## 1 Introduction  
The upper maxilla's transversal discrepancy is considered one of the most common cranio-facial deformations dentits have to treat during their clinical practice [1], and its incidence has been reported to be 8-22% [2]. The treatment of this kind of discrepancy must undergo an accurate diagnosis which consists in an attentive analysis of medical history, intra and extra oral photos, cast models, bidimensional radiological exams such as OPG and Tele X-rays and, in some cases, even three-dimensional radiographies like CBCT [3]. During the last century, new devices, diagnostic instruments and operative protocols were developed in order to enlarge the upper arch perimeter starting from [PERSON]'s Hyrax RPE in 1968 [3] to Ricketts Quad Helix SPE of 1975 [3], to [PERSON]'s and [PERSON]'s Miniscrew assisted rapid palatal expansion (MARPE) [4, 5] and eventually to the surgically assisted rapid palatal expansion (SARPE) [6]. Rapid Maximillary Expansion (RME) is the most consolidated technique used to correct maxillary deficiency: it determines an opening of the mid-palatine suture in growing patients using heavy and intermittent forces for a short time, and it is reached through teeth or tissues anchored devices [7]. On the contrary there are different appliances that determine a Slow Maximillary Expansion (SME), using lower and continuous forces applied for a longer period than RME [8]. The RME protocol consists in two activations a day, which correspond to 0.50 mm of transversal expansion a day. The patient must be monitored weekly, and the expansion must continue until an over-expansion is obtained: usually an edge-to-edge contact between the palatal cusps of the upper first molars and the vestibular cusps of the lower fist molars is expected [9]. Three months retention are also necessary for the bone to rearrange in the mid-palatal suture space. The aim of these kind of appliances is to expand the palatal suture the quickest way possible applying intense orthopedic forces (up to 9-10 kilos per half-palate) and to give more skeletal than dental expansion. This clinical approach is nowadays still well accepted and applied [9; 10; 11]. Nevertheless, it has been proven that RME has numerous side effects such as pain, relapse, molar uncontrolled buccal inclination, bone loss, gingival recession, ulcerations, necrosis of the palatal mucosa, root absorption, bite opening tendency, TMJ's microtraum [12; 13]. By contrast, SME uses decidedly lighter forces (around 450 gr up to 900 gr) resulting in less discomfort for the patient, less resistance of the skeletal structures and better bone formation within the median palatal suture [14]. The activations should be done twice a week, the end of the activations is reached around the 10 th-12 th week. The perceived pain is less than in the RME devices [13; 15]. In 2013 a new device for palatal expansion was designed: the Leaf Expander [16]. This device allows the maxilla's expansion thanks to the application of slight and continues forces with specific intensity and direction, which cause a dento-alveolar remodeling [17]. The structure of the Leaf Expander resembles a common RME or SME, but it differs in the active components' characteristics and the operating instructions. In fact, the screw doesn't directly act on the supporting teeth, but it determines a measured expansion of the upper jaw by compressing a NiTi leaf-like coil [16]. The new device indeed encompasses several characteristics that can be deemed optimal for a fixed orthodontic expansion appliance, such as: a remarkably reduced number of intraoral reactivation sessions; easily executable activations; absence of pain even in the initial phases of expansion; controlled bodily vestibularization of middle and posterior teeth; high control over the progression of movement; impossibility of device deactivation due to occlusal forces; development of light, predetermined, and continuous forces; possibility to precisely graduate the extent of movement; absence of risk of overexpression [18].