## 2 Materials and Methods  
This retrospective study was approved by the Institutional Review Board of Korea University Anam Hospital (2019 AN0288), and informed consent from patients for inclusion in the study was waived due to the retrospective nature of the study. Patients who were diagnosed with maxillary transverse deficiency and had undergone MARPE at the Department of Orthodontics at Korea University Anam Hospital, from September 2014 to March 2019, were reviewed for the study. The patients had agreed to undergo maxillary dental expansion in case of MARPE failure. The inclusion criteria were a maxillary transverse deficiency of \(>\)2 mm (according to the criteria suggested by [PERSON] et al. [19]) an age of 18 years and older, cervical-vertetbrae maturation index (CVMI) of 6 [20], and CBCT imaging performed before and after MARPE. Patients with cleft lip and palate were excluded. The study included 53 patients (27 males, 26 females, mean age 25.8 \(\pm\) 8.9 years, and range 18-56 years). Two types of bone-borne expanders for MARPE were used: (1) a modified Hyrax expander with soldered holes for microimplant placement on the palate and banded to the premolars and molars, and (2) a maxillary skeletal expander (MSE, Biomaterial Korea, Seoul, Korea) that had holes for microimplants incorporated in the expander and banded to the molars (Figure 1). A modified Hyrax expander was used in 2014-2016. The maxillary skeletal expander (MSE) was used in 2015-2019. After delivery of the appliances, microimplants were inserted under local anesthesia. Four microimplants (Jeil Medical Corporation, Seoul, Korea) with a diameter of 1.6 mm and lengths of 8 mm and 10 mm for anterior and posterior screws, respectively, were used for the modified Hyrax expanders; microimplants (Biomaterials Korea, Seoul, Korea) with a diameter of 1.5 mm and lengths of 11 mm for anterior and posterior screws were used for the MSEs. Patients started maxillary expansion one week after the placement of microimplants at a rate of 0.4-0.5 mm per day. After two weeks of activation, patients who showed only dental tipping of the posterior teeth and no anterior diastema were considered as failure, and the expansion was discontinued. A CBCT scan was obtained to observe the midpalatal sutures. If a split in the midpalatal suture was not observed in the CBCT scan, the patient was classified in the failed group (F group) and dental expansion was performed using removable appliances to treat the posterior crossbite. Patients who showed an anterior diastema continued maxillary expansion as planned. At the end of maxillary expansion, the patients underwent CBCT, and separation of the midpalatal suture was confirmed. These patients that completed treatment were allocated to the success group (S group). Following expansion, the patients continued with the treatment of their malocclusions using a fixed orthodontic appliance. All procedures were performed by the same orthodontisti (YK). CBCT images were obtained using 3D eXam (KaVo Dental GmbH, Biberach, Germany) with the following parameters: field of view of 17 \(\times\) 23 cm, at 120 kV, and 5 mAs, and pulsed scan time of 17.8 s. Scan data were reconstructed with a voxel size of 0.3 mm\({}^{3}\). In the patients belonging to the S group, the linear distances of maxillary widths were measured in the CBCT scans taken at pretreatment (T1) and after MARPE (T2) using Invivo5 software (Anatomage, San Jose, CA, USA). In the coronal plane passing through the function of the upper first molars, the distance between the function of the maxillary first molar, maxillary width at the level of the deepest palatal arch, the most inferior border of the zygomaticomaxillary sutures, and maximum nasal-cavity width were measured (Figure 2). The bone mineral density of the two main resistance areas of skeletal expansion, the zygomaticomaxillary suture (zygomaticomaxillary- suture density, ZMSD) and the midpalatal suture (MPSD), was measured using CBCT at T1. A modification of the method by [PERSON] et al. was used.