## 3 Discussion  
Digital technology is used to perform operations with accuracy. Since the introduction of computer-assisted surgery, CBCT has been used to create surgical guides that apply to implant placement, tumor resection and reconstruction, and orthognathic surgery performed by oral and maxillofacial surgeons. Recently, orthognathic surgery has been performed as a type of computer-assisted surgery, including preoperative planning by computer simulation and fabrication of surgical guides by 3D printing technology. Previous studies showed a clinically acceptable error in the range of 0.18 to 1.7 mm compared to preoperative planning data and postoperative results [8; 9; 10; 11; 12]. VR simulation is attracting modality because it has been shown to improve operative accuracy, efficiency, and outcomes [13]. [PERSON] and [PERSON] suggested a mixed reality-based system that synchronizes the movement of dental models in the real world and a 3D mesh model in the virtual world for orthognathic surgery [14]. [PERSON] et al. applied a VR-based simulation system for mandibular angle reduction surgery [15]. In this study, we could easily make a plan via VR simulation. VR simulation showed great potential in surgical planning, in terms of convenience, intuition, and immersiveness compared to a 2D monitor display and mouse input interface. AR navigation surgery was introduced with technological development in the field of AR tracking accuracy. The AR guide (to guide implant placement position) showed errors of 0.53, 0.50, 0.46, and 0.48 mm [16]. In \begin{table}
\begin{tabular}{c c} \hline \hline
**Bony Landmarks** & **Distance (mm)** \\ \hline ANS & 1.18 \\ PNS & 4.22 \\ Right MxM1 & 2.05 \\ Left MxM1 & 2.02 \\ Pog & 2.34 \\ Right MnM1 & 4.32 \\ Left MnM1 & 4.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Differences between preoperative planned 3D model and postoperative 3D model. Figure 6: The pre-planned 3D model and postoperative 3D model were fused and measured the differences at bony landmarks. addition, an error of 1.63 mm appeared when the surgeon placed it at 1.25 mm, according to experience, and the error was small when using the AR guide [17]. The technology to track a marker with AR technology showed an accuracy of a maximum of 1.03 mm, an average of 0.71 mm, and a standard deviation of 0.27 mm [18]. Studies suggest that AR systems are becoming comparable to traditional navigation techniques, with precision, and sufficient safety for routine clinical practice [6]. The deviation in our finding was measured somewhat higher than other AR-based navigation surgeries, especially the mandible position. This deviation is considered a simulation error, in that preoperative simulation could not reproduce the exact mandibular movement in terms of the condyle position. In addition, since we attempted to introduce AR navigation as a simple device in the surgical process, more accurate results can be obtained when depth cameras and marker trackers are used. When an AR navigation system is applied to the operation, we could experience a small delay when compared to a traditional navigation system. It is possible that this happens because (i) the 4K camera takes the scene and then sends it to the computer, along with numerous data; (ii) the computer needs to calculate the data, integrating the 3D models to display onto the monitor. This problem can be resolved by replacing with a high quality GPU. There are many important structures in the oral and maxillofacial areas, including nerves and blood vessels [19]. To preserve these anatomical structures, it is necessary to improve the accuracy in the surgical filed. When AR navigation is applied to the face with optical makers--metal objects, such as oral and nose piercings, can influence the tracking system [20]. Therefore, it is necessary to remove all face piercings during operation. AR technology is appropriate for this purpose and well-suited to the currently preferred minimally invasive philosophy in maxillofacial surgery [19].