Finite analysis of the mandible and TMJ with chin cup therapy

Objectives: To investigate stress distributions on the mandible and TMJ during the application of a chin cup force in different directions by means of a finite element method (FEM) from a living subject. Methods: A force of 7.84 newtons was applied to the menton area and distributed in 19 nodes in order to evaluate condylar and TMJ loading. An finite element model consisting of 7942 nodes and 41010 elements was developed for this purpose. Morphologic characteristics were obtained from a living subject, to whom a computerized tomography was taken in order to determine the geometry. Mechanical properties of anatomic components were taken from the literature, with cortical and cancellous bone being modeled as orthotropic, while dentin, periodontal ligament, articular disc and temporal bone were modelled as isotropic. Different force directions were evaluated with respect to a reference line traced on a cephalogram which was formed by the union of condyleon and gnathion. Forces were evaluated at 30°, 15°, 0°, -15° and -30° degrees with respect to said line. Results: Compressive stresses were observed in the mandible and TMJs. They were found to be higher for the condyle than the temporal bone. In the condyle they were mainly localized in the upper medial aspect. Stresses differed depending upon the force direction with the lowest being for the 30° degree (clockwise direction) and the highest for -30° degree (counterclockwise direction). Compressive stresses were registered in incisors labial alveolar process. Conclusions: Symmetric loading of the condyle and TMJ was observed and it was concluded that the disk dissipated the stresses received. Under the same chin cup force, changes in direction can alter the stresses the TMJ withstands. It is suggested that lingualization of lower incisors during chin cup therapy could be due to the loading pattern generated with this type of treatment.