Objective: The purpose of this study was to understand the differences in masseter muscle (MM) between the shifted and non-shifted sides in facial asymmetry patients, and the changes shown by MM after mandibular surgery. Methods: Pre- and post-operative CT scans were performed on 12 Class III patients with facial asymmetry who were treated by intraoral vertical ramus osteotomy and 10 subjects with normal occlusion. Using the V-works 4.0 program (Cybermed, Seoul, Korea), 3-dimensional images of the mandible, and MM were reconstructed, and evaluated. Results: In the asymmetry group, the MM angle between the shifted and non-shifted sides was only significantly different (p < 0.05). Compared with normal occlusion, the asymmetry group showed a significantly smaller volume and maximum cross-sectional area in both sides of MM (p < 0.05). After mandibular surgery, the angle of MM (p < 0.01) and differences in angle between the shifted and non-shifted sides of MM (p < 0.05) were significantly decreased. The thickness in the maximum cross-sectional area was significantly increased (p < 0.01). After surgery, MM in facial asymmetry patients was similarly changed to those in the normal occlusion group except for widths. Conclusions: MM in facial asymmetry was definitely different from those in normal occlusion. However, this study suggests that MM changed symmetrically in conjunction with the mandible after proper mandibular surgery.
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