## 4 Discussion  
Bimaxillary orthognathic surgery should be an optimal treatment option for patients with mandibular prognathism and asymmetry. However, to date, mandibular surgery has mainly been performed in amanner where the maxilla is repositioned and then adjusted accordingly [21]. For many years, and even to this day, the main objective of the surgical treatment for facial asymmetry has conventionally been to coincide the Me, which is a designated anatomical landmark in the symphysis area, with the postsurgical MSP, despite the recent developments for diagnosis, surgical plans, and simulations using 3D technologies [8; 9; 22]. Diagnostic and surgical methods for facial asymmetry face few clinical problems when the direction of the menton deviations, as measured as the difference in length between the left and right ramus, and the canting direction of the maxillary occlusal plane, coincide [9]. However, if there is an asymmetry in the body shape of the left and right mandibles, residual asymmetry often remains after surgery [9]. In other words, facial asymmetry mainly due to the right and left hemi-mandibular body may remain or be inadvertently increased after orthognathic surgery. This is not only because post-surgery changes in the soft tissue and adaptation to the skeletal structures are hard to anticipate precisely, but also because structural analysis of the hemi-mandible is not carefully considered [17; 23]. [PERSON] et al. [13] reported that condylar and body units both affected the asymmetrical differences in the mandible. Therefore, to reflect the morphological difference between the left and right hemi-mandible bodies, a 3D stereoscopic image mirroring technique is required. Furthermore, to optimize such mirroring, an optimal mandibular reference plane other than Me is needed. In a typical IVRO surgical procedure, the proximal segment containing the condylar region is overlapped with the distal segment of the mandible by cortex-to-cortex contact after the conventional double-slide osteotomy line is defined from the angle at the ante-lingular prominence to the sigmoid notch. To focus on the main contributing components of residual mandible asymmetry after IVRO surgery, the anterior segment of the hemi-mandible was sectioned, apart from the distal segment along the osteotomy line for IVRO surgery, and chosen for comparative analysis in this study. When thoroughly assessing studies that evaluated or analyzed the stereoscopic structure of the mandible in the 3D space using either mirroring or segmentation, it was very important to set up the reference plane which could distinguish the right and left hemi-mandibles [10; 11; 12; 13; 14; 16; 17]. In 2019, [PERSON] et al. [17] proposed the new concept of SI, which intuitively assessed the mandible asymmetry on 3D images by using the mirroring technique with AMP as the internal reference plane of the mandible. AMP is the plane connecting Me, B, and G. According to the literal definitions of these anatomical landmarks, B is the midpoint of greatest concavity on the anterior border of the symphysis and Me is the most inferior midpoint on the symphysis. Rather than the exact configuration that is usually used and defined as the landmark in 2D lateral and posteroanterior cephalogram analysis, here the words \"greatest\" and \"most\" refer to the highest possibility. Since the mirroring technique is very sensitive to slight alterations of the plane formed by the coordinates of these points in the 3D space, it is crucial to reduce empirical errors and bias as well as to increase reproducibility. Furthermore, the genial tubercle is an anatomical structure at the inner surface of the mandible symphysis region that shows great morphological variation among individuals, with possibly (1) two superior and two inferior tubercles; (2) two superior and one inferior tubercle; (3) the same as in (2), but with more thickened tubercles; (4) a common spine; (5) the absence of genial tubercles; and (6) only one superior pair of tubercles [24].