## 2 Case Presentation  
This study protocol was approved by the Institutional Ethics Committee (CUDHIRB 2007003). Written informed consent was obtained from the patient. A 24-year-old male visited the department of oral and maxillofacial surgery for orthognathic surgery. Radiographic optical markers (Marker, Dio Co., Ltd.; Busan, Korea) were attached to the patient's face; a cone beam CT was used (Hitachi, Marunouchi, Japan), and facial scan data were obtained with a handheld scanner (Artec Space Spider; Artec Group; Luxembourg). Skin and skull 3D models were reconstructed with Mimics software (18.0, Materialise, Leuven, Belgium), facial scan and oral scan 3D models were imported. With a registration system ( Mimics software), the facial scan model was registered to the 3D skin model generated from the CBCT, based on the optical markers (Figure 1). VR simulation was performed in virtual space with segmented and registered 3D models, by controlling with a controller (Vive Pro, HTC Vive, Taipei, Taiwan), wearing the head mount display (HMD). The maxilla position was adjusted for posterior impaction and advancement, and then the distal segment of the mandible was moved for the optimal position, matched with the maxillary position (Figure 2). In addition, the intermediate and final splint were designed for surgical application and then exported as STL files for 3D printing (Figure 3). Figure 1: Facial scan model and CT skin model were fused by registering according to the optical markers in virtual space. The 3D models were implemented with an AR guide system by Unity 3D software. We developed a four-point tracking algorithm, which was tested with a dental cast to evaluate the tracking accuracy. Tracking accuracy was 0.55 mm on average. The four point auto-tracking algorithm was applied for intraoperative AR display of the pre-planned 3D model with a 4k RGB camera (Figure 4). To apply the AR navigation system, first, we set the optical markers onto the patient's face; CT and face scanning was then carried out. The 3D models were registered to the real patient's face, with the four-point tracking algorithm registering manually during operation. Finally, the AR navigation system automatically tracked the markers and projected the 3D model onto the actual patient's operating field. A 4k RGB camera acquired the \"movie\" from the actual patient, and a 4k display monitor showed an augmented 3D model in real time (Figure 5). For accurate surgery, LeFort I osteotomy was performed according to the pre-planned osteotomy line under AR navigation. After the osteotomy was completed, the intermediate splint was applied and then the maxilla position was confirmed with AR navigation. After intermaximillary fixation, the maxilla was fixed with miniplates and then the final splint was applied for setting the mandibular position. Finally, the position of the repositioned maxilla and mandible were confirmed with AR navigation. Two weeks after operation, the CT scan and facial scan were taken for postoperative evaluation. Post-operative CT and facial scan data were registered to the preoperative 3D models, based on Mimics software. For comparison with the preoperative plan, reference points were set, such as anterior nasal spine (ANS), posterior nasal spine (PNS), right maxillary first molar mesiobuccal cusp (Right MxM1), left maxillary first molar mesiobuccal cusp (Left MxM1), pogonion point (Pog), right mandibular molar mesiobuccal cusp (Right MnM1), and left mandibular molar  
Figure 3: Intermediate splint and final splint were designed after simulation. Figure 2: Maxilla moved to the optimal position with the controller (**left**) and the mandible was moved to the appropriate position with the controller (**right**). mesiobuccal cusp (Left MnM1). The differences of the reference points were measured as three-dimensional linear distances (Figure 6).