Rapid prototyping manufacturing technology (RPMT), also known as direct CAD manufacturing and free-form manufacturing [1], originated in Japan in 1981 [2] and was a major breakthrough in the field of manufacturing technology for more than 20 years. 3] and was used in the field of dentistry around 1992 [4].
Harris et al [5] predicted that the fast and accurate characteristics of RPMT technology will make it have a broad application in oral and maxillofacial surgery.
1, Characteristics of RPMT [1, 2, 6]
RPMT is based on the computer description of object geometry, structure and connection state, which automatically and rapidly materializes design ideas into prototypes with certain structure and function or directly manufactures
RPMT is based on the computer description of the object geometry, structure and connection state, which can automatically and rapidly materialize the design idea into a prototype or directly manufactured object with certain structure and function, and can reduce the time of conversion from CAD model to physical model by more than 80%. With the development of diagnostic technology characterized by digital imaging technology (CT, MRI, 3D B-ultrasound, etc.), it is easy to reconstruct local cross-sectional images of the human body in 3D by computer, and these data can be transferred to the RPMT system to create the tissue entity of this part of the structure. It is particularly suitable for the direct fabrication of small batches, complex (e.g., grooves, convex shoulders, hollow, nested, etc. [7]), shaped objects; easy modularization and interchangeability of equipment with different process principles; remote fabrication services with the help of the Internet; wide range of available materials (e.g., resin, plastic, paper, paraffin, film, metal or ceramic powder, foil, silk, etc.); theoretically, the raw material utilization rate can reach Theoretically, the raw material utilization rate can reach 100%, and the manufacturing process is free of vibration and noise, making it an environmentally friendly and efficient manufacturing technology. Currently, special molding equipment is used with a maximum accuracy of 0.001 mm and a layer thickness of ±0.005 mm [2], molding parts up to 800 mm × 1600 mm × 500 mm in size (e.g., SSM-1600 from Tsinghua University) at a speed of several hours to tens of hours/piece [2].
2. The significance of RPMT in maxillofacial surgery applications
The purpose of maxillofacial surgery is not only to meet the physiological and functional needs of the patient’s facial organs, but also to maximize personalized restoration of the patient’s appearance, which requires: careful and thorough preoperative planning; explanation of postoperative results to the patient; and surgical operation as simple as possible, etc. The direct or indirect intervention of RPMT undoubtedly simplifies these problems, as it can play an important role in aiding diagnosis (fractures, joint ankylosis or even obstructed teeth [8]), planning, simulation of surgery (e.g., Gateno et al [9] used RPMT for prediction of distraction osteogenesis surgery), and treatment, making it possible to perform complex orthopedic procedures that were originally done through several operations in a single visit [7]. Statistics have been done [10]: after applying RPMT, the correct diagnosis rate increased by 29.60%, the operation accuracy increased by 36.23%, and the operation time increased by 17.63%. Not only that, the application of RPMT in the field of basic research such as tissue engineering in maxillofacial surgery has also been highly regarded in recent years as an important method for manufacturing cell carrier scaffolds.
Broadly speaking, the application of RPMT can be divided into 3 stages: primary stage, biological solid models for diagnosis and surgery; intermediate stage (compatible biological models), implants for therapeutic and rehabilitation engineering; and advanced stage (advanced biological models), artificial organs (“real” bone that can participate in metabolic processes).
3. Several rapid prototyping techniques applied in the field of maxillofacial surgery
RPMT is usually classified according to the principle of manufacturing process. The following are some of the more mature techniques applied in the field of maxillofacial surgery and their characteristics [1, 2, 6].
(1) Stereo printing and molding (Stereo Lithography Apparatus, SLA) is also known as photosensitive liquid phase curing, stereolithography, stereoscopic modeling, etc. This technology uses photosensitive resin as raw material, and a computer-controlled UV laser is used to scan the contour of each layered section of the predetermined prototype as a trajectory point by point, the resin in the scanned area is cured, and the table moves while a new layer of resin is placed, and so on until the manufacturing is completed. This technique is the most mature and is used in maxillofacial surgery to obtain visual prototypes and perform various data measurements, optimize the surgical design (e.g., determine the site of osteotomy, direction of bone movement, etc.) to reduce surgical risks, and provide valuable information for teaching and research. Hollister et al [12] used SLA to make a repair of a lateral mandibular joint defect in a small Yucatan pig with a body on top of which a surgical procedure was made for the repair of the lateral mandibular joint defect. SLA can also be used for microfabrication, and the Kyushu Institute of Technology in Japan has produced a model of approximately 50 μm [2]. The volume change in forming makes it more difficult to control.
(2) Laminated Object Manufacturing (LOM). LOM can produce large-size prototypes, low cost equipment and molding materials, no internal stress and deformation in the model, high precision, high strength and stiffness, and high volume variation in forming. The LOM process is now diversifying into different materials (e.g., sheet metal and ceramic materials) [2]. Qiu et al [13] used the LOM method to produce a paper-based physical model of the temporal bone, which can be used for the preoperative design of complex otolaryngological surgery and to simulate surgical operations.LOM can be used clinically for hard tissue replacement of craniofacial bone tissue defects caused by congenital, trauma, post-craniectomy decompression, infection, etc. For example, Ono et al [14] used HA ceramics via LOM to repair complex jaw defects in nine patients ( The shortcomings of LOM are that the material resistance and bond strength are very close to the chosen substrate and adhesive species, and waste separation is more time consuming.
(3) Selected Laser Sintering (SLS). It is precisely guided by the laser beam so that the material powder sintering or melting after solidification to form a three-dimensional prototype or model. Generally no binder is added and there is no subsequent treatment, so a high-strength model can be formed; no support is needed; the model accuracy is high (up to ±0.01mm when the particle size is less than 0.1mm); if wax powder is used, a precision casting wax model can be manufactured directly. The early SLS method was less competent for the fabrication of tissue engineering cell scaffolds because of the difficulty in removing the powder between the pores, and the current development of SLA has made it possible to adjust the internal microstructure (pores and pore size) of the sintered product by controlling the parameters. For example, Cheah et al [15] used the principle that the pore size of microtubules increases in the X-Z plane when the laser energy is reduced and the scanning speed is increased to fabricate a drug retardation device by controlling the formation of dense and porous zones, which is significant for defect reconstruction in the region of maxillofacial bone tumors; Tan et al [16] controlled different ratios of two materials on a commercial SLS device The shortcomings of SLS are the difficulty of precise control of absorbed power per unit area in sintering; sometimes the surface of the model is relatively rough and needs to be properly baked and cured and polished.
(4) Fused Deposition Modeling (FDM), also known as melt stacking method, melt extrusion into the mold. Is the use of hot melt nozzle, so that the semi-flowing state of the material according to the CAD layered data control path extrusion and deposition in the developed position of solidification molding, gradually deposited, solidified to form a model. This method does not use laser, has low cost and small device size.Eppley et al [7] performed cranial reconstruction in 13 patients by FDM to produce personalized pseudofixation, which greatly saved intraoperative operation time.Rohner et al [17] used FDM to produce a biodegradable polyhexanoate (PCL) scaffold, a carrier for bone marrow stromal cells. lactone (PCL) scaffold to repair maxillofacial bone defects in pigs with satisfactory osteogenic results. schantz et al [18] created a 15 mm defect in the skull of New Zealand white rabbits and used FDM to “replicate” the defect with some porosity using PCL, fibrin glue, etc.” Cao et al [19] proposed a symbiotic weight-bearing resorbable scaffold of chondrocytes and osteoblasts by FDM for the difficult repair of articular cartilage defects, and the results showed a great potential for the repair of osteochondral defects. The results showed great potential for repairing osteochondral defects. The disadvantage is that the precision is relatively low; there is also volume variation; the FDM method is only suitable for making scaffolds that do not add active substances such as growth factors during processing because of the need for heating.
(5) Normal temperature multi-insufflation forming (TMF). This method is to mix the materials that can be formed and cured after mixing into separate containers and mixed and formed by multi-injector jetting. Its outstanding advantages [20] are that it can eliminate the damage to the active material caused by high temperature, and the compounding of human bone growth factor is not done by the current popular method of post-compounding, but by special treatment, so that it can achieve multi-dimensional compounding at the rapid forming stage; and it has a slow-release effect, which greatly facilitates the healing of the bone in the defective area; the manufacturing process can change the material so as to produce a variety of different materials, colors, mechanical properties, thermal properties, and combinations. It is possible to change the material during the manufacturing process to produce a variety of composite or non-homogeneous materials and porous structures with different combinations of materials, colors, mechanical properties, and thermal properties, and to achieve functional gradient material stacking and forming. Therefore, it is promising to be the main process method for the preparation of bioengineered scaffolds (fine structure injection molding). In China, Yanchun Shi et al. of Tsinghua University [21], a porous cylinder with a diameter and height of 5 mm was fabricated by TMF technology from polylactic acid (PLA) with a molecular weight close to 100,000 compounded with HA, collagen and BMP, and was found to be significantly beneficial to bone tissue healing after being placed into the radial defect in dogs.
4. Outlook
In addition to the most mature techniques mentioned above, many other techniques have been practical. Such as three-dimensional spray bonding, photomask method, digital accumulation, and the latest direct shell method and other technologies. Currently, RPMT is being vigorously applied to the medical field abroad. There is still much room for development in terms of standardization of software and hardware, curvilinearization of slicing method, further intelligent processing, and integration of equipment technology. There is also a great potential for the use of RPMT to produce human tissues and organs to assist in diagnosis and surgical procedures. For maxillofacial surgery, the special characteristics of the “region” will make RPMT more “useful”.
References
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