Later orthodontic appliance reactivation stimulates immediate appearance of osteoclasts and linear tooth movement

Background: Delays in the appearance of osteoclasts at compression sites oc cur after orthodontic appliance reactivation, when this is done during both the period of osteoclast recruitment and the peak expansion in the osteocl ast population. This experiment examines osteoclasts and tooth movement in alveolar bone after appliance reactivation coinciding with alveolar bone fo rmation and the time when reactivation osteoclasts first appear (ie, 10 day s after initial appliance activation). Methods: Bilateral orthodontic appliances were activated to mesially tip ma xillary molars with 40 cN in 144 rats. After 10 days, all rats were randomi zed into two groups of 72. Group I had appliances reactivated in precisely the same manner as the first activation. Group II had appliances sham-react ivated. Nine to 12 rats were then sacrificed at 1, 3, 5, 7, 10, and 14 days in both groups (eg, day 1 represents an interval of 11 days after the firs t appliance activation and 1 day after either sham or real reactivation). O rthodontic movement was measured cephalometrically; changes in osteoclasts and root resorption were assessed at both compression and tension sites his tomorphometrically. Results: Teeth in the reactivated group (Group I) displayed linear tooth mo vement (62.6 mu m/day), and 0.9 mm tooth movement by day 10. Significant in creases in osteoclast numbers, osteoclast surface percentage, and surface p er individual osteoclast were evident in these animals by 1 day postreactiv ation (P < .01). Significant treatment-related increases in root resorption were not evident at compression sites at any time. Conclusions: These findings indicate that, after appliance reactivation dur ing the time when reactivation osteoclasts appear, a second cohort of osteo clasts can be recruited immediately, along with immediate and substantial t ooth movement and no greater risk of root resorption.