## 4 Disscussion  
This research aimed to study the efficacy of clear aligners regarding root movement by evaluating the torque before and after treatment and verifying whether the torque variations led to root resorption. One of the significant challenges for clear aligners is controlling root movement, including the labiolingual inclination, which is more effective in the upper arch than in the lower one. This may suggest that the inter-proximal enamel reduction is a reasonable solution to resolve lower crowding and preserve the roots' integrity. The torque variation was, as previously reported, higher and statistically significant in the upper and lower incisors: in these teeth, the mean torque variation was more important than 2\({}^{\circ}\). The torque increase in the canines was lower than in the incisors: it can be explained by their position at the curvature of the dental arch, which does not allow them to receive optimal force. Moreover, the mechanical efficiency for delivering effective bucally directed force by the aligner decreases from anterior to posterior sectors [23], which is mainly related to more complex root anatomy, thicker cortical plate, higher mastication loading, and greater soft tissue resistance from the cheeks in the posterior region [24]. In literature, the outcomes regarding the torque variation appear to be discordant: this is due to methodological heterogeneity. Some studies analyzed torque without planning for optimized attachments, auxiliaries, or power ridges [25]; others used different materials, even materials no longer used today [1]. Different material properties and aligner production processes affect the force levels and, thus, the predictability of tooth movements [26]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Tooth**} & \multicolumn{2}{c}{**Changes in Root**} & \multicolumn{2}{c}{**Changes in Root**} & \multirow{2}{*}{**Range (\%)**} & \multirow{2}{*}{**Tp**} & \multirow{2}{*}{**Confidence**} & \multirow{2}{*}{_p-Value**} \\  & **Length (mm)** & & & & & & **Interval** & \\  & **(Mean \(\pm\) SD)** & & & & & & \\ \hline Central incisor & \(-\)0.20 \(\pm\) 0.28 & \(-\)0.80–0 & \(-\)1.58 \(\pm\) 1.77 & \(-\)6.20–0 & 4.98 & 0.28–0.12 & * \\ Lateral incisor & \(-\)0.09 \(\pm\) 0.11 & \(-\)0.50–0 & \(-\)0.67 \(\pm\) 0.79 & \(-\)3.60–0 & 4.71 & 0.13–0.05 & * \\ Canine & \(-\)0.08 \(\pm\) 0.07 & \(-\)0.20–0 & \(-\)0.50 \(\pm\) 0.41 & \(-\)1.31–0 & 6.70 & 0.10–0.05 & * \\ First premolar & \(-\)0.003 \(\pm\) 0.01 & \(-\)0.10–0 & \(-\)0.02 \(\pm\) 0.13 & \(-\)0.71–0 & 1 & 0.004–0 & NS \\ Second premolar & \(-\)0.003 \(\pm\) 0.02 & \(-\)0.10–0 & \(-\)0.02 \(\pm\) 0.13 & \(-\)0.75–0 & 1 & 0.004–0 & NS \\ First molar & \(-\)0.003 \(\pm\) 0.02 & \(-\)0.10–0 & \(-\)0.03 \(\pm\) 0.14 & \(-\)0.80–0 & 1 & 0.004–0 & NS \\ \hline \hline \end{tabular} SD indicates standard deviation; and NS is not significant. * \(p\) < 0.05.  
\end{table} 
Table 5: Root length changes and paired _t_-test in the lower arch (\(\alpha\) = 0.05). Although clear aligners seem to meet all the criteria of an ideal orthodontic appliance, some biomechanical limits have yet to be overcome [27]. The predictability of tooth movement widely depends on different types of the tooth, dental movement, and arch. The aligners show the same biomechanical principles as other orthodontic appliances, but the material properties of clear aligners are probably responsible for their weak accuracy in applying torque. Indeed, an aligner's gingival margin is elastic, which would have difficulty controlling forces applied in this region [28]. To overcome this limit, some manufacturers have introduced power ridges, which generate a more significant moment than attachments, as described by [PERSON] et al. [29]: this represents a valid alternative for accurate root torque control, especially on the incisors.