## 6 Discussion  
During the past few years, several animal studies dealing with the same subject have been conducted, and the results found have favoured the use of PBMt to increase mini-screw stability [27; 28]. Human studies have also been conducted, but our study was the first to apply PBMt to assess the mini-screw stability for a period up to three months. It is common knowledge that the stability of mini-screws decreases with time, especially in the first two weeks [29]. However, if the expected results are achieved, this stability decrease does not have a negative influence on the treatment. Instead, it is a physiological phenomenon that arises from several factors, among which immediate or delayed loading of the mini-screws can be cited. It has been suggested that immediate loading be performed because the primary stability of the mini-screw is sufficiently capable of fulfilling its clinical use [3; 30]. In our clinical trial, we performed immediate loading, initially using light forces that were later increased progressively. In fact, [PERSON] et al. demonstrated in their study that the stability of orthodontic mini-screws decreases during the first three weeks but then started to increase between weeks 3 and 5 [9]. Similarly, according to our results shown in Table 1, the stability of the irradiated and non-irradiated mini-screws decreased significantly up to two months post-implantation (T3 and T4) where the changes of ISQ values became non-significant, and the stability curve of mini-screws appears to stabilize. This observation was found interesting because the three-month treatment and follow-up represented the originality of our study. On a clinical basis, this finding could guide us in terms of force magnitude by indicating light forces for the first two months post-implantation to start and then continuing with higher increments in order to rule out one of the risk factors known to jeopardize the clinical stability of the mini-screw. Furthermore, on the distal side of the L group mini-screws, another non-significant decrease of ISQ values were recorded between the third and twelfth weeks (T1 and T4) (Table 1). Knowing that the distal side of a mini-screw is opposed to the force applied, it is probably the one that undergoes most of the cellular changes and reactions. This is where the PBMt could potentially be the most effective. Similarly, in the L group between the third and sixth weeks of post-implantation (between T1 and T2), both the ISQ values recorded on the upper side of the mini-screws and the global ISQ values stopped decreasing significantly. As mentioned above, the first week following mini-screw insertion, the PBMt was in fact performed every other day and then carried out after two weeks (T1) during the first follow-up appointment. This likely means that the effect of PBMt was maximal during this period and that this positively influenced the stability of the mini-screws. Hence, PBMt seems to follow this rule: the more, the better. Results favouring the use of PBMt with mini-screws could be obtained if laser applications are more frequent. However, this frequency may seem impractical since orthodontic appointments are normally scheduled every three to four weeks. Our clinical trial showed no significant difference between the stability of irradiated and non-irradiated mini-screws. While several clinical studies have targeted the same subject, different protocols were used, and different results have been found. In fact, a study by [PERSON] et al [31]. evaluated the effect of PBMt on the stability and displacement of mini-screws with immediate and delayed loading. Two types of lasers were used: a red emission laser with a wavelength of 660 nm that was applied only on the same day of implantation, and a second infra-red laser with a wavelength of 808 nm that was applied every two to three days for two weeks. This study included 35 mini-screws and showed a lower loss in stability than in the ones that received PBMt.