Objective: The objective of this study is to evaluate the effects of attachments on maxillary molar distalization, distalization with simultaneous expansion, and the influence of the third molar on distalization in the clear aligners using Finite Element Method (FEM).
Material Methods: Six models were created using three different attachment designs on the second molars. Model I: vertical rectangular attachment; Model II: vertical rectangular attachment with the presence of the third molar; Model III: combined semi-elliptical attachment; Model IV: opposed semi-elliptical attachment with buccal and palatal parts. The first four models were used to evaluate distalization. Models V and VI were created by adding expansion to Models III and IV. The displacement amounts, clear aligner deformation, and stress distributions were examined using Ansys 19.2.
Results: The presence of the third molar reduced the maximum total displacement by 17%. The distal displacement and distal tipping in Models I and III were similar and greater than in model IV. Model IV provided the most parallel tooth movement, with the least distal and buccal tipping and dystopalatinal rotation. When comparing the last two models, distal and buccal displacement, as well as tipping, were greater in Model VI than in Model V.
Conclusion: The presence of a third molar reduced distalization and increased the deformation of the clear aligner. In both movements, the attachment that provided the most parallel movement also showed the least displacement. When choosing attachments, tooth rotation and inclination, bone support, and root health should be be taken into account.

Keywords: Attachment, Distalization, Expansion, Clear aligner