Le C Fort I osteotomy is the most commonly applied procedure in orthognathic surgery for the correction of various different types of maxillary deformities. Clinical results over the years have shown that the procedure is safe and reliable, but there are still some complications, even serious ones, due to various reasons, which limit the popularity of this technique in China. In this study, cone-beam CT (Conebeam computed tomography, CBCT) was applied to the diagnosis of maxillary orthognathic surgery, preoperative design and positioning to precisely guide the clinical osteotomy, and good results were achieved, and the application of CBCT-guided maxillary Le Fort I osteotomy in our hospital is summarized and analyzed. 1. Data and Methods 1.1 Study Subjects There were 84 patients with maxillary deformities, 38 males and 46 females, aged 16-41 years old, with an average age of 23.5 years old, admitted between March 2008 and October 2010. There were 58 patients with anterior-posterior maxillary developmental malformations, including 4 cases of simple maxillary retrusion, 32 cases of maxillary retrusion with mandibular protrusion, 3 cases of maxillary hypoplasia secondary to cleft lip and palate, 2 cases of maxillary retrusion due to anhidrotic ectodermal hypoplasia, 3 cases of simple maxillary protrusion, and 14 cases of bimaxillary protrusion; 10 patients with maxillary vertical developmental malformations, including 6 cases of open jaw malformation and 4 cases of long face syndrome. There were 16 patients with maxillary left-right developmental malformations, 12 patients with maxillary asymmetrical malformations, and 4 patients with maxillary plane deviation due to the first and second w arch syndrome. All patients were treated with Le Fort I osteotomy, partially combined with maxillary block osteotomy, or bilateral mandibular ascending sagittal splitting osteotomy, mandibular anterior subapical osteotomy, horizontal osteotomy chin shaping, etc. Among 84 patients, 71 patients underwent preoperative orthodontic treatment with orthodontic metal brackets and wire arch, and 7 patients started orthodontic treatment after 2 months postoperatively. 6 patients did not need orthodontic treatment. 1.2 Pre-operative examination and positioning All patients took preoperative surface tomography films, positioning cranial frontal and lateral radiographs, and routinely observed and positioned important anatomical structures of the jaws such as the height of the pterygomaxillary union and the position of the maxillary third obstructive teeth. The distance from the edge of the pyriform foramen to the root of the distal middle maxillary second molar was measured along the palatal plane according to the scaling of the image of the positioning cephalometric lateral film, and the depth of the medial wall osteotomy of the maxilla was initially located. CBCT (produced in Korea) was examined and scanned to finally obtain 3D reconstructed images of the maxilla and mandible, standard coronal, sagittal and axial views, multiplanar reconstructed views, serial longitudinal views, and arbitrary tomographic views. The images were edited using image processing tools, the measurement settings were equal to the actual distance, the transverse image was selected above and parallel to the palatal plane, and the distance from the rim of the pyriform foramen to the pterygopalatine canal, the distance from the zygomatic alveolar ridge to the pterygomaxillary union, and the length of the nasal septum were accurately measured. 1.3 Surgical method The procedure was performed under controlled low-pressure general anesthesia via nasotracheal intubation, with local anesthesia containing 1/100,000 epinephrine at low concentration in the area of the local mucosal incision. The mucoperiosteum was incised from the first molar on one side to the first molar on the opposite side at a point 5 mm above the gingival attachment of the maxillary vestibular sulcus, and the mucoperiosteum was peeled away to expose the anterior and lateral walls of the maxilla, and then peeled away from the zygomatic alveolar ridge to the posterior medial subperiosteum, ending at the pterygomaxillary junction. The mucoperiosteum of the nasal cavity was peeled off to expose the anterior nasal spine, the inferior margin of the pyriform foramen and the lateral margin. The osteotomy line is made in the anterior maxillary wall according to the preoperative design. The osteotomy line can be oblique or modified high step type, and the osteotomy of the anterior and lateral maxillary sinus walls can be completed first with a cleft drill or rifling, and then the precise osteotomy of the medial wall of the maxillary sinus and the posterior wall of the maxillary sinus can be completed with a thin-bladed bone chisel according to the depth and angle of CBCT. Using a curved bone chisel, the posterior maxillary wall was split inward and downward from the pterygomaxillary junction to the pterygopalatine junction. The nasal septum is completely separated from the anterior nasal crest posteriorly using a septal bone gouge. The anterior segment of the maxilla is pressed down from the anterior edge of the pyriform foramen using maxillary grip forceps or freehand to break and loosen the entire bone mass. The mucoperiosteum of the nasal floor is inspected and the tear is tightly sutured. The bone can be further osteotomized in pieces, the nasal septal bone ridge is trimmed, and the distance of movement is accurately positioned by relying on the jaw plate, and four miniature titanium plates are chosen to be fixed at the edge of the pear-shaped foramen and the zygomatic alveolar bone wall area respectively. Both sides of the nose were converged and the mucoperiosteal incision was closed with absorbable sutures. 2 .Results Under the guidance of CBCT image positioning, Le Fort I osteotomy was performed, and there was no one case of palatal descending artery and nerve injury, and no one patient had accidental fracture during the operation. During the preoperative examination of maxillary orthognathic surgery, the combination of conventional localized lateral cephalometric radiographs and curved tomography films could understand the cusp position of the maxillary third molar and the pterygomaxillary union, but could not show the bone condition of the posterior wall of the maxilla or accurately locate the descending palatal artery. Cross-sectional CBCT examination can clearly show the thickness of the posterior maxillary wall, the angle and thickness of the medial maxillary wall, and the location of the pterygopalatine canal. In three patients with maxillary sinus hypoplasia, the distance between the walls was much less than normal (Figure 3), and 11 patients with high maxillary third molar obstruction were accurately localized (Figure 4). Eight patients with maxillary sinus separation and four patients with nasal septal deviation were found. three patients were found to have maxillary sinus cysts. Sagittal CBCT examination allowed observation of the pterygomaxillary union and the situation in the maxillary sinus. However, due to the influence of local bone density and the influence of soft tissues after 3D reconstruction, the clarity of the pterygomaxillary junction was poor in some patients in the 3D reconstructed images, and it did not accurately show the exact location of the upper and lower points of the localized pterygomaxillary junction. A serious intraoperative complication of maxillary Le Fort I osteotomy is hemorrhage caused by vascular dissection, and the vessels mostly involved clinically are the descending palatine artery, internal maxillary artery and their branches, so it is especially important to accurately locate the descending palatine artery and understand the height of the pterygomaxillary junction before surgery. The current clinical localization is mainly based on the curved tomography film and localized lateral cephalometric film to locate the superior and inferior points of the pterygomaxillary junction. The depth of surgical osteotomy is also based on previous experience. The average distance from the edge of the pear-shaped foramen to the pterygopalatine canal was 35.25 mm in the domestic study, and the average distance from the zygomatic alveolar ridge to the pterygopalatine union was 25.47 mm. The average distance from the edge of the pear-shaped foramen to the pterygopalatine canal was 38.4 mm (34-42 mm) in men and 34.6 mm (28-43 mm) in women in the foreign study. Too shallow an osteotomy may leave too much bone connection and cause high fracture or poor force transmission in the posterior maxillary wall, leading to ocular symptoms; too deep may damage the descending palatine artery or break the pterygoid plate, leading to serious complications such as bleeding. This study utilizes the CT tomography function of the CBCT system and its measurement function with a 1:1 ratio between the projected illumination, which allows for actual measurement and thus preoperative accurate localization of the distance from the margin of the pyriform foramen to the pterygopalatine canal. Since the descending palatine artery runs obliquely downward from the pterygomaxillary fissure to the palatal foramen, the height of the horizontal osteotomy line varies, and the distance from the margin of the pyriform foramen to the pterygopalatine canal also varies, so we accurately predict the depth of the osteotomy based on the height of the preoperative design of the osteotomy line, and achieve good results intraoperatively. artery, three from the internal maxillary artery, and three of unknown origin, all of which occurred during dissection of the pterygomaxillary union. Therefore, it is more important to be able to accurately grasp the height and width of the pterygomaxillary union. The combination of cross-sectional and sagittal scans of CBCT can accurately locate the pterygomaxillary union and the adjacent situation of the medial descending palatine artery. However, the clarity of the pterygomaxillary junction is poor in some patients in the 3D reconstructed images due to factors such as local bone density effects and soft tissue effects after 3D reconstruction. Another frequent complication of maxillary Le Fort I osteotomy is accidental fracture. The two most frequent fractures are the fracture between the horizontal plate of the palatine side of the maxilla and the palatine bone, and the fracture above the intersection of the internal posterior wall of the maxillary sinus. The former is caused by excessive care to protect the descending palatine artery at the intersection of the posterior wall of the maxillary sinus, leaving too much of the bony connection here unbroken, while the latter is caused when the external posterior wall of the maxillary sinus is forcibly descended and broken due to inadequate separation. The common cause of both is inadequate osteotomy, or improper handling of the osteotomy site and angle. As the maxillary orthognathic patients themselves have jaw development deformity, individual differences are very large, such as patients with maxillary sinus hypoplasia, high arched palatal cover, the measurement of each bone wall is much lower than the normal value, there are two cases of patients with severe maxillary hypoplasia in this group data, the maxillary sinus cavity is very narrow, the wall of each maxillary bone is thick and short distance, if the conventional distance and angle osteotomy, there will be accidental fracture, there are reports in the literature in the past A very small number of patients have an excessively thick posterior maxillary wall, making it difficult to lower the osteotomy fracture, which in turn leads to serious intraoperative and postoperative complications. Patients with high obstruction of the third molar in the maxilla generally have a thick posterior maxillary wall and require intraoperative root amputation. The above cases remind us that the maxillary Le Fort I osteotomy should be individualized, and the application of CBCT can enable the surgeon to target not only the thickness of the posterior and interior walls of the maxilla, the separation of the maxillary sinus, and the deviation of the nasal septum, but also the location of the blood vessels, and for patients with maxillary hypoplasia secondary to cleft lip and palate, the alveolar ridge fissure, palatal fissure, and past bone graft can be determined by CBCT. In patients with cleft lip and palate secondary to maxillary hypoplasia, CBCT can be used to determine the alveolar ridge fissure, palatal fissure and past bone grafting, providing a more thorough basis for surgical planning. The maxillary sinus cyst is a common type of sinus cyst, with insidious onset, slow development, and no clinical symptoms in the early stage, making it difficult to be detected and even less likely to be treated early. The traditional preoperative examination method of Le Fort I osteotomy is to take surface tomography to observe the situation in the maxillary sinus, but the surface tomography is to project the maxillofacial bone into an unfolded planar image, with much anatomical overlap, and the size and shape of the maxillary sinus cyst is distorted more obviously, so this method is only used to determine the presence or absence of cysts, and small cysts are even difficult to detect. CT tomography, on the other hand, can clearly show a well-defined and complete round or round-like shadow with clear margins and a base close to the sinus wall, against the backdrop of gas in the sinus cavity. In the present case, the preoperative CBCT scan of three patients revealed a unilateral maxillary sinus occupying lesion with homogeneous density and a preliminary diagnosis of maxillary sinus cyst.