Effect of appliance reactivation after decay of initial activation on osteoclasts, tooth movement, and root resorption

Clinical orthodontists frequently reactivate appliances following decay. St udies of tooth movement and tissue responses following reactivations indica te that linear tooth movement and rapid recruitment of osteoclasts can be a chieved if reactivation is timed to coincide with the latter part of the bo ne remodeling cycle initiated by the first activation. Both can be delayed if reactivations are timed for the early part of the previous cycle. The ob jective of this study was to examine tooth movement, root resorption, and o steoclast recruitment following appliance reactivation after the first acti vation had decayed. Bilateral orthodontic appliances were activated with 40 cN in 144 rats to mesially tip the maxillary molars. After 16 days, rats w ere randomized into two groups of 72. In group 1, appliances were reactivat ed in precisely the same manner as the first activation In group 2, applian ces were sham-reactivated, Rats were sacrificed at 1, 3, 5, 7, 10, and 14 d ays. Orthodontic movement was measured cephalometrically; changes in osteoc lasts and root resorption were assessed at both compression and tension sit es histomorphometrically; tartrate-resistant acid phosphatase (TRAP) was me asured in alveolar bone and serum biochemically. Orthodontic tooth movement was linear in group 1, but osteoclasts required 3 to 5 days to appear. The re were no group- or time-related differences in root resorption. Bone TRAP levels were elevated in both groups but dropped significantly (p < 0.01) i n group 2 at day 7. Appliance reactivations that followed decay of the firs t activation produced efficient tooth movement without increased risk of ro ot resorption, but these changes were not accompanied by rapid osteoclast r ecruitment at compression sites. Timing appliance reactivations for the lat ter portion of the previous bone remodeling cycle could have significant cl inical advantages because the delay period seen in tooth movement following a single activation or short-term reactivation can be avoided.