Effect of orthodontic appliance reactivation during the period of peak expansion in the osteoclast population

Background: Delays in the appearance of osteoclasts at compression sites occur following orthodontic appliance reactivation when this is done during the period of osteoclast recruitment. This study examined changes in alveolar bone after appliance reactivation at a time coinciding with the peak expansion of the osteoclast population following the first appliance activation. Methods: Orthodontic appliances were activated with 40 g on maxillary molars followed by a reactivation with the same force after 4 days in one group and sham reactivation in the other. Rats were killed at 0, 1, 3, 6, and 10 days thereafter. Orthodontic movement was measured cephalometrically. TRAP and interleukin-1 alpha (IL-1 alpha) were measured biochemically, and changes in osteoclasts and root resorption were assessed at both compression and tension sites histomorphometrically. Results: Teeth in the reactivated group displayed more initial displacement than controls but no more tooth movement 10 days following appliance reactivation. Also, increases in osteoclast numbers and surface percent, as well as alveolar bone Tartrate-resistant acid phosphatase (TRAP), became evident in the treated animals only 10 days after reactivation. However, IL-la was elevated in alveolar bone within 1 hr following appliance reactivation but returned to baseline by day 1. There were no treatment-related difference in nuclear number per osteoclast or trabecular surface per osteoclast. Significant treatment-related increases in root resorption were evident at compression sites by day 10. Conclusions: These findings indicate that after appliance reactivation during the height of osteoclastic stimulation, a second cohort of osteoclasts can be recruited, but only after a delay of several days. This delay is not due to a failure to produce IL-1 alpha in the tissues. (C) 1998 Wiley-Liss, Inc.