In the past literature, superimposition of cephalometric radiography was often used to observe condyle changes after orthognathic surgery. However, it is now possible to observe changes in the position, angle, and shape of the condyle using 3D images in CT . There are two methods for superimposing images: an anatomical structure-based approach and a voxel-based approach . In this study, the changes in the position and angle of the condyle were observed by superimposing based on the anatomical structures.
Many studies have reported that the position of the mandibular condyle changes after orthognathic surgery. Choi et al.  reported that the condylar head of patients who underwent orthognathic surgery tended to rotate inward in the axial view and return to its original position 6 months after surgery. Lee et al.  reported that the condylar head rotated at an average of 4.00° in the axial plane after BSSRO. The authors also reported that condylar position changes greater than an average of 4° require caution as they may reduce surgical stability or lead to iatrogenic TMJ disorders. Edward Ellis III et al.  reported that during orthognathic surgery in patients with severe facial asymmetry, the position of the mandibular condyle is changed due to bone interference between the mesial and distal regions, which may cause TMJ problems. Therefore, he said it was important to properly position the mandibular condyle in the mandibular fossa during the surgical procedure.
For this reason, various studies on bone fixation methods using wires, devices, and plates have been reported to position the condyle within the physiological range during orthognathic surgery [11, 15, 16]. In addition, various modified osteotomy methods have been studied to remove bony interference . Yang et al.  evaluated the usefulness of grinding technique and posterior bending osteotomy to prevent bony interference during orthognathic surgery in patients with facial asymmetry of 4 mm or more. According to the study, in the presence of large bony interference, there was a statistically significant tendency for the condyle to rotate inward in the group using the grinding technique. Kim et al.  recommended the use of the DSO procedure to minimize the recurrence of patients with facial asymmetry. They reported that, as a result of long-term follow-up, the postoperative recurrence rate was lower in the group with BSSRO and DSO than in the group treated with BSSRO alone, and more stable surgical results were obtained. Based on these studies, in this study, DSO, which can easily remove bone interference, was additionally selected and studied in patients with mild facial asymmetry among patients who planned BSSRO surgery, which has a wider contact area between segments than IVRO.
Although DSO is a useful procedure, care must be taken to prevent nerve damage during DSO because the inferior alveolar nerve is close and it is not routinely performed during orthognathic surgery. However, the risk of nerve damage associated with DSO has not yet been precisely identified .
We hypothesized that the change in displacement from the deviation side to the lateral direction would be minimized in Group II, but there was no statistically significant difference between the two groups. In general, during BSSRO surgery, the surgeon should strive to remove most of the interference site between the proximal and distal bone segments to allow for passive contact. Although it is a time-consuming step, there is no doubt that it must be considered, especially in mandibular surgery for patients with facial asymmetry, since reducing bone interference minimizes the mandibular condylar torque. In this study, when interference was removed using DSO, there was no significant difference in displacement between the deviation aspects of Group I and Group II. Therefore, it was assumed that it is not necessary to perform BSSRO along with DSO for patients with mild facial asymmetry with an average asymmetry of about 4 mm (minimum 3 mm, maximum 7 mm) even for patients with facial asymmetry. However, it is considered that additional research is needed on cases where facial asymmetry is excessive by more than 7 mm.
The tendency of inward rotation of the mandibular condyle in DS and NDS was clearly confirmed in all patients as in other studies. This means that when the mandible is repositioned to its normal position, some interference is inevitably generated below the proximal segment including the condyle. However, since it does not affect the manifestation of clinical symptoms, rotation of the mandibular condyle by 5–6 degrees is considered to be within the physiological range.
According to the results of this study, DSO is not an essential procedure in patients with mild facial asymmetry. But attention must be paid to nerve damage when BSSRO is performed on a patient with excessive facial asymmetry of 7 mm or more, which is expected to have severe bone fragment interference. One option for preventing nerve damage is using intraoral vertical ramus osteotomy (IVRO) procedure. IVRO relieves symptoms in patients with TMJ disorders by restoring the TMJ and disc relationship . Furthermore, the procedure is simpler than SSRO, with a shorter operation time and lower risk of nerve damage. However, there are complications such as condylar displacement or luxation [21, 22].
In order to evaluate the stabilization of the condylar position according to each surgical method, a long-term study on the position change of the mandibular condyle, symptoms, and disorders of TMJ after surgery for IVRO and BSSRO with DSO is additionally needed.