## 2 Materials and Methods  
Ninety Caucasian patients, 43 males and 47 females (mean age 7.5 \(\pm\) 1.5 years), presenting with maxillary transverse deficiency and posterior crossbite, were selected from those treated at the Department of Biomedical Surgical and Dental Sciences, University of Milan from June 2018 until December 2019. The study was approved by the Ethic Committee, Fondazione IRCCS Ospedale Maggiore-Policlinico, Milan (Prot. n\({}^{\circ}\)573).  
Inclusion criteria were: early or midmixed dentition with upper primary second molars (E) preserved, cervical vertebral stage 1 through 3 (CVS methods 1-3) [26], Angle Class I or Class II malocclusion, no previous orthodontic treatments, transverse maxillary hypoplasia, digital records obtained before activation and at appliance removal. Exclusion criteria were: presence of craniofacial deformities, tooth extraction or surgical treatment, temporomandibular disorders, absence or caries of E.  
Sixty-nine patients (33 males/36 females), mean age 7.6 \(\pm\) 1.7 years old, treated with SME (Leaf Expander) were selected according to the inclusion criteria. The appliances were banded by a unique operator on the E. The expanders presented a 6 mm screw (450 g) with an anterior arm till canines; no other orthodontic treatment was applied before the removal of the appliance. The expander was activated as described previously by [PERSON] et al. [19] (Figure 1 and Table 1).  
Figure 1: Test Group: Leaf Expander banded on primary second molars with an anterior arm till the canine before (**a**), after activation (**b**) and to the removal (**c**).  
The RME control group consisted of 21 subjects (10 males/11 females); mean age \(7.4\pm 1.2\) years old (Table 1). These appliances were banded by a unique operator on the primary second molars. The expander (Haas type) presented an anterior arm till canines and a posterior till the first permanent molars (Figure 2). Screw activation was done three times per day for ten days (0.2 mm \(\times\) 3 \(\times\) 10).  
The expansion, in both groups, was considered clinically obtained when the intercuspation of the lingual cusp of the first permanent upper molar laid on the occlusal aspect of the vestibular cusp of the lower first permanent molar.  
Dental casts were done pretreatment and at appliance removal with a mean comprehensive treatment duration (active phase and retention) of 9 to 11 months and were processed by means of an intraoral scanner (Trios 3, 3 Shape D250 laser, 3 Shape, Copenhagen, Denmark). The sample comprised 180 cast models (90 of the maxillary and 90 of the mandibular arch).  
Digital landmarks were traced by means of 3 Shape Ortho Analyzer software (version 1.7).  
Each patient was randomly identified with a number and eight linear variables were considered, traced and analyzed by a single blinded examiner.  
Permanent interpolar, primary canine, and first and second interpolar width were measured for all subjects. The reference points were set for the molars at the meso-palatal/lingual cusps and the intercanine width was traced between the two canine cusps tips (Figures 3-6). When either the deciduous or the permanent teeth were missing or not fully erupted, the measurements for that variable were not traced.