## 4 Discussion
In addition, operation time may increase due to the complicated preoperative preparation, such as docking of the robot, adequate positioning of the robot and patient, and preoperative registration. A lack of specially designed surgical instrument for orthognathic surgery is another limitation. In robot-assisted orthognathic surgery, it may be ideal that the mobilized bone segment is moved to the target position using an automatic robotic assistant [21]. However, autonomous bone repositioning may be accomplished only when the bone interference between the bone segments is completely removed. Although surgeons can identify the location and amount of bone interference using 3D simulation surgery, it is difficult to immediately remove bone interference due to the different osteotomy lines and unpredictable pterygomaxillary separation compared to virtual simulation surgery. Thus,repeatedly repositioning the maxilla to the target position for the identification of bone interference is inevitable, but it is time-consuming even though when repeatedly repositioning the maxilla can be achieved automatically. Increasing the working speed of the robot arm may be directly related to safety due to unexpected soft tissue injury including neurovascular bundles. Furthermore, the location and amount of bone interference may also vary depending on the path of reaching the maxilla to the target position. In addition, since tactile sensation is not provided to surgeon, it is difficult to precisely identify bone interference using only the eyes during autonomous bone repositioning. Finally, the autonomous execution of robot assistants requires more complex technology due to the registration between the robot space and the physical space. For these reasons, in the system developed in this study, the maxilla was moved to the target position using the robot arm's hands-on mode via image-guided navigation. The bone interference, that occurred during the repositioning of the maxilla, was identified and removed easily using the specialized end-effector with a single-axis slider and the rotary instrument. Specifically, bone interference was identified easily with the sliding up of the maxilla, and sufficient work space for removal of the bone interference using the rotary instrument was obtained with the sliding down of the maxilla. After the complete removal of bone interference, the slider reached its initial position, and it was confirmed based on the navigation whether the maxilla also reached its target position. There were several reports studied on the maxillary repositioning accuracy in navigation assisted orthognathic surgery. In the study by [PERSON], et al. [25], the mean deviation between the planned and postoperative maxillary position was 1.1 \(\pm\) 0.9 mm after maxillary repositioning using OTS. [PERSON], et al. [23] reported that RMSD between the preoperative planning and intraoperative guidance was 1.16 mm immediately after repositioning of the maxillomandibular complex using OTS and image-guided navigation. For navigation using the electromagnetic (EM) tracking, [PERSON], et al. [32] reported that the mean absolute deviation ascertained by EM tracking was 0.9 mm after maxillary repositioning using EM navigation in phantom skulls. In a clinical pilot study using EM tracking by the same study group, the mean absolute deviations of the maxilla were 1.0 mm on the \(x\)-axis, 0.9 mm on the \(y\)-axis, and 1.2 mm on the \(z\)-axis in the intraoperative navigation, and the RMSD was 2.1 mm [26]. Unlike navigation-assisted orthognathic surgery, where a number of surgical accuracy assessments have been reported, there are few studies on surgical accuracy for robot and navigation assisted surgery. In the present study, to reproduce the actual surgical procedures, we performed virtual surgery according to the four different surgical plans and performed maxillary orthognathic surgery including the removal of bone interference and repositioning of the maxilla using the robot arm and navigation assisted orthognathic surgical system.