# Clinical Applications of Low-Intensity Pulsed Ultrasound and Its Underlying Mechanisms in Dentistry
## 1 Introduction
Interestingly, applying LIPUS, LLLT, LIPUS, and LLLT in mice respectively, the bone remodeling effect was conspicuous in the LIPUS and LLLT group while lowest in the LIPUS group [57]. This outcome suggests that the combination of LIPUS and LLLT is more effective in facilitating OTM. OITRR usually occurs in orthodontic treatment as a side effect [62]. A study showed that almost all patients and 55%-91% teeth occurred root resorption during orthodontic treatment [63]. However, OITRR has few clinical symptoms and a clinically available approach for treating OITRR has not been discovered. So as OITRR is examined, it's often severe [64]. Under most circumstances, only the surface layers of cementum are assimilated and then repaired by cementoblasts during OTM. Once the repair ability of the cementum was suppressed, apical root resorption becomes worse [65]. LIPUS was proven to impose restrictions on OITRR. Applying LIPUS 20 min/day in beagle dogs that received OTM, cementum in premolars near the apex became thick, indicating the potential of LIPUS in reducing OITRR [66]. In beagle dogs, the LIPUS inhibiting effect of resorption in the middle and the apical thirds of the root was prominent [67]. Similarly, in rats, the number of osteoclasts, resorption lacunae, and resorption area ratio in the group with LIPUS stimulation were all lower than in the group only with orthodontic appliances [68]. Concerning the mechanism of LIPUS inhibiting OTIRR, one of the most acknowledged viewpoints is that OPG and RANKL play an important role. RANKL can combine with its receptor RANK on the surface of osteoclast precursors and then promoted the growths and differentiations of osteoclasts [69]. OPG is an inhibitor of RANKL which can reduce the absorption of alveolar bone and promote the apoptosis of osteoclast [70]. However, studies discovered that LIPUS contributed to bone repair and remodeling through increasing the proportion of OPG/RANKL (Figure 5). The increased OPG/RANKL enhanced the ability to inhibit osteoclasts and then reduce the destruction of cement, thereby suppressing OTIRR [62; 68]. In addition to LIPUS, photobiomodulation (PBM) also has an effect on OTIRR. Actually, a control experiment manifested that the function of LIPUS and PBM in OTIRR had no significant difference [68]. However, at present, research on the combination of LIPUS and PBM in OITRR is limited. More control experiments are needed to explore whether the cooperation of LIPUS and PBM is more efficient in OITRR. In addition to OTM and OTIRR, LIPUS was demonstrated to play a crucial role in relieving pain. Through applying LIPUS, osphyalgia can be reduced [71]. In the field of the maxillofacial region, LIPUS is also effective. It was proved that LIPUS is an efficient method to relieve pain after orthognathic surgery [72]. Based on these results, LIPUS was tried to use in orthodontic pain. However, through applying LIPUS to patients who have Angle's Class II division 1 malocclusion every three weeks, the orthodontic pain showed no improvement, which indicated that LIPUS had no analgesic effect in these patients [73]. Since there are limited studies about LIPUS and orthodontic pain, further research is needed to explore their relationship. In general, LIPUS has a vital effect on enhancing OTM and reducing OITRR. Actually, the frequency of LIPUS used in OTM is usually 1.5 MHz. Since different frequencies may generate diverse outcomes, it's necessary to study ulteriorly to explore the best frequency and the usable range of LIPUS. The bright spot of LIPUS is that it can promote the treatment of malocclusion and OITRR at the same time and prevent the further development of OITRR during orthodontic treatment. So, more attempts to design and manufacture a more convenient LIPUS device through interdisciplinary cooperation are urgent. Figure 4: LIPUS has an effect on both the tension zone and the pressure zone in the OTM process. During OTM, the tension zone happens to bone remodeling while the pressure zone occurs bone resorption.