## 4 Discussion  
The success of the treatment of the maxillary contraction using slow or rapid palatal expanders is at this point well documented in the scientific literature [24; 28; 29; 30; 31]. It was reported in the literature that the range of obtainable expansion with both slow and rapid protocol is 2.49-3.58 mm for the inter-molar width and 2.27-2.64 mm for the inter-canine width [32]. Only few studies investigated the effects of the aligners on the maxillary transversal expansion [24; 33; 34; 35] and only two studies on Invaslign First were published [23; 24]. The correct management of phase 1 in orthodontics is very important to intercept malocclusion at an early stage, and the use of clear aligners, as well as representing a more aesthetic and comfortable alternative to traditional treatment, it also appears to be able to solve different issues at the same time, with movements on the three space dimensions simultaneously [24]. The expansion of the maxilla using aligners represents nowadays one the most debated topics in the orthodontics scientific community. In the permanent dentition, the expansion of the upper arch can be done with Invaslign, but the increase in width is obtained mainly by vestibular tipping of the crowns [25]. According to [PERSON] et al. [34] the mean accuracy for the upper arch expansion movements is around 72.8%: 82.9% at the cusps level and 62.7% at the gingival margins level. [PERSON] and [PERSON] [33] declared that the efficiency of the coronal expansion is between 68.31% and 79.75% whereas the efficiency of the bodily expansion movement of the first upper permanent molars is around 36.35%. [PERSON] et al. [35] affirm that the amount of expansion at the end of treatment is not  
Figure 5: Effects on inclination and rotation of the first upper molars in both Leaf Expander and Invaslign First. Statistically significant values are marked with (*). predictable because of the difference between the planned virtual set up and the final 3D digital models. In both the protocols used in this study, the exerted forces are similar, with a range of 350-450 g. As for the deciduous canines, first and second deciduous molars, no substantial difference was observed between the two protocols. On the contrary, the expansion at the gingival level was significantly higher in the Leaf Expander cases for the deciduous canines and deciduous first and second molars. Regarding the first permanent upper molars, the expansion difference was statistically significant for the LE cases both at dental and gingival level. For what concerns the rotation of the first permanent molars, IF was found to be more effective in the derotation movements (28.4\({}^{\circ}\)\(\pm\) 3.2\({}^{\circ}\) in IF sample vs. 15.1\({}^{\circ}\)\(\pm\) 1.6\({}^{\circ}\) in LE sample): this is in agreement with [PERSON] article, in which the author underline the effectiveness of Invisalign First in derotating and distalyzing molars, resulting in much more significative stable molar class [23; 36]. For what concerns the inclination of the permanent molars, IF gave a higher vestibular tipping of the crown then LE (6.5\({}^{\circ}\)\(\pm\) 3.8\({}^{\circ}\) in IF vs. 3.1\({}^{\circ}\)\(\pm\) 4.1\({}^{\circ}\) in LE), in accordance with what has already been observed in literature [25]. The interpretation of these data suggests that the transversal expansion in the molar area is more dental rather than skeletal in the Invisalign First sample whereas Leaf Expander give a more bodily vestibularization which makes this last device preferable and limits the use of Invisalign First for light transversal discrepancies (3 mm maximum) [24]. Since both protocols use light forces, a great advantage in both the approaches was the reduced discomfort for the patients in the first days and weeks right after the first activations [37; 38]. The use of a slow expansion protocol leads to an extension of the total duration of the treatment which, in fact, goes from 8 to 10 months.