Stress distributions at the periodontal ligament and displacements of the maxillary first molar under various molar angulation and rotation: Three dimensional finite element study

Kwon, Dae Woo; Son, Woo Sung; Yang, Hoon Chul

Abstract

The purpose of this study was to evaluate the stress distributions at the periodontal ligament (PDL) and displacements of the maxillary first molar when mesially directed force was applied under various molar angulations and rotations. A three dimensional finite element model of the maxillary first molar and its periodontal ligament was made. Upright position, mesially angulated position by 20degrees and distally angulated position of the same degree were simulated to investigate the effect of molar angulation. An anteriorly directed force of 200g, countertipping moment of 1,800gm-mm (9:1 moment/force ratio) and counterrotation moment of 1,000gm-mm (5:1 moment/force ratio) were applied in each situation. To evaluate the effect of molar rotation on the stress distribution, mesial-in rotation by 20degrees and the same amount of distal-in rotation were simulated. The same force and moments were applied in each situation. The results were as follows: In all situations, there was no significant difference in mesially directed tooth displacement. Also, any differences in stress distributions could not be found, in other words, there were no different mesial movements. Stress distributions and tooth displacement of the 20degrees mesially angulated situation were very similar with those of the 20degrees distal-in rotated situation. The same phenomenon was obserned between the 20degrees distally angulated situation and 20degrees mesial-in rotated situation. When the tooth was mesially angulated, or distal-in rotated, mesially directed force made the tooth rotate in the coronal plane, with its roots moving buccally, and its crown moving lingually. When the tooth was distally angulated, or mesial-in rotated, mesially directed force made the tooth rotate in the coronal plane, with its roots moving lingually, and its crown moving buccally. When force is applied to an angulated or rotated molar, the orthodontist should understand that additional torque control is needed to prevent unwanted tooth rotation in the coronal plane.