## 2 Materials and Methods
### Materials and Treatment Protocol  
MSE appliances (MSE II expander, BioMaterials Korea, Seoul, South Korea) and four mini-implants (1.5 mm diameter and 11.0 mm length) were used for maxillary expansion in this study [28] (Figure 1). Type II appliances expand by 0.8 mm in 6 turns, and the body size is 13.5 \(\times\) 14.5 mm. It is located between the maxillary first molars in the paramidsagittal region. MSE contains four holes and four arms that are soldered to the bands of the maxillary first permanent molars. Four 1.5 \(\times\) 11.0 mm mini-implants are applied to fenestrate the palatal and nasal floor in order to create higher expanding skeletal stresses. The appliance used in this study had a maximum expansion of 8.0 mm.  
The MSE treatment protocol applied in our clinic is as follows (Figure 2). To fabricate the MSE device, a separation ring is inserted anteriorly and posteriorly to the maxillary first molars. If the separation ring cannot be inserted well due to tight interdental contact, a brass wire is inserted. A band is fitted 7 days after separation. After taking a pickup impression, an MSE is placed as close to the palate as possible on the working model. The MSE arm is soldered to the maxillary first molar band. The activation rate is 1 turn per day for early teens (under 15 years), 2 turns per day for late teens (over 15, under 20 years), and 4 turns per day for adult patients (over 20 years) until diastema appears. Once a diastema was observed, the number of activations was halved, and the expansion continued until sufficient expansion was obtained. After implantation, the patient was asked to visit the clinic every 7 to 10 days to observe whether a diastema had occurred. After the diastema occurs, the number of expansions is reduced by half. Existing opinions on palatal expansion are mainly described based on the cusps of the maxillary and mandibular teeth, and it is recommended to expand enough to overcorrection. Similarly, in adult patients, the amount of expansion can be recommended based on the cusp, but it is necessary to balance the width of the maxilla with a narrow palatal arch and the width of the mandible, which is relatively normal when viewed from the frontal face of the patient.  
Active expansion is usually performed for about a month, and thereafter, the MSE is maintained so that bone deposition can occur at the expansion site. It has been reported that sufficient new bone is deposited in the expanded area 3 to 6 months after palatal expansion, and clinically, when observing the median palatine suture using CT in some patients, the new bone generated between the expanded median palatine sutures was reported. It is expected that it will take several months to fully mature. Therefore, unless there is a special reason, it is recommended to maintain the MSE for 6 months to 1 year. Six months after the end of the expansion, a CBCT is taken to check whether the expansion is well maintained and if there is sufficient bone formation.  
Figure 1: (**A**) MSE II appliances. (**B**) Initial state. (**C**) After expansion.  
During CBCT imaging, all patients were instructed to sit upright with their imagined Frankfort horizontal plane parallel to the floor. To ensure their position during the CBCT scan, their heads were secured with a chin cup and ear rods. The CBCT scanner was adjusted to 80 kVp and 7.0 mA for adults and 80 kVp and 3.0 mA for adolescents (Alphard-3030; ASAHI Roentgen IND, Kyoto, Japan). The scan time was 17 s, and the voxel size was 0.39 mm. INFINITT PACS software was used to import the images as digital imaging and communications in medicine files after they had been taken (INFINITT Healthcare Co, Ltd., Seoul, Korea).