## 2 Materials and Methods  
This retrospective study was approved by the Institutional Review Board of Kyung Hee University Hospital at Gangdong (KHNMC 2023-01-031). Patients who underwent orthodontic treatment at the Department of Orthodontics, Kyung Hee University Hospital at Gangdong from 1 March 2017 to 31 December 2022 and in whom MARPE was used as the first treatment device were enrolled in this study (Figures 1 and 2). Based on previous studies, the number of participants per group should be 15-20 [9; 12]. A minimum sample size of 15 patients was required (G*Power 3.1.9.6, Dusseldorf, Germany), considering an alpha of 0.05, power of 80%, and effect size of 0.80 to detect differences in palatal expansion using a paired \(t\)-test [13]. Figure 1: Clinical application of miniscrew-assisted rapid palatal expansion (MARPE). (**A**) before MARPE expansion; (**B**) after RPE expansion; (**C**) removal of dental connections after a month of expansion completion. The inclusion criteria were as follows: (1) CBCT was performed before expansion (T1) and after palatal expansion treatment (T2); (2) adolescent patients aged 12-19 years; and (3) four miniscerews without failure during treatment were used with the same MARPE appliance. The exclusion criteria were as follows: (1) craniofacial deformity, such as a cleft palate; (2) palatal expansion that did not occur as planned after MARPE; and (3) history of orthodontic treatment and/or orthognathic surgery. The study group comprised 20 patients (7 male and 13 female patients) with a mean age of 15.40 \(\pm\) 2.07 years. In this study, the same type of MARPE appliance was used in all patients for rapid palatal expansion. The MARPE device (MSE; BioMaterials Korea, Seoul, Republic of Korea) has four screw holes in the jackscrew body and four retention arms soldered to the maxillary first molars. Following MARPE cementation, four self-drilling bone screws (1.5 mm diameter and 9-11 mm length) were inserted perpendicularly under local infiltration anesthesia, and the miniscerew lengths were determined considering the palatal bone depth for fenestration of the palatal bone to facilitate greater expansion forces. The 9-11 mm length was selected by considering the 2 mm height of the insertion slots, 1-2 mm space between the appliance and the palatal surface, 1-2 mm gingival thickness, and 5-6 mm bone engagement. The antero-posterior position of the MARPE was set the same as the maxillary first molar in all patients. Activation was two turns per day for 2 weeks or one turn per day for 4 weeks. Patients below age 12 received one turn per day activation protocol. Additionally, with the patients who complained about much pain when activating MARPE, one turn per day activation protocol was applied even though the patient was older than the age 12. In our samples, only one patient received one turn per day expansion protocol while all the other patients received two turns per day expansion protocol. Both one turn and two turns per day activation protocols fall in rapid expansion, and it was assumed that there might be no significant difference between the two protocols. The expansion continued until the palatal cusps of the maxillary first molars were in contact with the buccal cusps of the mandibular molars. If there was hard resistance against activation, a 2-week rest was provided, and activation was resumed. Figure 2: Flowchart of participant recruitment. After completion of the expansion, the expansion screw was fixed with flowable resin to secure its expanded state. One month after the completion of the expansion, four dental connections of the MARPE were removed, and CBCT was taken to ensure the expansion of the midpalatal suture. In this study, the mean duration of expansion was 22 \(\pm\) 3.1 days (range, 13-63 days). The mean duration from completion of palatal expansion to CBCT taking at T2 stage was 44.6 \(\pm\) 4.0 days (range, 27-97 days).