## 1 Introduction  
A precise transfer of surgical plans in the operation field is one of the most important factors to achieve successful surgical outcome [1]. In orthognathic surgery, an intermediate splint that is fabricated through a cast model surgery has been widely used to reposition bone segments according to the established surgical plan [2]. However, model surgery and manual fabrication of the splint require several error-prone procedures, which lead to inaccuracies compared with the surgical plan [2,3]. Even though three-dimensional (3D) virtual simulation surgery and computer-aided design/computer-aided manufacturing (CAD/CAM) technology can eliminate traditional laboratory procedures for intermediate splints, innate splint limitations persist [4; 5; 6]. An intermediate splint can provide information about relationships between the osteotomized maxilla and the mandible, however, the vertical position of the maxilla should be determined by the surgeon [5; 7]. The patient's condylar condition also can influence the accuracy of maxillary repositioning in traditional maxillary repositioning methods using an intermediate splint. As an alternative to an intermediate splint, a 3D printed surgical guide or plate was developed and applied for orthognathic surgery to overcome the limitations of an intermediate splint and improve surgical accuracy [8; 9; 10; 11]. Compared to intermediate splint method, there are obvious advantages to this method; however, we often encounter situations in which the surgical guide or plate is not available during surgery [7; 11; 12]. Because the surgical guide or plate is prefabricated according to the preoperatively established surgical plan and 3D simulation surgery, it is difficult to manage misdiagnoses, unpredictable soft tissue responses, or mal-produced surgical devices. Furthermore, for patients with a short maxillary height or a thin maxillary sinus wall, it is difficult to obtain a sufficient surface area for the device and rigidity for stable fixation [7]. When thorough stabilization of the maxilla cannot be achieved due to the screw loosening or a thin maxillary wall, we should make screw holes in other maxillary surface, not in the area that was determined preoperatively based on the simulation surgery. As robotic technology has been applied for medical fields, medical robots have been developed for various surgical procedures, such as laparoscopic surgery, neurosurgery, orthopedic surgery or maxillofacial surgery for head and neck tumors [13; 14; 15; 16; 17]. Minimally invasive surgery using medical robots can result in reduced pain and complication rates, faster recovery, and shorter hospital stays [18; 19]. Their use can also overcome limitations associated with human-conducted surgeries, such as degrees of freedom and the surgeon's tremor or fatigue. In orthognathic surgery, the robot technology can be helpful to reposition the bone segments without any prefabricated surgical guide or template and to hold the bone segment in the target position stably during fixation. In addition, it may be possible to cope with situations that require changing the surgical plan due to diagnostic errors or aesthetic problems found intraoperatively. Despite these possible advantages, the application of a robot surgical system to the orthognathic surgery has been relatively delayed and little consideration has been given to anatomic features and surgical procedures related with orthognathic surgery [20]. Previously, we developed the image-guided robotic system for autonomous repositioning of the bone segment to the desired position and reported acceptable repositioning accuracy [21]. In our previous study, however, maxillary segment was moved only to the direction where bone interference did not occur, because bone grinding system by robot has not been developed.