Comparison of the stress direction on the TMJ in patients with class I, II, and III skeletal relationships.

OBJECTIVE To assess the relationship between skeletal morphology and stress direction on the temporomandibular joint (TMJ) by a two-dimensional rigid body spring model (RBSM). DESIGN Lateral cephalograms were analyzed and the information was processed with a fortran analysis program. SETTING AND SAMPLE POPULATION The subjects were 149 patients (54 men and 95 women, mean age 21.8 +/- 5.9 years) from Kanazawa University Hospital and the School of Dentistry, Virginia Commonwealth University. Of the 149 cases, 48 were skeletal class I, 54 were class II, and 47 were class III. The patients had no TMJ symptoms or abnormalities. OUTCOME MEASURE The force vector on the condyle, its direction (Ph angle), the degree of the vector (Ph) and the displacement vector (u, v), and the rotational angle (theta) of the mandibular body were calculated by RBSM. RESULTS The direction of the force vector (Ph angle) on the condyle was 24.83 degrees +/- 4.67 degrees in the class II group, 21.04 degrees +/- 5.59 degrees in the class I group, and 19.58 +/- 7.57 degrees in the class III group. The Ph angle of the class II group was significantly larger than those of the class I and III groups (p < 0.05). CONCLUSIONS This study suggests that differences in skeletal patterns induce differences in stress distribution on the TMJ; the morphology of the TMJ was also associated with stress direction and distribution on the condyle.