Simplified stress analysis on the temporomandibular joint in Class III patients with and without mandibular asymmetry using a rigid body spring model.

OBJECTIVE Aim of this study was to investigate the differences in stress on the temporomandibular joint (TMJ) between Class III patients with and without mandibular asymmetry using a rigid body spring model (RBSM). DESIGN Menton (Me), the centre point of occlusal force on the line that connected the bilateral buccal cusps of the second molars and the most lateral, superior and medial points of the condyle were plotted on frontal cephalograms, and stress on the condyles was calculated with the 2-dimensional RBSM program of fortran. SETTING AND SAMPLE POPULATION Eighty Japanese patients with diagnosed mandibular prognathism were divided into two groups, a symmetry group and asymmetry group on the basis of the Mx-Md midline position. OUTCOME MEASURE The degree (force partition) of the resultant force, the direction (angulation) and displacement (X, Y) of each condyle were calculated. The horizontal displacement vector (u), the vertical displacement vector (v) and rotation angle (theta) of the mandibular body at Menton were also calculated. RESULTS There were significant differences between the deviated and non-deviated sides of both groups regarding resultant force (symmetry group: p = 0.0372, asymmetry group: p = 0.0054), X (symmetry group: p < 0.0001, asymmetry group: p = 0.0001) and Y (symmetry group: p = 0.0354, asymmetry group: p = 0.0043). For angulation, there was a significant difference between the deviated and non-deviated sides in the asymmetry group (p = 0.0095). CONCLUSION The results of this study suggest that difference in stress angulation on the condyles could be associated with asymmetry in mandibular prognathism.