## 1 Introduction  
Transverse maxillary hypoplasia is one of the most frequent problems in interceptive orthodontics and one of the most studied topics in orthodontics. This condition is frequently related to as unilateral or bilateral cross-bite and/or antero-superior crowding. Posterior cross-bite is a common malocclusion in children with deciduous or mixed condition with a prevalence between 8% and 22% [1]. Frequently, transverse deficit is also associated with a lateral shift of the mandible and/or a space deficit involving superior permanent canines, raising the risk of impacted canines. Even if cross-bite occurs in 6-30% of the general population, spontaneous resolution is very low (0-9%) despite the elimination of oral habits and other etiological factors [2, 3, 4, 5]. Maximillary expansion is considered the gold standard to treat this type of malocclusion. It can be achieved using different types of appliances depending on the clinical defects that have to be treated [6, 7, 8]. The possibility to achieve palatal expansion decreases with age, so this treatment is to be executed as soon as possible, or until the median palatal suture remains fibrous. The suture starts to obliterate at around 14-16 years old, and past 13-14 years in women and 15-16 years in men it is difficult to achieve orthopaedic expansion. If maxillary expansion is executed with proper timing, it results in a fast, predictable and minimally invasive treatment. There are several orthodontic appliances leading to a maxillary expansion, which differentiate themselves for three main reasons: (a) patient's age (with deciduous, mixed, or permanent dentition); (b) type of applied force (light/orthodontic or heavy/orthopaedic); and (c) time of force applications (continuous/discontinuous). The main difference concerns the type of resulting expansion: Rapid Maximly Expansion (RME) or Slow Maximillary Expansion (SME) [9; 10; 11]. RME is the gold standard for the treatment of posterior cross-bite because the opening of the median palatal suture and the maxillary transverse diameter is increased [12; 13; 14; 15]. The main disadvantages of RME are discomfort and pain for the young patient and the need for compliance by parents and patients together for the activation of the appliance. In the literature, among early orthodontics treatments RME is considered one of the most painful therapies in terms of symptoms reported by patients (up to 98%) [16; 17; 18]. Pain could be related to rapid expansion protocol, because the expressed force could reach up to 10 lbs for each screw activation (0.2 or 0.25 mm) [19]. Since 1970, several authors have suggested that slow expansion is effective in opening the mid-palatal suture in growing patients, thereby reducing pain and discomfort. Some studies, including that of [PERSON] et al. [20], showed that slow maxillary expansion through a nickel-titanium expander may also have orthopaedic effects in deciduous or mixed dentition. Studies to date that evaluate nickel-titanium maxillary expanders are few in number [21; 22] and the most of them are focused _\"on the memory screw appliance\"_[23]. A nickel-titanium expander is able to exert a seamless, light and continuous force, thus producing maxillary expansion while maintaining tissue integrity [24; 25] during midpalate suture remodelling [7; 26; 27]. The characteristics of the reactivable and pre-activated NiTi leaf spring expanders are as follows:  
**Leaf Expander**  
The L.E. is an orthodontic appliance first constructed in 2013 by the development of a previous appliance, E.L.A (Espansore Lento Ammortizzato) [20], at the Centre of Bioresearch Leone in Sesto Fiorentino, Florence, Italy. The L.E. has similar features to a conventional rapid palatal expander, but instead of the central screw, it has a double nickel-titanium leaf spring (Figure 1) which returns to its original shape during deactivation, resulting in calibrated expansion of the upper arch [21]. Over the years, different models of the L.E.