## 2 Materials and Methods  
The study group consisted of 95 patients who were treated between 2007 and 2021. The inclusion criteria for the study were age above 10 years; permanent dentition; transversal maxillary deficiency above 5 mm referring to the lower dental arch, visible as unilateral or bilateral cross-bite; Class II malocclusion (with maxillary deficiency) with upper airways constriction visible on lateral cephalograms; Class III malocclusion (with maxillary deficiency); and normal health status. The diagnosis of a sagittal malocclusion was confirmed via cephalometric analysis by [PERSON]. In the lower dental arch, the patients had a Little's irregularity index between 2 and 3 mm. The exclusion criteria were previous orthodontic treatment, craniofacial deformities (including cleft lip or palate) and missing permanent teeth (except for the third molars). The sample size was verified using a standard sample size calculator for two-sample \(t\)-tests for an estimated effect size of 0.83, power of 80%, level of significance (alpha) of 0.05, and the required sample size per group yielded 26. All patients included were treated during one orthodontic practice by the same experienced orthodontist (MKM). The appliances used were bonded acrylic Hass-type expanders, with a Hyrax screw (Dentaurum, Phorzheim, Germany) bonded on permanent teeth from the first premolar to the first or second molar with a chemical-cure adhesive (Excel, Reliance, USA) and a fixed appliance with MBT 0.022 slot brackets ([PERSON] 2007, Henry Schein Orthodontics, Melville, NY, USA) as presented in Figure 1.  
The semi-rapid maxillary expansion (SRME) protocol included one quarter turn of the screw per day [22; 23], carried out just before bedtime. It was recommended to activate the screw with a slow movement. Opening of the palatal suture was checked clinically by the presence of a diastema and radiologically via intraoral radiographs (Figure 2a,b). The minimum expansion included 10 turns of the screw, and the maximum was 28 turns. Expansion was considered as sufficient when the palatal cusps of the upper permanent first molars approximated the buccal cusps of the lower permanent first molars (stop of activation). The appliance was kept on the teeth as a passive retainer for 5 months. The course of the study is presented in Figure 3.  
The questionnaires, delivered one month after debonding the Haas-type expanders, concerned pain and other discomfort related to the RME and fixed appliance procedures, as presented in Figure 4.  
Figure 1: Appliances used in the present study (bonded acrylic Hass-type expander and fixed appliance—Carriere brackets). Figure 2: (a) 25-year-old female patient at the day of blocking the screw. (b) 33-year-old female patient (32y 10m) at the day of blocking the screw. No patient refused to fill in the questionnaire, and drop-out during the treatment phases described was noted only for one 11-year-old boy (who moved to another country just after bonding a Haas-type appliance). Informed consent was signed by all patients and/or their relatives before the proposed treatment. The distributions of the study group, according to age, sex and type of sagittal skeletal classes, are presented in Table 1.  
The age ranged from 10y 1m to 41y 11m. The cohort was almost evenly split between adults (equal and above 18 years) and pediatric patients (between the age of 10 to under 18 years), with a preponderance of females in both age groups. The difference in sex distribution, with 68.63% females in the adult subgroup compared to 56.82% in the pediatric subgroup, was statistically insignificant (\(p\) = 0.234). The distribution of skeletal classes enrolled in the study was 9.47% Class I patients, 18.95% Class II patients and 71.58% Class III patients, as presented in Table 1.  
In order to assess pain, an NRS--numerical rating scale--from 0 to 10 was used, where 0 was no pain, and 10 was an extreme amount of pain [21]. The patients were asked to report the highest pain perceived. Figure 3: The course of the study.