## 4 Discussion

Table 7: T-Test for independent samples comparing the net (\(\Delta\text{T}1-\text{T}0\)) interdental width (mm) increase after Leaf (n\({}^{\circ}\)69) or RPE treatment (n\({}^{\circ}\)21)—mandibular arch. From these reviews it has been reported that there is not a significative difference comparing dentoalveolar transversal effects of the SME with the RME. Contrariwise in our study we have noticed significant differences comparing the test group with the RME. The Leaf Expander produced a significative greater expansion for maxillary intercanine and primary interimolar width. However, the interdental change for the first permanent molars was greater in the RME group. This might be explained also considering that the RME was banded with a posterior arm that is not placed on the Leaf Expander. Despite this, all the interdental widths in the test group were significantly improved when comparing the pre and post-treatment values. Minimal data has been already published about the SME effects on the mandibular arch: an increase of 0.49 mm was previously found on the lower molars and considered as clinically not relevant, thus indicating a lesser spontaneous adaptation of the lower dentition [22, 30]. Indeed, in the test group we observed a significant increment of all the interdental width that was higher than those previously reported for the SME and comparable with those of the RME. Moreover, unlike previous studies, deciduous teeth such as primary molars were included in our study. Almost all the lower interdental width in the test group increased more than 1 mm (except for the lower first primary molar +0.93 mm). Mandibular interimolar width (1.19 mm) and intercanine (0.9 mm) inter second primary molar (0.7 mm) were found to be increased in the patients treated with Hyrax or Haas expander in previous studies [28]. The results of the control group were in accordance with this data confirming a spontaneous response of the lower arch after RME. The interdental width increment in the lower arch should be considered as the result gained from the increase of tongue pressure and the reduction of lip pressure and new occlusal intercuspation [32]. Moreover, the tongue lowers its position due to the presence of the expander (the Leaf Expander has the same structure as the RME of the control group) influencing and promoting the enlargement in the position of the lower teeth. Another important consideration is that the new intercuspation during the action of chewing might also promote expansive forces in the mandibular dental arch. Patients were selected in order to be far from their puberty growth peek and the measurements have been taken after a short period (not more than 11 months) from baseline in order to be able to consider the expansion obtained as the direct consequence of the treatments with no or insignificant influence of the spontaneous physiological growth. Posterior crossbite correction and stability of the expansion by other SME such as the Quad-Helyx has shown more than 90% of stability during the retention phase confirming the long-term effectiveness of SME [31, 33]. The main limitation of the study is the lack of a long-term follow-up and the retrospective design of the study. Randomized controlled clinical trials are needed to confirm our results.