ALVEOLAR BONE TURNOVER AND TOOTH MOVEMENT IN MALE-RATS AFTER REMOVAL OF ORTHODONTIC APPLIANCES

The purpose of this study was to acquire tooth movement, histomorphome tric and biochemical data on oral tissues that had previously been loa ded with calibrated orthodontic forces. One hundred and forty-four mal e Sprague-Dawley rats were randomly divided into two groups: Group I, orthodontic appliances placed for 16 days to mesially move maxillary f irst molars with an initial force of 40 gm, and group II, sham orthodo ntic treatment. Seven to twelve rats were killed at each of six times after removal of appliance. Tooth movement was measured cephalometrica lly, alveolar bone turnover by histomorphometry, and tissue phosphatas e levels biochemically. Treated molars moved distally more rapidly tha n the shams (13.9 vs 5.0 mu m/day). The appliance removal group had a persistent 10-fold elevation in root resorption on the mesial (p < 0.0 001), as well as early elevations in osteoclasts on the mesial and ost eoblasts on the distal (p < 0.001) that returned to control by 3 to 5 days. Acid, alkaline phosphatase, and tartrate-resistant acid phosphat ase (TRAP) remained elevated in the tissues until 10 days (p < 0.0001) . Changes in the dynamic measures of bone formation were characterized by low rates at days 1 and 3 (p < 0.01), elevating thereafter on the mesial and the converse on the distal. Orthodontic tooth movement rela pses, and bone remodeling continues for several days after removal of appliance consistent with the direction of loading, orthodontic treatm ent stimulates root resorption at sites that were loaded in pressure w ithout detectable recovery, and root resorption does not increase at t he tension sites.