Ceramic-Based Materials
Key Terms
Ceramic, CAD-CAM Ceramic that is formulated for the production of the whole or part of a ceramic prosthesis through the use of a computer-aided design, computer-aided manufacturing process.
Ceramic, castable Glass or other ceramic specially formulated to be cast into a refractory mold as a core coping or framework for a ceramic prosthesis.
Ceramic, core Opaque or semitranslucent crystalline dental ceramic material that provides sufficient strength, toughness, and stiffness to support overlying layers of veneering ceramics.
Ceramic, dental Inorganic compound with nonmetallic properties typically consisting of oxygen and one or more metallic (or semimetallic) and/or nonmetallic elements, such as aluminum, calcium, lithium, magnesium, potassium, silicon, sodium, tin, titanium, and zirconium. Ceramic frits that are provided in a powder form for veneering (layering) ceramics are typically composed of a mixture of glass and crystalline particles.
Ceramic, glaze Specially formulated ceramic powder that, when mixed with a liquid, applied to a ceramic surface, and heated to an appropriate temperature for a sufficient time, forms a smooth, glassy layer on a dental ceramic surface.
Ceramic, hot-pressed (pressable ceramic) Ceramic that can be heated to a flow temperature and forced under isostatic pressure to fill a cavity in a refractory mold.
Ceramic, stain Mixture of one or more pigmented metal oxides and a low-fusing glass that can modify the shade of the ceramic-based restoration; this mixture is dispersed in an aqueous medium, applied to the surface of porcelain or other dental ceramic, and heated to the vitrification temperature of this material for a specific time.
Glass-ceramic Ceramic consisting of at least one glass phase and at least one crystalline phase that is produced by a controlled crystallization of the glass.
Glass-infiltrated core ceramic A partially sintered core ceramic with a porous structure that is densified by the capillary inflow of a molten glass.
Green state Term referring to an as-pressed or minimally sintered condition prior to final sintering.
Metal-ceramic prosthesis A partial crown, full crown, or fixed partial denture made with a metal substructure to which porcelain is bonded for aesthetic enhancement via an intermediate metal oxide layer. The terms porcelain fused to metal (PFM), porcelain bonded to metal (PBM), porcelain to metal (PTM), and ceramometal are also used to describe these prostheses, but metal-ceramic (MC) is the term accepted internationally.
Porcelain A relatively dense, white ceramic material produced by sintering a mixture of feldspar, kaolin, quartz, and other substances, strictly referring to those containing kaolin.
Porcelain, feldspathic Ceramic composed of a glass matrix phase and one or more crystalline phases, such as leucite (KAlSi 2 O 6 ), sanidine (KAlSi 3 O 8 ), and apatite [Ca 5 (PO 4 ) 3 (F, Cl, OH)]. Most commercial dental porcelains designed for metal-ceramic restorations are partially crystallized feldspathic glasses that consist of tetragonal leucite (K 2 O•Al 2 O 3 •4SiO 2 ) crystals in a glass-phase matrix.
Sintering Process of heating closely packed particles below the melting point of the main component to densify and strengthen a structure as a result of bonding, diffusion, and flow phenomena.
Spinel or spinelle Crystalline mineral composed of mixed oxides such as MgO•Al 2 O 3 .
Thermal compatibility Condition of low transient and residual tensile stress in ceramic adjacent to a metal or ceramic core that is associated with a small difference in the coefficients of thermal contraction between the core material and the veneering ceramic.
Transformation toughening A ceramic-strengthening mechanism that occurs through transformation of crystalline structure. For example, the stress-activated tetragonal ( t ) to monoclinic ( m ) transformation in yttria-stabilized zirconia causes slight volume expansion at the crack tip that keeps the crack from propagating and raises the stress needed to keep the crack propagation to fracture the prosthesis.
Which property best describes the fracture resistance of dental ceramics?
Dental ceramics consist of silicate glasses, porcelains, glass-ceramics, or highly crystalline solids. They are nonmetallic, inorganic structures, primarily containing compounds of oxygen with one or more metallic or semimetallic elements (aluminum, boron, calcium, cerium, lithium, magnesium, phosphorus, potassium, silicon, sodium, titanium, and zirconium). Many dental ceramics contain a crystal phase and a silicate glass matrix phase. Silicate glasses differ from nonsilicate glasses in that silicon is the central divalent cation that is bound to four relatively large oxygen anions that link in a random order to other tetrahedra to form polymeric-type (SiO 2 ) n chains. Their structures are characterized by chains of (SiO 4 ) –4 tetrahedra in which Si 4+ cations are positioned at the center of each tetrahedron, with O – anions at each of the four corners ( Figure 10-1 ). The resulting structure is not closely packed and exhibits both covalent and ionic bonds. The SiO 4 tetrahedra are linked together by sharing their corners and not their edges or faces. They are arranged as linked chains of tetrahedra, each of which contains two oxygen atoms for every silicon atom. In industry, the term porcelain is generally associated with ceramics produced with a significant amount of kaolinite [Al 2 Si 2 O 5 (OH) 4 or Al 2 O 3 •2SiO 2 •2H 2 O]. Kaolinite is a form of kaolin, which is a type of clay. None of the modern low-fusing or ultralow-fusing porcelains contains any clay product such as kaolinite.
General Properties of Ceramic Materials
The properties of ceramics are customized for dental applications by precisely controlling the types and amounts of the components used in their production. Ceramics exhibit chemical, mechanical, physical, and thermal properties that distinguish them from metals, acrylic resins, and resin-based composites. Most ceramics are characterized by their biocompatibility, aesthetic potential, refractory nature, high hardness, low to moderate fracture toughness, excellent wear resistance, susceptibility to tensile fracture, and chemical inertness.
Chemical Properties
Chemical inertness is an important characteristic because this ensures that the chemically stable surface of dental restorations does not release potentially harmful elements and reduces the risk of surface roughening with increased abrasiveness or increased susceptibility to bacterial adhesion over time. The chemical inertness also makes ceramics more resistant to corrosion than plastics. Ceramics do not react readily with most liquids, gases, alkalis, and weak acids. They also remain relatively stable over long time periods, although they have been shown to undergo corrosion in simulated oral environments.
Acidulated phosphate fluoride (APF), one of the most commonly used fluoride gels for caries control, is known to etch glass by selective leaching of sodium ions, thereby disrupting the silica network. When glazed feldspathic porcelain is exposed to 1.23% APF or 8% stannous fluoride, surface roughness is produced within 4 minutes. As shown in Figure 10-2 , a 30-minute exposure to 1.23% APF gel resulted in preferential attack of the glass phase (areas with white precipitate particles) of a body porcelain. However, the use of neutral gel, such as 0.4% stannous fluoride and 2% sodium fluoride, has no significant effect on the ceramic surface. Dentists should be aware of these long-term clinical effects of fluorides on ceramic and composite restorations and avoid the use of APF gels when composites and ceramics are present. APF gels should not be used on glazed porcelain surfaces. If the use of such a gel is needed, the surface of these restorations should be protected with petroleum jelly, cocoa butter, or wax.
If tensile strength is not a reliable property of dental ceramics, which property is a better measure of the material’s fracture resistance?
Mechanical Properties
Ceramics exhibit good to excellent strength and fracture toughness. Zirconium dioxide is one of the strongest and toughest ceramics and has a flexural strength similar to that of steel, but this material’s fracture toughness is much lower than that of steel. Although ceramics are strong and heat-resistant, these materials are brittle and may fracture without warning when flexed excessively or when quickly heated and cooled (i.e., under thermal shock conditions).
Resistance to Tensile Fracture
The susceptibility to tensile fracture is a drawback, particularly when flaws and tensile stress coexist in the same region of a ceramic prosthesis. Strength is not an inherent property of ceramics because this property varies with specimen size, specimen length, specimen shape, loading rate, surface preparation methods, and the environment. For example, the strength of silica-based ceramics increases with a decrease in specimen thickness, an increase in stressing rate, and a smoother surface. Such variability indicates that strength is not necessarily a bulk property because surface conditions can significantly alter the mean strength and the spread of experimental values, as indicated by the Weibull modulus and the coefficient of variation ( Chapter 4, Weibull Modulus ). Therefore the tensile strength of ceramics is not a useful parameter to describe the fracture resistance.
The general belief is that dental ceramics fail primarily because of their brittle nature. From a fracture mechanics point of view, the major reason for the fracture of ceramics is their low resistance to crack growth. Two fracture-mechanics properties that better explain this behavior are fracture toughness and critical strain energy release rate. Fracture toughness, which is designated as K Ic , describes the critical stress intensity factor based on a Mode I (see Figure 4-14, C ) crack opening under tensile stress. For dental ceramics, K Ic varies from 0.8 MPa∙m 1/2 for feldspathic porcelain to 8 MPa∙m 1/2 or more for yttria-stabilized zirconia to 12 MPa∙m 1/2 or more for ceria-stabilized alumina-zirconia ceramic. In comparison, K Ic values are 0.7 to 1.3 MPa∙m 1/2 for enamel, 3.1 MPa∙m 1/2 for dentin, and 0.8 to 2.5 MPa∙m 1/2 for resin composites.
The critical strain energy release rate, which is designated as G Ic , is a measure of the strain energy that is released per unit increase in crack area as a ceramic with flaws or cracks is loaded progressively. The subscript I refers to the crack-opening mode under tension as expressed for K Ic . Comparative values of K Ic (MPa∙m 1/2 ) versus G Ic (kJ/m 2 ) for different types of materials are as follows: (1) ductile metals, 150 versus 50 to 200; (2) brittle metals, 25 versus 1 to 5; and (3) soda-lime glass, 0.8 versus 0.5.
Abrasiveness to Enamel
In spite of their overall excellence in meeting the ideal requirements of a prosthetic material, dental ceramics can cause catastrophic wear of opposing tooth structure under certain conditions because of their hardness. Wear is a form of fracture that occurs microscopically on the surface. The most extreme damage occurs because of bruxing, premature occlusal contacts, and/or inadequate occlusal adjustments. When cuspid-guided disocclusion is ensured, the wear of opposing enamel and dentin will be greatly reduced. The abrasive wear of opposing tooth structure can be reduced further by periodically refinishing the occlusal surface after frequent exposures to carbonated beverages and/or acidulated phosphate fluoride.
The microfracture is the dominant mechanism responsible for the damage that a roughened ceramic surface can cause to tooth enamel surfaces. Enamel is susceptible to this kind of microfracture ( Figure 10-3 ) through four specific mechanisms: (1) asperities extending from the ceramic surface that produce high localized stresses and microfracture; (2) gouging that results from high stresses and large hardness differences between two surfaces or particles extending from these surfaces; (3) impact or erosion that occurs through the action of abrasive particles carried in a flowing liquid, such as saliva; and (4) contact stress microfracture that increases localized tensile stress and also enhances the damage caused by asperities, gouging, and impact or erosion. Because of microfracture mechanisms, polishing the ceramic surface periodically to reduce the height of asperities and to minimize enamel wear rates is necessary and highly recommended.
The abrasiveness of ceramics against enamel is affected by numerous factors and properties of the crystal-phase particles and the glass matrix (if present). These include ceramic properties such as hardness, tensile strength, fracture toughness, fatigue resistance, particle–glass bonding, particle–glass interface integrity, and chemical durability. The oral environment also affects the abrasiveness, such as frequency of exposure to corrosive chemical agents (acidulated phosphate fluoride, carbonated beverages), abrasiveness of foods, residual stress, subsurface quality (voids or other imperfections), magnitude and orientation of applied forces, chewing patterns, bruxing frequency, contact area, lubrication by saliva, and duration of exposure to abrasive particles. All of these factors can lead to wear of the ceramic, with the production of roughened surfaces that can in turn abrade the opposing enamel. As a general rule, the larger the hardness difference between two sliding surfaces, the greater the degree of wear. However, this simple principle does not explain the wide variation in wear rates that is exhibited by different patients under apparently similar conditions.
How can the potential abrasive damage of tooth enamel that opposes ceramic surfaces be minimized?
Biological Properties
By virtue of their chemical inertness and resistance to corrosion, ceramics are very biocompatible restorative materials because they do not release any harmful agents to the oral environment. There have been reports of traces of lead in feldspathic-based ceramic powders up to 250 ppm. The presence of lead stems from the use of the natural mineral feldspar in the ceramic. The American Dental Association (ADA) Specification 69 states that the maximum allowable concentration of lead in dental ceramic powder is 300 ppm. The main issue is not the total amount of lead in the powder but the leaching potential of lead or solubility of the ceramic restoration in the oral cavity. International Organization for Standardization (ISO 6872) has established a solubility testing standard for dental ceramics but no specific values for the maximum allowable release of lead. Solubility testing standards are considered harsher than the conditions in the oral environment and are representative of a lifetime of use. The amount of lead released after exposure to 4% acetic acid at 80 °C for 16 hours was not detectable according to several independent studies. The limits of detection (LOD) established for these studies were at 5 and 10 ppb (μg/L), at which no lead was detected. If lead was indeed released, the level was below the 5 ppb level and is considered insignificant.
Physical Properties
Two other important attributes of dental ceramics are their potential for matching the appearance of natural teeth and their low electrical and thermal conductivities.
Color-Matching Ability and Aesthetic Qualities
Dental ceramics are excellent in matching the appearance of natural teeth because they can display translucency, color, and chroma. Color phenomena and terminology are discussed in Chapter 3, Color and Optical Effects . Perfect color matching is extremely difficult and demands exceptional skill and experience on the part of the dentist and lab technician. The structure of the tooth influences its color. Dentin is opaquer than enamel and reflects light very well. Enamel represents a predominantly crystalline layer over the dentin and is composed of tiny prisms or rods cemented together by an organic substance. The indices of refraction of the rods and the cementing substance are different. As a result, light rays are dispersed by varying proportions of absorption, transmission, scattering, and reflection to produce a resulting translucent effect and a sensation of depth as the scattered light ray reaches the eye. As light strikes the tooth surface, part of the light is reflected, and the remainder penetrates the enamel and is scattered. Any light reaching the dentin is either absorbed or partially reflected to the eye and partially scattered within the enamel. If dentin is not present, as in the tip of an incisor, some of the light rays may be transmitted into the oral cavity. As a result, this area may appear to be more translucent than that toward the gingival area. Because the law of energy conservation must apply, the following relationship shows the five energy components that are derived from the energy (E) of the incident light: E incident = E scattered + E reflected + E absorbed + E transmitted + E fluoresced .
Although some of the absorbed light may be converted into heat, some may be transmitted back to the eye as fluorescent energy. Light rays can also be dispersed, giving a color or shade that varies in different teeth. The dispersion can vary with the wavelength of the light. Therefore the appearance of the teeth may vary according to whether they are viewed in direct sunlight, reflected daylight, tungsten light, or fluorescent light. This phenomenon is called metamerism. The dentist and laboratory technician must reproduce the aesthetic characteristics sufficiently such that the appearance of a ceramic prosthesis is discernable only to the trained eye.
Specimens of ceramic shades are produced and distributed in a specific order in shade guides by dental ceramic manufacturers to assist dentists and lab technicians in selecting optimum ceramic shades and for communicating the desired prosthesis appearance to each other. Shade guides made of porcelain are used most often by dentists to describe the desired appearance of a natural tooth or ceramic prosthesis. However, there are several deficiencies of shade guides. Shade-guide tabs are much thicker than the thickness of ceramic that is used for dental crowns or veneers, and they are more translucent than teeth and ceramic crowns that are backed by a nontranslucent dentin substructure, veneering ceramics that are backed by an opaque core ceramic, or a metal framework. Much of the incident light is transmitted through a tab. In contrast, most of the incident light on a crown is reflected except at the incisal edge and at proximal incisal areas. Furthermore, the necks of shade tabs are made from a deeper hue, and this region tends to distract the matching ability of the observer in the gingival third of the tab. To avoid this situation, some clinicians grind away the neck area of a set of shade tabs.
The production of color sensation with a pigment is a physically different phenomenon from that obtained by optical reflection, refraction, and dispersion. The color of a pigment is determined by selective absorption and selective reflection. For example, if white light is reflected from a red surface, all the light with a wavelength different from that of red is absorbed, and only the red light is reflected. If a red hue is present in a ceramic crown, but the red hue of the same wavelength is not present in the light beam, the tooth will appear as a different shade. If the tooth or restoration surface is rough, most of the light will be scattered, and little will penetrate the structure.
Conductivity
Ceramics also offer freedom from galvanic effects (low electrical conductivity). Because the metal atoms transfer their outermost electrons to the nonmetallic atoms and stabilize their highly mobile electrons, ceramics are also excellent thermal (low thermal conductivity and low thermal diffusivity) and electrical insulators. Thus tooth sensitivity associated with metallic restorations that are electrical and thermal conductors is minimized.
Which two inventions dramatically increased the success and survival probability of metal-ceramic (MC) restorations?
History of Dental Ceramics
The first porcelain tooth material was patented in 1789 by de Chemant, a French dentist, and Duchateau, a French pharmacist. This product was an improved version of “mineral paste teeth” but was not used to produce individual teeth because there was no effective way to attach the teeth to a denture base material. In 1808, Fonzi, an Italian dentist, invented a “terrometallic” porcelain tooth that was held in place by a platinum pin or frame. Planteau, a French dentist, introduced porcelain teeth to the United States in 1817, and Peale, an artist, developed a baking process in Philadelphia for these teeth in 1822. Commercial production of these teeth began in 1825 by Stockton. In England, Ash developed an improved version of the porcelain tooth in 1837. In 1844, the nephew of Stockton founded the S.S. White Company, which led to further refinement of the design and the mass production of porcelain denture teeth.
Dr. Charles Land, the grandfather of aviator Charles Lindbergh, introduced one of the first ceramic crowns to dentistry in 1903 using a platinum foil matrix (also known as coping ) and high-fusing feldspathic porcelain. These crowns exhibited excellent aesthetics, but the low flexural strength of porcelain resulted in a high incidence of fractures. Although feldspathic porcelains with reliable chemical bonding have been used in MC prostheses, they were considered to be too weak to use reliably in the construction of all-ceramic crowns without a tougher ceramic core, a cast-metal core, or metal-foil coping. Furthermore, their firing shrinkage resulted in significant discrepancies in fit and adaptation of margins unless correction firings were performed.
In 1959, Weinstein et al. filed U.S. Patent 3,052,982, which described formulations of feldspathic porcelains with a wide range of expansion coefficients that also bonded chemically to and were thermally compatible with the existing alloys of the time. The first commercial porcelain was developed by VITA Zahnfabrik around 1963. Although the first VITA porcelain products were known for their aesthetic properties, the subsequent introduction of the more versatile Ceramco porcelain (Dentsply Sirona) exhibited thermal-expansion behavior that allowed this porcelain to be used safely with a wider variety of alloys. Significant developments in the areas of MC properties, design, and performance, such as opalescence, specialized internal staining techniques, greening-resistant porcelains, porcelain butt-joint margins, and shoulder porcelains, have significantly enhanced the overall appearance and “vitality” of MC crowns and bridges and the clinical survivability of these restorations.
McLean and Hughes in 1965 introduced fracture-resistant all-porcelain crowns made of dental aluminous core ceramic, a glass matrix containing 40 to 50 wt% Al 2 O 3 fillers. A feldspathic porcelain veneer was required to achieve acceptable aesthetics because of the chalky-white appearance of the aluminous porcelain core material. A 5-year fracture rate report showed only 2% for anterior crowns but an unacceptable 15% when aluminous porcelain was used for molar crowns. Relatively large sintering shrinkage (about 15% to 20%) of the core material and the use of a 20- to 25-μm-thick platinum foil have made marginal adaptation difficult to achieve except by highly skilled laboratory technicians. The principal indication for the use of aluminous porcelain crowns is the restoration of maxillary anterior crowns when aesthetics is of paramount importance.
Since the introduction of aluminous porcelain crowns and methods to produce durable MC crowns in the 1960s, improvements in the composition of ceramics and the method of forming all-ceramic crowns have greatly enhanced the ability to produce more accurate-fitting and fracture-resistant crowns made entirely of ceramic material. An all-ceramic system developed by controlling the crystallization of a glass (Dicor) was demonstrated by Adair and Grossman in 1984 and, later, a machinable glass-ceramic version (Dicor MGC), which had a tetrasilicic fluormica crystal volume of approximately 70%. In the early 1990s, a pressable glass-ceramic (IPS Empress), which contained approximately 34 vol% leucite, was introduced. A more fracture resistant, pressable glass-ceramic (IPS Empress 2) containing approximately 70 vol% of lithia disilicate crystals was introduced in the late 1990s. This core ceramic has been used for three-unit fixed dental prostheses (FDPs) as far posterior as the second premolar. Other systems based on Al 2 O 3 and zirconia will be discussed later. Significant progress has been made toward the goal of developing less abrasive veneering ceramics.
In the succeeding sections, the discussion will focus on MC systems, all-ceramic systems, ceramic-strengthening mechanisms, and indications for the use of ceramic materials in greater detail.
Metal-Ceramic Systems
Several clinical studies have confirmed the high overall survival percentages of MC prostheses. In this chapter, the term metal-ceramic or MC is used synonymously with porcelain fused to metal (PFM), although the former term is the most internationally accepted descriptor for these types of prosthetic material systems. One clinical study revealed that the fracture rate of MC crowns and bridges made from a high noble alloy was as low as 2.3% after 7.5 years. The most outstanding advantage of MC restorations is their resistance to fracture. With metal occlusal surfaces, the fracture rate in posterior sites could be reduced further. Depending on the ceramic material used, another potential advantage of MC over all-ceramic restorations is that less tooth structure needs to be removed to provide the proper bulk for the crown, especially if metal alone is used on occlusal and lingual surfaces and porcelain butt-joint margins are used on facial and buccal surfaces. Such designs also cause less wear of antagonist enamel than occurs when enamel is opposed by a ceramic surface.
A dark line at the facial margin of an MC crown that is associated with a metal collar or metal margin is of significant concern when gingival recession occurs. This adverse aesthetic result can be minimized by designing the crown with a ceramic margin or by using a very thin knife-edge margin of metal that is veneered with opaque shoulder porcelain ( Figure 10-4 ). This ceramic margin should be polished and/or glazed to avoid a rough surface at the margin.
One of the most frequently mentioned disadvantages is the potential for metal allergy. Such allergic reactions are very rare, except, possibly, when nickel-based alloys are used. Nonetheless, MC crowns are decreasing in popularity for use in anterior restorations. An improvement in the properties of all-ceramic crowns offers a greater potential for success in matching the appearance of the adjacent natural tooth, especially when a relatively high degree of translucency is desired. Indications may vary by patients, dentist preference, occlusion, and so forth. MC crowns are more commonly used for multiunit or posterior FDPs.
The following section focuses on the categories of ceramics, requirements of metal components, bonding of ceramic to metal, and fabrication of MC prostheses.
Which components of ceramics can cause excessive wear of tooth enamel?
Ceramic Types
Conventional dental porcelain is a vitreous ceramic based on a silica (SiO 2 ) network and potash feldspar (K 2 O•Al 2 O 3 •6SiO 2 ) or soda feldspar (Na 2 O•Al 2 O 3 •6SiO 2 ) or both. The ternary-phase diagram in Figure 10-5 for the K 2 O-Al 2 O 3 -SiO 2 system shows the approximate composition ranges of feldspathic porcelain products that are used for MC prostheses and for denture teeth. The feldspars used for dental porcelains are relatively pure and colorless. Thus pigments must be added to produce the hues of natural teeth or the color appearance of tooth-colored restorative materials that may exist in adjacent teeth. Opacifiers and glass modifiers to control the fusion temperature, sintering temperature, coefficient of thermal contraction, and solubility are also added. These ingredients are mixed together and fired to a molten state to complete the necessary chemical reaction (fusing), and then quenched in water. The resultant product, which is called frit, is then ground to fine powders for application.
Feldspathic Porcelains
Feldspathic porcelains contain a variety of oxides including an SiO 2 matrix (52 to 65 wt%), Al 2 O 3 (11 to 20 wt%), K 2 O (10 to 15 wt%), Na 2 O (4 to 15 wt%), and certain additives (e.g., B 2 O 3 , CeO 2 , Li 2 O, TiO 2 , and Y 2 O 3 ). These ceramics are called porcelains because they contain a glass matrix and one or more crystal phases, although the term porcelain traditionally refers to products that were produced from kaolinite (Al 2 O 3 •2SiO 2 •2H 2 O), which is a type of clay. The compositions of the veneering (layering) ceramics used for MC restorations ( Table 10-1 ) generally correspond to those used previously for veneering aluminous porcelain core ceramic (see History of Dental Ceramics section). For MC porcelains, specific concentrations of soda, potash, and/or leucite are necessary to reduce the sintering temperature and raise the thermal expansion coefficient to a level compatible with that of the metal coping. The opaque porcelains also contain relatively large amounts of metallic oxide opacifiers to conceal the underlying metal and minimize the thickness of the opaque layer.
| Component | Low-Fusing Vacuum Porcelain | Metal-Ceramic Porcelain | HIP Glass-Ceramic | FAP GLASS-CERAMIC | ||||
|---|---|---|---|---|---|---|---|---|
| Aluminous Porcelain | Dentin | Enamel | Low-FusingDentin Enamel | Ultralow-Fusing | IPS e.max Press (Based on Li 2 O•2SiO 2 ) | IPS e.max Ceram Veneer Ceramic | ||
| SiO 2 | 35.0 | 66.5 | 64.7 | 59.2 | 63.5 | 60–70 | 57–80 | 45–70 |
| Al 2 O 3 | 53.7 | 13.5 | 13.9 | 18.5 | 18.9 | 5–10 | 0–5 | 5–22 |
| CaO | 1.1 | 2.1 | 1.8 | — | — | 1–3.0 | — | 1–11 |
| Na 2 O | 2.8 | 4.2 | 4.8 | 4.8 | 5.0 | 10–15 | — | 4–13 |
| K 2 O | 4.2 | 7.1 | 7.5 | 11.8 | 12.3 | 10–13 | 0–13 | 3–9 |
| B 2 O 3 | 3.2 | 6.6 | 7.3 | 4.6 | 0.1 | 0–1.0 | — | — |
| ZnO | — | — | — | 0.6 | 0.1 | — | 0–8 | — |
| ZrO 2 | — | — | — | 0.4 | 0.1 | 0–1.0 | 0–8 | — |
| BaO, Y 2 O 3 | — | — | — | — | — | 0–0.2 | — | — |
| SnO 2 | — | — | — | — | — | 0–0.2 | — | — |
| Li 2 O | — | — | — | — | — | 0–1.0 | 11–19 | — |
| F | — | — | — | — | — | 0–1.0 | 0.1–2.5 | |
| P 2 O 5 | — | — | — | — | — | — | 0–11 | 0.5–6.5 |
| Sb 2 O 3 | — | — | — | — | — | 0–1.0 | — | — |
| CeO 2 | — | — | — | — | — | 0–0.2 | — | — |
| TiO 2 | — | — | — | — | — | 1–3.0 | — | — |
| Pigments/other | — | — | — | — | — | — | 0–8/0–10 | 0–3 |
| Sintering/firing temperature (°C) | 980 | 980 | 950 | 900 | 900 | 650–700 | 945 | 750 |
Feldspar has the tendency to form crystalline leucite (K 2 O•Al 2 O 3 •4SiO 2 ) when melted. Leucite is a potassium-aluminum-silicate mineral with a high coefficient of thermal expansion (20 to 25 × 10 –6 /K) compared with that of feldspar porcelain (8.6 × 10 –6 /K). When feldspar is heated at temperatures between 1150 °C and 1530 °C, this material undergoes incongruent melting to form leucite crystals in a liquid glass. Incongruent melting is the process by which one material melts to form a liquid plus a different crystalline material. The formation of leucite during melting controls the thermal expansion of the porcelain during bonding to a metal coping. Leucite has been added in felspar porcelains to control their coefficients of thermal contraction.
Feldspathic porcelains cannot be classified as glass-ceramics because crystal formation does not occur through controlled nucleation, crystal formation, and growth. There are four types of feldspathic porcelains: (1) ultralow- and low-fusing ceramics, (2) low-fusing specialty ceramics (shoulder porcelains and wash-coat ceramics), (3) ceramic stains, and (4) ceramic glazes (autoglaze and add-on glaze).
The particle type and size of crystal fillers greatly influence the potential abrasiveness of the ceramic prosthesis. Thus the abrasiveness of the finished surface will depend on the presence or absence of crystalline fillers. When the opaque porcelain of MC restorations becomes exposed as a result of the loss of veneer, the excessive wear of enamel may occur by direct two-body contact with the opaque porcelain. When one examines the research literature investigating the potential for abrasive damage of enamel, emphasis should be placed on clinical studies that clearly describe the preparation history of the outer surface layer.
Veneering ceramics (“porcelains”) for metals have higher expansion and contraction coefficients than the ceramics used to veneer alumina or zirconia core ceramics. They should not be subjected to nonessential repeated firings because this may lead to devitrification and an increased risk of cloudiness within the porcelain, in addition to potential changes in coefficient of thermal expansion (α e ) and coefficient of thermal contraction (α c ). A proper matching of these thermal properties of the alloy and porcelain is imperative to reduce the risk for chipping or cracking of the ceramic veneers, either during cooling from the sintering or glazing temperatures or at some later time in clinical service.
Ultralow-Fusing Ceramics for Metal-Ceramic Prostheses
In 1992, Duceram LFC (low-fusing ceramic; Dentsply Sirona) was marketed as an ultralow-fusing ceramic for metal-ceramic prostheses with three unique features: (1) this ceramic is based on a hydrothermal glass in which water is incorporated into the silicate glass structure to produce nonbridging hydroxyl groups that disrupt the glass network, thereby decreasing the glass transition temperature, viscosity, and firing temperature and increasing the coefficient of thermal expansion (CTE) to allow use as a veneer for certain low-expansion metals; (2) these types of ceramics are also claimed to be “self-healing” through a process of forming a 1-μm-thick hydrothermal layer along the ceramic surface; and (3) the extremely small size of the crystal particles (400 to 500 nm) enhances the opalescence of the ceramic by reflecting blue light hues from the surface and yellow hues from the interior of the ceramic. Other ultralow-fusing ceramics (sintering temperatures below 850 °C) that are now commonly referred to as low-fusing ceramics have been introduced with veneering glasses that are claimed to be kinder to opposing tooth enamel, either because they are predominantly a glass phase material or because they contain very small crystal particles.
Most of the ultralow-fusing ceramics have microstructures that exhibit either a well-distributed dispersion of small crystal particles or few to no crystals. Wear studies are promising in several cases relative to the enamel wear caused by these ceramics, although not all ultralow-fusing ceramics exhibit this decreased level of abrasiveness. Ultralow-fusing ceramics contain less Al 2 O 3 and increased concentrations of CaO, K 2 O, Li 2 O, and Na 2 O ( Table 10-1 ).
Ultralow-fusing dentin and enamel ceramics may be easier to polish and may yield smoother and less abrasive surfaces than conventional low-fusing and medium-fusing porcelains. Because of their lower concentration of leucite crystals compared with conventional porcelains, they have lower expansion and contraction coefficients. Their lower sintering temperatures provide opportunities for use with alloys that have lower fusion temperatures, like Type 2 and 4 gold alloys, whose compositions must be modified to ensure proper chemical bonding and matched thermal expansion and contraction coefficients.
Glass Modifiers
The sintering temperature of crystalline silica is too high for use in veneering aesthetic layers onto dental casting alloys. At such temperatures, the alloys would melt. In addition, the coefficient of thermal contraction of crystalline silica is too low for these alloys. Bonds between silica tetrahedra can be broken by the addition of alkali metal ions such as sodium, potassium, and calcium. These ions are associated with the oxygen atoms at the corners of the tetrahedra and interruption of oxygen-silicon bonds. As a result, the three-dimensional (3-D) silica network contains many linear chains of silica tetrahedra that are able to move more easily at lower temperatures than the atoms that are locked into the 3-D structure of silica tetrahedra. This ease of movement is responsible for the increased fluidity (decreased viscosity), lower softening temperature, and increased thermal expansion conferred by glass modifiers. However, too high of a modifier concentration reduces the chemical durability (resistance to attack by water, acids, and alkalis) of the glass. In addition, if too many tetrahedra are disrupted, the glass may crystallize (devitrify) during porcelain-firing operations. Hence, a balance between a suitable melting range and good chemical durability must be maintained.
Boric oxide (B 2 O 3 ) can behave as a glass modifier to decrease viscosity, lower the softening temperature, and form a glass network. Because boric oxide forms a separate lattice interspersed with the silica lattice, this oxide still interrupts the more rigid silica network and lowers the softening point of the glass. Water, which is not an intentional addition to dental porcelain, can become a modifier. The hydronium ion, H 3 O + , can replace sodium or other metal ions in a ceramic that contains glass modifiers. This fact accounts for the phenomenon of “slow crack growth” of ceramics that are exposed to tensile stresses and moist environments. Water also may account for the occasional long-term failure of porcelain restorations after several years of service. The role of alumina (Al 2 O 3 ) in glass formation is complex. Alumina is not a true glass former but can take part in the glass network to alter the softening point and viscosity.
Manufacturers employ glass modifiers to produce dental porcelains with varying firing temperatures. Dental porcelains are classified according to their firing temperatures. A typical classification is given in Table 10-2 . The medium-fusing and high-fusing types are used for the production of denture teeth. The low-fusing and ultralow-fusing porcelains are used as veneering ceramics for crown and bridge construction. Some of the ultralow-fusing porcelains are used for titanium and titanium alloys because of their low-contraction coefficients that closely match those of the metals, and because the low firing temperatures reduce the risk for growth of the metal oxide. However, some of these ultralow-fusing porcelains contain enough leucite to raise their coefficients of thermal contraction as high as those of conventional low-fusing porcelains.
| Class | Applications | Sintering Temperature Range |
|---|---|---|
| High fusing | Denture teeth and fully sintered alumina and zirconia core ceramics | >1300 °C (>2372 °F) |
| Medium fusing | Denture teeth, presintered zirconia | 1101–1300 °C (2013–2372 °F) |
| Low fusing | Crown and bridge veneer ceramic | 850–1100 °C (1563–2012 °F) |
| Ultralow fusing | Crown and bridge veneer ceramic | <850 °C (<1562 °F) |
Glazes and Stain Ceramics
To ensure adequate chemical durability, a self-glaze of porcelain is preferred to an add-on glaze. A thin external layer of glassy material is formed during a firing procedure at a temperature and time that cause localized softening of the glass phase. The add-on glaze slurry material that is applied to the porcelain surface contains more glass modifiers and thus has a lower firing temperature. Keep in mind that higher proportions of glass modifier tend to reduce the resistance of the applied glazes to leaching by oral fluids.
How is the degree of sintering controlled, and what parameter defines complete sintering?
The aesthetics of porcelains for MC and ceramic prostheses, veneers, and denture teeth may be enhanced through the application of stains and glazes to provide a more lifelike appearance. Stains are simply tinted glazes that are also subject to the same chemical durability problems. However, most of the currently available glazes appear to have adequate durability if they are produced in thicknesses of 50 μm or more. To make shaded veneering ceramic to simulate stain on natural teeth, coloring pigments are first fused with feldspar, ground to fine powder, and then blended with the unpigmented powdered frit to provide the proper hue and chroma. Examples of metallic oxides and their respective color contributions to porcelain include iron or nickel oxide (brown), copper oxide (green), titanium oxide (yellowish brown), manganese oxide (lavender), and cobalt oxide (blue). Opacity may be achieved by the addition of cerium oxide, zirconium oxide, titanium oxide, or tin oxide.
One method for ensuring that the applied characterizing stains will be permanent is to use them internally. Internal staining and characterization can produce a lifelike result, particularly when simulated enamel craze lines and other features are built into the porcelain rather than merely applied to the surface. The disadvantage of internal staining and characterization is that the porcelain must be stripped completely if the color or characterization is unacceptable.
Autoglazed feldspathic porcelain is stronger than unglazed porcelain. The glaze is effective in sealing surface flaws and reducing stress concentrations. Figure 10-6 shows an MC crown with a properly fired autoglazed porcelain surface. If the glaze is removed by grinding, the transverse strength may be only half that of the sample with the glaze layer intact. However, the results of recent studies indicate that porcelains with highly polished surfaces have strengths comparable to those of specimens that were polished and glazed. This observation is of clinical importance. After the porcelain restoration is cemented in the mouth, the common practice for the dentist is to adjust the occlusion by grinding the surface of the porcelain with a diamond bur. This procedure weakens the porcelain, with a roughened surface that can cause increased wear of enamel. There are commercial finishing and polishing kits available for various types of ceramics ( Chapter 16, Ceramic Restorations ). A smoother surface also reduces the abrasion-causing damage to opposing teeth or restorations.
Glazing of feldspathic porcelain is believed to eliminate surface flaws and produce a smoother surface. However, an optimum method of producing the smoothest surface in the shortest time has not been established. Fine polishing of a roughened surface, followed by glazing will logically produce a smoother surface than polishing alone, sandblasting, followed by glazing, or diamond grinding, followed by glazing. However, even though one polishes and/or glazes a porcelain veneer surface, the surface will either slowly or markedly break down in the presence of liquids in our everyday diets, including acids such as citric acid and acetic acid. Studies have shown that these veneers will corrode over time with constant exposure to fluctuations in dietary pH changes coupled with occlusal wear.
Requirements of Metal Component
Many alloys are available to be veneered with low-fusing and ultralow-fusing porcelains. The compositions of alloys control the castability, bonding ability to porcelain, and the magnitude of stresses that develop in the porcelains during cooling from the sintering temperature. A list of typical alloy types sold by one manufacturer is presented in Table 10-3 , along with the relevant properties of the respective alloy. The reader is referred to Chapter 9, Requirements of Alloys for Metal-Ceramic Applications , for a description of other systems and the effects and purposes of the constituent metals.
| Components and Properties | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Alloy Product | Principal Elements | Noble Metal (wt%) | ADA Class | 2% Proof Stress (MPa) | Elongation (%) | Hardness (VHN) | Elastic Modulus (GPa) | CTE (25–500 °C) 10 -6 /K | CTE (25–600 °C) 10 -6 /K |
| Brite Gold | Au-Pt-In | 99 | HN | 230 | 15.0 | 100 | 79 | 14.8 | 15.0 |
| Brite GoldXH | Au-Pt | 97.9 | HN | 355–427 | 11.0 | 180 | 107 | 14.4 | 14.7 |
| Golden Ceramic | Au-Pt-Pd | 97.4 | HN | 360 | 12.0 | 165 | 64 | 14.6 | 14.7 |
| Aquarius Hard | Au-Pt-Pd | 97.2 | HN | 455 | 12.0 | 205 | 88 | 14.5 | 14.8 |
| Aquarius | Au-Pt-In | 97–98 | HN | 320 | 12.0 | 160 | 79 | 14.6 | 14.8 |
| IPS d.SIGN 98 | Au-Pt | 98.0 | HN | 510 | 8.0 | 220 | 80 | 14.3 | 14.6 |
| Y | Au-Pt-Pd | 96.8 | HN | 320 | 12.0 | 160 | 81 | 14.6 | 14.8 |
| Aquarius XH | Au-Pt-Pd | 96.8 | HN | 510 | 7.0 | 220 | 83 | 14.1 | 14.4 |
| Y-2 | Au-Pt-Pd | 94.6 | HN | 380 | 12.0 | 155 | 83 | 15.0 | 15.1 |
| Y-Lite | Au-Pd-Ag | 93.8 | HN | 500 | 14.0 | 225 | 88 | 13.9 | 14.1 |
| Sagittarius | Au-Pd-Pt | 95.8 | HN | 580 | 10.0 | 245 | 94 | 14.0 | 14.3 |
| Y-1 | Au-Pd-Pt | 87.4 | HN | 340 | 15.0 | 185 | 99 | 14.8 | 15.0 |
| IPS d.SIGN 96 | Au-Pd-Pt | 87.7 | HN | 405 | 15.0 | 214 | 92 | 14.3 | 14;5 |
| IPS d.SIGN 91 | Au-Pd-In | 90.6 | HN | 570 | 31.0 | 250 | 136 | 14.2 | 14.4 |
| W | Au-Pd-Ag | 80.4 | HN | 455 | 21.0 | 220 | 113 | 14.2 | 14.5 |
| W-5 | Au-Pd-Ag | 78.2 | HN | 530 | 20.0 | 255 | 118 | 14.0 | 14.2 |
| Lodestar | Au-Pd-In | 90.0 | HN | 495 | 20.0 | 240 | 98 | 14.1 | 14.3 |
| W-3 | Au-Pd-In | 88.3 | HN | 495 | 17.0 | 225 | 128 | 13.9 | 14.1 |
| W-2 | Au-Pd-Ag | 85.3 | HN | 640 | 20.0 | 205 | 113 | 14.2 | 14.6 |
| Evolution Lite | Au-Pd-In-Ag | 89.6 | HN | 375 | 11.0 | 280 | 130 | 14.2 | 14.5 |
| Capricorn | Pd-In-Ga-Au | 84.1 | N | 525 | 21.0 | 260 | 101 | 14.3 | 14.5 |
| IPS d.SIGN 84 | Pd-In-Au-Ag | 84.2 | N | 485 | 29.0 | 295 | 117 | 13.8 | 14.0 |
| IPS d.SIGN 67 | Pd-Ag-Sn-Au | 66.7 | N | 545 | 15.0 | 240 | 104 | 13.9 | 14.2 |
| Spartan Plus | Pd-Cu-Ga-Au | 80.8 | N | 795 | 20.0 | 310 | 97 | 14.3 | 14.6 |
| Capricorn 15 | Pd-Ag-Au-Pd | 66.9 | N | 490 | 21 | 255 | 101 | 14.3 | 14.5 |
| Aries | Pd-Ag-Sn | 63.7 | N | 415 | 46.0 | 185 | 98 | 14.7 | 14.8 |
| IPS d.SIGN 59 | Pd-Ag-Sn | 59.2 | N | 490 | 14.0 | 230 | 139 | 14.5 | 14.8 |
| IPS d.SIGN 53 | Pd-Ag-Sn-In | 53.8 | N | 545 | 13.0 | 250 | 132 | 14.8 | 14.9 |
| W-1 | Pd-Ag-Sn | 53.3 | N | 485 | 11.0 | 240 | 114 | 15.2 | 15.4 |
| Calisto CP+ | Co-Pd-Cr-Mo | 25.0 | N | 640 | 10.0 | 365 | 180 | 14.4 | 14.9 |
| Pisces Plus | Ni-Cr-W | 0 | PB | 600 | 10.0 | 280 | 183 | 14.1 | 14.4 |
| IPS d.SIGN 15 | Ni-Cr-Mo | 0 | PB | 340 | 13.0 | 200 | 200 | 13.9 | 14.2 |
| IPS d.SIGN 30 | Co-Cr-Mo | 0 | PB | 520 | 6.0 | 375 | 234 | 14.5 | 14.7 |
The metal should have a higher melting range, with solidus temperature greater than the sintering temperature of the ceramic to prevent sag, creep (see Figure 9-2), or melting of the coping or framework during sintering and/or glazing. This deformation does not occur at oral temperatures. In addition, for gold alloys, a small amount (about 1%) of base-metal elements, such as iron, indium, and tin, is added to form a surface oxide layer during the so-called “degassing” treatment. Degassing is a misnomer because the primary purpose is to produce an adherent metal oxide on the surface to facilitate bonding to porcelain. Other properties of alloys of particular importance are the elastic modulus and proof strength (yield strength), which should be high enough to resist deformation, and the CTE should be closely matched to those of the ceramics. These properties, along with the oxide layer, are discussed in the following section.
What three conditions control the durability of ceramic bonding to an oxidized metal coping?
Bonding Porcelain to Metal
A durable bond between the ceramic and the metal is the primary requirement for the success of an MC restoration. When the ceramic powder is brought to sintering temperature, it melts and wets the metal surfaces. The liquid-phase ceramic then fills the surface roughness of the metal coping. Meanwhile, the liquid-glass phase reacts with metal oxide and forms an intermediate layer that adheres strongly to the ceramic and the metal coping. When the sintered MC prothesis is cooled to room temperature, the bond between the metal and the ceramic may remain intact or become separated, depending on the extent of difference between their CTEs. These phenomena, occurring during the sintering and cooling cycle, constitute the three factors that control the durability of MC bonding: mechanical interlocking, chemical bonding, and thermal compatibility.
Recall from the discussion in Chapters 2 (Mechanical Interlocking) and 6 (Mechanisms of Adhesion) that mechanical interlocking occurs when there is surface roughness. There is some evidence to support MC bonding through a mechanical interlocking mechanism. One Pd-Ag alloy was found to form metal nodules on the surface via a creep mechanism. These nodules presumably provide sufficient mechanical retention for clinical use because this alloy had been used for many years without noted problems (see Figure 9-3).
What condition is required of cast metals to achieve ionic and/or covalent bonding to veneering porcelain?
As described, metal oxides work like coupling agents in bonding ceramic to metal. The oxidation behavior of these alloys largely determines their potential for bonding with porcelain. Research into the nature of MC adherence has indicated that those alloys that form adherent oxides during the oxidation cycle also form a good bond to porcelain, whereas those alloys with poorly adherent oxides or poor bond of porcelain to the oxide ( Figure 10-7 ) form poor wetting. Although the thickness of the oxide is thought to play a role, there is insufficient evidence to support this theory. Rather, the quality of the oxide and this oxide’s adhesion to the metal substrate appear to be the most important factors. This porcelain–metal bond is primarily ionic but is likely to have a covalent character as well and is capable of forming on smooth surfaces where little opportunity exists for mechanical interlocking.
There is a slight difference in the CTE between the metal and ceramic; usually metal has a higher value than that of the ceramic. During cooling to room temperature, the dimension of the metal coping becomes smaller relative to the ceramic, and vice versa. This means that some adjustment in the dimension at the interface must occur to keep both metal and ceramic bonded together as they cool down. The metal coping needs to be stretched somehow, and the ceramic needs to be compressed. A possible scenario is that the ceramic stretches the metal coping as the metal coping compresses the ceramic. That arrangement results in stresses developed around the interface. For example, a difference in the coefficients of thermal contraction of 1.7 × 10 –6 /K can produce a shear stress of 280 MPa in porcelain next to the MC interface when the porcelain is cooled from 954 °C to room temperature. If the shear resistance to failure is far less than 280 MPa, these stresses, resulting from thermal contraction, would likely cause spontaneous bond failure or cracking of the veneering ceramic.
The tensile stresses induced within the restoration by occlusal forces would be added to the residual thermal stresses. However, for MC systems that have an average contraction coefficient difference of 0.5 × 10 –6 /K or less (between 600 °C and room temperature), fracture is unlikely to occur except in cases of extreme stress concentration or extremely high intraoral forces. These are known as thermally compatible systems . Many restorations made from metal and porcelain combinations having contraction coefficient differences between 0.5 and 1.0 × 10 –6 /K are known to survive for many years. Most patients generate typical bite forces of 400 to 800 N between molar teeth and much lower forces between premolars and between anterior teeth. Thus a rather small number of patients will have bite force capabilities that are likely to cause fracture of MC crowns or bridges even when residual thermal incompatibility stresses are present. As a general rule, lower forces are generated by younger children, female patients, a more closed bite, occlusion between natural teeth, a denture, and fixed partial dentures.
The general term compatibility has been used in ISO 9693-1:2012 Dentistry—Compatibility Testing—Part 1: MC systems, which describe a debonding/crack-initiation strength test, to identify the ability of the MC system to demonstrate both acceptable adhesion of ceramic to metal oxide and freedom from the crack formation associated with stresses caused by thermal expansion and contraction differences.
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