To evaluate the length and orientation of masseter in different types of malocclusions using Cone Beam Computed Tomography (CBCT).

Samples of 180 patients seeking orthodontic treatment at the University of Nevada, Las Vegas School of Dentistry, were included in the study. Pre-treatment multi-slice CBCT scans of these patients were divided into three anteroposterior groups: Class I subjects with ANB angle 0° to 5°; Class II subjects with ANB angle >5°; and Class III subjects with ANB angle <0°. CBCT scans were also divided into three vertical groups: normodivergent subjects with mandibular plane angle 22° to 30°; hyperdivergent subjects with mandibular plane angle >30°, hypodivergent subjects with mandibular plane angle <22°. The masseter was identified and landmarks were placed on the anterior border, at the origin and insertion of the muscle in 3-D mode of the Dolphin Imaging 10.5 Premium software. The Frankfort Horizontal Plane was used as a reference plane and an angular measurement was obtained by intersection of a line produced by the masseter landmarks to calculate the orientation of the muscles. The length of the masseter was measured and data were analyzed using ANOVA and matched pairs test.

ANOVA found significant differences in muscle length among the three vertical groups for both the left and right muscles. Paired t test showed significantly shorter muscle length for the hypodivergent group (43.3±4.0mm) compared to the normodivergent group (45.6±4.5mm, P<0.05) and shorter muscle length for the hyperdivergent group (42.3±4.7mm) compared to the hypodivergent group, P<0.05. No significant differences were found in muscle length among the three anteroposterior groups. However, significant differences in muscle orientation angle were found among the three anteroposterior groups (P<0.05). Class II subjects were found to have the most acute orientation angle (67.2±6.6°) and Class III had the most obtuse orientation angle (81.6±6.8°).

These results suggest that certain types of malocclusion may have different masseter lengths and orientations and these differences may have implications for the mechanical advantage in bite force. For example, Class III individuals may have greater bite force than Class II individuals because the muscle fibers are oriented more along the arch of closure.