## 4 Discussion  
The aim of the study was to evaluate the influence of slot size (0.018\" compared to 0.022\") and low-friction on patient pain and OHRQoL during the first phases of orthodontic treatment. Due to the design of the study and the planned objectives, it was not possible to carry out a controlled study, since there was no control group without orthodontic treatment. Currently, there are limited published articles in the scientific literature that analyze oral pain and OHRQoL comparing conventional and low-friction brackets and, most importantly, the influence of slot size on these variables. The participants of this study were reasonably comparable, since no significant differences in terms of sex and age were observed. Published studies looking at conventional brackets versus low-friction brackets (Synergy(r)) are in vitro studies [17; 19]. In this study, we used the VAS. The VAS is a useful tool to quantify pain perceived by patients and is widely used in various publications [19; 20; 21; 22]. We observed that the peak of pain described by patients occurred between the first and second day after starting orthodontic treatment. The level of pain reached minimum values 7 days after starting treatment. These results are in line with those of other studies [4; 6; 19; 20; 22; 23]. This can be understood in terms of the synthesis of various biochemical mediators (such as cytokines and prostaglandamins) relative to hyperalgesia, with peaks occurring 24 h after applying force to the teeth [24; 25]. Seven days after starting treatment, we observed that the level of pain decreased until reaching minimum values. This coincides with other studies describing similar values to those obtained in this study [19; 20; 26]. Most of the published studies that quantify pain and the impact of treatment on OHRQoL have used brackets with a 0.022\" slot [19; 20; 22; 23], with only a few studies using brackets with a 0.018\" slot [27]; hence the justification for analyzing the influence of slot size in this study. [PERSON] et al. analyzed the influence of slot size (0.018\" with 0.022\") on pain during orthodontic treatment, and concluded that there were no statistically significant differences between the two study groups. In contrast to our study, these authors evaluated pain 6 months after starting treatment [28]. The patient's perception of orthodontic treatment is an important factor to consider. Before and during orthodontic treatment there are changes in the patients' quality of life. A multifactorial concept that helps us to measure these changes is the OHRQoL. To measure these psychosocial factors, there are many questionnaires. The Oral Health Impact Profile-14 (OHIP-14) is the most used questionnaire to evaluate OHRQoL and was used in this work [10; 29; 30; 31; 32]. Physical pain is the domain of OHRQoL that patients describe as having the greatest impact, followed by psychological discomfort. Social disability and handicap domains have the lowest scores on the OHIP-14 questionnaire. Our results coincide with those observed by other authors, especially during the first phases of orthodontic treatment [29; 30; 33]. Other authors did not observe