The aim of the current study was to evaluate maxillary and mandibular arch widths' response to five different appliances and clinical protocols (Rapid maxillary expander RME, Leaf Expander 450g, Leaf Expander 900g, Self-expander 450g, Self-expander 900g) for the correction of the maxillary deficiency.

Maxillary expansion with fixed appliance is a well-known and consolidated practice in clinical orthodontics but current findings of "evidence-based dentistry" have not yet identified a better clinical expansion protocol. This issue is due both to the several expansion screws available on the market and to the different screw activation protocols, which could be grouped in rapid and slow, with several customizations. The comparison between slow and rapid expansion is a hotly debated topic in the literature and a recent systematic review have shown that both rapid and slow expansion protocols are clinically effective on the primary outcome (the resolution of the maxillary deficiency and crossbite with a significant increase of skeletal transversal maxillary dimension). Based on these results, the choice of appliance based on its ability to solve the maxillary constriction may not be any more the main selection criteria. The choice of the orthodontist should also be based on the timing and on a "patient-oriented" device, that minimizes the side effects, such as e.g. appliance breakages, functional impairments, and pain perception.

The aim of the present study is to investigate and analyze five different maxillary expansion appliances to identify an effective and efficient clinical protocol for the maxillary expansion.

Inclusion Criteria:

• prepubertal phase of development (cervical stage CS1 or 2 in cervical vertebral maturation or corresponding stage of third Finger middle phalanx maturation index).
• early or intermediate mixed dentition stage with fully erupted upper and lower first permanent molars.
• presence of the second upper deciduous molars available as an anchoring tooth. The second deciduous molar was considered available as anchoring tooth when the root had the same length as the clinical crown at the radiographic examination (Quinzi V, Federici Canova F, Rizzo FA, Marzo G, Rosa M, Primozic J. Factors related to maxillary expander loss due to anchoring deciduous molars exfoliation during treatment in the mixed dentition phase. Eur J Orthod. 2021 Jun 8;43(3):332-337. doi: 10.1093/ejo/cjaa061)
• Posterior transverse interarch discrepancy (PTID) of at least 3 mm. PTID was calculated on dental casts with a caliper as the difference between the maxillary intermolar width (distance between the central fossae of right and left permanent first maxillary molars) and the mandibular intermolar width (distance between the tips of the distobuccal cusps of right and left permanent first mandibular molars) (Tollaro, I., Baccetti, T., Franchi, L. and Tanasescu, C.D. 1996 Role of posterior transverse interarch discrepancy in Class II, Division 1 malocclusion during the mixed dentition phase. AJODO, 110, 417-422).

Exclusion Criteria:

• age older than 12 years,
• pubertal or postpubertal stage of development (CS 3-6),
• late deciduous or late mixed dentition, 4. agenesis of upper second premolars (assessed on initial panoramic radiograph),
• Class III malocclusion,
• cleft lip and/or palate and craniofacial syndromes