[Stress distribution in mandible induced by occlusal force in different horizontal mandibular positions]

The objective of this study was to investigate the stress distribution in the mandible induced by occlusal forces in a various different horizontal mandibular positions identified as neutroclusion, distoclusion, mesioclusion, and laterally shifted occlusion. A three dimensional Finite Element Method (3D-FEM) was applied to an artificial model of a simulated mandible constructed as a homogeneous, isotropic and elastic body. This model was divided into 480 elements with 785 nodes. The right and left heads of the mandibular condyles and the occlusal surfaces of the posterior teeth of the model were restrained. These restraints acted against the loading of the masticatory forces which consisted of a pair of masseter, temporal, and also the lateral and medial pterygoid muscles. The stress distributions analyzed by 3D-FEM were graphically demonstrated on a three dimensional display system. The data obtained were summarized as follows: I. Neutroclusion (1) Tensile stress appeared along the anterior border of the ramus, and compressive stress appeared along the angular portion of the ramus. (2) Compressive and tensile stress appeared on the external and the internal surfaces of the anterior portion of the mandibular body. II. Distoclusion and mesioclusion (1) As the position of the mandible was moved distally, the absolute values of the stress tended to increase on the same areas which is the external and the internal surfaces of the anterior portion of the mandibular body. (2) Compressive stress which appeared on the occlusal surfaces of the posterior teeth was very great in distoclusion. At the same time, the stress that appeared on the heads of the mandibular condyle was also great in mesioclusion. III. Right laterally shifted occlusion (1) Tensile stress appeared along the left sides of both of the left and right rami, and compressive stress appeared along the right side of both of the left and right rami. (2) A great amount of stress appeared on the left occlusal surfaces of the posterior teeth and on the left portion of the mandibular body. From these results, it was determined probably that the mandibular morphogenesis was influenced by a series of stress factors which were induced by the forces of the masticatory muscles.