## 4 Discussion
In Asia, most patients request mandibular angle reduction during surgery due to the preference for a slim and oval face [20]. Thus, the 3D gonion point would be altered after surgical contouring of the mandibular angle. In mandibular variables on the deviated side, a significant difference was observed in the distances between the bilateral gonion, glenoid fossa, condyle, and medial and lateral condylar poles and the coronal plane between groups A and S. The distance of the deviated side variables to the coronal plane was smaller in group A than in group S. On the opposite side, the distances between the bilateral medial and lateral condylar poles to the MSP were significantly different between the two groups. The distance in group A is greater than that in group S. The different results between the deviated and opposite side variables indicate that the preoperative condyle position in the sagittal and transverse directions might relate to a higher asymmetry index after surgery. One reason is that the condyle and glenoid fossa might be anatomical limitations, because we could not move or change their original position during surgery. This should be considered during 3D planning. In clinical situations, when the distance of the CoL-MSP is greater in the opposite side than in the deviated side, the ramus axis is affected, causing facial asymmetry, even when the midline landmarks are perfectly aligned. In such cases, the patient should be informed preoperatively of a greater chance of residual asymmetry. The mandible border contouring or fat transplantation are choices for further asymmetry correction [8]. Some studies have discussed different surgical techniques to improve the outcome of ramus symmetry. The greatest advantage of intraoral vertical ramus osteotomy (IVRO) over BSSO is a lower incidence of injury to the inferior alveolar nerve [28,29]. Other advantages of IVRO include technical simplicity, shorter operation time, and less adverse effects related to the temporomandibular joint [30]. However, BSSO provides better bony interface with proximal and distal segments, and easier use of rigid fixation, thus minimizing the need for intermaximillary fixation [31]. Therefore, there is larger interference between the proximal and distal segments in IVRO during mandibular setback, resulting in a more significant increase of the total ramus angle, compared with that in BSSO [32]. However, [PERSON] et al. had a different opinion: they reported a series of three cases with severe rotational asymmetry, using unilateral IVRO on the shorter side combined with sagittal split ramus osteotomies on the contralateral side for a greater setback [33]. They proposed that IVRO causes less rotational displacement of the proximal segment on the deviated side. The condylar segments that were displaced or rotated during surgery might return to their original physiologic position. The outcome and stability of the two different combined osteotomies on the mandible, in correction of face asymmetry, require further investigation. Many factors are related to residual facial asymmetry, such as anatomical factors, surgical factors, soft relapse, soft tissue response and adaptation, and asymmetric dynamic facial movement. However, in this study, we only discussed anatomical factors. This study also had the following limitations: first, we only discussed skeletal and dental variables, while the information of real facial soft tissue appearance was lacking. Second, the 3D reference planes were designed by skeletal landmarks; the setting of the 3D reference plane may be different from real patients. In clinical situations, the whole face,including eyes and ears, should be considered when deciding the reference plane. Finally, considerable interindividual variation may exist in the etiology of facial asymmetry, and thus, variables accounting for each type of asymmetry classification. Further investigations might be needed to develop different surgical consideration and modification for outcome improvement in correction of facial asymmetry.