Abstract: Objective　To analyze the orthodontic efficiency of buccal and lingual attachments with different sizes by finite element method in order to determine the optimal position and size of the attachments. Methods　Experiment 1: a normal occlusion sample was selected to establish the model. Attachments were divided into five groups: the control group, the buccal horizontal rectangular attachment group, the buccal vertical rectangular attachment group, the lingual horizontal rectangular attachment group and the lingual vertical rectangular attachment group. Except for the control group, the other 4 groups contained 3 subgroups (grouped by rectangular attachment length 3/4/5 mm). The long axis of the maxillary first premolar was set as the rotation central axis, and a 2° clockwise rotation (occlusion view) displacement was applied to carry out simulation operation to observe the stress distribution and displacement changes. In experiment 2, the attachment was modified and divided into 4 groups: the control group, the lingual b (2/3) rectangular attachment group, the lingual b (1/2) rectangular attachment group and the lingual b (1/3) rectangular attachment group. The method was the same as experiment 1. Results　In experiment 1, the periodontal membrane stress, the maximum displacement of the crown and the torsion efficiency of orthodontic force showed the maximum stress with the 3 mm attachment size in the lingual vertical group, and it decreased with the increase of the attachment size units in each group. The vertical group was larger than the horizontal group, and the expression of the torsion degree efficiency was the smallest in the group without the attachment. In the experiment 2, the periodontal membrane stress and the maximum displacement of the crown of the b (2/3) lingual vertical group were the closest to that of the 3 mm lingual vertical group, while the highest expression of torsion efficiency was the 3 mm lingual vertical group, and decreased with the decrease of the size. Conclusion　The 3 mm lingual vertical group shows more beneficial to correct torsional premolars, and the b (2/3) lingual vertical attachment could be used as the improved specification.

Key words: tooth abnormalities, orthodontic appliances, bicuspid, finite element analysis, lingual attachment