Computer-aided design and fabrication of dental restorations
Current systems and future possibilities
Joerg R. Strub, DMD, PhD,
E. Dianne Rekow, DDS, PhD and
Siegbert Witkowski, MDT, CDT
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ABSTRACT
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Background. For more than 20 years, researchers have been trying to automate conventional manual processes in dental technology with the hope of producing higher- and more uniform-quality materials, standardizing manufacturing processes and reducing production costs.
Methods. The authors review existing computer-aided design (CAD)/computer-aided manufacturing (CAM) systems, describing the components of CAD/CAM technologies and addressing the limitations of current systems, and suggest possibilities for future systems.
Conclusions. Existing dental CAD/CAM systems vary dramatically in their capabilities; each has distinct advantages and limitations. None can yet acquire data directly in the mouth and produce the full spectrum of restoration types (with the breadth of material choices) that can be created by traditional techniques. Emerging technologies may expand dramatically the capabilities of future systems, but they also may require a different type of training to use them to their full effectiveness.
Clinical Implications. In the future, automatically fabricated, fully esthetic restorations might be produced more quickly and have longer lifetimes than restorations currently produced with CAD/CAM systems.
Key Words: CAD/CAM; scanning software; data exchange; dental materials; hardware; future trends
Computer-aided design/ computer-aided manufacturing (CAD/CAM) technology was introduced to the dental community in the early 1980s. Since then, the technology has evolved in two directions. One is the intraoperatory application for one-appointment restoration fabrication using prefabricated ceramic monoblocks.1 In parallel, CAD/CAM systems for commercial production centers and dental laboratories emerged, expanding the range of materials that could be used and the restoration types that could be produced. In this article, we provide an overview of current CAD/CAM systems, describe components of CAD/CAM technologies and suggest future possibilities.
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BACKGROUND
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The first attempts to automate the production of dental restorations began more than 20 years ago (and are summarized in the supplement that accompanies this issue of JADA2). The hope and expectation was that automation could achieve the following:
- – produce higher- and more uniform-quality material by using commercially formed blocks of material;
- – standardize restoration-shaping processes;
- – reduce production costs.
The use of high-strength structural materials like alumina- and zirconia-based ceramics for restoration cores and frameworks, which can be shaped only by CAD/CAM systems, has both increased the lifetime of restorations and expanded the demand for CAD/CAM-produced restorations.3 As a result, the number of CAD/CAM systems currently available to the dental community has increased substantially within the last few years (Table
).4-6
All CAD/CAM systems have three functional components: data capture or scanning to capture and record data about the oral environment (tooth preparation, adjacent teeth and occluding tooth geometry); CAD to design the restoration to fit the preparation and to perform according to conventional dental requirements; and CAM to fabricate the restoration (Figure 1, page 1292).
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Figure 1. Overview of computer-aided design/computer-aided manufacturing systems for dentistry. In-Ceram is manufactured by Vita Zahnfabrik, Bad Säckingen, Germany.
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DATA CAPTURE
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Data capture differs remarkably between commercially available dental CAD/CAM systems.6 An intraoral digital 3-D scanning device (digitizer) is an integral component of the CEREC system (CEREC 3D, Sirona Dental Systems GmbH, Bensheim, Germany). The Evolution 4D system, currently under development by D4D Technologies (Richardson, Texas), also is expected to have intraoral data capture capabilities. Other commercially available CAD/CAM systems capture data from models, using mechanical or optical digitizers of various types. With few exceptions, these high-precision digitizers use technologies that prevent them from being used intraorally.
Mechanical digitizers, for instance, must map the entire surface of a prepared tooth while accurately maintaining the relative position of the device to the tooth. Many optical digitizers are exceptionally sensitive to any motion. Slight movement of a patient during data acquisition with either of these types of scanner would compromise the quality of the data, ultimately leading to a restoration that would not fit. In most cases, the scanner used to capture data is an integral part of the CAD/CAM system and operates only in combination with dedicated CAD software.
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RESTORATION DESIGN
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Several CAD software programs are available commercially for designing virtual 3-D dental restorations on a computer screen. Some of these programs can design restorations nearly matching the excellence of restorations produced by master dental technicians. The degree of interaction needed from the CAD/CAM system operator to design a restoration 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. Like the data acquisition systems, the software programs usually are proprietary to the CAD/CAM system and cannot be interchanged among systems.
When the design of the restoration is complete, the CAD software transforms the virtual model into a specific set of commands. These, in turn, drive the CAM unit, which fabricates the designed restoration.
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RESTORATION FABRICATION
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CAM uses computer-generated paths to shape a part. A diverse set of technologies has been used to create dental restorations (Figure 1
). Early systems relied almost exclusively on cutting the restoration from a prefabricated block with the use of burs, diamonds or diamond disks.7 This approach, in which material is removed to create the desired shape, is termed a "subtractive method"; material is subtracted from a block to leave the desired shaped part (the restoration).8
Subtractive fabrication can create complete shapes effectively, but at the expense of material being wasted. Approximately 90 percent of a prefabricated block is removed to create a typical dental restoration. As an alternative, "additive" CAM approaches like those used in rapid prototyping (also called "solid free-form fabrication") technologies are beginning to be used in dental CAD/CAM systems.9,10 Selective laser sintering is one of the technologies that can be used to fabricate either ceramic or metal restorations (Medi-facturing, Bego Medical AG, Bremen, Germany; Hint ELs, Hint-ELs, Griesheim, Germany) (Figure 2, page 1293). In this method, the computer design of the part (the dental restoration) generates a path much like a cutting tool path in existing CAD/CAM systems. However, instead of cutting, the system sinters material along the path, building a part from a "bath" of ceramic or metal powder and adding material continually until the complex part is complete. No excess material remains.
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Figure 2. A platform of dental restorations produced by selective laser sintering of a powdered alloy (photograph reprinted with permission of Hint-ELs, Griesheim, Germany).
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Some commercially available CAD/CAM systems use a combination of additive and subtractive CAM approaches. In one (Procera, Nobel Bio-care, Göteburg, Sweden), an enlarged metal die first is milled based on the 3-D data for the prepared tooth with the use of the subtractive approach. (This enlargement takes into account shrinkage associated with sintering the final restoration to achieve its final strength.) Powder is compacted under pressure onto the metal die, creating an oversized block by means of an additive approach; the block then is milled away to create the outer contours of the restoration. The oversized restoration is removed from the die and sintered to make the material as dense as possible and to shrink it to its correct size.
Another combined CAM approach (Wol-Ceram, Wol-Dent, Ludwigshafen, Germany) involves the application of a slurry of alumina powder directly to a master die using an additive electrophoretic dispersion method, which creates a coping. The operator trims away by hand excess material extending beyond the margin. The outer contour of the restoration is shaped using a subtractive CAM approach. The operator then removes the coping from the die and infiltrates glass (Figure 3).
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Figure 3. Dispersed ceramic oxide framework powder (In-Ceram Classic Alumina, Vita Zahnfabrik, Bad Säckingen, Germany) formed around four stone dies and a pontic area by using an additive electrophoresis dispersion computer-aided manufacturing method.
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An additive approach also has been used to generate copings and frameworks for bridges from pure alumina oxide and zirconia-based ceramics with superfine nanodispered ceramic particles smaller than 100 nanometers (ce.inovation, Inocermic, Hermsdorf, Germany). This system is housed in a production center, and details of the fabrication have not been disclosed. Brick and colleagues11 reported that it produces frameworks with high strength.
A different additive rapid prototyping technique, 3-D printing, is being used to design and then print a wax pattern of a restoration (WaxPro printer of the Pro 50 system, Cynovad, Saint-Laurent, Quebec, Canada).12 Operating like an inkjet printer, the machine builds wax patterns of frameworks and full crowns. The wax pattern subsequently is cast or pressed in the same manner as manually waxed restorations would be. An advanced printing unit (Cynovad) prints a resin-type material instead of the wax (Figure 4). This system has an expanded capability beyond that of most CAD/CAM systems for dental restorations; it also can be used to fabricate auricular prostheses.13
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Figure 4. Resin-type casting patterns of dental frameworks built up by a printing system based on a computer-aided design (Neo, Cynovad, Saint-Laurent, Quebec, Canada). The framework structure (in blue) is built on a supporting structure (white), which is removed later.
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OTHER CAD/CAM SYSTEMS IN DENTISTRY
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CAD/CAM systems have been created for dental applications other than producing restorations. One system (SL, Perfactory, Envisiontec GmbH, Gladbeck, Germany) uses stereolithography, another additive process to produce 3-D dental components from acrylics.14 Three-dimensional occlusal splints and similar components are created by selectively light-curing sequential layers of acrylic monomer in a liquid.
In addition, CAD/CAM systems have been developed to fabricate surgical templates (custom drill guides) to guide dental implant placement15 (SurgiGuide, Materialise, Leuven, Belgium) (Figure 5) and working models, permiting restorations to be inserted immediately after implants have been placed16 (Nobel Guide software, Nobel Biocare). Both systems use data captured from computerized tomographic scans in conjunction with CAD software to determine the most ideal restoration placement, and CAM technologies generate the templates and working models.
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Figure 5. A computer-aided design custom-made drilling guide for implant placement fabricated using stereolithography computer-aided manufacturing techniques (SurgiGuide, Materialise, Leuven, Belgium).
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MATERIALS
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Using CAD/CAM systems, operators can fabricate restorations from an array of materials. These include ceramics, metal alloys and various composites. The ceramics currently being used for restorations are predominantly alumina-(including those subsequently infiltrated with glass), zirconia- and porcelain-based ceramics.
The combination of materials that can be used and restoration types that can be produced by different systems vary (and are summarized in the table). Some CAD/CAM systems can fabricate a final restoration with some materials (although subsequent characterization of the esthetics and/or polishing may be needed). For instance, with porcelain-based ceramics shaped using the CEREC system, acceptable strength and esthetics can be achieved (for at least some clinical indications) without further processing. Crowns, inlays, onlays and veneers can be fabricated in a single appointment. Other ceramics, such as alumina- and zirconia-based ceramics, are extremely strong but not esthetic, requiring subsequent veneering using traditional hand-stacking methods to achieve acceptable esthetics.
Completely dense zirconia-based materials are extremely difficult to machine. Consequently, many CAD/CAM systems using subtractive milling or grinding operations shape restorations from this material in its partially sintered form. The restoration then is subjected to a heat treatment (sintered to make the material completely dense, which creates an exceptionally strong restoration). The sintering process permits color and shading to be customized (using manual operations) before the final sintering stage, thereby improving the esthetics of the restoration. The CAM system machines the presintered restoration to be oversized, which compensates for the shrinkage that occurs when increasing the density of the restoration, ultimately yielding an accurately fitting restoration. The process is sensitive, however, to possible deformation of the restoration and marginal adaptation, especially with long-span frameworks,17 as shown in Figure 6.
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Figure 6. A. Implants and abutments in the maxillary jaw. B. A computer-aided design/computer-aided manufacturing–produced zirconia-based bridge framework (Precident DCS system, DCS, Allschwil, Switzerland) and crowns (Procera, Nobel Biocare, Göte-burg, Sweden). C. Frameworks of the bridges veneered with Vita D ceramic (no longer on the market and replaced by VM9) (Vita Zahn-fabrik, Bad Säckingen, Germany) and crowns veneered with Nobel Rondo (Nobel Biocare) permanently cemented in place. (Photographs reproduced with permission of the Department of Prostho-dontics, University Hospital Freiburg, Germany, courtesy of Dr. Ch. Stappert and MDT O. Bothe.)
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BUSINESS MODELS FOR PRODUCING CAD/CAM RESTORATIONS
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As might be expected, based on the number of CAD/CAM systems available and the broad range in size and cost, different business models for producing CAD/CAM restorations have emerged. These include in-office systems, dental laboratory systems, dental laboratories working in collaboration with a production center, and a network or open-concept business model.
In-office system model.
The first, and so far only, commercially available in-office system is the CEREC system (Sirona). With this system, all three steps involved in the automated production of restorations can be accomplished in a dental office. The dentist can prepare a tooth and, by selecting appropriate materials, can fabricate a restoration and seat it within a single appointment. The supplement to this issue of JADA summarizes the evolution of this system and the performance of restorations produced by it as it reaches its 20th anniversary.
Dental laboratory.
The dental laboratory model is similar to that used in producing conventional restorations. The dental office sends an impression or model of the prepared and opposing teeth to the laboratory, and the laboratory fabricates the restoration. The only difference with this CAD/CAM technology is that at least part of the fabrication is automated. Unfortunately, the cost of many of these CAD/CAM systems is high, often precluding all but a few of the largest laboratories from offering this service.
Dental laboratory–production center model.
In the dental laboratory–production center model, the dental laboratory has the data acquisition and design software available to it.18 The laboratory technician scans models and designs the restorations, making optimal use of his or her skills. The laboratory sends the finished design to a production center, where it is converted into appropriate commands to drive the CAM component of a CAD/CAM system. This model minimizes the cost to the laboratory and has the potential to improve fabrication efficiencies.
Network or open-concept model.
The network or open-concept model is similar to the dental laboratory–production center model, but in this model multiple commercial laboratories and/or production centers collaborate. The dental laboratories have data acquisition and design capabilities and the production center and/or other dental laboratories have the CAM capabilities. In general, only limited types of materials can be fabricated with any one CAM system. With this network model, greater flexibility with regard to material choices is possible; the same restoration design can be produced from a broader array of materials. In the most open concept, a standard file format (similar to that used in solid free-form fabrication) facilitates transfer of design data to any number of different CAM systems, permitting interesting and more flexible material choices and pricing strategies.
Only a few manufacturers of digitizers and software programs offer networking or open-concept possibilities. Most dental CAD/CAM systems operate as closed-data systems. That is, all components are linked by a unique data format, precluding data from one system from being used to shape a restoration with a different system.19,20 The notable exceptions are the ZENO Tec (Wieland Dental +Technik GmbH, Pforzheim, Germany) and Hint ELs (Hint-ELs) systems.
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CAD/CAM SYSTEMS OF THE FUTURE
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No automated system currently offers the flexibility with regard to restoration types and material choices that is possible with traditional fabrication methods. However, new and emerging technologies will continue to push the boundaries we face today. An emphasis on intraoral data acquisition scanners and digitizers is likely. This could lead ultimately to the elimination of impressions and stone models. It is likely that future digitizers or scanners will be more robust, facilitating accurate data capture despite the differences in foundation restorations within teeth, as well as differences in saliva and, perhaps, soft tissue. This means that data pertaining to the prepared, adjacent and opposing teeth could be sent directly to a CAD/CAM system without being interpreted by a technician or clinician.
CAD software is relatively mature and probably will not change dramatically. However, likely enhancements may include a simpler user interface and integration of virtual articulators, which would facilitate automatic design of the occlusal surface.
CAM changes.
The CAM component of dental CAD/CAM systems likely will undergo the most remarkable changes. A major challenge that has not been addressed completely in existing systems is the completely automated, economical, high-precision production of restorations. High-speed machining is being adapted, permitting faster removal of material.9 This reduces machining time and could reduce production costs. Femtosecond lasers have been introduced for cutting dental materials, including zirconia-based ceramics.21 Direct shell production uses a rapid prototyping process similar to selective laser sintering to create ceramic investments in the shape needed, without a wax pattern.22
Direct-write assembly.
Other systems may shape parts using additive techniques such as selective laser sintering, stereolithography and 3-D printing, as described above. Another rapid prototyping approach that has shown much promise is direct-write assembly.23 With this system, the material from which the part is made is incorporated into special inks. The ink is delivered through specialized nozzles along the "tool path," defining the designed restoration to create the complex 3-D part. As the ink leaves the nozzle, it freezes instantaneously into the desired shape; however, for high-strength parts such as ceramic dental restorations, the materials need to be made more dense. This technology could expand the breadth of material choices, eliminate damage induced during subtractive shaping operations and minimize the amount of material needed to produce a restoration.
Esthetics.
One limitation of current CAD/CAM systems is their inability to incorporate esthetic veneers with strong (but relatively unesthetic) cores and frameworks. Lasers have been shown to sinter translucent veneering silicate ceramics after they have been applied to a core using a plotter system and direct shell production casting.22 Other approaches such as direct-write assembly also may be able to improve esthetics by applying an esthetic outer layer onto a strong core layer within a single additive CAM process.
Many new technologies are being applied in industrial fields, resulting in the creation of complex 3-D parts from an array of materials. In the future, practical application of these technologies to dentistry may provide unexpected paradigm shifts in fabrication approaches and materials options.
As more scanning and fabrication technologies are introduced to fabricate restorations, it is likely that more cooperative networks and open systems will be used.20 People with special expertise may be required to select and combine the components of open CAD/CAM systems. Informed decisions will be needed to optimize the choice of materials, hardware for shaping the materials and specific dental indications.
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CONCLUSIONS
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Existing CAD/CAM systems vary dramatically in their capabilities, each bringing distinct advantages, as well as limitations. None can yet acquire data directly in the mouth and produce the full spectrum of restoration types (with the breadth of material choices) that can be created with traditional techniques. Emerging technologies may expand dramatically the capabilities of future systems, but they also may require a different type of training to use them to their full capacity.
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FOOTNOTES
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Dr. Strub is a professor and the chair, University Hospital Freiburg, School of Dentistry, Department of Prosthodontics, Hugstetter Street 55, 79106 Freiburg, Germany, e-mail "joerg.strub{at}uniklinik-freiburg.de". Address reprint requests to Dr. Strub.
Dr. Rekow is a professor and the chair, Department of Basic Science and Craniofacial Biology, and the director, Translational Research, New York University College of Dentistry, New York City.
Mr. Witkowski is the head, Dental Technology, University Hospital Freiburg, School of Dentistry, Department of Prosthodontics, Germany.
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ABSTRACT
BACKGROUND
