## 4 Discussion
LLLT accelerates the bone remodeling process by stimulating both osteoblastic and osteocl activity during OTM [42]. 4 HR facilitates OTM in ovariectomized rat models [29]. In our study, 4 HR increased the bone turnover markers, which were RANKL, OPG, Runx2, and AP, in the experimental group more than in the control group (Figure 4). An increased bone turnover rate might inhibit root resorption (Figure 2). The main physiological role of the RANKL/RANK/OPG system is regulating bone remodeling. Functional osteoclast formation and bone resorption are dependent on the RANKL/OPG ratio expressed by osteoblastic cells and RANK expression by osteoclast precursor cells [43]. The OPG/RANKL/RANK system may influence root resorption during OTM, as well [15]. Odontoclasts are multinucleated cells located on root dentin being resorbed. RANKL expression was detected not only in osteoclasts but also in odontoclasts, so there seems to be a common regulatory mechanism between osteoclasts and odontoclasts [44]. If root resorption is more severe, the expression of RANKL increases [45]. The amount of RANKL is an important factor for root resorption, but the OPG/RANKL ratio and the concentration of OPG are also important for hard-tissue repair [40]. In this study, the expression of both RANKL and OPG may have contributed in inhibiting root resorption in the experimental group (Figures 2 and 3). Rats have been used frequently as a model to study OTM, despite the morphological and physiological differences in the periodontal ligament and alveolar bone compared to humans [46]. In this study, the experimental teeth included the lower first molar. Although anterior teeth are known to be the most vulnerable teeth for root resorption [47], since the anterior teeth of the rats continue to grow, they are not suitable as an experimental model for root resorption. As heavier forces were applied to teeth, greater root resorption occurred [48]. A large force of 120 g was used to examine the effect on root resorption in this study. Because a human molar is approximately 20 times larger than a rat molar [49], the effect of a 120 g force on a rat molar is comparable with that of a very heavy force of 2400 g on a human molar. This is a force that is large enough to cause root resorption. In this study, it was found that the root resorption induced by a very heavy force was alleviated by 4 HR (Figures 2 and 3). Further research is needed to investigate the effect of 4 HR on root resorption under normal orthodontic force. Root resorption during OTM is closely associated with the OPG-to-RANKL ratio [50]. In this study, both OPG and RANKL were shown to be elevated in their expression by 4 HR administration (Figure 4). Considering that 4 HR inhibits the NF-\(\kappa\)B pathway [22; 26], the effects on osteoclasts and cementoclasts by the elevated expression of RANKL are restricted by hampering its downstream signaling and by the blocking of 4 HR. TGF-\(\beta\)1 inhibits mineralized tissue resorption by elevating the OPG level [51]. Interestingly, 4 HR is a strong inducer of TGF-\(\beta\)1 [21; 51]. Cementum is a barrier to root resorption, and its resorption is dependent on the OPG-to- RANKL ratio [50]. In this study, the experimental group showed a higher OPG-to-RANKL ratio compared to the control group (\(p<0.05\) for the female group, Figure 4b). Despite the elevation of both bone-formation and bone-resorption markers in the alveolus supports in a previous study [29], the systemic effects of 4 HR injection were not evaluated, and the mechanisms of how 4 HR affects the signaling pathway of bone metabolism is still unclear. Further study is necessary to elucidate the in vivo mechanism of 4 HR's activity. 4 HR has been accorded a GRAS (generally recognized as safe) status as an oral health care agent [52]. There are some issues for the estrogenic activity of 4 HR [53], but a recent study reported that 4 HR behaves differently from other xeno-estrogens [54]. The limitations of this study were as follows: First, rats have a continuous erupting incisor and no premolar.