# Three-Dimensional Change of Lip after Two-Jaw Surgery in Facial Asymmetry Using Facial Scanner
## 4 Discussion
It seems that these results might be reflective of the present study cohort, which comprised patients with both class I and class III asymmetry (Table 1). Although the downward movement of the cheliions was more pronounced in class III patients because of the mandibular setback, in class I patients, the amount of maxillary impaction, including the non-deviated side, was relatively higher than the amount of mandibular setback. Accordingly, there was a tendency toward upward movement in the cheliions of some class I patients. In the current study, there were significant correlations between the backward movement of U6-D and the downward movement of the cheliions (Table 6). In terms of surgical movement of the maxilla in patients with facial asymmetry, the cant correction was performed by impaction of the non-deviated side more than that of the deviated side. According to this differential impaction of the maxilla, the upper dental midline could be shifted to the non-deviated side, and it can be considered that the yawing of the maxilla was planned to compensate for this shift and to establish symmetry. As a result, U6-D moved backward more than U6-ND, suggesting that there were significant correlations. The orbicularis oris muscle itself is not connected to the jawbone, instead displaying a structure resembling a rubber band [31]. Muscles originating from the maxilla and mandible adhere to the skin of the lips, resulting in changes in the orientation of those muscle fibers in cases with deviations of the mandible and maxilla [32]. Therefore, these muscles appear to pull the orbicularis oris muscle and the skin of the lips, changing the morphology of the upper and lower lips [33]. Due to these characteristics of the orbicularis muscle, not only the vertical movement of the hard tissue but also the 3D movement, including in the anteroposterior and horizontal directions, could affect the vertical position of the chelion. Despite the greater amount of setback on the non-deviated side in the mandible during orthognathic surgery (Table 3; Mf-ND, \(-7.25\) mm > Mf-D, \(-6.14\) mm), the backward movement of the Ch-D exceeded that of the Ch-ND (Table 3, Ch-D, \(-2.27\) mm > Ch-ND, \(-1.69\) mm). As a result, there was a tendency for improvement of the anterior-posterior lip asymmetry, although the change was not statistically significant (Table 4; 0.75 mm vs. 0.17 mm). These results appear to stem from the characteristics of the soft tissue. When the differential setback of the mandible is performed in patients with class III skeletal asymmetry, the period soft tissues on the non-deviated side can be more redundant, and consequently, Ch-D moves more backward than Ch-ND. The present study has some limitations. The position of the chelions was influenced not only by the hard tissue position but also by soft tissue factors, such as muscle tonicity and tissue thickness. The soft tissue changes after orthognathic surgery could be diverse owing to the biologic differences in these factors. In this study, to evaluate the 3D change in lip asymmetry after two-jaw surgery, FS and 3D CT were used. When taking 3D images with an FS, errors may occur due to the scanning ability of the equipment, and there is a possibility that the scanning accuracy may be further reduced in tissues with complex geometric curvature, such as around the nostrils. In addition, in this study, the areas and landmarks selected as references in the process for soft tissue superposition may cause more errors, because they have insufficient constraints, especially in the axial plane direction. Furthermore, there could be a slight registration error between the FS and CT images due to patients' facial expression, posture, or motion. In addition, the limited number of patients included in the study should be considered. Further studies of different skeletal patterns would illuminate the effects of two-jaw surgery on the correction of lip asymmetry.