Janko HODOLIČ1, Tatjana PUŠKAR2, Igor BEŠIĆ3
1,3University of Novi Sad, Faculty of Technical Sciences, Trg Dositeja Obradovića 6, 21000 Novi Sad
2University of Novi Sad, Medical Faculty, Hajduk Veljkova 3, 21000 Novi Sad, 21000 Novi Sad
, ,
Abstract:This article presents recent developments in dental computer aided design and manufacturing systems for fabrication of custom inlays, onlays, crowns and fixed partial dentures from perspective of different materials, manufacturing technologies and digitization methods. The use of dental CAD/CAM systems is promising in terms of minimising time and effort made by dentists, technicians and patients for restoring and maintaining patients oral function and aesthetic, while delivering high quality end result. Components and methodologies of the existing commercial dental CAD/CAM systems as well as systems in development are analyzed and discussed in detail.
Key words: CAD, CAM, crowns, inlays, onlays, bridges, digitizing
- INTRODUCTION
Automation of the production process or computer aided manufacturing (CAM) can be regarded in two ways: as a computer aided system which purpose is to support activities in manufacturing; or as software which is used to develop control programs for the numerically controlled systems.
CAM systems evolved historically in the industrial metal manufacturing environment with beginnings in 1950’s as concept of numerically controlled machine tools. A major milestone in the development was in 1970’s with introduction of microprocessors. This enabled broader and more intensive development of CAM. In the 1980’s complex systems for automation of product design – computer aided design (CAD) and manufacturing emerged. These systems were named CAD/CAM systems (IDEAS; CADAM, CATIA). In the 1990’s six major corporations were dominating available CAD/CAM choices: Computervision, EDS/Unigraphics, SDRC, PTC, Matra Datavision and Dassault Systems. Software systems developed by these corporations were made from modules for various areas in engineering and are still dominating the choice of CAM software solution until present [1].
With computer hardware getting more available, the CAM systems also made it to manufacturing areas and users other than industrial metalworking.
The first steps in dental CAM application were made in 1980’s with first concept of the CEREC system. The concept evolved to functional CAD/CAM system CEREC-1 five years later [2].
- DENTAL COMPUTER AIDED MANUFACTURING PROCESS MODEL
The overall quality of recovering patient’s oral function and maintaining their oral health has been largely dependant on technology and materials used for fabrication of crowns and fixed partial dentures (FPDs). The classical fabrication process relies heavily on manual labor and its quality is resultingin part from craftsmanship, skill and experience of dentists and dentist technicians.
In terms of the manufacturing engineering, fabrication of dental crowns and FPDs can be considered highly complex and low in production volume, driving the fabrication method selection towards CAM, i.e. the usage of CAM can be recognized as an immediate benefit for improving the quality of the restorations.
2.1.Traditional process
The traditional fabrication process is presented in detail in Figure 1. The process starts with the tooth or teeth preparation and ends with aesthetically and functionally restored tooth or teeth (restoration). The algorithm shows the process steps grouped to form three main phases: a) stone model fabrication, b) custom restoration design and manufacturing, and c) final restorationin vivo testing,fitting and cementing.
Stone model fabrication starts with placing viscous fluid into the mouth with prefabricated tray and, optionally, using customized tray. After some time, the liquid hardens to become an elastic solid. After removal from the mouth, the material retains shape of the preparation, and is referred to as the impression. Custom dental model cast (also known as work model) is than made based on the impression. A wax pattern is then applied to the surface of the model and hand-modeled to have desired shape of the future restoration. A lost wax or investment casting process is then used to produce metal restoration. This investment casting method is time consuming and comprised of many manual steps including manual post-processing or finishing. Post processing includes: veneering, thermal processing, grinding and pre delivery quality control. This functional quality control is being done utilizing specialized mechanical device (articulator) to check interaction of the restored teeth to adjacent teeth and the occluding teeth.
Fig.1. Traditional wax modeling based fabrication process flow for metal ceramic crowns
Final adjustments are being made in vivo and require extensive experience.
2.2.Computer aided manufacturing process
With the simultaneous introduction of numerically controlled machining and fast digitization techniques a major breakthrough has been achieved in: application of new materials, reducing labor, cost effectiveness and quality control[3]. This concept has been named dental CAD/CAM.
Figure 2 shows the flow of a typical dental CAD/CAM fabrication process. The wax and investment casting phase has beenreplaced with three new functional components: data capture, restoration design and fabrication.
Digitization is a data aquisition process of the oral environment (tooth preparation, adjecent teeth and occludiong teeth geometry). This data capturing step differs between commercially available systems[4]. Extraoral 3D scanning systems capture data from models, using mechanical or optical methods. With few exeptions, extraoral digitizers use technologies that prevent them from being used intraorally [5].
Intraoral digitizers capture data from the patients mouth directly. When having this capability, the model making and impression taking phase is avoided (Figure 2). Consequently, this digitization technique is also refered to as digital impression taking.
Fig.2. CAD/CAM fabrication process flow in case of intra oral digitization
The device used for data aquisition is an integral part of the CAD/CAM system and can be used only in combination with the CAD software[5]. In contrast to these closed systems, there are systems available that allow such type of component interchange. These systems are refered to as open systems. For the digitizer component, this usualy means that the digitized result can be exported in one of the common data formats: ASCII, DXF, IGES, STEP or STL for use in CAD software.
The CAD software is used for restoration design based on data captured in digitizing stage. The puprose of the dental CAD software is to enable individual design of the restoration. This digital design replaces traditional restoration shaping in wax.
There are many CAD software choices available for this activity [6]. Like the data acquisition systems, software component is usually proprietary and cannot be interchanged among systems.
In case of the full crown restoration, the restoration design consists of inner face and the outer face. The inner face is modeled based on the preparation digitization, while the outer face is retrieved from the CAD software internal database of teeth shapes. The line where those two surfaces meet is called margin line.The gap between the preparation and restoration must be designed to leave space for adhesive material (cement) as shown in Figure 3. This gap can be viewed as one of the components of error budget for the digitization, design andfabrication process. Acceptable marginal opening for full crowns for instanceis 50μmup to 75 μm [7].
The degree of interaction in the CAD stage varies, ranging from substantial to no required user operations. Even in the most automated systems, the user generally has the option to modify the automatically designed restoration to fit his or her preferences [5].
Fig.3. The position of the cement space relative to model and restoration [8]
Finally the CAD modeled shape is transformed to physical crown or a bridge by means of computer aided manufacturing. During the last years, a spectacular amount of production systems has been developed and made commercially available. These advanced systems use a subtractive or an additive approach.
The subtractive approach usually implies usage of dedicated milling or grinding NC systems. The NC codes are calculated by the CAM software and transferred to these systems. One approach is to have fully automated system capable of one-appointment restoration fabrication. The other possibility is that the dentists handle only the data capturing and CAD issues, while central production centers deal with the CAM issues and NC milling .
Fig.4. An example of calculated milling tool path resulting from the dedicatedCAM software module [9]
Grinding/milling systems subtract the material from prefabricated ceramic blocks. These ceramic blocks are machined mostly in pre-sintered state because of their mechanical properties. This means that the restorations need to be heat treated in furnace after machining. One example is dense zirconia-based ceramics which is very hard for machining [10]. The NC milling approach is also being successfully used in machining of metal alloys and composites.
Rapid prototyping processes like selective laser sintering (SLS), stereolithography (SLA), ink jet printing (IJP) and 3D-Printing are successfully used to produce wax or resin based master patterns for mould fabrication [11].Rapid manufacturing approach such as selective laser melting (SLM) is used for direct manufacturing of crowns/bridges from cobalt-chromium and precious metal alloys. The CAM task here is to position the parts virtually in the build volume of the SLM machine, slice this positioned CAD model and set the optimal process parameters.
- EXISTING SYSTEMS
The commercially available systems vary dramatically in their advantages and limitations. Table 1 gives an overview of few selected systems.
The CEREC system was the first successful implementation of integrated intraoral digitizer and numerically controlled grinding system for restoration fabrication. The development started 30 years ago and is continued with the latest version that uses the “step-bur” diamond tool for 5-axis grinding (Figure 6) and a blue light fringe projection system for intraoral digitizing (Figure 5). This highly integrated system is intended to be single-visit chairside restoration system.
Table 1 Dental CAD/Cam systems overview
CAD/CAM system (reference) /Data acquisition method
/Material options
/Production technology
CEREC[12] / intraoral/extraoral (optical – blue light) / Al, Zr ceramics, composites / NC milling
LAVA
[13] / intraoral/extraoral (optical – blue light) / Zr ceramics / NC milling
Medifacturing
[14][15] / extraoral (optical – white light) / Zr cer., Titanium alloys / SLM
Evolution 4D
[16] / intraoral (optical-laser) / * / NC milling
* The system is in development; no information about the material options has been specified.
Fig.5. CEREC acquisition center: intraorally projected fringe pattern of short frequency visible light [12]
Fig.6. CEREC: 5-axis double tool milling system [12]
Lava system is a two option system with intraoral or extraoral digitizing capability. The both options can be integrated with the dedicated Lava CNC milling unit, or used in standalone mode (Figure 7).
Fig.7. LAVA intra oral scanner (left), 3shape extraoral scanner(right) [13][14]
TheBEGOMedifacturing uses alternative method of direct restoration fabrication - SLM. The available materials include:titanium alloys, precious metals and CoCr alloys. The digitizing option is the “open” 3shapeextraoral scanner (Figure 7). Sirona infiniDent uses the similar SLM approach. Because of the high price of the laser sintering machines, they are rarely seen in in-office dental applications (such as CEREC). The pragmatic approach taken by both companies is to centralize the production of the restorations while leaving data capturing and CAD issues to the dentists.
Evolution 4D CAD/CAM system uses intraoral laser stripe scanning system for data capturing. The E4D milling system is a double tool with a tool magazine. This complete system is still in development stage.
- FUTURE TRENDS AND CHALLANGES
As already mentioned in the article, there are several directions to develop CAD/CAM systems in dentistry. One of them is further integration of the CAD/CAM system, including intraoral digitization phase, design phase and manufacturing phase in the dental office. With doing so, a single-visit treatment approach is made available. While this approach has advantages of shorter treatment and patients convenience, it has drawbacks in terms of skills required for its deployment (dentists should have additional training in CAD/CAM) and the price of investment and maintenance. This approach implies further improvements of intraoral scanners (which are less accurate then their extraoral counterparts). Higher automation and simplification CAD user interface may lead to an integration of virtual articulators, which would facilitate automatic design of the occlusal surface.
Because of these limitations, centralized approach has been introduced. By using this alternative approach, only digital equipment needed in dental office is a digitizer. The digitized result is sent from the office electronically and final physical restoration is being received after fabrication in centralized manufacturing center. This business model is being used in both CNC milling and SLM manufacturing approaches.
An introduction of industrial CT digitization of cast models is likely, because it has already been used in orthodontic applications (Invisalign [17]). Utilization of medical CT technology can also be an interesting development since the optical scanning or impression taking techniques can’t capturesub gingival structures.
Another opportunity could also be in improving of the accuracy of the restoration, tackling errors being made in digitization, design and fabrication stage. This may enable, e. g., fabrication of thicker and more durable restorations while retaining the same preparation depth and decreasing the cement space.
- CONCLUSION
The application of dental CAD/CAM is promising not only in the fields of dentalprosthetics, but also in other fields of dentistry. The available CAD/CAM systems have been contributing to improvement of patient’s quality of life. Emerging technologies and further engineering developments will expand capabilities of future systems and also may lead to less required training for their use in full capacity.
The automated CAD/CAM concept taken from the industrial metalworking may completely substitute the traditional manually intensive wax based method.
ACKNOWLEDGEMENT
Results of investigation presented in this paper are part of the research realised in the framework of the project “Research and development of modelling methods and approaches in manufacturing of dental recoveries with the application of modern technologies and computer aided systems“ – TR 035020, financed by the Ministry of Science and Technological Development of the Republic of Serbia.
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