Using 3D-CT to analyze the masseter muscle in facial asymmetry before and after orthognathic surgery

The purpose of this chapter is to understand the differences between the masseter muscle (MM) on the deviated and non-deviated sides in patients with facial asymmetry, and the changes shown by the MM after 2-jaw surgery (Le fort I osteotomy + Intraoral vertical ramus osteotomy). Pre- and post-operative CT scans were performed on 12 Class III patients with facial asymmetry who had been treated with Le fort I osteotomy with intraoral vertical ramus osteotomy (IVRO) and 10 subjects with normal occlusion. Using V-works 4.0 software (Cybermed, Seoul, Korea), 3-dimensional (3D) images of the mandible and MM were reconstructed and evaluated. In the asymmetry group, the MM angle, formed by the anterior border of the MM and the Franfort Horizontal Plane between the deviated and non-deviated sides was significantly different (p<0.05). Compared to the normal occlusion group, the asymmetry group showed a significantly smaller volume and maximum cross-sectional area on both sides of the MM (p<0.05). After 2-jaw surgery, the angle of the MM (p<0.01) and differences in the angle between the deviated and non-deviated sides of MM (p<0.05) were significantly decreased. The thickness at the maximum cross-sectional area was significantly increased (p<0.01). After surgery, the MM in facial asymmetry patients changed to resemble that of the normal occlusion group except in width. The MM in facial asymmetry patients was definitely different from that in those who had normal occlusion. However, this study suggests that the MM changed symmetrically in conjunction with the mandible after surgery combined with Lefort I osteotomy and IVRO.