone remodeling parameters on tooth movement into extraction sites at different time was performed by histomorphometric measurements and computer image analysis.

RESULTS1. The bone turnover had two bone modeling processes: resorption under pressure and formation in tension. 2. The bone resorption was more active on the mesial surface (pressure side) than that on the distal surface (tension side). While bone formation was more active on the distal surface (tension side) than that on the mesial surface (pressure side). 3. Both the resorptive parameters and the formative parameters across time were manifested by a peak at day 7 after tooth extraction.

CONCLUSION1. Orthodontic retraction into extraction sites should be initiated at an early stage after tooth extraction, so that the advantage of bone remodeling in extraction sites was used. 2. The optimal time of tooth movement into extraction sites clinically was about a week after tooth extraction.