Indirect Millable Non-Zirconia Ceramics
A chairside and laboratory brief.
In today’s $11 billion US dental prosthetics market, crowns are among the prostheses registering high growth rates, not only in the number seated annually, but also in the percentage that are metal-free.1 Likewise, the steady growth witnessed in the dental CAD/CAM market cannot be missed, with advances in both hard and soft technology a companying innovations in materials, all evidenced by the continual stream of new products to market.
The fact is, crowns are the third most common dental procedure.2 Knowing how those crowns are fabricated and what they are fabricated from is undeniably imperative for every dental technician determined to stay relevant in the marketplace. While porcelain-fused-to-metal (PFM) and all-metal restorations are by no means obsolete, the industry-wide effort currently underway to improve esthetics over these metallic options is fueling unprecedented growth in all-ceramic materials. (The growth is not exclusive to crowns, but includes inlays, onlays, veneers, bridges, implant structures, and beyond.)
Also not to be overlooked are: the rising demand for “teeth while you wait;” the need to cut costs and increase efficiency to remain profitable; and the need to maintain or increase the quality, precision, and esthetics of every dental restoration. Dental technicians have every reason to look carefully at the non-zirconia based ceramic materials available and the systems used to fabricate these restorations, both chairside and in the laboratory.
The former pioneered chairside milling capabilities beginning in the late 1980s, and with 27,000 dentists and 20 million patients backing the system worldwide, it continues to corner the chairside CAD/CAM market.3 CEREC AC is a closed digital system (ie, file types cannot be shared with other systems) and some materials used in fabrication remain proprietary.
E4D Dentist, which is newer to market, offers powder-free scanning, and the E4D Sky (beta) digital system permits data transfer to laboratories via the network as well as the ability to convert to an open file format.
While there is plenty to compare and contrast between systems, both follow the same basic methodology: a tooth preparation for an inlay, onlay, veneer, or single crown is scanned; next, the restoration is digitally designed (CAD), then fabricated (CAM) from a monolithic material block—all in as little as one patient appointment.
An important aside: Technicians are naturally more adept and efficient than dentists or their assistants at designing restorations that are morphologically correct and esthetically pleasing. Many dentists with chairside units (most are without an in-house technician) willingly concede this and are only too glad to send scan data to the technician, who designs the restoration, then sends it back chairside for milling. For this reason, technicians should not write off the percentage of dentists who currently operate a chairside CAD/CAM system or those intending to purchase one.
Laboratory-based CAD/CAM systems—inLab® MC XL (Sirona) and E4D Labworks™ (D4D) are two among many—are powerful workhorses covering a wide range of clinical indications far beyond those of a chairside system. Laboratory-based systems handle larger and more complex restorations, mill more units simultaneously, and support a broad spectrum of metals, zirconia-based materials, and all-ceramics, although only the latter are of interest for discussion here.
If a dentist has chairside digital impression scanning capability, the scanned tooth data is sent to the laboratory. Otherwise, the technician scans the model or impression and digital design, and fabrication ensues for everything from physical models to custom implant abutments to final restorations of any size, shape, and purpose.
For a clearer look into the not-so-distant-future, the road to open architecture in CAD/CAM dentistry—the ability of different systems to “talk” to each other by permitting scan files and design files to be shared at various stages of CAD and CAM—is slowly being paved. New software systems built on an open global standard software platform—a groundbreaking collaboration between 3M ESPE (www.3mespe.com), Straumann, (www.straumann.us), and Dental Wings (www.dental-wings.com)—intentionally offers customers the option of third-party milling. Eventually, the technician will accept scan data from a dentist or generate scan data in the laboratory, design in an open platform software, and mill on the machine of choice—and yes, choose the material block best suited to the restoration at hand. As systems become open, materials will become less machine-specific. Flexibility, simplicity, and convenience will be enhanced while a laboratory’s overall investment, cost of operation, and time will be minimized.
That said, while any current or future systems—chairside or laboratory-based—can guarantee precision and conformation to the inputs provided by the operator, the clinical success of the restoration depends heavily upon the material used.
Considering bicompatibility, strength, esthetics, longevity, machinability, cost, and speed of fabrication, each material exhibits unique features designed for specific clinical scenarios. A full armament for CAD/CAM restoration fabrication may contain products from several of the following materials: feldspathic glass ceramic, leucite-reinforced glass ceramic, lithium disilicate ceramic, resin nano-ceramic, and composite resin (Table 1).
All oxide-based glass ceramics exhibit four essential features—excellent bicompatibility, ease of fabrication of complex shapes, sufficient
mechanical and corrosion resistance, and esthetic appeal.4
It is essential for the technician to know that ceramics must be adhesively bonded rather than cemented conventionally, not only for retention of the restoration, but also for increased fracture resistance.
Feldspathic Glass Ceramic
Two fine-grained, homogenous feldspathic glass ceramics are VITABLOCS® Mark II (Vident, www.vident.com) and CEREC Blocs (Sirona). Both exhibit relatively high translucency and flexural strength (154 MPa). Their fine particle size (average 4 µm) and uniform embedding of feldspar into the glass matrix account for abrasion properties and a high-gloss finish similar to enamel (Figure 1).
VITABLOCS Mark II has been on the market since 1991 and reports a clinical survival rate after 10 years for bonded restorations of 95%. More than 16 million restorations have been fabricated to date. The Mark II line is available in the 10 most common Vita 3D Master shades. VITA Triluxe copies the optical characteristics of a natural tooth with three shades and three translucencies within each block. Now VITA Real Life offers a 3-D radial gradient of color and translucency from the inside to the outside of the block to mimic the natural transition from dentin to enamel.
CEREC Blocs, on the market since 2007, offer six shades and three degrees of color saturation, as well as a polychromatic block, CEREC PC, a three-layer structure with varying pigmentation, and translucency.4
Leucite-Reinforced Glass Ceramic
Leucite-reinforced glass ceramic is a feldspathic glass ceramic with leucite added to modify the coefficient of thermal expansion and impart increased fracture strength.
IPS Empress® CAD (Ivoclar Vivadent, www.ivoclarvivadent.us), on the market since 2006, is 35% to 45% leucite-reinforced glass ceramic with a fine particle size of 1 to 5 µm, and is often chosen for its optical properties and strength values (160 MPa) (Figure 2). IPS Empress CAD’s variety of blocks and shades surpasses that of any other material discussed here. Nine shades (A through D), three Chromascop, and four bleach shades are available, all in two translucencies. Further, the Multi block, which is available in five shades, offers a smooth gradient of color and translucency ranging from cervical to incisal.
Lithium-Disilicate High Strength Ceramic
In 2006, Ivoclar Vivadent introduced lithium-disilicate IPS e.max® CAD, a material with two to three times the flexural strength of the glass ceramics previously mentioned (360 MPa). IPS e.max CAD is often termed the “blue block” for its blue-violet, partially crystallized soft state, which is 40% by volume lithium metasilicate crystals that are 0.2-1 µm in size. It mills easily and can be stained, glazed, and crystallized in a single firing. The final product has a fine 1.5-µm particle size and is 70% crystals by volume within the glass
matrix (Figure 3).
IPS e.max CAD is available in 16 shades (A through D), two translucencies, and four bleach shades. Where feldspathic glass ceramic and its leucite-reinforced cousin must be adhesively bonded, lithium-disilicate’s high strength permits conventional cementation with a resin-modified or traditional glass ionomer cement.
Resin Nano-Ceramic (RNC)
The combination of nanotechnology and ceramics has given birth to this new category in CAD/CAM materials with a product exhibiting the handling properties of a composite material with the high shine of porcelain. Lava™ Ultimate (3M ESPE) embeds silica (20 µm particles) and zirconia (4 to 11 µm particles) into a cross-linked polymer matrix (Figure 4). As the marketing literature states, it is mostly ceramic (80% approximately) with “a little” resin. This combination results in a manufacturer-reported 204-MPa flexural strength and the ability to retain a high-gloss finish over time. It is easily contoured and can be adapted with light-cure restoratives.
Available in eight shades, and in low and high translucency, Lava Ultimate can be milled on any of the afore-mentioned chairside or laboratory-based CAD/CAM systems. 3M ESPE has also launched an industry-standard-setting, 10-year manufacturer’s warranty for a replacement of the Lava Ultimate should it fail clinically.
Composite resins, available for final restorations and for provisional restorations, exhibit solid strength and wear resistance, but their excellent handling properties are what make them desirable. Refining the margins and the proximal and occlusal contacts, and polishing intraorally are two examples. Further, repairs can be made with hybrid composites.
Paradigm™ MZ100 (3M ESPE) is a polymer composite material made from Z100 Restorative (3M ESPE), which relies on a proprietary processing technique to maximize the degree of cross-linking and a thorough cure. Paradigm MZ100 uses zirconia-silica ceramic particles as filler (85% by weight) and has a fine particle size of 0.6 µm (Figure 5). It is strong, wear-resistant, radiopaque, and available in two sizes and six shades, one of which is a translucent enamel shade.
Relevance for Today
There is no argument that the laboratory industry is fighting on several fronts to survive in today’s changing dental landscape. Skilled labor is either being outsourced overseas or replaced with devices, and “normal” or basic, repetitive procedures are being digitized via CAD/CAM operatives. In addition, materials are evolving at a pace as fast or faster than the fabrication technology itself.
Is this a threat or an opportunity? In response, laboratory owners can create new working models in-house, continue to push for the development of open CAD/CAM systems at every stage, and expand (with care) the range of materials used—all with the end-goal of operating at a higher level of productivity and creativity while remaining relevant in the marketplace and being uniquely prepared for the future of restorative care.
1. PRNewsire, New York, October 10, 2011. http://www.prnewswire.com/news-releases/us-market-for-dental-prosthetics-and-cadcam-devices-2011-131441713.html. Accessed March 26, 2012.
2. Key Dental Facts. American Dental Association, Survey Center. September 2008: 4.
3. CAD/CAM at a glance: CEREC – setting the standards in dentistry. 2010. http://www.sirona.com/.comaXL/index.php?site=SIRONA_E_support_downloads. Accessed March 26, 2012.
4. Chu S, Ahmad I. A historical perspective of synthetic ceramic and traditional feldspathic porcelain. Pract Proced Aesthet Dent. 2005;17(9): 593-598.
About the Author
David Gratton, DDS, MS, is an associate professor in the Department of Prosthodontics at the University of Iowa College of Dentistry.