970 nm low-level laser affects bone metabolism in orthodontic tooth movement

Abstract Objective During orthodontic tooth movement (OTM), the speed of movement depends on the rate of bone turnover. In this study, we used a rat model to investigate the effect of 970 nm low-level laser therapy (LLLT) on OTM under different dose and frequency protocols. Methods We first compared the OTM rates between the OTM only control and the OTM + LLLT group (1250 J/cm 2 ) in Experiment 1 and showed that LLLT significantly increased OTM. In Experiment 2, we employed 3 different LLLT protocols: the low-dose group and the high-dose group receiving 5 doses of 750 J/cm 2 and 15,000 J/cm 2 of LLLT every 3 days, respectively, and the early high-dose group which received 5 daily doses at 15,000 J/cm 2 at the beginning of the experiments. The OTM-only control group received no LLLT. Tooth movement rate was measured through sequential silicone impressions. MicroCT was also performed to evaluate bone de-mineralization rate. Bone histmorphometry was used to compare the bone turnover rate between LLLT group and control group. Finally, TRAP, Osteocalcin, and VEGF expression is evaluated by immunohistochemistry (IHC) in tissue sections. Results When LLLT treatment was given every three days, both the 1250 J/cm 2 and 15,000 J/cm 2 groups showed significantly increased OTM compared to the control group. No significant difference was observed in the 750 J/cm 2 group, or in the early irradiation group, when compared with controls, although 750 J/cm 2 showed the same trend of accelerating OTM. The MicroCT result of rat maxilla demonstrated that LLLT increased bone remodeling and showed decreased bone mineral density and bone volume/total volume in the furcation areas of the maxillary first molars at the end of experiment. LLLT without OTM increased bone turnover as evidenced by fluorochrome incorporation. Immunohistochemistry analyses revealed high osteocalcin expression at later stages of OTM in the LLLT group, while VEGF expression was highly induced in the LLLT + OTM group at an early stage. Conclusion Our results suggest that the 970 nm LLLT increases the rate of OTM in a dose-sensitive and frequency-dependent manner. Further animal and human studies are needed to determine the optimal timing and dosage of LLLT for OTM acceleration.