# Comparison of Root Parallelism in Extraction Cases Treated with Clear Aligners vs. Fixed Appliances
## 4 Results
However, the results of the present study matched those reported by [PERSON] et al. [18] that both the \"Invisalign\" appliance and fixed appliance achieved good root angulation. This result may be attributed to the innovation in the \"Invisalign\" system, supported by big data and the improvement in the aligner materials. In addition, judging from the average angle difference (aligner 7.00\({}^{\circ}\)\(\pm\) 5.30\({}^{\circ}\); fixed 10.81\({}^{\circ}\)\(\pm\) 9.16\({}^{\circ}\)), the fixed appliance might have a greater range in root angulation adjustment than the aligner, which was basically consistent with the conclusion of a systematic evaluation from [PERSON] et al. [22], who reported the likelihood of achieving \"great improvement\" in a malocclusion with fixed appliances. Nevertheless, since, the root control performance of clear aligner and fixed appliances remains controversial, we believe that further studies (for example, research on the mechanical properties of materials and 3D printing) are needed to verify the root-movement control capacity of both orthodontic appliances. The categories of the nonparallel angulation of the root on both sides of the tooth extraction space after treatment with an aligner and a fixed appliance were different: the former mainly showed root apical divergence, while the latter mainly showed root apical convergence. Lack of root parallelism has long been reported as a possible factor for the relapse of extraction spaces [21]. The nonparallel root can lead to an uneven distribution of the bone between the roots. If the distance of the root apical is too large (root apical divergence), the interproximal contacts can move toward the crown, possibly resulting in a black triangle, which is not only unaesthetic but also easily leads to food impaction and thus affects periodontal health [23; 24]. Furthermore, if the distance of the root apical was too small (root apical convergence), i.e., a width of bone and tissue \(<\) 0.8 mm, it easily causes the risk of periodontitis [25]. Therefore, it is very important to control the root parallelism with an orthodontic system. A clear aligner appliance, which mainly showed root apical convergence, may be prone to relapse, and this inference is consistent with the results of [PERSON] et al. [15]. This indicates the necessity to analyze root parallelism near the tooth extraction side before finishing the clear aligner appliance treatment, to guide the axial inclination adjustment of the teeth in refinement. To avoid the influence of measurement error, we set the angulation change outside this range (between \(-\)7\({}^{\circ}\) and 7\({}^{\circ}\)) to be considered as effective tipping change. The results of this study showed that the dental crown of the maxillary molars had a tendency of mesial tipping after treatment with either appliance. This phenomenon may be caused by the blocking of the maxillary bone structure, the loss of anchorage, and the contraction-deformation of aligner, which is consistent with clinical observations. However, the dental crown of the mandibular first molars had a tendency toward distal tipping after treatment with either appliance. The dental crown of the mandibular second molars showed no obvious tipping change after clear aligner treatment, while it showed a tendency toward distal tipping after fixed appliance treatment. Comparing the mean value of the angulation changes, the difference was statistically significant between the clear aligner and the fixed appliance. In the treatment of class I and class II cases, when close to the tooth extraction spaces, the mandibular molar may need more mesial movement, while the root movement of the mandibular molars was more obvious than the crown due to the pull of the supra-alveolar fibers. This may explain why the mandibular molars likely tended toward distal inclination. Overall, the clear aligner requires both effective attachments and completed aligner coverage to load the orthodontic forces that can be calculated and designed by computer.