The Design Manufacture And Accuracy Evaluation Of Customized Implant Surgical Guides Based On CNC Technology

BackgroundSince Professor Branemark brought forward the "Osseointegration" theory in the middle of last century, and dental implant technology has been developed quickly. The implant dentures are make up with implants and superstructures. Compared with traditional dentures, implant dentures are easy to clean and more comfortable, do not hurt the adjacent teeth, have higher chewing efficiency better aesthetic effect, which are known as "the third teeth of mankind". Therefore, implant dentures are widely used in dental and orthopedic surgery to replace missing teeth or repair skull defections. Because of the important and complex anatomic structures, surgeons need to determine precisely about the position, depth and orientation of implant, so pre-operative design becomes the key factor to influence the effect of implantations. The main methods of dental implant placement are free-hand implantation and surgical guide assisted implantation. According to the types of surgical guides, the surgical guide assisted implantations were divided into two categories:conventional surgical guides and computer aided design/computer aided manufacturing (CAD/CAM) surgical guides. Free-hand implantations were mainly relied on surgeon’s hand feeling and experience. Due to the missing of internal information of bones, the accuracy of conventional surgical guides is far from satisfaction. On the contrary, more optimal implant position can be used in surgery with CAD/CAM surgical guides.At present, the clinical routine use of CAD/CAM surgical guides is made by rapid prototyping (RP) technology. The data used in three-dimensional (3D) reconstruction of teeth and jaws is collected before surgery using CT/CBCT. Then reverse engineering (RE) software is used to extract the surface contour of 3D reconstruction models and the optimal implant positions were decided. After that, customized implant surgical guides were designed with CAD technology. Ultimately, the surgical guides were realized by RP machines. The RP surgical guides not only improve accuracy and safety, but also are easily to be mastered with short learning curve. As a result, worldwide attentions were obtained. However, due to its high cost, long manufacturing cycle, materials such as resin with unclear toxicological property, high temperature and high pressure disinfection, the clinical use of RP surgical guides were limited.In summary, this research focuses on how to construct customized implant surgical guides with high accuracy and no biological safety problem. A novel method of producing customized implant surgical guide was discussed, aiming at finding a novel approach to the help manufacturing customized implant surgical guide.Product molding technology in reverse engineering, in contrast to the addition technology such as rapid prototyping, subtraction technology such as computer numerical control (CNC) technology, is usually more mature. CNC technology was invented in 1960s, after several generations of development. CNC machine can manufacture components with complex structure. CNC processing has the characteristics of high speed, high efficiency, high precision, high flexibility, stable and reliable processing quality and so on.Therefore, this study included 3D reconstruction technology, RE technology, CNC technology and RP technology to design and manufacture customized CNC and RP implant surgical guide. Then the accuracy of in vitro implantations assisted with those two kinds of surgical guides were evaluated using SPSS vl9.0 software.ObjectiveThis study is to design and manufacture customized implant surgical guide using CNC and RP technology, and evaluate the accuracy of these two kinds of surgical guides.Materials and MethodsEthics StatementEthical approval was obtained from the Human Research Ethics Committee, Southern Medical University, Guangzhou, China. The subjects gave informed consent. And all consent wrote in nature regarding body donation for research.Materials20 specimens of corpse mandibleSelf-curing resin powder、Self-curing resin monomerMethods1. Grouping and Pretreatment20 specimens were randomly divided into two groups.10 for RP surgical guide, 10 for CNC surgical guide.All alveolar fossa were filled with self-curing resin.2. ImagingAll specimens were scanned with a Somatom Definition CT 64-channel scanner (Siemens, German). The scan slice thickness was set as 5mm and the reconstruction slice thickness was set as 0.6mm.3. Customized implant surgical guides design and manufacture3.1 Three-dimensional design of customized implant surgical guidesThe data of specimens were processed and edited by Mimics software v14.11 (Materialise Corp, Belgium). Three dimensional (3D) models of mandible was reconstructed from the CT images. Virtual guide pins contained optimal position and orientation were designed by MedCAD module. Then the 3D mandible model and virtual guide pins were saved in STL format and were exported to Geomagic studio v12 (Raindrop Geomagic Corp, US) software. The initial templates were designd by the CAD software NX UG v6.0 (Siemens PLM software, German) and saved in STL format to be imported into Geomagic Studio 12 software. In Geomagic software, the guides were moved, so that the template surface could be in contact with the top surface of mandible, and the virtual pins could go through the surgical guides. Though Boolean subtraction operation, final surgical guides with customized mandible surface and optimal guide pins were designed.3. The design and manufacture of customized implant surgical guides3.2 Customized implant surgical guides manufacture by CNC machineVirtual surgical guides were saved as IGS format, and imported to NX UG v6.0 software for processing design to meet the production requirements of VMC650 four-axis CNC machine (Baoji Machine Tool, China). Firstly, the outline of surgical guides on the blank were cut. Secondly, the irregular surfaces combined with mandible were processed. Thirdly, the drill holes were produced with a 2.0mm twist bits. Only one clamping was took with one whole process. The process designs were saved as TXT format and imported into VMC650 four-axis CNC machine (Baoji Machine, China). The 70mm × 40mm × 15mm stainless steel blanks were champed on the control desk and cut as designed. After processing was complete, the surgical guides were cleaned and set aside.3.3 Customized implant surgical guides manufacture by RP machineThe virtual template data in STL format were imported into Materialise Magics VI3 software (Materialise Corp., Belgium) to run the virtual cutting process. The cutting thickness was set as 0.05mm. Then the virtual designs were transferred to RS6000 Stereo lithography appearance rapid prototyping machine (RPM) to be printed. After printing was complete, the surgical guides were cleaned and set aside.4. Surgical guide assisted cadaveric carbon fiber insertions and accuracy evaluation of simulated implantations.4.1 Cadaveric carbon fiber InsertionPre-manufactured surgical guides were layered on mandibles and tooth surfaces. The guides were fixed by operator’s left hand to keep the stability. A 2.0mm-diameter pioneer drill was drilled into mandibles assisted with surgical guides. After the drill, carbon fiber robs with 2.0mm diameter were inserted into the drilled canal.4.2 Registration and evaluation of accuracyPost-operative images were collected with the same CT scanner.3D models of post-operative mandible with carbon fiber inserted were reconstructed with Mimics v14.11 software using the same segmentation and reconstruction strategy.Accuracy of the simulated insertions with surgical guides were evaluated using Geomagic studio v12 software. The pre-operation and post-operation 3D models of mandibles were imported into the Geomagic software. On the pre-operation model defined the midpoint of bilateral mental foramens as coordinated origin; the plane passing bilateral mental foramens and the middle and low 1/3 point of mandible symphyseal was defined as horizontal plane; the straight line formed by bilateral mental foramens was defined as X (coronal) axis; the straight line on the horizontal plane through the coordinated origin and perpendicular to the X axis was defined as Y (sagittal) axis; the straight line through the coordinated origin and perpendicular to both X and Y axes was defined as Z (vertical) axis. The X axis deviations of the left side of coordinated origin were recorded as positive values and the deviations of the right deviations were recorded as negative values; the Y axis deviations of the posterior and anterior were recorded as positive and negative values; the Y axis deviations of the superior and inferior were recorded as positive and negative values, respectively. The post-operation model was registered to the pre-operation model. Two models were cut with the horizontal plane to gain the cross-sections of implants. Then circles were polyfitted from the cross-sections and the coordination of center of circle were recorded to evaluate the deviation of implantation. All Y axis coordinates were zero.4.3 Statistical analysisIndependent-sample T test was used to analyze the absolute deviations. A P value< 0.05 was considered as statistically significant.Results1. High quality scan images were obtained with Siemens CT scanner, three-dimensional models were created by Mimics v14.11 software.2. "Virtual implant surgical guides were designed through Mimics v14.11, Geomagic studio v12, NX UG v6.0 software3. CNC surgical guides were manufactured with VMC four-axis machine. The surfaces of the guides were smooth and continuous. The surgical guides had strength and no deformation. During the in vitro implantation, all guides were obtained stable retention without tilting and swing.4. RP surgical guides were printed with RS6000 SLM machine. The surfaces of the guides were smooth and continuous. The surgical guides had a certain strength and no deformation. During the in vitro implantation, all guides were obtained stable retention without tilting and swing.5. A total 120 carbon fiber robs were inserted into 20 specimens (60 robs in CNC group and 60 robs in RP group) assisted with surgical guides. The absolute deviations of CNC group in horizontal plane were (0.133±0.094) mm, The absolute deviations of RP group in sagittal plane were (0.689±0.301) mm. Calculation showed a significant difference between the absolute deviations towards left and right in horizontal plane (P=0.000). The absolute deviations of CNC group in sagittal plane were (0.269±0.159) mm, The absolute deviations of RP group in sagittal plane were (0.935±0.261) mm. Calculation showed a significant difference towards anterior and posterior in the sagittal plane (P=0.000).ConclusionThe stainless steel customized surgical guides were manufactured by CNC technology and the photopolymer resin customized surgical guides were manufactured by RP technology, respectively. In vitro implantations showed that CNC surgical guides had higher accuracy than RP surgical guides.The present study offers some evidences that CNC metal surgical guides have the potentials to be developed as novel clinical applications with further research and improvement.