Contemporary Ceramics and Cements

Abstract

The demand for improved esthetics and concerns about the biocompatibility of metals have caused a revolution in the area of all-ceramic restorations. Ceramic materials are evolving with higher strength and increased translucency to provide improved esthetic restorations. Additionally, monolithic ceramic restorations are rapidly growing in popularity, rapidly replacing ceramometal and layered restorations. As dental materials continue to evolve, confusion regarding material choice and cementation, bonding, and surface treatment is prevalent. Clinicians are seeking ways to navigate both material choice and cementation protocols. Unfortunately, there is not an ideal combination of materials that are suitable for every clinical situation. The goal of this Contemporary Issues article is to provide a brief understanding of available materials on the market and discuss how the properties of both the ceramic and cement perform together. Modern all-ceramic systems can be categorized based on their glass and crystalline content. A ceramic with high glass content will exhibit excellent esthetics whereas a ceramic with high crystalline content will provide superior strength. Ceramics can be classified into three categories based on composition: porcelains (feldspathic), glass ceramics (leucite-reinforced, lithium silicate, and lithium disilicate), and polycrystalline ceramics (zirconia and alumina). Feldspathic porcelains (containing mostly glass) are composed of potassium feldspar, quartz, and kaolin. Because of their high glass content, they have superior translucency and esthetics. They have the lowest flexural strength among ceramics, ranging from 70 to 110 MPa, making them highly brittle. They are most commonly used as a veneering material on a stronger ceramic or metal core. Leucite-reinforced ceramics (IPS Empress Esthetic, Ivoclar Vivadent, Amherst, NY, USA; Cerpress SL, American Dental Supply, Allentown, PA, USA) offer excellent esthetics and translucency. The addition of leucite crystals to the glass matrix slightly improves mechanical properties, making these ceramics ideal for veneers, inlays/onlays, and anterior crowns. Lithium disilicate ceramics (IPS e.max, Ivoclar Vivadent) consists of approximately 70% lithium disilicate crystals that are embedded in a glassy matrix, resulting in a relatively translucent material that is 2 to 3 times stronger than leucite-reinforced ceramics (360–400 MPa). The combination of strength and esthetics makes them one of the most versatile materials on the market, allowing them to be used for veneers, inlays/onlays, anterior/posterior crowns, three-unit anterior fixed partial dentures, and implant restorations. Zirconia-based ceramics (mostly crystals) are the highest strength ceramics because of their crystalline structure. The lack of a glass component within the matrix results in an opaque restoration, compromising esthetics in some cases. Traditional zirconia-based ceramics utilize zirconia as a base framework. Similar to the fabrication of a Porcelain-fused-to-metal (PFM) crown, the framework is veneered with ceramic, providing esthetics to the inherently opaque restoration. The success of zirconia-based ceramics as a strong framework led to the development of full-contour monolithic restorations. New zirconia-based ceramics have been developed that are preshaded with improved translucency and are designed for use as monolithic restorations (Lava Plus, 3M ESPE, St. Paul, MN, USA; NexxZr T, Sagemax, Federal Way, WA, USA; Zenostar, Ivoclar Vivadent/Wieland Dental, Amherst, NY, USA; Bruxzir, Glidewell Dental Laboratories, Newport Beach, CA, USA). Zirconia-based ceramics continue to gain popularity because of high strength, fracture toughness, biocompatibility, longevity, and versatility. They are increasing in popularity because of minimal tooth preparation (similar to cast gold), low cost, and use in multiple connected units. The current clinical challenge with zirconia ceramics is that they lack the esthetics of other more translucent materials. These full-contour restorations offer the strength of zirconia and when stained and glazed properly, better esthetics than previous full-contour zirconia material. Figure 1 illustrates properties of ceramics as a whole for consideration in case selection. As a general rule: As flexural strength increases, esthetics decreases. Reprinted by permission of Dental Consultants Inc. Newer materials on the market include lithium silicate (Obsidian, Glidewell Dental Laboratories) and “resin nano ceramics” (Lava Ultimate, 3M ESPE; Enamic, Vident, Brea, CA, USA). The latter are available as a computer-aided design and computer-aided manufacturing(CAD/CAM) block, both for chairside milling and laboratory use. The block is fabricated by combining resin nanoparticles with zirconia particles. With a flexural strength of ∼200 MPa, the material is best used for inlay/onlays, anterior/posterior crowns, and implant-supported crowns. The material is not as brittle as a conventional ceramic; therefore, it mills easily and does not require an additional firing step. Lithium silicate is a glass ceramic material indicated for the fabrication of full-contour crowns, three-unit anterior fixed partial dentures, veneers, inlays, and onlays. The reported flexural strength falls between leucite reinforced and lithium disilicate. Obsidian can be pressed or milled, or as a veneering ceramic to metal. The adhesive systems that allow newer ceramics to bond to tooth structure have rapidly evolved as new ceramics are introduced. There are a number of new resin cements on the market, some of which combine existing materials for convenience and simplified procedures, and some that have entirely new chemistry; however, we have yet to see a true universal cement for all indications. The introduction, improvement, and growing popularity of all-ceramic restorations has, in turn, spurred a surge in popularity of resin cements that address the many of the shortcomings of traditional luting cements. They exhibit high bond strength to tooth structure, superior esthetics, and the lowest solubility of the available cements. Currently, resin cements can be classified into three categories: self-adhesive, adhesive, and esthetic resin. Self-adhesive cements (RelyX Unicem, 3M ESPE; Panavia SA, Kuraray America, Inc., New York, NY, USA) require no separate etching or priming of tooth structure. Adhesive cements (Multilink Automix, Ivoclar Vivadent; Duo-Link Universal, Bisco, Inc., Schaumburg, IL, USA) bond to the tooth through the use of self-etching primers. Esthetic resin cements (NX3, Kerr Corporation, Orange, CA, USA; Calibra, Dentsply Caulk, Milford, DE, USA) bond based on an etch-and-rinse adhesive. Characteristics of each are outlined in Table 1. There are many types of cement to choose from, and often there is more than one viable option. An easy place to start in the decision-making process is the strength of the ceramic in conjunction with the retentiveness of the preparation. Generally, when esthetics is of high concern, low- to medium-strength ceramics (feldspathic, leucite-reinforced, lithium silicate, and lithium disilicate) are selected. In these cases, using high-strength cement (adhesive resin or esthetic resin) will add strength to the restoration. When cementing a veneer, esthetic resin cements provide a variety of shades, translucencies, and try-in pastes. Light-cured resin cements are less likely to change color when cured and are recommended for veneers. When using high-strength ceramic, such as zirconia, with a retentive preparation, low-strength cement such as a self-adhesive resin cement or resin-modified glass ionomer (RMGI) can be used because it is not necessary to rely on the cement for additional strength. Zirconia-based ceramics are not glass ceramics; therefore, the methods of mechanical and chemical bonding (hydrofluoric acid-etch and silanation) used on glass ceramics are not applicable. To form a strong bond to zirconia-based ceramics, the bonded surface must be mechanically roughened, free of contaminants, and chemically primed prior to cementation. For zirconia-based restorations, micromechanical roughening is accomplished through sandblasting (airborne particle abrasion). Laboratory studies at The Dental Advisor (Ann Arbor, MI, USA) have found that sandblasting the intaglio surface with 50 um alumina oxide at low pressure (2 bar) increases the surface area, resulting in better resin bond strengths. Contaminants after try-in can be removed with the use of a surface cleaner (Ivoclean, Ivoclar Vivadent). Zirconia-based ceramics require specific primers to promote the chemical bond at the non-silica oxide–cement interface when the retention/resistance from is compromised. These primers (Z-Prime Plus, Bisco, Inc.) contain an acidic monomer (10-MDP (10-Methacryloyloxydecyl dihydrogen phosphate)) and are compatible with dual-cured resin cements. Some ceramic primers (Clearfil Ceramic Primer, Kuraray America, Inc.; Monobond Plus, Ivoclar Vivadent; Scotchbond Universal Adhesive, 3M ESPE) will bond to both silica- and zirconia-based restorations. Zirconia-based restorations with good retention can be cemented with traditional crown and bridge cements (such as RMGI) or self-adhesive resin cements. If retention is not ideal, then the zirconia-based restoration should be primed with ceramic primer and bonded using esthetic or adhesive resin cement (Figure 2). Guidelines for cement selection based upon strength of ceramic and retentiveness of preparation. Reprinted by permission of Dental Consultants Inc. As dental materials evolve, there is a continual push toward strong yet esthetic restorations. Just in the last decade, we have witnessed tremendous progress in terms of both strength and esthetics. Where once low-strength, silica-based ceramics were the only option for esthetic restorations, we now have all-ceramic options that provide us four times the strength. With the success of lithium disilicate ceramics, it is clear that the dental industry is moving toward acceptance of monolithic restorations. As such, full-contour zirconia-based crowns and bridges are being manufactured, offering higher translucency and high strength. We will continue to see full contour zirconia restorations grow in popularity because of lower cost, higher strength, and conservative preparation. As materials change, the evolution of a truly universal cement will emerge, providing the clinician with every setting option (light-, dual-, and self-cure) contained in one kit.