Relationship between Mandibular Asymmetry and Articular Disc Position Relative to the Mandibular Condyle

Objective: To investigate the relationship between the length of the mandible and articular disc position in the temporomandibular joint using information obtained with magnetic resonance (MR) imaging and helical computed tomography (CT).<BR>Materials and Methods: Ninety patients with jaw deformity who were candidates for surgical orthodontic treatment were examined with MRI and CT. One-hundred and eighty joints of 78 patients were classified into the following three categories according to the articular disc position observed on MR imaging: superior positioned type (type S), anterior positioned type (type A), and slightly posterior positioned type (type P). Each patient was assigned to one of six groups, according to the combination of disc positions on both TMJ's: S/S, A/A, P/P, S/A, A/P, and S/P. For 3D CT measurement of the mandibular length, the apex of the mandibular condyle (Co), the narrowest point of the mandibular incisure (In), the most lateral point on the mandibular angle (An), mental foramen (Fo), and Pogonion (Po) were assigned as reference points. Differences between Co-Po distances on both sides were evaluated as the reference for mandibular asymmetry, and correlated with articular disc position in each group. In the asymmetry group, differences between Co-In, In-An, An-Fo, and Fo-Po distances on both sides were also evaluated and correlated with articular disc position.<BR>Results: Differences between Co-Po distances on both sides were greatest in the A/P group, followed by the S/A, S/P, A/A, S/S, and P/P groups in descending order. In every case of S/A group and P/A group, mandibular midline displacement was observed on the side of the type A disc position. In the S/P group, the mandibular midline of each case shifted toward the side of the type S disc position. The difference between Co-In distances on both sides was greatest, especially in the A/P group. Of all cases of the S/A, A/P, and S/P groups, Co-In distances were longer on the side of the type P disc position, and were shorter on the type A side.<BR>Conclusion: The present investigation indicated that mandibular asymmetries were much greater in groups with different types of articular disc position in both joints than in groups with the same types of disc position in both joints. In groups with different types of articular disc position, it was suggested that the shorter side of the mandible might relate to the anteriorly positioned articular disc, and the longer side of the mandible might relate to the posteriorly positioned articular disc.