Material parameters and stress profiles within the periodontal ligament.

Levels and profiles of initial stress in the periodontal ligament after application of various force systems were studied. Two finite-element models, based on sections of human autopsy material, were developed to simulate one full and one partial mandible. The validity of the finite-element model was improved by identification of material parameters; the mechanical properties of the tissue were described by means of strain-gauge measurements of initial tooth movements in human autopsy material. The multiple modeling technique, in which data from a coarse global model are transferred to a more detailed one, was used to identify bone structure and boundary conditions. Parameters known to influence the results were varied to establish the validity of the finite-element model. Iterative calculation methods were used to gain stable results. However, optimizing features of the bone structure and boundary conditions did not influence the results significantly. The elastic stiffness of the periodontal ligament was determined to 0.07 MPa and tau = 0.49 (tau being the Poisson's ratio). Stress profiles were obtained for various force systems--as in tipping, translation, and root movement. As we expected, there was a marked variation in the stress distribution from cervix to apex when tipping forces were applied. Bodily movement of the tooth produced an almost uniform stress distribution; root movement produced stress patterns opposite to those observed during tipping; and masticatory forces alone produced stress patterns almost identical to those achieved by masticatory force in combination with orthodontic forces.