A study using Three-Dimensional Cone Beam Computed Tomography to evaluate masseter muscle morphology in observed skeletal malocclusions

Objectives. To evaluate the masseter muscle, in both orientation and length, in the anterior- posterior and vertical dimension using 3D Cone Beam Computed Tomography. Methods. One hundred and eighty two DICOM files were analyzed using Dolphin imaging 10.5 Premium licensed to West Virginia School of Dentistry Department of Orthodontics. These DICOM file were divided into anterior- posterior groups using ANB angle. The groups were Group I determined to be subjects with normal skeletal growth pattern, Class I ANB angle range 0°--5°, Class II ANB angle > 5°, and Class III ANB angle < 0°. Overjet will be recorded for all subjects groups. The DICOM files were also divided into vertical groups consisting of normodivergent with mandibular plane angle (FH/MP 22° to 30°), and hyperdivergent with mandibular plane angle > 30°, hypodivergent with mandibular plane angle < 22°. The masseter muscle was identified and landmarks placed on the anterior border, at the origin and insertion of the muscle, in 3D mode of the Dolphin software. The Frankfort Horizontal Plane was used as a reference plane and an angular measurement was obtained with intersection of a line produced by the masseter muscle landmarks, thus calculating the orientation of the masseter muscle. The length of the masseter muscle was also calculated in millimeters using the landmarks. Results. The right and left masseter muscle length and orientation was compared for all subjects in this study (n=182) using ANOVA and matched pairs test and no significance was found. The masseter muscle length was determined to be not significant in the anterior- poster dimension but statistical significance was seen in the vertical dimension between hyperdivergent and hypodivergent groups and hypodivergent and normodivergent groups. The masseter muscle orientation angle was determined to be not significant in the vertical dimension but significant in the anterior- posterior dimension between the Class I, Class II, and Class III groups. Conclusions. The masseter muscle orientation angle is statistically significant in the anterior- posterior dimension with Class II having the most acute orientation angle and Class III having the most obtuse orientation angle. This shows that there is a mechanical advantage in bite force for Class III individuals versus Class II individuals mainly because muscle fibers are oriented more along the arch of closure enabling these individual to produce more force in biting. Other studies should be done to determine if the orientation of the masseter muscle changes after orthognathic surgery is completed and if this effects stability.