## 1 Introduction  
The smile is one of the most important criteria in the judgement of facial attractiveness, and a well-balanced smile is particularly important for self-esteem and social integration [1, 2]. In orthodontics, the smile is extensively studied, with the assumption that soft tissue form largely defines facial aesthetics [3, 4]. The anterior aesthetic zone is framed by three components: dental, gingival and soft tissue components [5]. The idea of a perfect smile remains not determined. A smile can be considered \"attractive\" when a \"balanced\" and harmonious inter-relationship exists between the teeth, gingiva and lips, interplaying within the dynamic display zone of the face [5, 6]. Moreover, upper incisal edges and canine tips that are set in harmony with the lower lip incurvation, as well as correspondence of the upper central incisors to the facial midline, are supposedly desirable features in an attractive smile. Lastly, the position and inclination of the upper incisor provide support to the upper lip, influencing smile appeal as well as the amount of upper gingival show; up to 3 mm upon smiling was found to be the accepted norm for an attractive smile [7]. The soft tissue component of a smile comprises the lips at rest and at the time of smiling, thus representing the frame of each exclusive smile [7]. Maxillary constriction is very common in orthodontic patients during the mixed dentition stage and can affect occlusion, facial development and smile aesthetics. A constricted maxilla is becoming more frequent nowadays and it is also proven that environmental factors--like diet consistency and/or mastication--could play a significant role in the growth of the maxilla and the mandible [8; 9; 10; 11]. However, maxillary expansion is required not only to correct transverse deficiencies but also to expand the perimeter of the arch [12], and it can be performed as Slow Maximly Expansion (SME) or Rapid Maximly Expansion (RME). The differences between these two treatments concern the intensity and time of application of forces [13]. Substantial differences are also highlighted in the dental and skeletal structures involved in the two different approaches. In fact, different studies have evidenced changes in facial soft tissues consequent to skeletal modification with RME [14; 15]. Furthermore, as reported by [PERSON] et al. [15] and [PERSON] and [PERSON] in 2007 [16], an improvement in smile aesthetics was registered after RME, with an evident decrease in the buccal corridors [15; 16; 17]; additionally, [PERSON] et al. [18] underlined that RME was beneficial for smile aesthetics through increases in the transverse smile dimension and exposures of maxillary central and lateral incisors [17; 18; 19]. Moreover, a constricted maxilla can be associated with several problems that include occlusal or functional difficulties such as narrowing of the pharyngeal airway, increased nasal resistance and alterations in tongue posture that results in a retrogolasal airway narrowing and mouth breathing. Therefore, early treatment of this malocclusion through palatal expansion is intensely recommended [20]. On the other hand, CAs is a viable alternative system to conventional orthodontic therapy in the correction of mild-to-moderate malocclusions [21; 22; 23; 24; 25]. The introduction of CAs in young patients provides an answer to the ever-growing demand for aesthetic and comfortable treatments, even in children. This device can be used to perform phase 1 orthodontic treatments, including the correction of a narrow maxillary arch [26; 27]. Numerous authors [27; 28] have recently analyzed the effectiveness of CAs in maxillary arch expansion. These studies reported encouraging results comparable to those obtained with RME therapy, with a consistent amount of bodily expansion for amines, deciduous molars and permanent first molars [24; 27; 28]. Other authors have also investigated the effect of these two expansion techniques on the craniofacial region and on the airway.