## 1 Introduction  
Transverse discrepancy between the maxilla and the mandible is a common orthodontic malocclusion that is often associated with mono- or bilateral posterior crossbite [1]. Posterior crossbite is a discrepancy in the buccolingual relationship between the upper and lower teeth, when the lower teeth--unilaterally or bilaterally--are in a buccal or labial position in relation to the upper teeth. The nature of the crossbite might be dental, skeletal, or functional alone, or in combination [2].  
The etiology of lateral posterior crossbite could be related to genetic components and/or to environmental factors, such as thumb and/or pacifier sucking, mouth breathing, and low tongue posture [2,3]. The prevalence of this malocclusion in children is between 1% and 16%, and it is more common in Caucasian ethnic groups, which generally exhibit a higher incidence than African and Asian populations [2]. The early treatment of lateral posterior crossbite is a widely accepted practice, since this condition is related to the onset of temporomandibular disorders [4] and to adverse effects on the development of teeth and jaws, including disturbances in muscle activity [5,6]. In growing patients, the orthodontic approach to palatal contraction involves an orthopedic procedure, rapid maxillary expansion, that involves the opening of the palatal median suture [3,7]. Various types of orthodontic devices can be used to expand the upper maxilla, and the initial data systematically collected have shown that there is not sufficient scientific evidence to prove the most effective treatment method [3]. Some studies published over the years have recorded an insufficient sample size, bias and confounding variables, lack of analysis of measurement errors, and measurements obtained without the use of blinding techniques. Furthermore, most of the studies have presented deficient statistical methods [3,8,9]. Some authors [6,10] have reported short-term side effects after the use of palatal expanders, such as an increase in vertical dimensions due to post-mandibular rotation and an increase in the angle of the mandibular plane. On the other hand, some long-term studies suggest that increases in the angle of the mandibular plane could be merely a transitory effect of expansion [11,12]. Therefore, the effects of maxillary expansion on the vertical plane are still controversial and not easily predictable. In recent times, advances in traditional maxillary expansion approaches have included bone-anchored palatal expanders (bone-borne expanders) as a valid option in growing patients, usually adolescents or children exhibiting a good level of cooperation [13]. In pre-pubertal age patients, the circummaximillary sutures have not fused and temporary anchorage devices are unnecessary in achieving expansion [14]. So, despite the popularity of the bone-borne approach, traditional expanders still remain the gold standard for treatment as an early orthodontic intervention when rapid maxillary expansion is indicated. However, current evidence is insufficient to support their use in young adolescents, as outcomes are not sufficiently superior compared to other expanders [15].  
The aim of this study was to compare two different maxillary expander devices--a two-band expander (RME) and a splint resin palatal expander (SRPE)--to evaluate the vertical and sagittal cephalometric variations in the upper and lower jaw and the effects on the dental arches.