Importance of the early phase of orthodontic force application in the induction of root resorption.

OBJECTIVE: To investigate the effectiveness of early and short-term administration of lithium (Li) during orthodontic force application (OFA) in preventing orthodontically induced root resorption (OIRR) and verify the importance of the early phase of OFA in the induction of OIRR. MATERIALS AND METHODS: Bilateral maxillary first molars of 10-week-old male Wistar rats were moved for 14 days using a closed coil spring inserted between the first molar and the incisor. The rats were randomly grouped into three groups: a group receiving Li for the first 4 days, a group receiving Li daily for 14 days, and a control group receiving a vehicle (saline). Orthodontic tooth movement (OTM) was measured using microcomputed tomography on day 14. The OIRR, osteoclasts, and odontoclasts were evaluated via histological analysis. Immunohistochemical staining for the receptor-activated NF-kB ligand and osteoprotegerin was also performed. RESULTS: The OTM distance did not differ among the three groups, and the pattern of OTM changed from tipping to bodily movement for both Li groups. Early and short-term administration of Li suppressed OIRR on day 14 as effectively as long-term administration for 14 days. The observed odontoclasts on days 4 and 14 were significantly reduced in both Li groups. Osteoprotegerin expression was significantly increased on day 14 in both groups receiving Li relative to the vehicle group. CONCLUSIONS: Early and short-term Li administration effectively suppressed OIRR. This suggests that the early phase of OFA plays an important role in the induction of OIRR.