## 4 Discussion

Table 1: Description of the included studies. In the meta-analysis conducted by [PERSON] et al. [9], it is possible to conclude that SME could provide a more effective maxillary expansion than RME in the molar region. Similarly, the values obtained after rapid palatal expansion reported by [PERSON] et al. [37] (deciduous upper interimolar width increase of 3.4 mm on average) are found to be lower in comparison to those of [PERSON] et al. [20]. This reflects the study conducted by [PERSON] et al. [38] in which the interimolar width at the level of the second deciduous molars results in a screw expansion of 55% after treatment with RPE, which is therefore lower than that achieved with the slow nickel-titanium spring expander. In a very similar vein, based on CT observations, [PERSON] et al. [39] state that RME is no more effective than SME in posterior cross-bits correction. On the contrary, the review by [PERSON] et al. [40] does not find particularly significant data or results concerning SME; in particular, they emphasise the limited bibliography available and the need to conduct further research on this subject. In another study, [PERSON] et al. [34] compare dental and orthopaedic effects of the slow nickel-titanium spring expander with the effects of the rapid palatal expander, based on posteroanterior cephalometric studies at the beginning of the treatment (T1) and after nine months (T2). Different parameters were analysed among a group of 30 subjects divided into two groups: nasal, maxillary, mandibular and upper interimolar widths. The authors conclude that no statistically significant differences were found between the different groups, confirming the efficacy of SME in the correction of transverse maxillary deficit. In addition, [PERSON] and [PERSON] et al. [21] analysed a sample of 10 patients, treated with slow a nickel-titanium maxillary expander bonded on deciduous teeth (E). Based on a digital scan of the models at the beginning of the treatment (T1) and at the removal of the palatal expander (T2), they show the complete correction of posterior cross-bits in all patients in four months, with relevant increase in dento-alveolar transverse diameters and anterior mandibular arch (+1 mm). In fact, many studies illustrate important changes in the transverse diameters of the maxilla at all ages, not only as an orthopaedic effect in younger patients when early intervention is performed, through light forces acting on still active sutures [40; 41; 42]. Indeed, the results of the slow nickel-titanium expander [35] are similar to those obtained with conventional rapid palatal expander, as we find in the study by [PERSON] et al. [43]. In fact, the increase in nasal width would correspond to 1/3 of the screw opening. [PERSON] et al. [43] also find rapid expansion after a 37% (3 mm) increase in screw activation. [PERSON] et al. [33] underline that the main limitation of their study is that the skeletal effects of the slow nickel-titanium spring expander on the palatine suture have not yet been understood; in fact, they point out the need for further research. [PERSON] et al. [44] in their recent study found a suture reaction after treatment with the slow nickel-titanium spring expander. Despite the reduced number of published articles due to the recent introduction of the nickel-titanium slow spring expander, most authors found that the effects of the L.E. are clinically and radiographically comparable to those achieved with the classic rapid palatal expander. As we saw, some studies showed significant changes in maxillary transverse diameters in all age ranges, as well as orthopaedic effects in younger subjects when early treatment was performed on still-active sutures. Thus, clinical results have demonstrated the effectiveness, efficiency and ease of use of these types of devices; therefore, these devices represent a valuable alternative for maxillary transverse deficit treatment in deciduous/mixed dentition.