## 2 Materials and Methods  
This study was approved by the Institutional Review Board at the Kyung Hee University Hospital at Gangdong (IRB number: KHNNMC 2022-06-014), which waived the requirement for written informed consent because of its retrospective nature. The study population included consecutive patients who visited the Department of Orthodontics at Kyung Hee University Hospital in Gangdong between March 2007 and April 2022 for orthodontic treatment. The inclusion criteria for this retrospective study were as follows: (1) dental Class II relationship before treatment and dental Class I relationship after treatment, (2) lateral cephalography acquired before and after treatment, (3) use of a buccal or palatal TAD appliance for maxillary molar distalization, and (4) patients aged above 15 years and those with cessation of maxillofacial growth changes. The exclusion criteria were as follows: (1) missing congenital teeth, (2) use of Class II elastics or orthopedic appliances, (3) therapeutic intrusion or extrusion of posterior teeth that may have affected the occlusal plane angle, (4) distalization appliance designs other than the designated ones, and (4) patients aged under 15 years or those showing growth changes on the lateral cephalogram. After screening using these eligibility criteria, 51 study participants (14 men and 37 women; mean age, 22.24 \(\pm\) 6.66 years; range, 14-44 years at pre-treatment; Table 1) were enrolled. Patients were divided into three groups according to the location of the TADs and device design. In group 1, the TADs were inserted into the posterior interradicular buccal area for maxillary molar distalization (Figures 1 and 2). The TADs, 7.0 mm in length and 1.6 mm in coronal diameter (Orlus; Ortholution, Seoul, Korea), were placed between the maxillary second premolars and first molars. Subsequently, a 0.019 \(\times\) 0.025 inch stainless steel rectangular archive using crimpable hooks located between the maxillary lateral incisor and canine was placed on the buccal fixed appliances. An elastomeric power chain (Ormco, Glendora, CA, USA) was used from each miniscrew to the hook for molar distalization with a distal driving force of 150 g and replaced at 4-week intervals. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variables** & **Group 1** & **Group 2** & **Group 3** & **\(p\)-Value** \\ \hline Sex (male/female) & 5/14 & 4/12 & 5/11 & 0.92 \({}^{\star}\) \\ Mean age (year) & 21.79 \(\pm\) 6.91 & 25.19 \(\pm\) 6.82 & 19.83 \(\pm\) 5.57 & 0.07 \({}^{\ddagger}\) \\ Range of age (year) & 15–44 & 16–42 & 14–35 & \\ \hline \hline \end{tabular} Values are presented as number only or mean \(\pm\) standard deviation. \({}^{\star}\) x2 test was performed. \({}^{\dagger}\) ANOVA was performed.  
\end{table} 
Table 1: Demographic features of the subjects. Figure 1: Schematic diagrams of maxillary molar distalization TAD appliances (**A**), group 1; (**B**) group 2; (**C**) group 3; From the left: before activation and after activation. In group 2, two miniscrew-type TADs (1.6 mm diameter \(\times\) 6 mm length, 0.0215 \(\times\) 0.025 inch slot size) (Machined-surface miniscrews, Jeil Medical, Seoul, Korea) were used as a rigid bone anchor. The screw was bilaterally inserted into the median palatal suture, and a lingual arch appliance (round 0.032 inch stainless steel wire) was placed on the palate, connecting the right and left first molars, and attached to a palatal bar (rectangular 0.021 \(\times\) 0.025 inch stainless steel archwire) engaged through the head slot of the two palatal TADs (Figures 1 and 3). Closed-coil nickel-titanium springs were applied with a distal driving force of 150 g, and an elastomeric power chain (Ormco, Glendora, CA, USA) was replaced at 4-week intervals for molar distalization. Fixed buccal appliances with thicker than rectangular 0.016 \(\times\) 0.022 inch stainless steel archwires were simultaneously used during molar distalization.