## 4 Discussion  
The use of interradicular buccal skeletal anchorage, in Class II non-extraction treatments with aligners, could reduce issues of patient compliance, which provides a consistent intraoral anchorage unit and allows the desired tooth movements, reducing unwanted movement of the anterior teeth. The results of the present FEM study support the use of TADs as anchorage units. Previous in vitro and in vivo studies by our team demonstrated that a proper treatment design, including attachments on all of the distalizing teeth and the use of Class II elastics, could provide an anteroposterior correction of about 3 mm in a reliable way [6; 9; 10]. However, it was demonstrated that the worst undesired effect of the mechanics was represented by huge posterior anchorage loss when starting premolar distalization [26]. Therefore, the use of TADs could be the solution to avoid posterior anchorage loss. The use of Class I elastics could be more acceptable by patients with respect to Class II interact elastics, because they are less visible and, therefore, more esthetic, therefore reducing patient compliance issues. Furthermore, Class II elastics reduce lingual tipping of the anterior teeth but aggravated mesial tipping of the posterior teeth [27]. It must be reminded that clear aligners provide a good control of the lower incisors' final position, despite the use of Class II elastics [28]. Regarding the performance of the analyzed configurations in determining the programmed distalization movement of the second premolars and first molars, the configurations that guarantee the best result are those in which the rubber band is parallel to the occlusal plane. In these configurations, it is possible to detect the greater component of displacement in the distal direction of the first molar and second premolar compared to those with the elastic positioned on the same element but at different angles. This is confirmed by the value of the contact pressure between the teeth and the aligner on the mesial surface of the attachments of the second premolar, which is the maximum when the elastic is parallel, and progressively decreases as the angle, with respect to the occlusal plane, increases. Regarding anterior anchorage loss, the greatest proclination of the incisors is observed in configurations with the elastic positioned at the level of the canine. In fact, these configurations present a total displacement of the incisal edge of the central incisor that is approximately 30% greater than those with the rubber band on the premolar. Looking at the displacement vectors, the lateral incisor is less affected by this side effect. Considering this, configurations with the rubber band positioned on the canine may be more indicated in Class II Division 2 malocclusions, where the side effect of proclination of the central incisors, which is greater in these configurations, may help correct their initial retroclination. The choice of elastic angulation with respect to the occlusal plane should be reduced to the configuration with the elastic parallel and at 30\({}^{\circ}\). The 30\({}^{\circ}\) elastic results in a greater intrusive component at the canine level and is, therefore, preferable in cases where this movement is required [Table 3]. From analyzing the configurations with the elastic on the first premolar, the one with the elastic parallel to the occlusal plane results in the least vestibular movement of the canine and is, therefore, the only one to be considered. Given the lesser extent of proclination at the incisor level compared to configurations with the rubber band on the canine, it may be preferable in Class II Division 1, where incisors are already proclined. Class II elastics have been used in various clinical trials to avoid pro-inclination of the incisors during sequential distalization with aligners [9; 10; 21; 28]. In FEM simulations previously conducted by [PERSON] et al.