INTRODUCTION: The orthodontic application of a force or moment leads to a strain distribution in the periodontal ligament. The corresponding local deformation at the interface periodontal ligament-alveolar bone causes a distribution of normal and shearing tension that is thought to be the local stimulus for bone remodeling leading to orthodontic tooth movement. Although researchers have examined initial tooth movement and the biomechanical properties of the periodontal ligament, few have reported human, in-vivo studies. For such examinations, very small tooth displacements must be measured with extremely high resolution and accuracy. METHODS: By a highly sensitive goniometer (resolution <0.001 degrees ), rotations of premolars in 22 subjects with natural spacing and 14 subjects with pronounced anterior crowding were measured in vivo. The rotations followed sets of pure moment-time functions starting at zero and reaching specified final values within +/- 1.43 Ncm. RESULTS: Usually, thresholds were seen that had to be overcome by moment to produce measurable rotations of the tooth in the mesial or distal direction. The subjects with pronounced anterior crowding showed distinct asymmetry: Thresholds for rotations to the distal direction were significantly higher than for those to the mesial direction. CONCLUSIONS: The cases of symmetric thresholds indicated a shear thinning viscosity in the periodontium under increasing moment. In the case of asymmetric thresholds, the findings suggested a mesially directed pretension of the gingival fiber apparatus that might be connected with the mesial drift.
INTRODUCTION: The orthodontic application of a force or moment leads to a strain distribution in the periodontal ligament. The corresponding local deformation at the interface periodontal ligament-alveolar bone causes a distribution of normal and shearing tension that is thought to be the local stimulus for bone remodeling leading to orthodontic tooth movement. Although researchers have examined initial tooth movement and the biomechanical properties of the periodontal ligament, few have reported human, in-vivo studies. For such examinations, very small tooth displacements must be measured with extremely high resolution and accuracy. METHODS: By a highly sensitive goniometer (resolution <0.001 degrees ), rotations of premolars in 22 subjects with natural spacing and 14 subjects with pronounced anterior crowding were measured in vivo. The rotations followed sets of pure moment-time functions starting at zero and reaching specified final values within +/- 1.43 Ncm. RESULTS: Usually, thresholds were seen that had to be overcome by moment to produce measurable rotations of the tooth in the mesial or distal direction. The subjects with pronounced anterior crowding showed distinct asymmetry: Thresholds for rotations to the distal direction were significantly higher than for those to the mesial direction. CONCLUSIONS: The cases of symmetric thresholds indicated a shear thinning viscosity in the periodontium under increasing moment. In the case of asymmetric thresholds, the findings suggested a mesially directed pretension of the gingival fiber apparatus that might be connected with the mesial drift.