# Long-Term CBCT Evaluation of Mandibular Third Molar Changes after Distalization in Adolescents
## 2 Materials and Methods  
This retrospective study was approved by the Institutional Review Board at the Catholic University of Korea (IRB number KC21 RISI0767).  
The study sample comprised 65 third molars from 35 subjects, divided into two groups. The treatment group included 26 third molars from 14 adolescent patients with Class I bimaxillary protrusion (mean age, 12.8 years), who underwent bilateral maxillary distalization with a modified C-palatal plate (MCPP) and mandibular distalization with Class III elastics at the Department of Orthodontics, Seoul St. Mary's Hospital, The Catholic University of Korea, between January 2009 and December 2013. CBCT images were scanned at pre- and post-molar-distalization (mean duration, 15.8 months) for evaluation during pre- and post-treatment. Long-term data (mean period, 6.1 years) were extracted from the CBCT images. In addition, there were 21 Class I subjects in the control group with 39 mandibular third molars, of which CBCT images were been taken for purposes other than this research, such as for periodontic treatment or due to pathologic disease (Table 1).  
The inclusion criteria of the treated group for this study were: (1) age range of 12 to 14 years, (2) Class I molar with bimaxillary protrusion, (3) unilateral or bilateral development of mandibular third molars, and (4) retention period of more than 4 years. For the comparison with the third-molar position at retention (T3) in treated group, control group included 21 participants with normal occlusion, (1) mean age 21.3 \(\pm\) 1.6 years, (2) Class I occlusion with a normal overbite and overjet range of 1 to 3 mm, (3) crowding \(\leq\) 2 mm, (4) no asymmetry or crossbite, and (5) the presence of third molars.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Treated Group (n = 14)**} & \multicolumn{2}{c}{**Control Group (n = 21)**} \\  & **(26 Third Molars in 14 Patients)** & \multicolumn{2}{c}{**(39 Third Molars in 21 Patients)**} \\ \hline \multicolumn{2}{c}{**Predistalization (T1)**} & **Postdistalization (T2)** & **Retention (T3)** & **Mean \(\pm\) SD** \\ \hline \multicolumn{2}{c}{**Mean \(\pm\) SD**} & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** & **Mean \(\pm\) SD** \\ \hline Age (y) & 12.8 \(\pm\) 1.2 & 14.4 \(\pm\) 1.3 & 20.1 \(\pm\) 1.4 \({}^{\dagger}\) & 21.3 \(\pm\) 1.6 \({}^{\dagger}\) \\ \hline \multirow{2}{*}{Gender} & Male (n = 7) & Male (n = 11) \\  & Female (n = 7) & Female (n = 10) \\ \hline \multirow{3}{*}{CVM} & T1 & T2 & T3 & Control \\  & CS2 (n = 5) & CS3 (n = 6) & CS5 (n = 3) \\ \cline{1-1}  & CS3 (n = 9) & CS4 (n = 8) & CS6 (n = 11) & (n = 17) \\ \hline \hline \end{tabular} Control group was applied only for the comparison with third molar position at retention (T3) in treated group. \({}^{\dagger}\): \(t\)-test between treated group (T3) and control group, \(p>0.05\).  
\end{table}
Table 1: Baseline characteristics of subjects in the treated and control groups.  
An i-CAT computed tomography scanner (Imaging Science International, Hatfield, PA, USA), which had 120 kVp, 47.7 mA, a standard voxel size of 0.4 mm, and a 200 \(\times\) 400-mm field of view, was used for all patients. An MCPP was used to distalize the maxillary dentition, while Class III elastics were applied to distalize the mandibular dentition.