Investigation of the effect of different attachments and different gingival lenghts of aligners on upper molar distalization

The aim of this study was to investigate the effect of different attachment shapes and aligner lengths on the distalization movement of the maxillary first molar using Finite Element Analysis (FEA). The eight groups of the study were; without attachment, vertical attachment, vertical attachment angled 25 degree, double horizontal attachment and combined with two different aligner lengths for each group; aligner ending at the gingival margin and 2 mm above the gingival margin. The amount of displacement of the maxillary first molar and the von Mises stress values developed in the periodontal ligament (PDL), crown and roots of the maxillary first molar were evaluated by FEA. Buccolingual and mesiodistal tipping of the maxillary first molar occurred mostly on models without attachment with both aligner lenghts. The least tipping movement was found in models with the vertical attachment angled 25degree. When all the landmarks moved distally, the palatal root tip moved mesially in all models. The least mesial movement of the palatal root was observed in the double horizontal attachment models. Increasing the aligner length generally increased the amount of distal movement and von Mises stresses on the PDL and the roots. It provided significantly more controlled root movement in the models without attachments. The use of different attachments and different lengths of aligners significantly affects the transmission of stress and the actual movement. Although the same amount of displacement was applied to all models, the distal movement on the Y-axis occured more with the aligner ending 2 mm above the gingiva. This shows that stress transmission can occur more easily in aligners 2 mm above the gingiva. Models with vertical attachment angled 25 degree and double horizontal attachment showed the most controlled and effective distalization movement. It has also been observed that as the length of the aligner increases, the von Mises stress in the roots of the first molar and the periodontal ligament also increase.