Abstract: BACKGROUND: Orthodontic treatment to move the mandibular second molar in place of the missing first molar is a good method for preserving natural teeth. However, molars often exhibit mesial tipping when using clear aligners. The impact of attachments and overcorrection on the molar mesialization with clear aligners is unclear.
OBJECTIVE: To investigate the effects of attachments and overcorrection on displacement and biomechanics of the mandibular second molar mesialization.
METHODS: A finite element model with the left mandibular first molar missing was constructed using a volunteer's cone-beam CT and intraoral scan data. The experiment was divided into four groups based on the position of attachments on the left mandibular second molar: without attachment, buccal attachment, lingual attachment, and buccal-lingual attachment, with overcorrection degrees (0°, 1°, 2°, 3°, and 4°) for each group, totaling 20 models. The calculation and analysis of the displacement trends and stress were performed using Abaqus software.
RESULTS AND CONCLUSION: (1) Mesial tipping and intrusion of the molar, distal tipping of the canines and permolars, and linguaing of the incisors occurred during molar mesialization. (2) The displacement of molars increased when attachments were used, with the maximum displacement occurring when buccal-lingual attachments were applied. Molars tended to tip towards the side without attachments when attachments were designed unilaterally. However, the inclination of molars did not decrease when attachments were used. (3) Molars exhibited reduced mesial tipping and intrusion, approaching overall movement, and the stress of periodontal ligament was relieved and more evenly distributed during overcorrection treatment, and this effect became more pronounced with increased degrees of overcorrection. (4) Attachments can enhance the effectiveness of overcorrection, with the best performance observed when buccal-lingual attachments are used. Using attachments and overcorrection together helps control molar mesial tipping and intrusion, but overall molar movement is not achieved.

Key words: clear aligner, finite element, molar, mesialization, attachments, overcorrection