Effect of moment to force ratios on stress patterns and levels in the PDL.

This study was conducted to investigate the effect of moment to force (M/F) ratios on stress distributions in the PDL. Three-dimensional finite element method (FEM) was applied to stress analysis, using a three-dimensional model of the upper central incisor. Five force systems were established to produce different M/F ratios with a constant 100 g lingual force and/or varying labial crown couples, applied at a point on the labial crown surface, 4 mm gingival to the incisal edge. Stresses were determined in the center of the PDL for eight apicogingival levels and at sixteen points around the root. Stress patterns and levels in the PDL changed in response to varying M/F ratios, however, stress values were invariable at the level of the center of resistance. M/F ratio for translation of a tooth produced the most uniform pattern of stress distributions and the minimum stress levels. It is found that the stress level induced in tooth translation is approximately 0.29 times as that in simple tipping of a tooth. Thus, it is shown that the M/F ratio is an important determinant for controlling the stress patterns and levels in the PDL and for achieving optimal tooth movement.