## 5 Discussion
used an RME device with a stainless-steel expander cemented to the maxillary first molars, comparing two protocols of expansion, and concluded that patients treated with two activations per day reported significantly higher pain levels compared to patients treated with one activation per day. The difference was more than twice as much between groups with one turn vs. two turns daily [16]. Similar findings were reported in other studies [5; 6; 17; 18; 34], whereas [PERSON] et al. reported no correlation between pain and activation protocols [35]. However, a more detailed insight into three protocols, described by [PERSON] et al., reveals that the first study group with a protocol of six turns daily had been treated with a memory screw combining intermittent and continuous force modules, while the second and third study groups had been treated with another type of appliance, i.e., the conventional Hyrax screw. Moreover, the groups with Hyrax turned the screw in the same protocol two times a day during the most painful period, i.e., the first ten turns [16; 28]. Thus, the results, despite the three different protocols, were not statistically different. The third factor is the time of screw expansion. The Haas-type acrylic splint appliance in our study was expanded in the evening, just before going to bed, in order to reduce the pain sensation. In general, pain usually starts 4 h after the onset of the orthodontic force [13]; thus, screw turning before going to bed drives the pain into sleep time. Additionally, pain during the RME procedure might be related to the level of _cortisol_, a critical regulator of metabolism [36] that has its lowest level after lights off, at the onset of sleep, with peak levels occurring in every individual around waking [37; 38]. An inhibition of proliferation of the periosteal cells, which gives rise to osteoblasts, is a primary effect of cortisol on bone growth [39] and down-regulation of the synthesis of collagen [40]. Therefore, small amounts of cortisol are necessary for normal bone development to reduce the initial resistance to expansion, while glucocorticoid excess inhibits bone formation and calcium absorption from the gut and increases bone resorption and osteoporosis [36; 41; 42]. Thus, there is a circadian turnover of the bone, with bone resorption in the morning and increasing formation at night [43]. Therefore, pain sensation during RME is opposite to the pain sensation related to treatment with fixed appliances, which is proved to be lower in the morning than in the evening [44] and achieves the lowest level during the afternoon [14]. In the present study, the average pain level reported on the NRS in the whole group of patients during SRME screw activation was compared with the pain during FA therapy, considered as a perfectly matching control group, and was equally moderate for both procedures. However, a comparison carried out between two age groups revealed that pain levels during the SRME screw activation of the Haas-type acrylic appliance in adolescent patients was more painful (NRS = 5.00) compared to adults (NRS = 3.00) with statistical significance (\(p\) = 0.009), contrary to the common opinion that for adults, the RME procedure might be more painful. A similar comparison between adolescents and adults carried out for lower FA therapy resulted in a similar moderate grade level for both age groups. During the leveling phase of orthodontic treatment, higher pain levels were reported in the lower arch compared to the upper arch at 6 h and at day 7 [45]. Lower pain levels were reported in the evening than in the morning for days 2-6, independent of patients' age, sex and irregularity [45], which is in disagreement with the study by [PERSON] et al. [44]. The finding that adolescents experience greater discomfort during the initial stages of SRME treatment may be attributed to a lower tolerance for pain or the higher sensitivity of developing tissues in midpalatal sutures during maxillary expansion.