Development of splintless orthognathic surgery methods

Abstract

Background Le Fort I osteotomy is widely used in orthognathic surgery to reposition the maxilla to a balanced and optimal position. Correct repositioning of the maxilla to the planned position is essential for achieving a successful surgical outcome in orthognathic surgery. In order to transfer a surgical plan to the actual operation, most surgeons use intermediate splints. This involves several error-prone procedures, including making a cast model, face bow transfer, model surgery, and fabrication of acrylic splints. In addition to these errors during preoperative preparation, malpositioning of the maxilla can also occur during surgery due to the perioperative condylar displacement. Thus, a template-based orthognathic surgery and a robot arm with navigation assisted orthognathic surgery were designed to replace the conventional maxillary repositioning methods that involves the use of intermediate splint. First, we developed the template-based orthognathic surgery and evaluated the accuracy of the positioning of the CAD/CAM based templates on dry skull, because its precise positioning is prerequisite for successful outcome. For the robot arm with navigation assisted surgery system, we evaluated the validity and reliability of the system for various surgical plans using phantom skulls.

Materials and methods I. Using the computed tomography (CT) data, CAD/CAM based maxillary surgical templates for orthognathic surgery were developed. Maxillary surgical templates consisted of 2 templates: the osteotomy guide template and the repositioning guide template. The osteotomy guide template had several specialized structures for exact and passive positioning according to the simulation surgery and guide the osteotomy and removal of bony interference. The repositioning guide template guided the maxilla to the planned position using wedge-shaped male and female structures. To determine the accuracy of the template positioning, deviation and fitness of the template were evaluated after positioning of the template on the dry skull.

II. The robot arm with navigation assisted orthognathic surgery was developed and consisted of a robot arm with six degrees of freedom, an image-guided navigation system including an optical tracking camera and a tracking tool, a display and a computer. The end-effector of the robot arm was designed to connect the robot arm to the surgical splint and to enable identification and removal of any bony interference. The position of robot arm was tracked in real time, and the robot arm and osteotomized maxilla were moved to the planned position using image-guided navigation. During the fixation, the osteotomized maxilla was held by the end-effector of the robot arm. In order to evaluate the accuracy of robot arm with navigation assisted orthognathic surgery, experiments were conducted using 12 full skull models. A total of 12 experiments involving four different surgical plans were conducted, with each surgical plan being implemented three times. Using intraoperative navigation and postoperative CT data, deviations between the planned and actual positions of the maxilla were calculated.

Results I. Regarding the deviation of the CAD/CAM based surgical templates, the mean absolute deviation was 0.41 ± 0.30 mm in medial-lateral direction (p < 0.001), 0.55 ± 0.59 mm in anterior-posterior direction (p < 0.001) and 0.69 ± 0.59 mm in superior-inferior direction (p < 0.001). The deviation was statistically significant in all directions. The mean root mean square deviation (RMSD) between the planned and the actual position of the template was 1.07 ± 0.76 mm. With respect to the fitness of the template, the mean distance between the inner surface of the template and the underlying bone was 0.76 ± 0.24 mm (range, 0 to 2.38 mm).

II. All experiments conducted using the developed robot arm with navigation assisted surgery system were successful. In the assessment of maxillary repositioning during intraoperative navigation, the mean absolute deviations were 0.10 ± 0.12 mm, 0.08 ± 0.07 mm, and 0.10 ± 0.10 mm in the medio-lateral, antero-posterior, and supero-inferior directions, respectively. The mean RMSD was 0.19 ± 0.15 mm. During the stabilization of the maxilla with miniplates, the maxillary segment showed slight deviations of 0.22 ± 0.19 mm, 0.18 ± 0.12 mm, 0.18 ± 0.12 mm, and 0.37 ± 0.20 mm in medio-lateral direction (p < 0.001), antero-posterior direction (p = 0.303), supero-inferior direction (p = 0.426), and in three-dimensional space, respectively, as compared to the maxillary position before stabilization. The final surgical outcome assessed using postoperative CT data showed that the mean deviations between the planned and actual positions were 0.42 ± 0.35 mm, 0.37 ± 0.25 mm, and 0.38 ± 0.32 mm in the medio-lateral (p = 0.169), antero-posterior (p = 0.006), and supero-inferior directions (p = 0.003), respectively, and the mean RMSD was 0.79 ± 0.35 mm. The error in superimposition for the comparison between the preoperative planned and the actual postoperative 3D model was 0.05 ± 0.01 mm. Surgical accuracy was similar in all regions of the maxilla.

Conclusion This study suggests that various surface-based CAD/CAM templates can be positioned based on prior simulation without the help of occlusal splints, and can serve as an alternative to traditional methods that involve the use of intermediate splints for the transfer of surgical plans to the actual operation. However, these templates have some limitations with regard to their universal clinical applicability. The robot arm with navigation assisted orthognathic surgery system allows us to overcome the limitations of template-based orthognathic surgery

the maxilla can be repositioned and stabilized successfully, with accuracy comparable to or better that that of other methods of surgical plan transfer.