Biomechanical responses of tooth associated with different root lengths and alveolar bone heights: changes of stress distributions in the PDL.

The purpose of the present study was to elucidate the nature of stress distributions in the PDL varied by different root lengths and alveolar bone heights. A three-dimensional model of the upper central incisor was constructed for the finite element method (FEM). The model was modified to produce various root lengths and alveolar bone heights. A lingually directed 100 g horizontal force was applied at a point on the labial crown surface. Stress distributions were determined in the center of the PDL for various apicogingival levels. Stress levels in the PDL gradually decreased with a longer root. Rates of changes in stress levels to those with an original root length were approximately 1.5 at maximum and 0.8 at minimum. Patterns of stress distributions were varied by different alveolar bone heights in both the qualitative and quantitative aspects; i.e., apicogingival level of stress transition shifted more apical, and stress levels also increased following a reduction of the alveolar bone in the apicogingival direction, approaching about eight times with a half alveolar bone height as the original. It is found that the root length and alveolar bone height affect stress distributions in the PDL. Thus, it is shown that an orthodontic force application should be determined on the basis of anatomical variations in root length and alveolar bone height to induce an optimal stress level in the PDL, which is a key to desirable tooth movement.