Evaluation of masseter muscle morphology in different types of malocclusions using cone beam computed tomography.
OBJECTIVE
To evaluate the length and orientation of masseter in different types of malocclusions using Cone Beam Computed Tomography (CBCT).


METHODS
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.


RESULTS
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 ± 4.0 mm) compared to the normodivergent group (45.6 ± 4.5 mm, P < 0.05) and shorter muscle length for the hyperdivergent group (42.3 ± 4.7 mm) 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°).


CONCLUSIONS
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.
107 Watterson Trail #103, Louisville, Kentucky KY 40243, USA.
Michael P
MP
Becht
Department of Orthodontics, University of Nevada, Las Vegas School of Dentistry, 1001 Shadow Lane MS 7410, Las Vegas, Nevada NV 89106, USA.
James
J
Mah
Department of Orthodontics, West Virginia University School of Dentistry, Health Science Center North, P.O. Box 9480, Morgantown, West Virginia WV 26506, USA.
Chris
C
Martin
Department of Diagnostic Services, West Virginia University School of Dentistry, Health Science Center North, P.O. Box 9480, Morgantown, West Virginia WV 26506, USA.
Thomas
T
Razmus
Department of Statistics, West Virginia University School of Dentistry, Health Science Center North, P.O. Box 9480, Morgantown, West Virginia WV 26506, USA.
Erdogan
E
Gunel
Department of Orthodontics, West Virginia University School of Dentistry, Health Science Center North, P.O. Box 9480, Morgantown, West Virginia WV 26506, USA. Electronic address: pngan@hsc.wvu.edu.
Peter
P
Ngan
D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D002508:Cephalometry / Q000379:methods; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D003766:Dental Occlusion; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D008310:Malocclusion / Q000000981:diagnostic imaging*; D008312:Malocclusion, Angle Class II / Q000000981:diagnostic imaging; D008313:Malocclusion, Angle Class III / Q000000981:diagnostic imaging; D008334:Mandible / Q000000981:diagnostic imaging; D008406:Masseter Muscle / Q000000981:diagnostic imaging*; D008437:Maxilla / Q000000981:diagnostic imaging; D061330:Multidetector Computed Tomography / Q000379:methods; D009295:Nasal Bone / Q000000981:diagnostic imaging; D015203:Reproducibility of Results; D014716:Vertical Dimension
Conebeam computed tomography (CBCT); Malocclusions; Muscle morphology