Bone turnover rate in rats does not influence root resorption induced by orthodontic treatment

The aim of this study was to determine, in a rat model, whether a state of high or low bone turnover had an effect on the rate and type of tooth movement and on the incidence of root resorption induced by orthodontic treatment. The maxillary left first molar was moved mesially for 21 days in 52 6-month-old Wistar rats. They were divided into three groups: group 1 (n = 19) with normal bone turnover, group 2 (n = 16) with high bone turnover, and group 3 (n = 17) with low bone turnover. The contralateral side was left untreated to act as a control. The different metabolic rates were created by inducing hyper- and hypothyroidism. The amount of tooth movement was measured using an electronic calliper and the location of the centre of rotation (CRot) was determined after microcomputer tomographic scanning and subsequent three-dimensional reconstruction. Histomorphometric evaluation of root resorption was performed on undecalcified 7 microm thick sections of the maxilla and the differences between treated and untreated sides were evaluated. The results showed that high bone turnover increased the amount of tooth movement compared with the normal or low bone turnover state. There was no statistical difference in the location of the CRot. The treated side presented more root resorption than the untreated side, but this difference was not influenced by the metabolic rates. On the contrary, the untreated side in the low bone turnover group showed more root resorption, suggesting that in subjects where a decreased bone turnover rate is expected, the risk of root resorption could be increased.