Orthodontic bone remodeling in relation to appliance decay

This study examined alveolar bone turnover and orthodontic tooth movement after appliance decay. One group of rats (N = 54) received orthodontic force (40 g initial activation) while the other (N = 36) was sham-treated. Groups of six rats were sacrificed at various times following activation. Tooth movement and appliance decay were monitored cephalometrically, and bone turnover was monitored locally by histomorphometry and phosphatase chemistries and systemically by serum phosphatase and osteocalcin changes. A significant association was found between spring forces assessed by direct measurement and by cephalometric images (R2 = 0.784; p = 0.02). The cephalometric method indicated appliances were at least 93% deactivated by day 16. Tooth movement continued beyond the point of appliance decay (p < 0.001). This was accompanied by a dramatic decline in osteoblast surface (p < 0.0001) and an increase in osteoclast surface to control levels (p < 0.001). A significant peak in bone formation rate was also noted around appliance decay (p = 0.005). Serum acid phosphatase and osteocalcin also increased after appliance decay (p < 0.05), but alkaline phosphatase did not. Bone acid phosphatase was characterized by a peak after appliance decay (p = 0.0004), but alkaline phosphatase remained depressed (p < 0.0001). These data demonstrate that significant amounts of alveolar bone turnover continue for an indeterminant period following appliance decay.