A Panorama of Dental CAD/CAM Restorative Systems

Perng-Ru Liu, DDS, MS, DMD; and Milton E. Essig, DMD

Computer-aided design (CAD) and computer-aided manufacture (CAM) technologic systems use computers to collect information, and design, and manufacture a wide range of products. These systems have been in general use in industry for many years, but dental CAD/CAM applications were not available until the 1980s. The earliest attempt to apply CAD/CAM technology to dentistry began in the 1970s with John Young, DDS, and Bruce Altschuler, DDS, in the United States, Francois Duret, DDS, MD, in France, and Werner Mormann, BMD, DDS, and Marco Brandestini, PhD, in Switzerland.

Young and Altschuler first introduced the idea of using optical instrumentation to develop an intraoral grid-surface mapping system in 1977.¹ In 1984, Duret developed the Duret system, which was later marketed as the Sopha Bioconcept system (Sopha Bioconcept, Inc, Los Angeles, CA), demonstrating the ability of CAD/CAM to generate single-unit, full-coverage restorations. However, this system was not successful in the dental market because of its complexity and cost. The first commercially viable dental CAD/CAM system was CEREC® (Sirona Dental Systems, LLC, Charlotte, NC), developed by Mormann and Brandestini.

The American Dental Association specifies that a dental restoration must fit its abutment within 50 μm.² This requirement demands that CAD/CAM systems have a very accurate data collection technique, sufficient computing power to process and design complex restorations, and a very precise milling system.

During the past two decades, exciting new developments have led to the success of contemporary dental CAD/CAM technology. A series of methods have been used to collect three-dimensional (3D) data of the prepared tooth, from optical cameras to contact digitization to and laser scanning. Replacement of conventional milling discs with a variety of diamond burs has resulted in major improvements in milling technology. Another vital factor has been the development of alumina (aluminum oxide) and zirconia (zirconium oxide) ceramic materials, which possess excellent machinability and physical strength.³

Integration of these technologic advances has resulted in the introduction of three categories of highly sophisticated CAD/CAM systems: in-office systems, laboratory-based systems, and milling center systems. The in-office systems include CEREC® 3D and E4D Dentist™ (D4D Technologies LLC, Dallas, TX). The laboratory systems, such as CEREC inLab®, DCS Precident (Popp Dental Laboratory, Inc, Greendale, WI), Cercon® (DENTSPLY Ceramco, York, PA), Everest® (KaVo Dental Corp, Lake Zurich, IL), and DentaCAD (Hint-ELs® Canada Inc, Ontario), are comprised of a scanner, a CAD design unit, and a CAM mill unit for generating restorations on-site. The milling center systems offer outsourcing services and allow laboratories entry into CAD/CAM technology at a level that is most comfortable. Dental laboratories can either send working casts to an authorized milling center for scanning, design, and milling, or purchase a scanner and send data files to the milling center for milling and finishing. The current milling center systems are Procera® (Nobel Biocare USA, LLC, Yorba Linda, CA), Lava™ (3M ESPE, St Paul, MN), and TurboDent (U-Best Dental Technology, Anaheim, CA).

CAD/CAM technology provides several advantages for dental laboratories. CAD/CAM systems offer automation of fabrication procedures with increased quality in a shorter period of time. Dental CAD/CAM systems have the potential to minimize inaccuracies in technique and reduce hazards of infectious cross-contamination associated with conventional multistage fabrication of indirect restorations. However, capital costs of these CAD/CAM systems are quite high and rapid large-scale production of good quality restorations is necessary to achieve financial viability.

Restorative Materials for CAD/CAM

CAD/CAM systems based on machining of presintered alumina or zirconia blocks in combination with specially designed veneer ceramics satisfy the demand for all-ceramic posterior crowns and fixed partial dentures. Many ceramic materials are available for use as CAD/CAM restorations (Table 1). Common ceramic materials used in earlier dental CAD/CAM restorations have been machinable glass ceramics, such as Dicor (DENTSPLY Caulk, Milford, DE) or Vitablocks® Mark II (Vident, Bera, CA ). Of these, Vitablocks Mark II remains available for clinical applications while Dicor is no longer manufactured. Currently IPS e.max® ProCAD and IPS Empress® CAD (Ivoclar Vivadent, Inc, Amherst, NY), which have a fine leucite crystal structure, and IPS e.max® CAD (Ivoclar Vivadent, Inc), which is a lithium disilicate ceramic, are used more commonly because of their strength.³ Although monochromatic, these ceramic materials offered excellent esthetics, biocompatibility, great color stability, low thermal conductivity, and outstanding wear resistance⁴ (Figure 1). They have been successfully used as inlays,^5,6 onlays,⁶ veneers,⁷ and crowns.⁸ However, Dicor and Vitablocks Mark II are not strong enough to sustain occlusal loading when used for posterior crowns.⁹ For this reason, alumina and zirconia materials now are being used widely as dental restorative materials.

These ceramic agents may not be cost-effective without the aid of CAD/CAM technology. For instance, Vita® In-Ceram® (Vident), first described by Degrange and colleagues,10 has been shown to have good flexural strength and good clinical performance.^11,12 However, the manufacture of a conventional In-Ceram restoration can take up to 14 hours.¹³ By milling copings from presintered alumina or zirconia blocks in 20 minutes and reducing the glass infiltration time from 4 hours to 40 minutes, CEREC inLab decreases fabrication time by 90%. After milling, Vita In-Ceram Spinell, Alumina, and Zirconia blocks are glass infiltrated to fill fine porosities. Other machinable, presintered ceramic materials are sintered to full density, eliminating the need for extensive use of milling tools.

Zirconia is strong and has high biocompatibility. However, fully sintered zirconia materials can be difficult to mill, requiring 3 hours for a single restoration. Milling a restoration from a presintered or partially sintered solid block is easier and less time-consuming, creates less tool loading and wear, and provides higher precision. Under stress, the stable zirconia tetragonal phase may be transformed to the monoclinic phase, with a 3% to 4% volume increase.^14,15 This dimensional change creates compressive stresses that inhibit crack propagation. This phenomenon, called “transformation toughening,” actively opposes cracking and gives zirconia its reputation as the “smart ceramic.” The quality of transformation toughness and its affect on other properties is unknown.

Zirconia copings are laminated with low-fusing porcelain to provide esthetics and to reduce wear of the opposing dentition. Its opacity is very useful in masking discolored abutment teeth. If the abutment lacks adequate reduction, the restoration may look opaque (Figure 2A and Figure 2B). Because they are not normally etchable or bondable, abutments require good retention and resistance form. Alumina and zirconia restorations may be cemented with either conventional methods or adhesive bonding techniques. Conventional conditioning required by leucite ceramics (eg, hydrofluoric acid etch) is not needed. Microetching with aluminum oxide particles on cementation surfaces removes contamination and promotes retention for pure aluminum oxide ceramic.¹⁶ Two in vitro studies recommended that a resin composite containing an adhesive phosphate monomer in combination with a silane coupling/bonding agent can achieve superior long-term shear bond strength to the intaglio surface of Procera AllCeram and Procera AllZirkon restorations.^17,18 The radiopacity of zirconia also can facilitate the radiographic screening for marginal discrepancy and dental caries.

CAD/CAM technology has introduced new dimensions for restoring dental implants. Three systems—Procera, TurboDent, and Everest—currently offer custom-made titanium or zirconia implant abutments and implant-retained overdenture bars. CAD/CAM systems also can be applied to restorations requiring metal, such as a crown coping. The DCS system can fabricate crown copings from titanium alloy with excellent precision.¹⁹ Several articles have reported the extension of CAD/CAM technology to the fabrication of maxillofacial prostheses, such as the artificial ear.^20-23

Review of Common CAD/CAM Systems

CAD/CAM systems may be categorized as in-office, laboratory-based, or milling center systems (Table 2).

In-Office Systems CEREC

Among all dental CAD/CAM systems, Sirona, with their CEREC line of products, is the only manufacturer that currently provides both in-office and laboratory-based systems. With CEREC 1 and CEREC 2, the operator takes an optical scan of the prepared tooth with a charged-coupled device (CCD) camera, and the system automatically generates a 3D digital image on the monitor. Then, the restoration is designed and milled. With the newer CEREC 3D, the operator records multiple images within seconds, enabling clinicians to prepare multiple teeth in the same quadrant and create a virtual cast for the entire quadrant. On the virtual model, the operator designs the contour of the restoration and electronically transmits the data to a remote milling unit for fabrication. While the system is milling the first restoration, the operator can seat the restoration virtually into the virtual cast to provide the adjacent contact to design the next restoration. An in vitro evaluation of CAD/CAM ceramic crowns that compared the marginal adaptation of CEREC 2 with that of CEREC 3D concluded that crown adaptation for CEREC 3D (47.5 μm ± 19.5 μm) was significantly better compared with CEREC 2 (97 μm ± 33.8 μm).²⁴ One recent long-term evaluation of 200 CEREC 1 CAD/CAM inlays and onlays indicated that the success rate was 88.7% after 17 years. The reasons for failure were ceramic fractures (62%), caries (19%), tooth fractures (14%), and endodontic problems (5%).²⁵

CEREC inLab is a laboratory-based system for which working dies are laser-scanned and a digital image of the virtual model is displayed on a computer screen. After designing the coping or framework, the laboratory technician inserts the appropriate ceramic block into the CEREC inLab machine for milling. A wide range of high strength ceramic blocks are available for the inLab system, which include Vita In-Ceram blocs and two sintered ceramics: inCoris ZI (zirconium oxide) and inCoris AL (aluminium oxide) (Sirona.Dental Systems, LLC) After milling, the technician manually inspects and verifies the fit of the milled coping or framework on the die and working cast. Subsequently, the coping will be adjusted to maximize adaptation to the die. The coping or framework then is either glass-infiltrated (Vita In-Ceram) or sintered (zirconium oxide or aluminium oxide), and the veneering porcelain is added (Figure 3).

The most recent developments of CEREC technology include using a step bur that eliminates the need to overmill, the Biogeneric software for easy and friendly design, a faster and quieter milling unit (CEREC inLab MC XL), and CEREC Connect, which is a Web-based communication platform between in-office and CEREC inLab systems.

E4D Dentist

The E4D Dentist system is a newly developed in-office CAD/CAM system. In most clinical situations, digital 3D impressions of the tooth preparation can be obtained through use of its high-speed IntraOral Digitizer (an intraoral laser scanner) without reflective agents. The operator performs multiple scans from various angles to maximize collection of data points, which allows the software to re-create true morphology. The Design Center and milling unit allow dentists to create inlays, onlays, veneers, and crowns in one appointment.

The manufacturer, D4D Technologies LLC, collaborates with three major corporations in the dental industry. Sales, marketing, and distribution are handled by Henry Schein, Inc (Melville, NY), while restorative materials are supplied by 3M ESPE and Ivoclar Vivadent, Inc (Amherst, NY).

Laboratory-Based Systems DCS Precident

The DCS Precident system is comprised of a Preciscan laser scanner and Precimill CAM multitool milling center. The DCS Dentform software automatically suggests connector sizes and pontic forms for bridges. It can scan 14 dies simultaneously and mill up to 30 framework units in a single, fully automated operation. Materials used with DCS include porcelain, glass ceramic, Vita In-Ceram, dense zirconia, metals, and fiber-reinforced composites. This system is one of the few CAD/CAM systems that can mill titanium and fully dense sintered zirconia.

An in vitro study was conducted evaluating the marginal fit of alumina- and zirconia-based three-, four-, and five-unit posterior fixed partial dentures machined by the DCS Precident system. Measuring marginal discrepancies between 60 μm and 70 μm, the study concluded that the system easily met the requirement of a discrepancy less than 100 μm.²⁶ Another study evaluated the DCS system for fabricating titanium copings. The mean values of marginal fit for the individual crowns ranged from 21.2 μm ± 14.6 μm to 81.6 μm ± 25.1 μm. The mean value for all crowns was 47 μm ± 31.5 μm.¹⁹

Everest

Introduced in 2002, the Everest system consists of scan, engine, and therm components. The operator fixes a reflection-free gypsum cast into the scanning unit where it is scanned by a CCD camera in a 1:1 ratio, with an accuracy of measurement of 20 μm. The system automatically generates a digital 3D model by computing 15-point photographs. The operator then designs the restoration on the virtual 3D model with Windows®-based software. The machining unit has five-axis movement that is capable of producing detailed morphology and precise margins from a variety of materials including leucite-reinforced glass ceramics, partially and fully sintered zirconia, and titanium. Partially sintered zirconia frameworks require additional heat processing in its furnace. The marginal adaptation for Everest crowns was reported as 32.79 (± 6.82) µm and 33.72 (± 6.69) µm.²⁷

Cercon

Also launched in 2002, the Cercon system initially was referred to as a CAM system because it did not have a CAD component. At that time, the operator needed to make a wax pattern (coping) with a minimum thickness of 0.4 mm. Subsequently, the system scanned the wax pattern and the Cercon Brain milling unit milled a zirconia coping from proprietary presintered zirconia blanks. The coping then was sintered in the Cercon Heat furnace (1,350oC) for 6 to 8 hours. A low-fusing, leucite-free Cercon Ceram S veneering porcelain was used to provide the esthetic contour. In 2005, DENTSPLY Ceramco introduced the Cercon Eye 3D laser optical scanner and Cercon Art CAD design software. Now, as a complete CAD/CAM system, Cercon can produce single units and bridges up to nine units from presintered zirconia milling blocks that are offered in white and ivory shades without any infiltration required. In an in vitro study, the marginal adaptation for Cercon all-ceramic crowns and fixed partial dentures was reported as 31.3 μm and 29.3 μm, respectively.²⁸

Milling Center Systems Procera

Procera/AllCeram was introduced in 1994 and, according to company data, has produced 8 million units. The system uses an innovative concept for generating alumina and zirconia copings. Two types of scanners are available to dental laboratories—Procera Piccolo for single-unit restorations and Procera Forte for single- and multiple-unit restorations. First, the scanning stylus is used to create 3D images of the master dies, which are sent to the processing center via modem. The processing center (located in New Jersey or Sweden) then generates enlarged dies designed to compensate for the shrinkage of the ceramic material. Copings are manufactured by dry pressing high-purity alumina powder (> 99.9%) against the enlarged dies. These densely packed copings then are milled to the desired thickness. Subsequent sintering at 2,000°C imparts maximum density and strength to the milled copings. The copings are returned to the laboratory for a technician to apply the veneering porcelain and complete final occlusal adjustment and finishing on the working cast. The complete procedure for Procera coping fabrication is very technique-sensitive because the degree of die enlargement must precisely match the shrinkage produced by sintering the alumina or zirconia. The recommended preparation marginal design for a Procera/AllCeram restoration is a deep chamfer or shoulder with a rounded internal line angle and a well-defined cavosurface finish line with a recommended coping thickness of 0.4 mm to 0.6 mm.

Nobel Biocare USA LLC has introduced various implant abutments for its Procera system—titanium (1998), alumina (2002), and zirconia (2003). The system also is capable of generating alumina (two to four units) and zirconia (up to 14 units) bridge copings. However, the occlusal-cervical height of the abutment should be at least 3 mm, and the pontic space should be less than 11 mm.

According to recent research data, the average marginal gap for Procera/AllCeram restorations ranges from 54 μm to 64 μm.²⁹ Literature also confirms that Procera restorations have excellent clinical longevity and strength.³⁰ The flexural strength for Procera alumina is 687 MPa and for Procera zirconia is 1200 MPa.

Lava

Introduced in 2002, the Lava system uses a laser optical system to digitize information from multiple abutment margins and the edentulous ridge. Lava Design 4.0 CAD software automatically finds the margin and suggests a pontic. The framework is designed to be 20% larger to compensate for sintering shrinkage. After the design is complete, the system software recommends a properly sized semisintered zirconia block for milling. The block is bar coded to register the special design of the block. The computer-controlled precision milling unit can mill 21 copings or bridge frameworks automatically without supervision or manual intervention. Milled frameworks then undergo sintering to attain their final dimensions, density, and strength. The system also has eight different shades to color the framework for maximum esthetics. The shaded framework is returned to the laboratory for a technician to apply the veneering porcelain and complete the restoration. Starting in 2006, laboratories can purchase a stand-alone scanner, Lava Scan ST, and send the digital data to an authorized Lava Form milling center.

Hertlein and colleagues tested the marginal adaptation of yttria zirconia bridges processed with the Lava system for two milling times (75 minutes vs 56 minutes). They concluded that milling time does not affect marginal adaptation (61 ± 25 μm vs 59 ± 21 μm) for three-unit zirconia bridge frameworks.³¹

TurboDent

The TurboDent System (TDS) milling center, with its headquarters in Taiwan, began full production in 2005. With this system, the operator scans the stone model and wax-up with the TDS Scanner, and the dental prosthesis is designed by the operator using the TDS Designer. The TDS Designer is a design software package that includes a digital wax-up tool and a comprehensive library of wax-up designs, enabling various prostheses designs to be input and modified by the operator. The five-axis TDS Cutter is capable of milling a wide range of restorations, such as inlays, onlays, veneers, copings, bridge frameworks, custom implant abutments, and implant bars, from titanium or ceramic material. Like the Procera system, casts and dies may be scanned from anywhere in the world using the TDS Scanner and electronically transmitted to the milling center for design, fabrication, and finishing.

In 2007, TDS launched a new software module, TDS Implant Smart, that integrates computed tomography scan technology to create a blueprint to place the implants of choice virtually. Additionally, it allows the technician to provide a comprehensive range of services to include fabricating a surgical stent for implant placement, a customized titanium or zirconia implant abutment, temporary crown, and final restoration.

Although there is no literature regarding the marginal fit of the TDS, the manufacturer’s internal data suggested that its average marginal gap for a titanium coping is 15 µm.³²

Marginal Integrity of CAD/CAM Restorations

One of the most important criteria in evaluating fixed restorations is marginal integrity. Evaluating inlay restorations, Leinfelder and colleagues reported that marginal discrepancies greater than 100 μm resulted in extensive loss of the luting agent.³³ O’Neal and colleagues reported the possibility of wear resulting from contact of food particles with cement when gap dimension exceeded 100 μm.³⁴ Essig and colleagues conducted a 5-year evaluation of gap wear and reported that vertical wear is half of the horizontal gap. The wear of the gap increased dramatically in the first year, becoming stable after the second year.³⁵

McLean and von Fraunhofer proposed that an acceptable marginal discrepancy for full coverage restorations should be less than 120 μm.³⁶ Christensen³⁷ suggested a clinical goal of 25 μm to 40 μm for the marginal adaptation of cemented restorations. However, most clinicians agree that the marginal gap should be no greater than 50 μm to 100 μm.^38-40 Current research data indicate that most dental CAD/CAM systems currently produce restorations with acceptable marginal adaptation of less than 100 μm.^{24,26-29,31,32}

Future Directions for CAD/CAM Technology

Today, an increasing number of CAD/CAM manufacturers offer less expensive stand-alone scanners that allow dental laboratories to scan and design their own restorations and then send the data off-site for fabrication. One of the major goals for future CAD/CAM development will be the implementation of the open system concept.

The term “open-system” means that the digital data generated by the scanner is transferred in an industry-standard format and can be read by any manufacturer’s milling unit. Thus, if a laboratory has a scanner, it could send the imaging data to any desired system and not be locked into just one system. It may offer laboratory owners more versatility as most of the current stand-alone scanners are priced in the $30,000 range.

Conclusion

CAD/CAM systems have dramatically enhanced dentistry by providing high-quality restorations. The evolution of current systems and the introduction of new systems demonstrate increasing user-friendliness, expanded capabilities, improved quality, and greater range in complexity and application. New materials also are more esthetic, wear more like enamel, and are strong enough for full crowns and bridges.

Dental CAD/CAM technology is successful today because of the vision of many great pioneers. As Duret concluded in his 1991 article: “The systems will continue to improve in versatility, accuracy, and cost-effectiveness, and will be a part of routine dental practice by the beginning of the 21st century.”⁴¹

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About the Authors

class="body"Perng-Ru Liu, DDS, MS, DMD, is a professor and chair of the department of comprehensive dentistry at The University of Alabama at Birmingham School of Dentistry. Milton E. Essig, DMD, is a professor and associate chair in the department of comprehensive dentistry, at The University of Alabama at Birmingham School of Dentistry.