## 4 Discussion  
The aim of this study was to compare preadolescents' smile outcomes after two different early orthodontic treatments: CAs and traditional interceptive therapy with RS [33]. These two orthodontic approaches are different to each other: RME produces skeletal effects in growing subjects [41; 42], whereas CAs exclusively induce dentoulveolar changes [43]. Our aim was to compare the two approaches by focusing exclusively on the different smile outcomes. The evaluation of the orthodontic treatments on smile aesthetics is an important area of study, as smile improvement is a crucial motivation for patients [36]. Therefore, it is important to understand how the types of treatment affect the smile. Of the smile variables analyzed in this study, both groups showed an increase in smile width and a decrease in the buccal corridors. This suggests that both types of treatment induce improvements in smile aesthetics [14; 17; 18; 44]. Previous reports have demonstrated that the transversal dimension of the smile showed a statistically significant increase following skeletal expansion compared to the baseline [18], with increases in intercamine and intermolar width observed after expansion [3]. These results are justified by the fact that RME had an impact on the median and surrounding sutures of the maxilla, increasing the bone tissue support of the dental arches [44; 45]. At the same time, our results agree with previous studies that have reported dentoulveolar arch expansion with controlled movement of the teeth following the use of CAs [23; 24; 27; 33; 46]. These effects are comparable to those of an SME. In fact, CAs allow for digital planning of the upper arch expansion, using a combination of buccal dental tipping and bodily translation of the posterior teeth [24]. The post-treatment smile score comparisons showed an increase in smile width and a reduction in the buccal corridors, with statistically greater variation in the RS group than the CAs group. Many studies have confirmed that skeletal expansion is greater and more stable than dentoulveolar expansion [47]. Several authors have underlined the effectiveness of RME in arch expansion by opening the mid-palatal suture in growing individuals [41; 42] and also inducing a significant reduction in the buccal corridors [48]. Conversely, [PERSON] et al. in 2017 [46] demonstrated that the efficiency of expansion movement with CAs can be reduced when moving posteriorly into the arch, finding a statistically significant difference between digital planning and final outcome, with predictions worsening toward the posterior region of the arch. Expansion across the first permanent molars is significantly less predictable, which may be due to a CA's limited ability to exert orthodontic forces on the terminal tooth, as recently reported by [PERSON] et al. [34]. In support of this thesis, [PERSON] et al. [28] reported a better expansion, using CAs treatment, at the levels of the first and second deciduous molars compared to the first permanent molars. The forces generated with the CAs system led to an adaptation comparable to slow maxillary expanders [27]. Indeed, according to [PERSON] and [PERSON], greater success is obtained with CAs when not treating skeletally constricted arches [25]. The results of the dental variables analyzed in this study show differences between the two groups. In agreement with the previous literature, the RS group showed a decrease in OVJ as a consequence of increases in incisor lingualization and OVB. RME was fixed on the first molars, without any control on the other teeth. Therefore, the movements of extrusion and lingualization were made possible. [PERSON] et al. [19] reported that RME might guarantee more space for the upper central and lateral incisors, which are therefore free to align themselves and rotate mesially whilst also under the influence of the upper lip [49]. [PERSON] et al.