[Biomechanical study on orthodontic tooth movement by means of numerical simulation. Effects of principal stresses in periodontal membrane].

The effect of biomechanical factors on tooth movement has not been clarified in a quantitative manner. This study was designed to investigate the stresses affecting tooth movement, using a numerical simulation. The influence of decrease in Young's modulus of the periodontal membrane on canine retraction was also examined through the simulation. A two-dimensional finite element model was constructed based on the average anatomic morphology of Japanese canine. A numerical simulation program based on the finite element method was developed for the orthodontic tooth movement. The stresses in the periodontal membrane were evaluated. The principal stress of which absolute is larger was selected as a reference stress. Each nodal point at the alveolar bone-periodontal membrane interface was repositioned in the direction of reference stresses, in response to discrepancy between assumed thresholds and the reference stresses. Moment to force (M/F) ratios at the bracket position of this model were examined for evaluating force conditions. Simulation of tooth movement were executed under three force conditions with different M/F ratios for distal movements and two force conditions for vertical movements. Three types of canine retraction, tipping movement, bodily movement and root movement, were displayed with the numerical simulation. Extrusion and intrusion were also displayed. Analytic movements of the canine were close to the actual tooth movements that have been reported, utilizing the principal stresses with the thresholds of the maximum and minimum principal stresses being about +0.4 gf/mm2 and -0.4 gf/mm2. The decrease in the Young's modulus of the periodontal membrane changed bodily movement to tipping movement under the same force conditions. These results indicate that the value and the direction of the principal stress in the periodontal membrane are key determinants of tooth movement and this numerical simulation is useful for investigating the influence of the biomechanical factors on tooth movement.