## 1 Introduction  
The term transversal malocclusion refers to skeletal and/or dentoalveolar anomalies that may or may not include a posterior crossbite, defined as occlusion of a vestibular cusp in the fossa of a lower tooth [1, 2, 3]. A differential diagnosis is essential to determine the skeletal or dentoalveolar etiology of these malocclusions. Furthermore, we should also consider the possibility of a functional deviation generating a posterior crossbite [2]. Resolution of maxillomandibular discrepancies in the transverse plane requires compensatory coronal and/or radicular torque movements, basal expansion, or orthodontic treatment, combined with orthognathic surgery when the malocclusion is of skeletal origin in an adult patient. However, orthodontics is the sole approach to malocclusions of dentoalveolar origin, using fixed multibracket appliances or clear aligners systems [3, 4, 5, 6]. Today, clear aligners are a real orthodontic alternative to traditional appliances that offer greater comfort, better aesthetics, better oral hygiene, and a more positive patient experience. Furthermore, clear aligners are capable of correcting dentoalveolar crossbite, resulting in an adequate interarch transverse coordination. However, clear aligners might not be as effective as braces in producing adequate occlusal contacts, controlling teeth torque, and increasing the transverse dimension [7]. Numerous authors have investigated the expansion attained with conventional orthodontic systems and clear aligners [6, 7, 8, 9, 10, 11, 12, 13]. A prospective cohort study [6] concluded that low-friction self-ligating brackets produced significantly more transverse dentoalveolar width and perimeter of maxillary arch compared to clear aligners. Expansion is not only used to correct transversal problems, but also malocclusions with marked crowding as a means of gaining space [14; 15; 16; 17]. However, reports on the application of clear aligners to correct transverse malocclusions have not specified the working mechanics or sequencing utilized for this purpose. Furthermore, most studies were conducted using EX30, an older material that has been replaced by SmartTrack (Align Technology, Santa Clara, CA, USA), making evaluations of the updated system necessary. Many earlier studies had methodological errors, small sample sizes, inadequate statistical analyses, low or moderate risks of bias, or high levels of heterogeneity. [PERSON] [18] first introduced the idea of differential anchorage, proposing two distinct phases: dental crown inclination and root straightening. Researchers have developed sequences of movements or \"staging\" [17; 18; 19; 20] in relation to the correction of sagittal [21] and vertical malocclusions [14]; however, differential anchorage has not been defined or established as a treatment option to correct transverse malocclusions. Clear aligners offer an advantage over conventional orthodontic systems, in which an undesired movement results from a desired one, in accordance with [PERSON]'s law of an equal and opposite action for every action. The clear aligner system allows these undesired movements to be controlled and minimized, selecting teeth that should not be moved but rather act as support units within the same dental arch, enhancing anchorage [22]. Intra-arch differential anchorage with clear aligners, in which some teeth are supported to move others in the same arch, is useful for mesialization or distalization movements, as well as expansion, extrusion, and intrusion movements [14; 21; 22]. Similar to conventional orthodontic systems, inter-arch anchorage is accomplished through the use of elastics between the maxillary arch and the mandibular arch. This way, reaction forces in the opposing arch can be controlled. For correction of posterior crossbites, elastics are used from the palatal surfaces of the upper teeth to the buccal surfaces of the lower teeth to achieve this inter-arch anchorage.