# Comparison of Root Parallelism in Extraction Cases Treated with Clear Aligners vs. Fixed Appliances
## 4 Results  
For the subgroup P\({}_{1}\), there was no statistically significant difference in the T3-T5 angulation and angle difference value between the aligner and fixed groups (Table 1). In addition, no significant difference was found in the percentage of the root parallelism of the canine and second premolar after treatment (\(p\) = 0.191, Table 2). This indicated that the teeth on both sides of the extraction space remained parallel after orthodontic treatment for the cases with parallel roots, independent of whether a clear aligner or fixed appliance was used. For subgroup NP\({}_{1}\), no statistically significant difference was found in the T3-T5 angulation and the percentage of root parallelism between the aligner and fixed appliance groups. However, the angle difference value showed a statistically significant difference. This indicated that the same treatment outcome was obtained by both appliances for cases with nonparallel roots, but judging from the average angle difference value (aligner 7.00\({}^{\circ}\)\(\pm\) 5.30\({}^{\circ}\); fixed 10.81\({}^{\circ}\)\(\pm\) 9.16\({}^{\circ}\)), the fixed appliance might have a larger range in root angulation adjustment than the aligner (Tables 1 and 2). Table 3 showed the root-angulation categories of the amines and second premolars after the treatment between the aligner and fixed appliance groups. With regard to satisfactory parallelism (SP), no difference was found between the two groups. However, there was a significant difference between the two groups with regard to nonparallel insulation. As displayed in Figure 2, the aligner group mainly showed apical divergence (AD), while the fixed group mainly showed apical convergence (AC). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{Aligner (\(n\) = 28)} & \multicolumn{2}{c}{Fixed (\(n\) = 30)} & \\  & & & 85 Extraction Sites & & 120 Extraction Sites & \\ \hline Subgroup & Variable & Mean & SD & Mean & SD & \(p\) \\ \hline \multirow{4}{*}{P\({}_{1}\)} & t3–t5 angulation & 3.42\({}^{\circ}\) & 2.21\({}^{\circ}\) & 3.59\({}^{\circ}\) & 1.94\({}^{\circ}\) & 0.691 \\  & T3–T5 angulation & 6.43\({}^{\circ}\) & 4.60\({}^{\circ}\) & 5.20\({}^{\circ}\) & 3.55\({}^{\circ}\) & 0.142 \\  & Angle difference value & 3.93\({}^{\circ}\) & 3.60\({}^{\circ}\) & 3.15\({}^{\circ}\) & 2.69\({}^{\circ}\) & 0.264 \\  & t3–t5 angulation & 14.08\({}^{\circ}\) & 6.54\({}^{\circ}\) & 17.04\({}^{\circ}\) & 9.48\({}^{\circ}\) & 0.067 \\ NP\({}_{1}\) & T3–T5 angulation & 9.32\({}^{\circ}\) & 7.17\({}^{\circ}\) & 6.92\({}^{\circ}\) & 4.49\({}^{\circ}\) & 0.069 \\  & Angle difference value & 7.00\({}^{\circ}\) & 5.30\({}^{\circ}\) & 10.81\({}^{\circ}\) & 9.16\({}^{\circ}\) & 0.009 ** \\ \hline \hline \end{tabular} (1) Parallelism (P1) and nonparallelism (NP1) are the states of the canine and second premolar roots before treatment. (2) The t3–t5 angulation indicates pretreatment, the T3–T5 angulation indicates post-treatment, and the angle difference value indicates the absolute value of the t3–t5 angulation minus the T3–T5 angulation. (3) The t3–t5 angulation, and angle difference value all use absolute values. (4) ** Statistically significant at \(p\) \(<\) 0.01.  
\end{table} 
Table 1: Comparison of the angulation between the canine and second premolar in pretreatment and post-treatment (independent \(t\)-test).