The traditional surgical positioning method in orthognathic surgery is through ordinary X-ray of the maxillofacial region, but conventional radiological examination can only provide two-dimensional information, and the images overlap and have different degrees of magnification and distortion, which makes it difficult to provide the three-dimensional situation of the jaws, especially the cross-sectional situation of the jaws. Our hospital introduced the dental computer tomography (DCT) system in 2008.10, based on which 20 cases of bimaxillary orthognathic preoperative patients were routinely examined by DCT, and the application of DCT in orthognathic surgery is summarized and analyzed.
1.Data and methods
There were 20 orthognathic preoperative patients, 8 males and 12 females, aged 17-33 years old, with an average age of 23.5 years. All patients underwent preoperative orthodontic treatment with orthodontic metal brackets and wire arches, and all patients took preoperative surface tomography films, positioned cranial frontal and lateral radiographs, and routinely observed important anatomical structures of the jaws such as the height of the pterygomaxillary union and the position of the mandibular uvula. 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 to initially locate the osteotomy depth of the medial wall of the maxilla.
DCT was performed using the cone beam projection technique, with a single 360° rotation angle, the patient in a sitting position, and the scanning conditions were 85 kV, 8 mA, 24 s continuous exposure, bulb frequency 36 kHz, layer thickness 0.1-0.3 mm, image reconstruction time 180 s, and distance between detector and X-ray light focus 770.0 mm.
Data were acquired by amorphous silicon flat panel detector (FPD) with stereo pixel size of 270M, Voxel size voxel size 0.2mm×0.2mm×0.2mm , 3D picture cutting thin section 320, and finally 3D reconstructed images of upper and lower jaws, standard coronal, sagittal and axial views, multi-planar reconstructed views, serial longitudinal views, and arbitrary tomographic views were obtained. Image processing tools were used for image editing, measurement and mandibular neural tube coloring to locate important anatomical structures. The measurement was set equal to the actual distance, and the distance from the rim of the pyriform foramen to the pterygopalatine canal was accurately measured on the transverse image.
After the maxillary Le Fort I osteotomy was lowered, the distance from the margin of the foramen pear to the descending palatal artery was measured with a steel ruler under direct vision; after the sagittal splitting osteotomy of the mandibular ascending branch with the medial and lateral bone plates split, the exposure and damage of the inferior alveolar vascular nerve were observed, and then the measurement and localization of the important anatomical parts in the orthognathic area were analyzed and compared between conventional X-ray and DCT images. accuracy. The difference between the DCT measurements from the margin of the pyriform foramen to the palatal descending artery and the actual intraoperative values was statistically analyzed.
2. Results
When the bimaxillary orthognathic surgery was performed under the guidance of DCT image positioning, there was no intraoperative damage to the palatal descending artery and inferior alveolar vessels and nerves in one case, and no accidental fracture in one patient. Due to the presence of orthodontic metal brackets and wire arch on the teeth, there were still artifacts in the dentition area, but they did not affect the accurate positioning of important anatomical structures.
During the preoperative examination of maxillary orthognathic surgery, conventional positional cephalometric lateral radiographs combined with curved tomography could understand the apical position of the maxillary third molar and the pterygomaxillary union, but could not show the bone of the posterior wall of the maxilla or accurately locate the palatal descending artery. Cross-sectional DCT examination can clearly show the angle and thickness of the posterior maxillary wall, the thickness of the medial maxillary wall, and the location of the pterygopalatine canal. The application of its measurement tool can accurately measure the distance from the margin of the pyriform foramen to the pterygopalatine canal and locate the position of the descending palatine artery.
Statistical results showed that the difference between the distance from the margin of the pyriform foramen to the pterygopalatine canal measured by DCT and the actual clinical measurement was between 1 and 2 mm, with no significant difference, while the difference between the lateral cephalometric measurement and the actual clinical measurement was between 1 and 4 mm, but there was a significant difference between men and women. Sagittal DCT examination can observe the pterygomaxillary joint, but due to the influence of local bone density after 3D reconstruction and the influence of soft tissues, etc., the clarity of the pterygomaxillary joint is poor in some patients in the 3D reconstructed images, and it does not accurately show the exact position of the upper and lower points of the localized pterygomaxillary joint.
During the preoperative examination of mandibular orthognathic surgery, conventional positioning cephalometric lateral radiographs combined with curved tomography can accurately locate the position of mandibular uvula and chin foramen, the distance between the mandibular canal orifice and sigmoid notch, the two-dimensional relationship between the mandibular canal and the third molar, and the travel of the mandibular canal within the mandible. While the DCT image locates the position of the mandibular canal in the buccolingual direction within the mandible in different sections after coloring through the mandibular canal, the serial longitudinal views observe the ratio of bone cancellous density in different parts to avoid accidental fractures. The tomographic view and 3D reconstruction observed the anatomical morphology around the mandibular hyoid groove and the distribution of cancellous bone density in the mandibular ascending bone.
3, Discussion
The key to maxillary Le Fort I osteotomy is to avoid damage to the descending palatal artery and to accurately truncate each maxillary bone wall, especially the pterygomaxillary junction, the medial maxillary wall, and the posterior wall. The current clinical positioning is mainly based on the curved tomogram and the positioned lateral cephalometric film to locate the superior and inferior points of the pterygomaxillary joint. The depth of surgical osteotomy is also based on previous experience. The average distance from the edge of the pyriform foramen to the pterygopalatine canal in domestic studies was 35.25 mm, and the average distance from the zygomatic alveolar crest to the pterygomaxillary union was 25.47 mm [1]. In foreign studies, the average distance from the edge of the pyriform foramen to the pterygopalatine canal was 38.4 mm (34-42 mm) in men and 34.6 mm (28-43 mm) in women.
Too shallow an osteotomy can leave too much bone connection and cause high fracture or poor force transmission in the posterior maxillary wall, leading to ocular symptoms; too deep can damage the descending palatal artery or break the pterygoid plate, leading to serious complications such as bleeding. A very small number of patients have been reported to have excessive thickness of the posterior maxillary wall, making it difficult to lower the truncal fracture, which in turn leads to serious intraoperative and postoperative complications [3]. In this study, the CT tomography function of the DCT system was used to observe the thickness of the posterior and medial walls of the maxilla, the maxillary sinus separation, the localization of the maxillary high obstructing teeth, and the deviation of the nasal septum, and combined with its good measurement function with a 1:1 ratio between the projected object, the actual measurement could be performed, and thus the preoperative accurate localization of the pearly foramen margin to the pterygopalatine canal distance.
Since the descending palatal artery runs obliquely downward from the pterygomaxillary cleft to the palatal foramen, the height of the horizontal osteotomy line varies and so does the distance from the margin of the pyriform foramen to the pterygopalatine canal. For patients with cleft lip and palate secondary to maxillary hypoplasia, DCT can be used to determine the alveolar crest cleft, palatal bone fracture and past bone grafting, which can provide a more thorough basis for the design of surgical plan. However, due to the influence of local bone density after 3D reconstruction and the influence of soft tissues, the clarity of the pterygomaxillary connection is poor in some patients in 3D reconstructed images.
An important complication of mandibular ascending sagittal split osteotomy is injury to the inferior alveolar vessels and nerves, and the incidence of inferior alveolar nerve dysfunction after mandibular ascending sagittal split osteotomy has been reported in the literature to be 54% to 100% [4], most of which are temporary nerve injuries that can be recovered 3 to 6 months after surgery, but permanent nerve injuries can also remain. In recent years, despite improvements in technique and instrumentation, studies have found that about 20% of patients have no osteophytes between the mandibular canal and the outer plate of the bone, and the inferior alveolar nerve in these patients is highly susceptible to injury and is even considered a relative contraindication to sagittal split osteotomy. This requires a human-centered surgical operation that can be tailored to the specific anatomy of different patients to make the procedure more precise and perfect.
The DCT images can be used to visualize the layer-by-layer structure of the mandibular nerve canal through the coloring of the mandibular nerve canal, local tomographic anatomical imaging techniques, and clearly show the buccolingual position of the mandibular nerve canal at each site, suggesting the operator to avoid neurovascular injury during surgery. CT studies have shown that the mandibular ascending branch is thinner in patients with mandibular protrusion than in the normal population, and the ratio of osteophyte to osteophyte distribution, and the location and type of osteophyte distribution also differ significantly from the normal population.
In this study, with the help of DCT cross-sectional images, we can observe the distribution of osteophytes in the region of the mandibular ascending branch, locate the depth and position of the horizontal osteotomy line, and then predict the position of the posterior margin disconnection after the splitting of the ascending branch osteotomy to avoid accidental fractures. In some special cases, such as patients with short hemifacial and ankylosed joints, which result in underdevelopment of the mandibular ascending branch and require traction osteogenesis, DCT images can provide osteotomy guidance for traction osteogenesis treatment, and also have important guidance for individualized traction device setting.
DCT imaging system is a combination of conical CT (CBCT) and flat panel sensor, which can be reconstructed in three dimensions and imaged in three dimensions, and can be observed in three dimensions through different angles to achieve the true sense of 3D image synchronization and adjustment. Compared with traditional CT, CBCT scan range is flexible, can scan specific diagnostic area, also can scan the whole craniofacial; image accuracy is high, and the ratio between the projected object 1:1, can be actual measurement; scan time is short; radiation dose is small, under normal circumstances, only 75kV, 8mA, 24s are needed to complete a projection, and the same radiation dose as the general curved body layer machine. It is safe and reliable; image artifacts are reduced; and the requirement for head position is low.
CBCT and spiral CT are both volumetric scans. CBCT uses low-energy radiation cone X-ray beam scanning, and the radiation is synchronized with the sensor to rotate around the patient for 1 or less than 1 week to image, and the scanning process only takes 10 to several tens of seconds, and the Z-axis coverage is significantly larger than that of multi-layer spiral CT, reaching 7 to 15 cm; CBCT has a spatial limit resolution of 50 LP/cm, and the minimum layer thickness is 0.1 mm. The image quality of multi-layer spiral CT is affected by many factors such as pitch, exposure parameters, and reconstruction parameters, while CBCT only requires the selection of the correct exposure conditions, and there are no other influencing factors, and the image quality is stable.
Although the application of DCT is important for orthognathic preoperative examination, its image system still has some inherent problems that need to be considered, such as the appearance of artifacts in the dentition area in patients with metal brackets with wire arches undergoing preoperative orthodontics.