In recent years, the three-dimensional measurement and reconstruction of craniofacial structure has attracted more and more attention in the field of stomatology, which is of great significance to the study of craniofacial growth and development, the diagnostic design of orthodontic treatment, the surgical design of orthognathic surgery, the evaluation of postoperative curative effect and the prediction of surgical curative effect. As early as19th century, some scholars began to reconstruct and measure the three-dimensional structure of maxillofacial region. Because the craniofacial structure and function are more complicated than other parts of the body, it is difficult for traditional two-dimensional analysis methods to accurately measure and analyze the three-dimensional structure of the craniofacial. In recent 10 years, the rapid development of computer, electronics, optics and image processing has further promoted the research and application of three-dimensional measurement and three-dimensional reconstruction, and has been widely used in various fields such as medicine, space measurement and machining.
In the field of stomatology, the measurement and reconstruction methods of craniofacial three-dimensional structure can be summarized as follows:
1, biological stereo measurement:
Stereophotogrammetry is the earliest stereo measurement method used in stomatology, which is used to study the growth and change of dental and maxillofacial structure and the correction of deformity. From 1970s to 1980s, many scholars combined stereo measurement technology with computer graphics technology, and applied it to the diagnosis and treatment design of craniofacial deformities to study jaw deformities, facial asymmetry, facial changes after orthognathic surgery, facial growth and development, etc. However, this method needs a special camera, and the operation steps are complicated, so it is difficult to popularize.
2, moire method:
Moire topography is a non-contact measurement method, and its principle is that two groups of periodic grids overlap each other, rather than contacting the outline image of the organism. In the early 1970s, this method was used in medical field, and Moire contour stripes of human body were obtained. Then this method is applied to the measurement and correction of scoliosis and facial deformity. In the field of stomatology, Moire fringe contour method has been applied to craniofacial growth and development, facial symmetry, three-dimensional morphological analysis of temporomandibular joint and cleft lip and palate. This method is not in contact with the subject, and it is cheap and relatively simple to operate. Electronic scanning sampling technology has replaced the traditional grating observation method, obtained clearer moire fringes, that is, scanning moire mapping method, and established a real-time measurement method of moire fringes, which has been widely used in the study of craniofacial three-dimensional morphology.
The disadvantages of moire fringe contour method are: 1, which is not suitable for flat or steep planes; 2, the sensitivity is low, and the patient's face is coated with South African dye; 3. Man-machine dialogue is needed, and the head position should be strictly positioned, otherwise the moire film will be seriously distorted.
3. Morphological analysis method:
This is a method of recording with three-dimensional photos. In this method, the position of the head is fixed in a standard position by a recording device, in which three Cartesian reference planes are kept in a constant position. The facial model is established by using the patient's photos, X-rays and occlusal models, so that it can be analyzed truly and accurately on the three-dimensional plane. Because these devices are very complicated and expensive, and the technical requirements are very time-consuming, they cannot be used in daily medical practice.
4. Laser tracking technology:
This method replaces an objective face 3D reconstruction method. The laser beam is irradiated on the patient's face through the laser generator, and the reflected light can be obliquely received by the camera. 1987, Moss used the three-dimensional measurement system developed by He-Ne laser to obtain the three-dimensional information of the face. The system has the advantages of high resolution, non-contact and low radiation dose, and the accuracy can reach 0.5 mm; ; At the same time, the hard tissue structure was reconstructed by CT to observe the prognosis of orthognathic surgery. Macanns's research team designed the patient to rotate under computer control, and the curve of the laser beam irradiated on the face was recorded every 2.8 degrees, while the center of the face was recorded every 1.4 degrees. This method is much slower than traditional X-ray and photography, because it takes 15 seconds to scan the face. Changes in the whole face and posture during scanning will distort some scanning results.
5.CT-assisted 3D reconstruction:
In the mid-1980s, CT-aided three-dimensional images and skull structure models were introduced into maxillofacial surgery and applied to skull anatomy, craniofacial deformities, trauma, tumors and salivary glands. It can also be used to design orthognathic surgery plan and predict the three-dimensional shape of the face after surgery. This method can obtain digital images of different levels of human face, with a certain interval between each level, and these images can be reconstructed into three-dimensional images after computer processing. Its disadvantage is that patients need to be exposed to high-dose X-rays, which is not suitable for long-term observation and evaluation of orthognathic surgery. Because the CT planes are more than 5mm apart, some structures between them will not be displayed, the soft tissue situation will not be displayed well, and the required instruments and equipment are expensive.
6, stereo printing imaging technology:
The cross section is scanned by CT and digitized into three-dimensional images by computer in a certain order. Each layer is processed by computer, and a polymer layer is established according to the data of the computer, and the layers are cured and polymerized into a whole by ultraviolet light. Some parts in the middle of the layer can also be hollow, so that the final model is also hollow. This three-dimensional reconstruction model needs high technology, rich medical experience and skilled operation.
7, ultrasonic three-dimensional image:
Ultrasonic image is an image system based on ultrasonic reflection, so the reflected ultrasonic wave must be detected and converted into digital information. The biggest disadvantage of this method is that ultrasonic waves cannot pass through the air, whether it is reflection or propagation. This method also requires a special detector industry to generate three-dimensional images.
8, three-dimensional stereo camera technology:
This is a biological three-dimensional measurement system developed in recent years. Its principle is to obtain image data quickly (less than 1 sec) by using stereo camera and special flash system, and directly input the digital data of CCD into computer for processing. Because of its low price, it will have great application prospects in the field of stomatology. Different from other three-dimensional reconstruction, this method is a real three-dimensional reconstruction of soft tissue, which can display the face in the reconstructed three-dimensional model, rotate and enlarge it in the computer, and measure it according to the orthodontic design, and its accuracy can be controlled within 0.5 mm
Recently, the Orthodontics Department of Beijing Stomatological Hospital affiliated to Capital Medical University has developed a digital three-dimensional measurement and reconstruction system for maxillofacial soft tissue for clinical diagnosis and treatment. The system adopts advanced digital photography system, and uses four digital computers to collect three-dimensional facial information, and develops a software system that can be used for three-dimensional diagnosis and evaluation of orthodontic face.
The hardware system of the system consists of the following parts: coordinate control field, digital stereo camera system, computer and professional 3D graphics card (with polarized glasses). The digital close-range stereo photography system consists of four modified Kodak DC 265 digital cameras, two grating projectors, two external flashlights, a synchronous shooting control device, two large photography frames, a headrest and a lifting chair.
The software system includes three-dimensional reconstruction software of facial soft tissue and three-dimensional measurement software of facial tissue. In facial reconstruction, the digital camera is used to record the facial image of the subject. The two facial images taken by the digital camera are first stored in the computer, and then a professional 3D graphics card is used to synthesize an image. Professional 3D display cards store two images on two video memories, which can be displayed on the monitor screen respectively. You can see the stereoscopic graphics of this image by wearing the stereoscopic glasses of the graphics card.
Take three shots with four digital cameras: for the first time, use four cameras with external flash; The second shot was taken with two cameras, using the grating on the left; The third shot was taken with two cameras and the grating on the right. The purpose of using four digital cameras and two camera pairs is to photograph all the left and right sides of the face. Four photos taken by four cameras for the first time are used for three-dimensional reconstruction of facial soft tissue; The four photos taken for the second and third time are used to reconstruct the three-dimensional mesh database model of regional soft tissue. Using the display card and the developed software system, the reconstructed face can rotate continuously in the range of up, down, left and right.
The basic principle of three-dimensional measurement of facial soft tissue is that the stereo image pair of the photographed object has a strict perspective relationship with the real object. If the relationship between stereo image pairs is restored to the photographic state (that is, the relative orientation of image pairs is completed), the images of stereo image pairs are projected, and the beams with the same name cross each other, thus forming the same optical model as the real object. Based on this optical model, the three-dimensional measurement of the corresponding object is carried out to obtain the required measurement data, which is the basic principle of three-dimensional measurement of facial soft tissue.
In a nutshell, the main advantages and characteristics of the system mainly include the following aspects:
1, which can simultaneously reconstruct and measure the facial soft tissue, not only measure the measurement items, but also visually observe and store the three-dimensional data;
2, the operation is very simple, the operation time is short, the shooting process of four cameras can be completed in a few seconds, and the accuracy of center reconstruction and measurement can be kept unaffected by time;
3. The accuracy of three-dimensional measurement of facial soft tissue is less than 0.5mm, which is enough to meet the requirements and needs of three-dimensional measurement of facial soft tissue;
4. Three-dimensional reconstruction of facial soft tissue can be quickly completed in the computer, displayed and observed;
5. Subjects will not be exposed to any harmful rays (such as X-rays);
6. The required equipment is not expensive, there are no consumables, the cost is low, and it has a wide application prospect in clinical treatment.
With the rapid development of computer technology and digital photography technology, the three-dimensional reconstruction and measurement system will be further improved and developed, and the three-dimensional reconstruction and measurement of maxillofacial region will be more widely used to quantitatively evaluate the three-dimensional structure of orthodontic face, which is helpful to formulate effective orthodontic treatment plans and predict the treatment effect. In the field of orthognathic surgery, it is helpful to determine the surgical plan and predict the three-dimensional structural changes of facial appearance after operation, and has a wide application prospect for people to understand the growth and development of craniofacial structure and its regulation mechanism.