– predominantly glass;

– particle-filled glass;

– polycrystalline.

Second, dentists can consider virtually any ceramic within this spectrum to be a composite, meaning a composition of two or more entities. Although dentists usually reserve the term "composite" for particle-filled resins, they can generalize the concept to include dental ceramics in which the matrix usually is a glass that is lightly filled or heavily filled with particles (crystalline particles or glass particles that melt at high temperatures). For polycrystalline ceramics, which contain no glass, the matrix is aluminum oxide or zirconium oxide, and the fillers are not particles but modifying atoms called "dopants." Figure 2³ (page 6S) presents many commercially available ceramics from the particle-filled glass and poly-crystalline categories according to the matrix, filler concentration and type, and process used to make the restoration. Dentists can apply these concepts directly to gain insights into two general facts regarding dental ceramics:

View larger version (60K): [in this window] [in a new window]   Figure 2. Organizational chart of dental ceramics for all-ceramic systems according to matrix material, filler or dopant (atomic-level filler) and fabrication process. CEREC 3 is manufactured by Sirona Dental Systems GmbH, Bensheim, Germany; InLab, Sirona Dental Systems GmbH; Vitablocs Mark II, Vita Zahnfabrik, Bad Säckingen, Germany; IPS Empress CAD and IPS Empress Esthetic, Ivoclar Vivadent; Amherst, N.Y.; OPC, Pentron Ceramics, Somerset, N.J.; Cerinate, Den-Mat, Santa Maria, Calif.; Fortress, Mirage Dental Systems, Kansas City, Kan.; IPS e.max CAD and IPS e.max Press, Ivoclar Vivadent; In-Ceram Alumina, Spinell and Zirconia, Vita Zahnfabrik; Vita AL Cubes, Vita Zahnfabrik; Procera, Nobel Biocare, Göteborg, Sweden; Vita YZ Cubes, Vita Zahnfabrik; IPS e.max ZirCAD, Ivoclar Vivadent; Lava Zirconia, 3M ESPE, St. Paul, Minn.; Cercon Zirconia, Dentsply Prosthetics, York, Pa.; Procera, Nobel Biocare. (Adapted with permission of Quintessenz Verlags-GmbH, Berlin, from Kelly.3)

– Highly esthetic dental ceramics have a high glass content, and higher-strength substructure ceramics generally are crystalline.

– The history of the development of substructure ceramics involves an increase in crystalline content ranging from approximately 55 percent crystalline to fully polycrystalline.

Predominantly glass. Dental ceramics that best mimic the optical properties of enamel and dentin have a high glass content. Manufacturers use small amounts of filler particles to control optical effects such as opalescence, color and opacity. Most veneering materials come paired with their appropriate substructure ceramic, so it makes little sense to introduce them according to trade name (which is why I have not included them in Figure 2). I discuss predominantly glass-based ceramics in more detail elsewhere.¹

Particle-filled glass. Manufacturers add filler particles to the base glass composition to improve mechanical properties, such as strength and thermal expansion and contraction behavior. These fillers usually are crystalline, but they also can be particles of high-melting glasses that are stable at the firing temperatures of the ceramic.² Often, it is these filler particles that are dissolved during etching to create micromechanical retentive features enabling bonding. Particles can be added mechanically during manufacturing as powder or be precipitated within the starting glass by special nucleation and growth heating treatments; in the second case, such materials are termed "glass-ceramics."

The family of ceramics containing high concentrations of lithium disilicate crystals is an example of a glass-ceramic (IPS e.max Press and IPS e.max CAD, Ivoclar Vivadent, Amherst, N.Y.). Another family of particle-filled glass is unique in that it is a three-dimensional (3-D) interpenetrating-phase composite, in which the filler particles and glass are both continuous in space (In-Ceram, Vita Zahnfabrik, Bad Säckingen, Germany). The filler is alumina, magnesium aluminate spinel or a mixture of 70 percent alumina and 30 percent zirconia.

Polycrystalline. Polycrystalline ceramics contain no glass; all of the atoms are packed into regular crystalline arrays through which it is much more difficult to drive a crack than it is atoms in the less dense and irregular network found in glasses. Hence, polycrystalline ceramics generally are much tougher and stronger than glass-based ceramics. Well-fitting prostheses made from polycrystalline ceramics were not practical before the availability of computer-aided manufacturing.

In general, these computer-aided systems use a 3-D data set representing either the prepared tooth or a wax model of the desired substructure. The systems use this 3-D data set to create an enlarged die upon which ceramic powder (Procera, Nobel Biocare, Göteborg, Sweden) is packed or to machine an oversized part for firing by machining blocks of partially fired ceramic powder (ZirCAD, Ivoclar Vivadent; Cercon Zirconia, Dentsply Prosthetics, York, Pa.; Lava Zirconia, 3M ESPE, St. Paul, Minn.; Vita In-Ceram YZ, Vita Zahnfabrik). Both of these approaches rely on well-characterized ceramic powders (that is, tight control over particle sizes and packing density) for which firing shrinkages can be predicted accurately.^2,3

All-ceramic versus metal-ceramic systems. All-ceramic systems can provide a better esthetic result for a wider range of patients than can metal-ceramic systems, because a wider range of translucency and opacity (value) can be achieved. All-ceramic systems have no metal framework to be masked or metal margins exposed that produce an unattractive appearance. Often, it is acceptable to leave margins of all-ceramic prostheses supragingival or at the gingival margin, resulting in more predictable and less traumatic impression-making. Emergence profiles are less likely to be overcontoured, as is often the result with metal-ceramic prostheses owing to efforts by laboratory technicians to provide a thicker layer of porcelain to mask the opaque-metal surface in areas that often are underprepared.¹

Most ceramics are superior to metals with respect to corrosion, galvanism and biocompatibility. Researchers report recovering smaller amounts of plaque and adherence molecules from ceramic surfaces than from gold alloys or amalgam.^4–6 In addition, intraoral plaque of a qualitatively healthier composition can form on ceramic surfaces.^4–6

Nevertheless, to use all-ceramic systems successfully, the clinician must have a high level of knowledge both to maximize the esthetic result and to choose materials appropriately for structural longevity. Metal-ceramic systems are well-enough developed that little special knowledge is required for their routine use. Many practitioners may be unaware of the metal-ceramic systems their laboratories use, but any system generally will be as suitable for anterior single-unit restorations as it is for posterior multiunit prostheses. This forgiving restorative system has survival rates of about 74 percent at 15 years, and 53 percent of metal-ceramic restorations have been reported to be in service at 30 years, with the majority of problems being biological (that is, secondary caries, periodontal disease and endodontic failures).^7–9

Clinical success. In this supplement, Dr. Della Bona and I¹⁰ review the clinical literature regarding the performance of commercially available all-ceramic systems. This literature is at the level of systematic reviews of numerous studies of systems indicated for inlays, onlays and anterior single-unit restorations. Such evidence is among the highest available for making clinical practice decisions. Fewer systems and less evidence are available for the restoration of molars and for multiunit prostheses involving molars.

Factors for clinical success. Dr. Donovan¹¹ shares the peer-reviewed literature and practical experience in exploring how to incorporate all-ceramic systems into dental practice.

Why ceramics? Long-lived esthetics and bio-compatibility are the promise of all-ceramic systems. Drs. Spear and Holloway¹² describe how the use of all-ceramic restorations has become routine and provide an overview of indications for their use.

	CONCLUSION

TOP SIMPLIFYING CONCEPTS IN... CONCLUSION REFERENCES

 
All-ceramic systems are no longer experimental or suitable only for specialty practices. Clinical data and years of experience form the basis of the articles in this supplement, which are designed to provide interested clinicians with the background to begin integrating all-ceramic restorations into their routine practice.