Three-dimensional morphology of the masseter muscle in patients with mandibular prognathism

Objective: To compare the morphology of the masseter muscle in patients with mandibular prognathism with that of normal subjects, Methods: Three-dimensional X-ray computed tomography (CT) was performed on 69 patients with mandibular prognathism and compared with 91 normal subjects. The angle of the muscle direction in relation to the Frankfurt horizontal plane and the area and the ratio of length of the short to long axes (s/l ratio) on the section perpendicular to the muscle direction were measured. Results: The mean angle, area and s/l ratio in patients with mandibular prognathism was 76.6 degrees (s.d, 4.4 degrees), 318.3 mm(2) (s.d. 77.2 mm(2)) and 0.312 (s.d. 0.049), respectively. Those of the normal subjects were 65.1 degrees (s.d. 4.4 degrees), 368.3 mm(2) (s.d. 97.2 mm(2)) and 0.393 (s.d. 0.054), respectively, The angle was significantly larger, and the area and sjl ratio were significantly smaller than those of normal subjects (P<0.001). Conclusion: The morphology of the masseter muscle in mandibular prognathism is significantly different from that of normal subjects. Our results may be helpful in evaluating the results of orthognathic surgery.