versão impressa ISSN 0012-7353
Dyna rev.fac.nac.minas v.77 n.163 Medellín jul./set. 2010
CERAMICS FOR DENTAL RESTORATIONS - AN INTRODUCTION
UNA PERSPECTIVA DE LAS CERÁMICAS PARA RESTAURACIONES DENTALES
SANTIAGO ARANGO SANTANDER
Universidad Pontificia Bolivariana and Universidad Cooperativa de Colombia. Medellín, Colombia. firstname.lastname@example.org
ALEJANDRO PELÁEZ VARGAS
INEB - Instituto Nacional de Engenharia Biomédica and Universidade do Porto, FEUP, DEMM, Porto, Portugal. email@example.com
JORGE SALDARRIAGA ESCOBAR
Universidad Pontificia Bolivariana, Medellín, Colombia. firstname.lastname@example.org
FERNANDO JORGE MONTEIRO
INEB - Instituto Nacional de Engenharia Biomédica and Universidade do Porto, FEUP, DEMM, Porto, Portugal. email@example.com
LUIS FELIPE RESTREPO TAMAYO
Universidad CES, Facultad de Odontología, Medellín. Colombia. firstname.lastname@example.org
Received for review July 29th, 2009, accepted June 25th, 2010, final version July, 8th, 2010
ABSTRACT: Dental ceramics are the preferred materials for oral restoration due to some characteristics, such as adequate esthetics, high fracture strength and chemical stability. Currently, dental professionals have a large amount of ceramic systems to choose from, all of them having small differences regarding their chemistry, processing temperatures, mechanical strength and clinical applications. These differences lead to classification systems that are difficult to understand by professionals outside the dental field. The aim of this work is to review the current dental ceramic systems and present them from a compositional perspective to assure the comprehension of these materials by professionals who belong to the Biomedical Engineering field. A great effort was made to avoid classification methods imposed by manufacturers and to obtain a quick compilation of available information from the literature allowing the reader to have a general view of the state-of-the-art on dental ceramics.
KEYWORDS: Ceramic systems, feldspathic porcelain, leucite, zirconium oxide, glass-ceramics, mechanical and physical properties.
RESUMEN: Las cerámicas dentales son el material de elección para restauración oral debido a características como adecuada estética, elevada resistencia a la fractura y gran estabilidad química.En la actualidad, los profesionales de la Odontología tienen a su disposición una gran cantidad de sistemas cerámicos de restauración que presentan pequeñas o grandes diferencias en términos químicos, temperatura de procesamiento, resistencia mecánica y aplicación clínica, lo que lleva a sistemas de clasificación de las cerámicas que son poco útiles para profesionales ajenos a la odontología. El presente trabajo tuvo como objetivo hacer una revisión de los sistemas cerámicos de uso dental y presentarlos desde una perspectiva composicional para favorecer el entendimiento de profesionales ajenos a la Odontología. Se hizo un importante esfuerzo para evadir la clasificación que han impuesto las casas comerciales y tener así una rápida compilación de información disponible en la literatura que permita tener una visión general completa, pero no intensiva, del estado del arte de las cerámicas dentales.
PALABRAS CLAVE: Sistemas cerámicos, porcelana feldespática, leucita, alúmina, oxido de zirconio, vitrocerámica, propiedades físicas y mecánicas.
Dental ceramics are materials that are part of systems designed with the purpose of producing dental prostheses that in turn, are used to replace missing or damaged dental structures. The literature on this topic defines ceramics as inorganic, non-metallic materials made by man through the heating of raw minerals at high temperatures .
Ceramics and glasses are brittle, which means that they display high compressive strength but low tensile strength and may be fractured under very low strain (0.1% - 0.2%). As restorative materials, dental ceramics have disadvantages, mostly due to their inability to withstand functional forces that are present in the oral cavity, hence initially they found limited application in premolar and molar areas, although further development in these materials has enabled their use as posterior long-span fixed partial prosthetic restorations and structures over dental implants. All dental ceramics display low fracture toughness when compared to other dental materials, such as metals. .
The main objective of this work is to review ceramic dental materials, including their most relevant physical and mechanical properties. A brief historical review, including the evolution of these materials over time, a summary of different dental ceramic classifications, and the composition of dental ceramics will be presented.
2. HISTORICAL EVOLUTION OF DENTAL CERAMICS
Current dental ceramics are far from the early ceramics that started being used over 200 years ago. Early records of the first ceramics used as dental materials date back to 1774, when French apothecary Alexis Duchateau and Parisian dentist Nicholas Dubois de Chemant manufactured the first complete ceramic denture.
There are reports of complete dentures being manufactured earlier by French dentist Pierre Fauchard, although these dentures were fabricated in a different class of ceramic, namely enamel .
The initial use of ceramic materials in dentistry was in the obtention of complete dentures. Early in the 19th century, Italian dentist Giuseppangelo Fonzi was capable of manufacturing individual ceramic teeth attached to a metallic substructure which, in turn, was attached to complete dentures. The restoration of individual ceramic teeth in the oral cavity was delayed until the late 1800s, when Logan constructed ceramic teeth fused to metallic posts so that these posts could function as an intra-radicular retention for the restoration [4, 5].
The method to manufacture dental prostheses during the second half of the 20th century was through the fusion of ceramics and metallic structures that could function as a core. Metal-ceramic systems combine both the exceptional esthetic properties of ceramics and the extraordinary mechanical properties of metals . Some metals used as restorative materials in dentistry may constitute a problem for some patients. These problems may reveal themselves as allergies , gum staining [7, 8], and release of metallic ions into the gingival tissue  and the gingival fluid . These drawbacks, as well as the search for more esthetic materials by patients and dentists, have stimulated research and development of metal-free ceramic systems.
During the last 40 years, research has focused on improving metal-free systems and developing superior materials regarding esthetics and clinical performance to offer patients several alternatives to restore missing or damaged teeth.
3. CLASSIFICATION OF DENTAL CERAMICS
Dental ceramics may be classified according to several parameters, such as their use, manufacturing temperature, ceramic system, composition, microstructure, and translucency. The classification used in this article will be based on composition. Due to the fact that this review will discuss some mechanical properties exhibited by dental ceramic materials, table 1 summarizes the results of some mechanical tests published in the literature.
3.1 Feldspathic porcelains
Before discussing this division of ceramics, it is important to keep in mind that most ceramics have two different phases: the glassy phase and the crystalline phase. The glassy phase is often responsible for the esthetic behavior, while the crystalline phase is associated with mechanical strength. However, the crystalline phase (leucite in feldspathic porcelains) has a similar diffraction index as the glassy matrix; hence it also contributes towards the translucency of the whole structure. Therefore, both mechanical and esthetic properties are dependent on the ceramic composition .
Feldspathic porcelain is produced from the mixture of potassium feldspar, quartz, and kaolin. The latter improves plasticity and manipulation before the heating process. Other essential components are oxides of sodium, potassium, calcium, aluminum, and magnesium (employed to control ceramic's expansion coefficient and to try and match it with that of metals) , zinc, iron, copper, titanium, nickel, manganese, and cobalt (as pigments) and tin, zirconium, and titanium (as opacifiers) . Feldspathic porcelain is highly brittle, hence it must be used as a veneering material in metal-ceramic and metal-free ceramic systems. .
This type of ceramic is made of extremely fine powders to which de-ionized water or a special modeling liquid is added. A paste is thus formed to be tooth-shaped by a technician. Then, this paste must be taken to an oven set at a pre-established temperature and time depending on the ceramic being used. Currently, microprocessor-controlled ovens control all the thermal cycle steps and have optimized programs designed by manufacturers to process different types of ceramics. The heating process may be carried out either under vacuum or in air, however the process under vacuum reduces ceramic's porosity. [15, 16].
The objective is to bring the raw paste's particles together to form a paste that will have solidified upon cooling. During sintering, ceramics density will increase and this is associated to a volumetric contraction of 30 % to 40%,[17, 18] although other researchers claim that this contraction is between 20 % and 25% . During the paste formation process, it is essential to avoid bubbles or pores formation, since their presence will reduce the ceramic's final mechanical strength, given that pores act as crack initiators . The sintering process considerably reduces porosity since this procedure involves the melting/softening of the powder particle, causing reduction of surface energy, which renders pores to become spherical and volumetrically contracted, thus increasing densification .
Some methods to strengthen porcelain and avoid crack initiation and/or propagation have been reported by different authors. The most common methods are ionic exchange (chemical exchange of smaller sodium ions for larger potassium ions to create a compressive stress on the surface) [22-26], thermal tempering (forced cooling of ceramic in air) [23, 27, 28] or crack-propagation interruption by means of induction of residual compressive stresses on the surface (air-injection tempering) .
3.2 Leucite-reinforced feldspathic porcelain
Feldspar may form leucite, which is a potassium aluminum silicate mineral with a high thermal expansion coefficient . Leucite may be present in feldspathic porcelain in two forms: Firstly, the formation of this mineral through the incongruent melting of potasic feldspar (incongruent melting is a process by which a material is melted to form a liquid and a different crystalline material ). Secondly, the aggregate of leucite to ceramics as synthetic powder . Leucite-reinforced ceramics may be used to manufacture metal-ceramic restorations. The large difference between thermal expansion coefficient of metals and that of ceramics is one of the problems encountered when manufacturing metal-ceramic restorations [31, 32] as well as the difference in thermal expansion coefficients between layers of ceramic  when cooling to room temperature. This imbalance creates residual stress between both materials that may trigger failure at the interface.  Hence, a layer to layer compatibility involving ceramics and underlying metal is required .
Also, leucite content in ceramics is associated to an increase in crack propagation strength . A mechanism by which this increase is produced is the difference between leucite's thermal expansion coefficient (22 to 25 X 10-6/°C) and that of the glassy matrix (8 x 10-6/°C), resulting in the formation of tangent compressive stresses in the glass around leucite crystals, that undergo a phase transformation from cubic to tetragonal upon cooling . Sodium content has an effect on the tetragonal phase stability at high temperatures and lowers the thermal expansion coefficient, but it also decreases the ceramic's overall strength . Another mechanism of strength increase is explained by microcrack formation within and around leucite crystals when cooling. Both mechanisms are essential to prevent crack formation and propagation in the ceramic . Some researchers have suggested that ceramics reinforced with cubic leucite at room temperature display less flexural strength and less fracture strength than tetragonal-leucite ceramics .
Pinto et al assessed the effect that pH has on leucite-reinforced ceramics. These authors concluded that an acidic pH (3.5) produces a reduction in overall strength in leucite-reinforced ceramics .
3.3 Aluminous ceramics
Aluminous ceramics were first developed by MacLean and Hughes to manufacture prosthetic dental crowns . The principle of this type of ceramics is that a dispersion of high strength and high elastic modulus crystals exists within the glassy matrix to strengthen and harden the ceramic .
Alumina has the property of strengthening feldspathic porcelain, thus making it more fracture resistant. This fact may be observed when comparing aluminous ceramics strength to that of feldspathic porcelain  and when comparing hardness and strength values of aluminous ceramics with those of other ceramic systems . Alumina particle size has been proposed as a factor that improves particles agglomeration and some mechanical properties, such as fracture strength, of the partially-sintered alumina matrix .
The presence of alumina decreases glasses' inherent brittleness and the risk of de-vitrification, which is a process whereby ceramics become opaque and brittle as a result of their crystallization, due to loss of the glassy structure .
3.4 Glass-infiltrated composites
Currently, there is a ceramic system marketed as In-Ceram (Vita-Zahnfabrik, H Rauter GMBH & Co.KG, Bad Säckingen, Germany ) that includes glass-infiltrated porous sintered alumina.  This system, developed by Tyszblat, is comprised of an aluminous core that is further infiltrated with molten glass to fill-in the voids left by aluminous particles in order to obtain a restoration that exhibits better physical properties when compared to traditional ceramics (see table 1) and very similar to those of natural teeth .
3.5 Alumina Polycrystals
High-alumina ceramics contain a minimum of 95% pure alumina (aluminum oxide, Al203) . Andersson and Oden  described the use of a high-purity (99.9%), densely-sintered alumina ceramic for the manufacturing of dental restorations . Alumina displays a 15% to 20% contraction that must be compensated to manufacture such restorations. These investigators compensated its high contraction by using enlarged models of teeth and restoration manufacturing using a technique known as copy-milling to obtain a properly-sized finished restoration. The amount of expansion necessary to be applied to the models may be calculated from the sintering of compacted powder . Copy-milling is better described by Andersson elsewhere .
According to McLean , the first works on glass-ceramics were performed by MacCulloch, but his work did not receive much attention. Further investigations by Grossman and Adair [58, 59] concluded with the development of a tetrasilicic fluormica-containing ceramic system. According to both investigators, its composition is as follows: 45-70% SiO2, 8-20% MgO, 8-15% MgF2, 5-35% R2O+RO, where R2O has a range between 5-25% and is composed of at least one of the following oxides: 0-20% K2O, 0-23% Rb2O, and 0-25% Cs2O to improve translucency and RO, which has a range between 0-20%, and is composed of at least one of the following oxides: SrO, BaO, and CdO. Additional components may account for up to 10% of Sb2O5 and/or up to 5% of traditional glassy colorants [58, 59].
The thermal treatment known as ceramming  is composed of two processes: glass is heated up to a temperature where nuclei form
3.7 Leucite-reinforced glass-ceramics
Pressed glass-ceramics are materials containing high amounts of leucite crystals (35% by volume) . The basic component of this ceramic is feldspathic porcelain consisting of 63% SiO2, 19% Al2O3, 11% K2O, 4% Na2O, and traces of other oxides. Leucite crystals are added to the aluminum oxide [63, 64]. This material is manufactured using a process known as heat-pressing, which is performed in an investment mold. This mold is filled with the plasticized ceramic, thus avoiding the sintering process and the subsequent pore formation . This ceramic undergoes dispersion strengthening through the guided crystallization of leucite. Dispersion strengthening is a process by which the dispersed phase of a different material (such as alumina, leucite, zirconia, etc.) is used to stop crack propagation, since these crystalline phases are more difficult to penetrate by cracks [15, 66]. Leucite crystals are incorporated during ceramming, hence performing this process again is unnecessary when inducing crystal growth .
The method of dual ionic exchange has been proposed in the literature to increase the resistance of this ceramic to separation, as long as potassium and sodium ions are available in the ceramic for the exchange. Long-term chemical effects on the ceramic should be further studied .
3.8 Lithium disilicate-reinforced glass-ceramics
This material is also a heat-pressed glass-ceramic with a 60% content of lithium disilicate crystals, which form an intertwined structure after being pressed, hence fracture strength is increased [65, 68]. Li2O and SiO2 aid in the crystallization of the required phase of lithium disilicate, BaO and Cs2O stabilize residual glass and Al2O3 and B2O3 render this ceramic chemically durable . The crystallization process is comprised of two steps: nucleation (one hour at 645°C) and crystal growth (four hours at 850°C) .
3.9 Zirconium oxide ceramics
Since Garvin et al  published their work titled "Ceramic steel?";, zirconia has been considered a tough ceramic. Zirconia occurs as a natural mineral called baddeleyite. This mineral contains 80% to 90% zirconium oxide.
The major impurities are usually TiO2, SiO2, and Fe2O3. This oxide exists in three different crystal structures: monoclinic at room temperature, tetragonal at ~1200°C, and cubic at 2370°C. A phase transformation from tetragonal to monoclinic occurs during cooling. This change produces a volumetric expansion ranging from 3% to 4%, which in turn causes crack formation within the material. Oxides such as CaO, MgO, Y2O3, and CeO2 are added to pure zirconia to stabilize it in multiphase materials known as Partially-Stabilized Zirconia (PSZ). These materials basically consist of cubic zirconia as the major phase and precipitates of monoclinic and tetragonal as the minor phase at room temperature . At room temperature, the tetragonal phase is in metastable state, since transformation from tetragonal to monoclinic may be induced by external factors, such as pressure or temperature [72, 73]. The resulting compressive stress from the volumetric expansion developed at the vicinity of a crack and the energy needed to carry out such phase transformation, provide this ceramic with high fracture strength [72, 74]. This process is known as transformation toughening [71-73, 75]. It may generate a process known as ceramic aging, which is the spontaneous transformation from metastable tetragonal zirconia to monoclinic zirconia , therefore a decrease in mechanical properties may occur in the ceramic over time . An investigation carried out by Ardlin demonstrated that strength was not affected by aging, although the ceramic's crystalline structure and surface were, indeed, affected .
As mentioned above, pure zirconia may be stabilized with Y2O3. Yttrium oxide is used to retain the tetragonal phase at room temperature as much as possible after sintering and avoid the monoclinic phase, since the accumulation of the latter is associated with a decrease in strength, toughness and density .
The resulting stabilized ceramic is known as Tetragonal Zirconia Polycrystals (TZP) and when it is doped with 2-3% mol of yttrium oxide, this material is known as Yttrium Tetragonal Zirconia Polycrystals (3Y-TZP) . The amount of yttrium oxide in the ceramic has an essential role in the capacity of transformation of the tetragonal phase and, therefore, in the ceramic's toughness . This material consists of tetragonal grains, whose size is in the submicron scale, without a glassy phase at the edge of the crystals. The amount of tetragonal phase that remains at room temperature depends on many factors, such as grain size, yttrium oxide content, and the degree of constriction exerted upon them by the matrix . Most commercial Y-TZP powders are manufactured using a process known as co-precipitation . The objective of this procedure is to manufacture multicomponent ceramic oxides through the formation of intermediate precipitates with the final goal of achieving an intimate mixture of the components during precipitation and keeping chemical homogeneity during calcination .
Today's ceramic materials used in the dental field comprise a large and diverse group of materials that offers patients a number of alternatives when dealing with prosthetic treatments.
These ceramic systems have been developed over time seeking highly-esthetic but also functional materials. Current dental ceramic systems (leucite or lithium disilicate-reinforced glass-ceramics, high-alumina or zirconia ceramics) offer better physical and mechanical properties than those of older, more traditional systems.
Mechanical values summarized in table 1 show that zirconium oxide ceramics display the highest values of flexural strength under 3-point bending tests. When it comes to hardness, enamel and dentin display the lowest results when compared to all ceramic systems, and zirconium oxide ceramics exhibit the highest hardness value. The highest value for fracture toughness is showed by zirconium oxide ceramics, although dentin shows higher values than most ceramic systems and enamel displays lower values than dentin and ceramics.
Dental ceramics is an interesting area of research, since countless possibilities are open for research, such as esthetics, processing of new ceramics with biological properties (increasing its interaction with cells or reducing bacterial adherence) and surface modification processing.
The authors wish to thank the partial support given by: Universidad Pontificia Bolivariana (CIDI Project-700-04/05-01), the European Community Progam Alban (E05D050652CO), Portuguese Science and Technology Foundation (Scholarship FCT/ SFRH/ BD/ 36220/ 2007 and Grant FCT/ PTDC/ CTM/ 100120/ 2008 "Bonamidi";), CRUP-Acções Integradas Luso-Espanholas
 ROSENBLUM, M.A. and SCHULMAN, A. A review of all-ceramic restorations, J Am Dent Assoc. 128(3), 297-307, 1997. [ Links ]
 RIZKALLA, A.S. and JONES, D.W. Mechanical properties of commercial high strength ceramic core materials, Dent Mater. 20(2), 207-12, 2004. [ Links ]
 RIZKALLA, A.S. and JONES, D.W. Indentation fracture toughness and dynamic elastic moduli for commercial feldspathic dental porcelain materials, Dent Mater. 20(2), 198-206, 2004. [ Links ]
 JONES, D.W. Development of dental ceramics. An historical perspective, Dent Clin North Am. 29(4), 621-44, 1985. [ Links ]
 KELLY, J.R., NISHIMURA, I., and CAMPBELL, S.D. Ceramics in dentistry: historical roots and current perspectives, J Prosthet Dent. 75(1), 18-32, 1996. [ Links ]
 STEJSKAL, V.D., et al. Metal-specific lymphocytes: biomarkers of sensitivity in man, Neuro Endocrinol Lett. 20(5), 289-298, 1999. [ Links ]
 ARVIDSON, K. and WROBLEWSKI, R. Migration of metallic ions from screwposts into dentin and surrounding tissues, Scand J Dent Res. 86(3), 200-5, 1978. [ Links ]
 VENCLIKOVA, Z., et al. Metallic pigmentation of human teeth and gingiva: morphological and immunological aspects, Dent Mater J. 26(1), 96-104, 2007. [ Links ]
 BUMGARDNER, J.D. and LUCAS, L.C. Cellular response to metallic ions released from nickel-chromium dental alloys, J Dent Res. 74(8), 1521-7, 1995. [ Links ]
 MEHULIC, K., et al. The release of metal ions in the gingival fluid of prosthodontic patients, Acta Stomatol Croat. 39(1), 47-51, 2005. [ Links ]
 MARTÍNEZ, F., et al. Cerámicas dentales: clasificación y criterios de selección, RCOE. 12(4), 253-263, 2007. [ Links ]
 WEINSTEIN, M., WEINSTEIN, L., and WEINSTEIN, A., Porcelain covered-metal reinforced teeth, 1962. [ Links ]
 ANUSAVICE, K.J. Degradability of dental ceramics, Adv Dent Res. 6, 82-9, 1992. [ Links ]
 KNORPP, E.A., Control arrangement for dental furnaces, especially microprocessor-controlled preheating furnaces, 1991. [ Links ]
 ANUSAVICE, K.J., Phillips Ciencia de los materiales dentales. Undécima ed: Elsevier España, 2004. [ Links ]
 MEYER, J.M., O'BRIEN, W.J., and YU, C.U. Sintering of dental porcelain enamels, J Dent Res. 55(4), 696-9, 1976. [ Links ]
 DENRY, I.L. Recent advances in ceramics for dentistry, Crit Rev Oral Biol Med. 7(2), 134-43, 1996. [ Links ]
 CAMPBELL, S.D., et al. Dimensional and formation analysis of a restorative ceramic and how it works, J Prosthet Dent. 74(4), 332-40, 1995. [ Links ]
 CONRAD, H.J., SEONG, W.J., and PESUN, I.J. Current ceramic materials and systems with clinical recommendations: a systematic review, J Prosthet Dent. 98(5), 389-404, 2007. [ Links ]
 TINSCHERT, J., et al. Structural reliability of alumina-, feldspar-, leucite-, mica- and zirconia-based ceramics, J Dent. 28(7), 529-35, 2000. [ Links ]
 RAHAMAN, M.N., Ceramic processing and sintering (materials engineering). Second ed: CRC Press, 2003. [ Links ]
 ANUSAVICE, K.J. and HOJJATIE, B. Effect of thermal tempering on strength and crack propagation behavior of feldspathic porcelains, J Dent Res. 70(6), 1009-13, 1991. [ Links ]
 ANUSAVICE, K.J., SHEN, C., and LEE, R.B. Strengthening of feldspathic porcelain by ion exchange and tempering, J Dent Res. 71(5), 1134-8, 1992. [ Links ]
 DUNN, B., LEVY, M.N., and REISBICK, M.H. Improving the fracture resistance of dental ceramic, J Dent Res. 56(10), 1209-13, 1977. [ Links ]
 ROSA, V., et al. Effect of ion exchange on strength and slow crack growth of a dental porcelain, Dent Mater. 25(6), 736-43, 2009. [ Links ]
 SEHA MIRKELAM, M., et al. Effect of Tuf-Coat on Feldspathic porcelain materials, J Oral Rehabil. 32(1), 39-45, 2005. [ Links ]
 ANUSAVICE, K.J., GRAY, A., and SHEN, C. Influence of initial flaw size on crack growth in air-tempered porcelain, J Dent Res. 70(2), 131-6, 1991. [ Links ]
 ASAOKA, K., KUWAYAMA, N., and TESK, J.A. Influence of tempering method on residual stress in dental porcelain, J Dent Res. 71(9), 1623-7, 1992. [ Links ]
 DEHOFF, P.H., ANUSAVICE, K.J., and VONTIVILLU, S.B. Analysis of tempering stresses in metal-ceramic disks, J Dent Res. 75(2), 743-51, 1996. [ Links ]
 CESAR, P.F., et al. Correlation between fracture toughness and leucite content in dental porcelains, J Dent. 33, 721-729, 2005. [ Links ]
 BERTOLOTTI, R.L. Calculation of interfacial stress in porcelain-fused-to-metal systems, J Dent Res. 59(11), 1972-7, 1980. [ Links ]
 NIELSEN, J.P. and TUCCILLO, J.J. Calculation of interfacial stress in dental porcelain bonded to gold alloy substrate, J Dent Res. 51(4), 1043-7, 1972. [ Links ]
 FAIRHURST, C.W., et al. Porcelain-metal thermal compatibility, J Dent Res. 60(4), 815-9, 1981. [ Links ]
 VASQUEZ, V., et al. Mechanical and thermal cycling effects on the flexural strength of glass ceramics fused to titanium, Dent Mater J. 27(1), 7-15, 2008. [ Links ]
 SHEU, T., et al. Mechanical properties and thermal expansion behaviour in leucite containing materials, J Mater Sci. 29, 125-128, 1994. [ Links ]
 DENRY, I.L., et al. Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain, J Dent Res. 75(12), 1928-35, 1996. [ Links ]
 PINTO, M.M., et al. Influence of pH on slow crack growth of dental porcelains, Dent Mater. 24(6), 814-23, 2008. [ Links ]
 MCLEAN, J.W. Evolution of dental ceramics in the twentieth century, J Prosthet Dent. 85(1), 61-6, 2001. [ Links ]
 SHERRILL, C.A. and O'BRIEN, W.J. Transverse strength of aluminous and feldspathic porcelain, J Dent Res. 53, 683-690, 1974. [ Links ]
 AL-SHEHRI, S. Relative fracture toughness and hardness of dental ceramics, Saudi Dent J. 14(2), 67-72, 2002. [ Links ]
 CHAIYABUTR, Y., GIORDANO, R., and POBER, R. The effect of different powder particle size on mechanical properties of sintered alumina, resin- and glass-infused alumina, J Biomed Mater Res B Appl Biomater. 88(2), 502-8, 2009. [ Links ]
 ALVAREZ-FERNÁNDEZ, M.A., et al. Características generales y propiedades de las cerámicas sin metal, RCOE. 8(5), 525-546, 2003. [ Links ]
 SORENSEN, J.A., et al. In-Ceram fixed partial dentures: three-year clinical trial results, J Calif Dent Assoc. 26(3), 207-14, 1998. [ Links ]
 TYSZBLAT, M., Process for the preparation of a dental prosthesis by slight solid phase fritting of a metal oxide based infrastructure, 1988. [ Links ]
 ANDERSSON, M. and ODEN, A. A new all-ceramic crown. A dense-sintered, high-purity alumina coping with porcelain, Acta Odontol Scand. 51(1), 59-64, 1993. [ Links ]
 ANDERSSON, M., et al. Clinical results with titanium crowns fabricated with machine duplication and spark erosion, Acta Odontol Scand. 47(5), 279-86, 1989. [ Links ]
 OILO, G. Flexural strength and internal defects of some dental porcelains, Acta Odontol Scand. 46(5), 313-22, 1988. [ Links ]
 PELÁEZ-VARGAS, A., et al. Microhardness obtained with two ceramic paste preparation methods, J Dent Res. 87(Spec Iss B), 0294, 2007. [ Links ]
 DRUMMOND, J.L., et al. Mechanical property evaluation of pressable restorative ceramics, Dent Mater. 16(3), 226-33, 2000. [ Links ]
 GONZAGA, C.C., et al. Mechanical properties and porosity of dental glass-ceramics hot-pressed at different temperatures, Mat Res. 11(3), 301-306, 2008. [ Links ]
 ARDLIN, B.I. Transformation-toughened zirconia for dental inlays, crowns and bridges: chemical stability and effect of low-temperature aging on flexural strength and surface structure, Dent Mater. 18(8), 590-5, 2002. [ Links ]
 VLEUGELS, J., YUAN, Z.X., and VAN DER BIEST, O. Mechanical properties of Y2O3/Al2O3-coated Y-TZP ceramics, J Eur Ceram Soc. 22, 873-881, 2002. [ Links ]
 CRAIG, R.G. and PEYTON, F.A. The microharness of enamel and dentin, J Dent Res. 37, 661-668, 1958. [ Links ]
 HASSAN, R., CAPUTO, A.A., and BUNSHAH, R.F. Fracture toughness of human enamel, J Dent Res. 60(4), 820-7, 1981. [ Links ]
 RASMUSSEN, S.T. and PATCHIN, R.E. Fracture properties of human enamel and dentin in an aqueous environment, J Dent Res. 63(12), 1362-8, 1984. [ Links ]
 FUENTES, V., et al. Microhardness of superficial and deep sound human dentin, J Biomed Mater Res A. 66(4), 850-3, 2003. [ Links ]
 EL MOWAFY, O.M. and WATTS, D.C. Fracture toughness of human dentin, J Dent Res. 65(5), 677-81, 1986. [ Links ]
 ADAIR, P.J., Glass-ceramic dental products, 1982. [ Links ]
 GROSSMAN, D., Tetrasilicic mica glass-ceramic method, 1973. [ Links ]
 STOOKEY, S.D., Method of making ceramics and products thereof, 1956. [ Links ]
 DENRY, I.L. and ROSENSTIEL, S.F. Flexural strength and fracture toughness of Dicor glass-ceramic after embedment modification, J Dent Res. 72(3), 572-6, 1993. [ Links ]
 THOMPSON, J.Y., et al. Fracture surface characterization of clinically failed all-ceramic crowns, J Dent Res. 73(12), 1824-32, 1994. [ Links ]
 EL-MOWAFY, O. and BROCHU, J.F. Longevity and clinical performance of IPS-Empress ceramic restorations--a literature review, J Can Dent Assoc. 68(4), 233-7, 2002. [ Links ]
 PROBSTER, L., et al. In vitro evaluation of a glass-ceramic restorative material, J Oral Rehabil. 24(9), 636-45, 1997. [ Links ]
 SORENSEN, J.A., et al. IPS Empress crown system: three-year clinical trial results, J Calif Dent Assoc. 26(2), 130-6, 1998. [ Links ]
 KELLY, J.R., Dispersion strengthened composite, 1990. [ Links ]
 FISCHER, H. and MARX, R. Improvement of strength parameters of a leucite-reinforced glass ceramic by dual-ion exchange, J Dent Res. 80(1), 336-9, 2001. [ Links ]
 MANSOUR, Y.F., et al. Clinical performance of IPS-Empress 2 ceramic crowns inserted by general dental practitioners, J Contemp Dent Pract. 9(4), 9-16, 2008. [ Links ]
 BRODKIN, D., PANZERA, C., and PANZERA, P., Pressable lithium disilicate glass ceramics, 2002. [ Links ]
 GARVIE, R.C., HANNINK, R.H., and PASCOE, R.T. Ceramic steel?, Nature. 258(5537), 703-704, 1975. [ Links ]
 PICONI, C. and MACCAURO, G. Zirconia as a ceramic biomaterial, Biomaterials. 20(1), 1-25, 1999. [ Links ]
 GUAZZATO, M., et al. Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part II. Zirconia-based dental ceramics, Dent Mater. 20(5), 449-56, 2004. [ Links ]
 PILATHADKA, S., VAHALOVA, D., and VOSAHLO, T. The Zirconia: a new dental ceramic material. An overview, Prague Med Rep. 108(1), 5-12, 2007. [ Links ]
 DEVILLE, S., et al. Low-temperature ageing of zirconia-toughened alumina ceramics and its implication in biomedical implants, J Eur Ceram Soc. 23, 2975-2982, 2003. [ Links ]
 ZIVKO-BABIC, J., CAREK, A., and JAKOVAC, M. Zirconium oxide ceramics in prosthodontics, Acta Stomatol Croat. 38(1), 25-28, 2005. [ Links ]
 PICONI, C., et al. Y-TZP ceramics for artificial joint replacements, Biomaterials. 19(16), 1489-94, 1998. [ Links ]
 SEGAL, D. Chemical synthesis of ceramic materials, J Mater Chem. 7(8), 1297-1305, 1997. [ Links ]