Development of the model to calculate the mechanical stress which applied to the region of TMJ during functional jaw movement

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Mandibular movement with 6 degrees of freedom and jaw-closing and opening muscles were taken during mastication, open-close jaw movement and voluntary clenching in 10 normal subjects, 10 patients with temporomandibular joint (TMJ) internal derangement, and 10 patients with unilateral posterior crossbite, Morphological image data were also taken in all subjects. The real time scale was transformed to normalized time. CT of head including TMJ were taken in one of above subjects. 3-D finite element model was made in a computer by using CT images and medical fast imaging finite element model making program. Using this model and morphological image data in each subject, respective finite element model was made. It was assumed that CT values on the image were proportional to the bone mineral density and the elastic moduluses of each bone were proportional to the cube root of the bone mineral density. The mechanical parameters were maximum principal and von Mises stresses on the condylar, mandibular fossa, articular disc and mandible. The forces of aforementioned muscles were derived from the equation of equilibrium of moment. The model to calculate the mechanical stress to TMJ was made by using aforementioned data and model. We revealed for the first time that the displacement of the condyle on the chewing side was greater in the direction of the mandibular fossa than that on the non-chewing side, that disc displacement inhibited the condylar motion during mastication and open-close jaw movement, and that the condyle on the posterior crossbite side moved more in the direction of the mandibular fossa, during mastication. In all subjects, almost same results were calculated by using aforementioned model. Therefore, we confirmed that our model to calculate the mechanical stress to TMJ was valid.