## 4 Discussion  
The use of aligners might be extremely useful in the treatment of long cases such as impacted canine, considering the high prevalence of decays and TMD that can be present in some ages and the risk of demineralization due to braces [37; 38; 39]. Furthermore, to increase the predictability of the treatment, the analysis of the FEA results provided interesting information that could improve the design phase of orthodontic aligners, allowing the desired effects to be maximized and the unwanted effects to be minimized. The resulting parameters about the force system helped to compare advantages and disadvantages for each aligner configuration. Figure 11: Patient’s documentation at four-year follow-up: extraoral (**a**) and intraoral (**b**) photographs. There are several studies in the literature that allow the efficiency of combinations of attachments and other auxiliaries to be evaluated depending on desired movement. [PERSON] et al. show that the aligner with the overhanging attachment can effectively reduce crown tipping and prevent axial rotation for an intended distal displacement of the central incisor [40]. Thanks to studies that follow a similar methodology, we could state that the optimal solution for an attachment placed on the lower canine is a cylinder form attached to the lingual side of the tooth [41]. With this methodology, it is possible to evaluate the behavior of teeth not only in relation to the combination of auxiliaries, but also to the type of movement [42]. In this study, the final choice of a pre-activation point (PAP) and an attachment was justified by the numerical results obtained by FEA. The FEA results demonstrated that the different design choices have a strong influence on the force system delivered to the tooth. The amount of Moment z (Mz) delivered to the tooth increased almost two times when adding the pressure point and the attachment to the standard aligner, while the aligner with two attachments brought a lower Mz, but also undesired lower Mx and My moments. The maximum tooth displacement (0.85 mm) was gained with the pressure point and attachment aligner configuration, while the lowest (0.058 mm) was gained with the use of a couple of attachments. These results are also shown in Table 2.  
Referring to Table 2, it can be noticed that the configuration led to the highest moments and forces. The double attachment instead brought load values lower than the standard aligner. The effectiveness of the choice can also be observed in Figure 9. The standard aligner does not satisfy the clinical expectations, leading to a tipping along the \(x\)-axis. With the double attachment configuration, the center of rotation (blue circle) is closer to the Cres than with the other configurations, bringing a rotation of the tooth along its long axis (\(z\)). This could probably be related to the symmetric position of the attachments with respect to the tooth's long axis. The pressure point and attachment aligner configuration generated a vestibular movement as desired by the clinician, as shown in Figure 9 and confirmed by the higher force on the vestibular direction (\(x\)-axis) compared to the other aligners. The planning of an orthodontic aligner treatment should be split in two phases: the definition of the expected movement and the choice of auxiliary elements for each movement. Our aim was to demonstrate how the auxiliary element's features affect the interaction between the aligner and the target tooth. It was demonstrated that both an attachment and a pressure point could improve the effectiveness of the appliance. In particular, the pressure point showed a higher influence on the absolute load value.