# Upper Second Molar Distalization with Clear Aligners: A Finite Element Study
## 4 Discussion  
[PERSON] et al. and [PERSON] et al. demonstrated that upper molar distalization efficiency is strictly dependent on the use of attachments, as well as Class II elastics [6; 7]. Within this research, Class II elastics were not considered since the aim was to test the actual efficacy of aligners only in distalizing upper molars. Based on our results, one could state that attachments have an influence on the force level and tooth displacement. However, in all the analyzed configurations, the amount of molar distalization reached was not enough to establish that aligners alone are effective at performing this kind of movement. Such findings are consistent with what was demonstrated in previous clinical trials [6; 7]. This study is the first attempt to uncover the force system and biomechanical behavior of aligners during distalization. In our opinion, one shall focus on aligner deformation: the obtained results demonstrated that even with a full set of attachments, the majority of programmed tooth movement (0.2 mm) would be lost in meso-buocal deformation of aligners upon insertion. Even if limited in time, this information could help us interpret clinical conditions in which the amount of space between molars generated from the replacement of aligners, as well as the amount of sagittal correction, does not match the ClinCheck(r) simulation. Another effect of the mesial deformation of aligner upon insertion is the buccal shift of lateral incisor, which represents the maximum displacement area in NO ATT and ATT 3-7 setups, while in ATT 3-6 present almost an equal amount of movement with respect to the second molar. Thus, this biomechanical study seems to confirm the clinical experiences of several experts that report this tooth as the most difficult to control with orthodontic aligners. Figure 8: PDL pressure (lateral view). Figure 6: ATT 3–6 contact pressure. Figure 7: ATT 3–7 contact pressure. Figure 9: Upper second molar PDL pressure (oclusal view). In analyzing the obtained results, it should be highlighted that the configurations with attachments are more effective in terms of the amount of displacement and PDL stress compared to a NO ATT configuration. The best performing model for tooth displacement appears to be ATT 3-6, which also presents the highest-pressure value at the PDL level. A similar study by [PERSON] et al. that analyzed force systems during distalization reported that the presence of attachments increased the measured force on the upper molar, although the authors reported it to be not statistically significant [15]. The slight differences in terms of tooth displacement and PDL stress between ATT 3-7 and ATT 3-6 could be explained on the basis of different aligner stiffnesses resulting from the number of applied attachments. In the ATT 3-6 configuration, the attachment units from tooth 1.6 to tooth 1.3 should act as an anchorage against the desired force (producing molar distalization) and the undesired effect (incisors buccal flaring). However, this assumption is true to a limited extent since anchorage preservation on lateral incisor still remains poor. The second molar, on which the aligner is acting to produce the distalization movement, is covered by a more elastic aligner section with respect to the anchorage unit; therefore, more displacement could be expected. On the other hand, the increased number of attachments in ATT 3-7 aligner could reduce the amount of second molar distalization, compared to the previous configuration, since the aligner section covering this tooth presents increased stiffness due to its anchorage to a rectangular attachment. However, these results are in contrast with the one published by [PERSON] et al. [7] in their 2016 study. The obtained results resemble the type of tooth movement obtained by placing an open spring between second and first molar braces in conventional orthodontics; so, as it would be done with fixed appliances, anchorage reinforcement with auxiliaries is a mandatory choice to improve the efficiency of the system.