20. Cone-beam Computed Tomography Evaluation of Mandibular Symphysis

Mandibular symphysis thickness and morphology is an important factor in orthodontic treatment. In this study, we compared symphysis thickness with various facial types using cone-beam computed tomography (CBCT) and lateral cephalogram in 100 healthy subjects aged from 15 to 56 years. The subjects were classified as long face (46 subjects), normal face (37 subjects), or short face (17 subjects) according to the average values of their Frankfort-mandibular plane angle. CBCT images and lateral cephalograms were performed at the initial examination. The following parameters were measured on saggital sections corresponding to the upper line, middle line, and lower line of the mandibular symphysis. Correlations were determined by statistical analysis with the following results. Facial morphology was determined by measuring SNA, SNB, ANB, facial angle, convexity, mandibular plane angle, gonial angle and Y-axis. There were statistically significant differences in SNB, facial angle, mandibular plane angle, gonial angle and Y-axis among the three facial types, revealing a positive correlation between position and clockwise rotation of the mandible. The thickness of the symphysis at all three lines was greater in the lateral cephalogram than on CBCT, and the differences were statistically significant. The upper line and middle line correlated significantly with the three facial types on CBCT and lateral cephalogram, but no statistically significant difference was found in the lower line among the facial types. We concluded that CBCT is a more accurate way to evaluate mandibular symphysis morphology than lateral cephalogram. Our study confirmed that such structural morphology limits the type of orthodontic treatment that is feasible and could also influence the orthodontic diagnosis and treatment plan.