OBJECTIVETo investigate the displacement, the position of rotation center and the stress distribution of PDL under different loading force system (Moment/Force, M/F) by simulating clinical loading force system.

METHODSA three-dimensional finite element model of upper central incisor, which consists of 945 isoparametric elements and 1,245 nodes was developed. The displacement, the position of the rotation center and the stress distribution of PDL were analyzed under 13 types of loading force system.

RESULTS1. Different force system led to different types of tooth movement. When M/F= -9.15:1, -10.30 - -10.50:1 and -10.90:1, it brought the result of controlled tipping movement, the bodily movement and the root movement; 2. The graph of the center rotation was a hyperbolic asymptotic line: Mx/Fy = -10.50 (horizontal axis) and L = 6.75 (vertical axis). Moreover, a little change of M/F between -9.15 and -10.90 led to apparent change of the position of rotation center. 3. The maximum strain and stress during the tipping movement were 1.47 x 10(-2) MPa and -2.81 x 10(-2) MPa, and during the bodily movement the results were 1.10 x 10(-2) MPa and -1.86 x 10(-2) MPa, while during the root movement were 0.96 x 10(-2) MPa and -2.58 x 10(-2) MPa.

CONCLUSION1 . Different force system (M/F) leads to different types of tooth movement. 2. It is necessary to adjust the force system accurately to obtain prescient tooth movement, especially when M/F changes between -9.15:1 and -10.90:1. 3. This study suggested that the tooth movement style and the force system (M/F) should be controlled to protect the periodontal tissue.