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. [email protected]

ALEJANDRO PELÁEZ VARGAS INEB ­ Instituto Nacional de Engenharia Biomédica and Universidade do Porto, FEUP, DEMM, Porto, Portugal. a [email protected] JORGE SALDARRIAGA ESCOBAR Universidad Pontificia Bolivariana, Medellín, Colombia. jor ge.sa lda r r ia ga @upb.edu.co FERNANDO JORGE MONTEIRO INEB ­ Instituto Nacional de Engenharia Biomédica and Universidade do Porto, FEUP, DEMM, Porto, Portugal. [email protected] LUIS FELIPE RESTREPO TAMAYO Universidad CES, Facultad de Odontología, Medellín. Colombia. [email protected]

Received for review July 29 th, 2009, accepted June 25 th, 2010, final version July, 8 th, 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 state­of­the­art on dental ceramics.

KEYWORDS: Ceramic systems, feldspathic porcelain, leucite, zirconium oxide, glass­ceramics, 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.

Dyna, year 77, Nro. 163, pp. 26­36. Medellin, September, 2010. ISSN 0012­7353 27 Dyna 163, 2010

PALABRAS CLAVE: Sistemas cerámicos, porcelana feldespática, leucita, alúmina, oxido de zirconio, vitrocerámica, propiedades físicas y mecánicas.

1. INTRODUCTION There are reports of complete dentures being Dental ceramics are materials that are part of manufactured earlier by French dentist Pierre systems designed with the purpose of producing Fauchard, although these dentures were dental prostheses that in turn, are used to replace fabricated in a different class of ceramic, namely missing or damaged dental structures. The enamel [4]. literature on this topic defines ceramics as inorganic, non­metallic materials made by man The initial use of ceramic materials in dentistry through the heating of raw minerals at high was in the obtention of complete dentures. Early temperatures [1]. in the 19 th century, Italian dentist Giuseppangelo Fonzi was capable of manufacturing individual Ceramics and glasses are brittle, which means ceramic teeth attached to a metallic substructure that they display high compressive strength but which, in turn, was attached to complete low tensile strength and may be fractured under dentures. The restoration of individual ceramic very low strain (0.1% ­ 0.2%). As restorative teeth in the oral cavity was delayed until the late materials, dental ceramics have disadvantages, 1800s, when Logan constructed ceramic teeth mostly due to their inability to withstand fused to metallic posts so that these posts could functional forces that are present in the oral function as an intra­radicular retention for the cavity, hence initially they found limited restoration [4, 5]. application in premolar and molar areas, although further development in these materials The method to manufacture dental prostheses has enabled their use as posterior long­span fixed during the second half of the 20 th century was partial prosthetic restorations and structures over through the fusion of ceramics and metallic dental implants.[2] All dental ceramics display structures that could function as a core. Metal­ low fracture toughness when compared to other ceramic systems combine both the exceptional dental materials, such as metals. [3]. esthetic properties of ceramics and the extraordinary mechanical properties of metals The main objective of this work is to review [1]. Some metals used as restorative materials in ceramic dental materials, including their most dentistry may constitute a problem for some relevant physical and mechanical properties. A patients. These problems may reveal themselves brief historical review, including the evolution of as allergies [6], gum staining [7, 8], and release these materials over time, a summary of different of metallic ions into the gingival tissue [9] and dental ceramic classifications, and the the gingival fluid [10]. These drawbacks, as composition of dental ceramics will be well as the search for more esthetic materials by presented. patients and dentists, have stimulated research and development of metal­free ceramic systems.

2. HISTORICAL EVOLUTION OF During the last 40 years, research has focused on DENTAL CERAMICS improving metal­free systems and developing superior materials regarding esthetics and Current dental ceramics are far from the early clinical performance to offer patients several ceramics that started being used over 200 years alternatives to restore missing or damaged teeth. 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. Arango et al 28

3. CLASSIFICATION OF DENTAL the ceramic being used. Currently, CERAMICS microprocessor­controlled ovens control all the thermal cycle steps and have optimized Dental ceramics may be classified according to programs designed by manufacturers to process several parameters, such as their use, different types of ceramics[14]. The heating manufacturing temperature, ceramic system, process may be carried out either under vacuum composition, microstructure, and translucency. or in air, however the process under vacuum The classification used in this article will be reduces ceramic’s porosity. [15, 16]. based on composition. Due to the fact that this review will discuss some mechanical properties The objective is to bring the raw paste’s particles exhibited by dental ceramic materials, table 1 together to form a paste that will have solidified summarizes the results of some mechanical tests upon cooling.[17] During sintering, ceramics published in the literature. density will increase and this is associated to a volumetric contraction of 30 % to 40%,[17, 18] 3.1 Feldspathic porcelains although other researchers claim that this contraction is between 20 % and 25% [19]. Before discussing this division of ceramics, it is During the paste formation process, it is essential important to keep in mind that most ceramics to avoid bubbles or pores formation, since their have two different phases: the glassy phase and presence will reduce the ceramic’s final the crystalline phase. The glassy phase is often mechanical strength, given that pores act as responsible for the esthetic behavior, while the crack initiators [20]. The sintering process crystalline phase is associated with mechanical considerably reduces porosity since this strength. However, the crystalline phase (leucite procedure involves the melting/softening of the in feldspathic porcelains) has a similar powder particle, causing reduction of surface diffraction index as the glassy matrix; hence it energy, which renders pores to become spherical also contributes towards the translucency of the and volumetrically contracted, thus increasing whole structure. Therefore, both mechanical and densification [21]. esthetic properties are dependent on the ceramic composition [11]. Some methods to strengthen porcelain and avoid crack initiation and/or propagation have been Feldspathic porcelain is produced from the reported by different authors. The most common mixture of potassium feldspar, quartz, and methods are ionic exchange (chemical exchange kaolin. The latter improves plasticity and of smaller sodium ions for larger potassium ions manipulation before the heating process. Other to create a compressive stress on the surface) essential components are oxides of sodium, [22­26], thermal tempering (forced cooling of potassium, calcium, aluminum, and magnesium ceramic in air) [23, 27, 28] or crack­propagation (employed to control ceramic’s expansion interruption by means of induction of residual coefficient and to try and match it with that of compressive stresses on the surface (air­injection metals) [12], zinc, iron, copper, titanium, nickel, tempering) [29]. manganese, and cobalt (as pigments) and tin, zirconium, and titanium (as opacifiers) [13]. 3.2 Leucite­reinforced feldspathic Feldspathic porcelain is highly brittle, hence it porcelain must be used as a veneering material in metal­ ceramic and metal­free ceramic systems. [11]. Feldspar may form leucite, which is a potassium aluminum silicate mineral with a high thermal This type of ceramic is made of extremely fine expansion coefficient [15]. Leucite may be powders to which de­ionized water or a special present in feldspathic porcelain in two forms: modeling liquid is added. A paste is thus formed Firstly, the formation of this mineral through the to be tooth­shaped by a technician. Then, this incongruent melting of potasic feldspar paste must be taken to an oven set at a pre­ (incongruent melting is a process by which a established temperature and time depending on material is melted to form a liquid and a different 29 Dyna 163, 2010 crystalline material [15]). Secondly, the dental crowns [38]. The principle of this type of aggregate of leucite to ceramics as synthetic ceramics is that a dispersion of high strength and powder [30]. Leucite­reinforced ceramics may high elastic modulus crystals exists within the be used to manufacture metal­ceramic glassy matrix to strengthen and harden the restorations. The large difference between ceramic [38]. thermal expansion coefficient of metals and that of ceramics is one of the problems encountered Alumina has the property of strengthening when manufacturing metal­ceramic restorations feldspathic porcelain, thus making it more [31, 32] as well as the difference in thermal fracture resistant. This fact may be observed expansion coefficients between layers of ceramic when comparing aluminous ceramics strength to [33] when cooling to room temperature. This that of feldspathic porcelain [39] and when imbalance creates residual stress between both comparing hardness and strength values of materials that may trigger failure at the interface. aluminous ceramics with those of other ceramic [34] Hence, a layer to layer compatibility systems [40]. Alumina particle size has been involving ceramics and underlying metal is proposed as a factor that improves particles required [32]. agglomeration and some mechanical properties, such as fracture strength, of the partially­sintered Also, leucite content in ceramics is associated to alumina matrix [41]. an increase in crack propagation strength [30]. A mechanism by which this increase is produced is The presence of alumina decreases glasses’ the difference between leucite’s thermal inherent brittleness and the risk of de­ expansion coefficient (22 to 25 X 10 ­6/°C) and vitrification, which is a process whereby that of the glassy matrix (8 x 10 ­6/°C), resulting ceramics become opaque and brittle as a result of in the formation of tangent compressive stresses their crystallization, due to loss of the glassy in the glass around leucite crystals, that undergo structure [42]. a phase transformation from cubic to tetragonal upon cooling [17]. Sodium content has an effect 3.4 Glass­infiltrated composites on the tetragonal phase stability at high temperatures and lowers the thermal expansion Currently, there is a ceramic system marketed as coefficient, but it also decreases the ceramic’s In­Ceram (Vita­Zahnfabrik, H Rauter GMBH & overall strength [35]. Another mechanism of Co.KG, Bad Säckingen, Germany) that includes strength increase is explained by microcrack glass­infiltrated porous sintered alumina. [43] formation within and around leucite crystals This system, developed by Tyszblat, is when cooling. Both mechanisms are essential to comprised of an aluminous core that is further prevent crack formation and propagation in the infiltrated with molten glass to fill­in the voids ceramic [36]. Some researchers have suggested left by aluminous particles in order to obtain a that ceramics reinforced with cubic leucite at restoration that exhibits better physical room temperature display less flexural strength properties when compared to traditional and less fracture strength than tetragonal­leucite ceramics (see table 1) and very similar to those ceramics [36]. of natural teeth [44].

Pinto et al assessed the effect that pH has on 3.5 Alumina Polycrystals leucite­reinforced ceramics. These authors concluded that an acidic pH (3.5) produces a High­alumina ceramics contain a minimum of reduction in overall strength in leucite­reinforced 95% pure alumina (aluminum oxide, Al203) [38]. ceramics [37]. Andersson and Oden [45] described the use of a high­purity (99.9%), densely­sintered alumina 3.3 Aluminous ceramics ceramic for the manufacturing of dental restorations [45]. Alumina displays a 15% to Aluminous ceramics were first developed by 20% contraction that must be compensated to MacLean and Hughes to manufacture prosthetic manufacture such restorations. These Arango et al 30 investigators compensated its high contraction 3.6 Glass­ceramics by using enlarged models of teeth and restoration manufacturing using a technique known as copy­ According to McLean [38], the first works on milling to obtain a properly­sized finished glass­ceramics were performed by MacCulloch, restoration. The amount of expansion necessary but his work did not receive much attention. to be applied to the models may be calculated Further investigations by Grossman and Adair from the sintering of compacted powder [45]. [58, 59] concluded with the development of a Copy­milling is better described by Andersson tetrasilicic fluormica­containing ceramic system. elsewhere [46]. According to both investigators, its composition is as follows: 45­70% SiO2, 8­20% MgO, 8­15% Table 1. Reported values for ceramic systems, MgF2, 5­35% R2O+RO, where R2O has a range enamel and dentin subjected to several mechanical between 5­25% and is composed of at least one tests of the following oxides: 0­20% K2O, 0­23% 3­point Rb2O, and 0­25% Cs2O to improve translucency Material bending Hardness Fracture and RO, which has a range between 0­20%, and [MPa] [GPa] Toughness is composed of at least one of the following 712.9 ­ oxides: SrO, BaO, and CdO. Additional 763.9 1.0 ­ 1.49 Feldspathic 98 – 101 kg/mm2 MPa/m0 .5 components may account for up to 10% of Sb2O5 porcelain [47] 6.98 ­ 7.48 [49] and/or up to 5% of traditional glassy colorants GPa [48] [58, 59]. Alumina­ 1.95 116 ­ 117 3.7 – 5.2 reinforced MN/m 1/2 The thermal treatment known as ceramming [15] [47] [3] ceramic [40] is composed of two processes: glass is heated up 4.58 High­alumina 155 5 ­ 10.8 to a temperature where nuclei form (750° ­ MN/m 1/2 content ceramic [47] [3] 850°C) and this temperature is kept for a period [40] of time ranging from one to six hours so that 1.28 239.2 3.4 – 4.1 crystalline nuclei form in the glass (process Glass­ceramic MN/m 1/2 [47] [3] known as nucleation). Then, the temperature is [40] risen to the crystallization point (1000°­1150°C) Leucite­ 78.37 ­ 1.26 6.53 reinforced glass­ 133.56 MN/m 1/2 and this temperature is maintained for a period [40] ceramic [49] [40] ranging from one to six hours until the desired Lithium level of glazing is obtained (process known as disilicate­ 5.6 ­ 6.2 crystallization) [58­60]. Developers of this type na na reinforced glass­ [50] of ceramic system claim that fracture strength ceramic and hardness are better than those of traditional 1188­1274 ceramic systems [58, 59], although other authors 4.6 – 6.2 Zirconium oxide 800 – 1590 kg/mm2 MPa.m1 /2 have shown that after the ceramming process a ceramic [51] 11.64 GPa [52] surface layer that decreases its strength and [52] esthetic characteristics forms on this ceramic 343 kg/mm2 0.70 – 1.16 3 2 [61]. However, some researchers claim that Dental enamel na 3.36 GPa MN/m / fractures in this type of ceramic have an internal [53] [54] origin, but no explanations are provided as to 64.75 ­ 73.75 what particular factor causes fractures to appear 137.9 – 3.08 kg/mm2 in restorations manufactured with this material Dentin 220.63 MN.(m) ­1.5 0.63 – 0.72 [62]. However, there is an investigation that [55] [57] Gpa suggests that this ceramic’s strength and [56] hardness may be improved by performing a na. Data not available chemical shift on the surface layer (addition of lithium fluoride) [61]. 31 Dyna 163, 2010

3.7 Leucite­reinforced glass­ceramics 3.9 Zirconium oxide ceramics

Pressed glass­ceramics are materials containing Since Garvin et al [70] published their work high amounts of leucite crystals (35% by titled “Ceramic steel?”, zirconia has been volume) [15]. The basic component of this considered a tough ceramic. Zirconia occurs as a ceramic is feldspathic porcelain consisting of natural mineral called baddeleyite. This mineral 63% SiO2, 19% Al2O3, 11% K2O, 4% Na2O, and contains 80% to 90% zirconium oxide. traces of other oxides. Leucite crystals are added The major impurities are usually TiO2, SiO2, and to the aluminum oxide [63, 64]. This material is Fe2O3. This oxide exists in three different manufactured using a process known as heat­ crystal structures: monoclinic at room pressing, which is performed in an investment temperature, tetragonal at ~1200°C, and cubic at mold. This mold is filled with the plasticized 2370°C. A phase transformation from tetragonal ceramic, thus avoiding the sintering process and to monoclinic occurs during cooling. This the subsequent pore formation [65]. This change produces a volumetric expansion ranging ceramic undergoes dispersion strengthening from 3% to 4%, which in turn causes crack through the guided crystallization of leucite. formation within the material. Oxides such as Dispersion strengthening is a process by which CaO, MgO, Y2O3, and CeO2 are added to pure the dispersed phase of a different material (such zirconia to stabilize it in multiphase materials as alumina, leucite, zirconia, etc.) is used to stop known as Partially­Stabilized Zirconia (PSZ). crack propagation, since these crystalline phases These materials basically consist of cubic are more difficult to penetrate by cracks [15, 66]. zirconia as the major phase and precipitates of Leucite crystals are incorporated during monoclinic and tetragonal as the minor phase at ceramming, hence performing this process again room temperature [71]. At room temperature, is unnecessary when inducing crystal growth the tetragonal phase is in metastable state, since [64]. transformation from tetragonal to monoclinic may be induced by external factors, such as The method of dual ionic exchange has been pressure or temperature [72, 73]. The resulting proposed in the literature to increase the compressive stress from the volumetric resistance of this ceramic to separation, as long expansion developed at the vicinity of a crack as potassium and sodium ions are available in and the energy needed to carry out such phase the ceramic for the exchange. Long­term transformation, provide this ceramic with high chemical effects on the ceramic should be further fracture strength [72, 74]. This process is known studied [67]. as transformation toughening [71­73, 75]. It may generate a process known as ceramic aging, which is the spontaneous transformation from 3.8 Lithium disilicate­reinforced glass­ metastable tetragonal zirconia to monoclinic ceramics zirconia [71], therefore a decrease in mechanical properties may occur in the ceramic over time This material is also a heat­pressed glass­ [74]. An investigation carried out by Ardlin ceramic with a 60% content of lithium disilicate demonstrated that strength was not affected by crystals, which form an intertwined structure aging, although the ceramic’s crystalline after being pressed, hence fracture strength is structure and surface were, indeed, affected [51]. increased [65, 68]. Li2O and SiO2 aid in the crystallization of the required phase of lithium As mentioned above, pure zirconia may be disilicate, BaO and Cs2O stabilize residual glass stabilized with Y2O3. Yttrium oxide is used to and Al2O3 and B2O3 render this ceramic retain the tetragonal phase at room temperature chemically durable [69]. The crystallization as much as possible after sintering and avoid the process is comprised of two steps: nucleation monoclinic phase, since the accumulation of the (one hour at 645°C) and crystal growth (four latter is associated with a decrease in strength, hours at 850°C) [69]. toughness and density [76]. Arango et al 32

The resulting stabilized ceramic is known as toughness is showed by zirconium oxide Tetragonal Zirconia Polycrystals (TZP) and ceramics, although dentin shows higher values when it is doped with 2­3% mol of yttrium than most ceramic systems and enamel displays oxide, this material is known as Yttrium lower values than dentin and ceramics. Tetragonal Zirconia Polycrystals (3Y­TZP) [71]. Dental ceramics is an interesting area of The amount of yttrium oxide in the ceramic has research, since countless possibilities are open an essential role in the capacity of transformation for research, such as esthetics, processing of new of the tetragonal phase and, therefore, in the ceramics with biological properties (increasing ceramic’s toughness [52]. This material consists its interaction with cells or reducing bacterial of tetragonal grains, whose size is in the adherence) and surface modification processing. submicron scale, without a glassy phase at the edge of the crystals. The amount of tetragonal phase that remains at room temperature depends 5. ACKNOWLEDGMENTS on many factors, such as grain size, yttrium oxide content, and the degree of constriction The authors wish to thank the partial support exerted upon them by the matrix [71]. Most given by: Universidad Pontificia Bolivariana commercial Y­TZP powders are manufactured (CIDI Project­700­04/05­01), the European using a process known as co­precipitation [77]. Community Progam Alban (E05D050652CO), The objective of this procedure is to manufacture Portuguese Science and Technology Foundation multicomponent ceramic oxides through the (Scholarship FCT/ SFRH/ BD/ 36220/ 2007 and formation of intermediate precipitates with the Grant FCT/ PTDC/ CTM/ 100120/ 2008 final goal of achieving an intimate mixture of the “Bonamidi”), CRUP­Acções Integradas Luso­ components during precipitation and keeping Espanholas (46/09), Portugal. chemical homogeneity during calcination [77].

6. REFERENCES 4. CONCLUSIONS [1] ROSENBLUM, M.A. and Today’s ceramic materials used in the dental SCHULMAN, A. A review of all­ceramic field comprise a large and diverse group of restorations, J Am Dent Assoc. 128(3), 297­307, materials that offers patients a number of 1997. alternatives when dealing with prosthetic treatments. [2] RIZKALLA, A.S. and JONES, D.W. Mechanical properties of commercial high These ceramic systems have been developed strength ceramic core materials, Dent Mater. over time seeking highly­esthetic but also 20(2), 207­12, 2004. functional materials. Current dental ceramic systems (leucite or lithium disilicate­reinforced [3] RIZKALLA, A.S. and JONES, D.W. glass­ceramics, high­alumina or zirconia Indentation fracture toughness and dynamic ceramics) offer better physical and mechanical elastic moduli for commercial feldspathic dental properties than those of older, more traditional porcelain materials, Dent Mater. 20(2), 198­206, systems. 2004.

Mechanical values summarized in table 1 show [4] JONES, D.W. Development of dental that zirconium oxide ceramics display the ceramics. An historical perspective, Dent Clin highest values of flexural strength under 3­point North Am. 29(4), 621­44, 1985. bending tests. When it comes to hardness, enamel and dentin display the lowest results [5] KELLY, J.R., NISHIMURA, I., and when compared to all ceramic systems, and CAMPBELL, S.D. Ceramics in dentistry: zirconium oxide ceramics exhibit the highest historical roots and current perspectives, J hardness value. The highest value for fracture Prosthet Dent. 75(1), 18­32, 1996. 33 Dyna 163, 2010

[6] STEJSKAL, V.D., et al. Metal­specific [18] CAMPBELL, S.D., et al. Dimensional lymphocytes: biomarkers of sensitivity in man, and formation analysis of a restorative ceramic Neuro Endocrinol Lett. 20(5), 289­298, 1999. and how it works, J Prosthet Dent. 74(4), 332­ 40, 1995. [7] ARVIDSON, K. and WROBLEWSKI, R. Migration of metallic ions from screwposts [19] CONRAD, H.J., SEONG, W.J., and into dentin and surrounding tissues, Scand J PESUN, I.J. Current ceramic materials and Dent Res. 86(3), 200­5, 1978. systems with clinical recommendations: a systematic review, J Prosthet Dent. 98(5), 389­ [8] VENCLIKOVA, Z., et al. Metallic 404, 2007. pigmentation of human teeth and gingiva: morphological and immunological aspects, Dent [20] TINSCHERT, J., et al. Structural Mater J. 26(1), 96­104, 2007. reliability of alumina­, feldspar­, leucite­, mica­ and zirconia­based ceramics, J Dent. 28(7), 529­ [9] BUMGARDNER, J.D. and LUCAS, 35, 2000. L.C. Cellular response to metallic ions released from nickel­chromium dental alloys, J Dent Res. [21] RAHAMAN, M.N., Ceramic processing 74(8), 1521­7, 1995. and sintering (materials engineering). Second ed: CRC Press, 2003. [10] MEHULIC, K., et al. The release of metal ions in the gingival fluid of prosthodontic [22] ANUSAVICE, K.J. and HOJJATIE, B. patients, Acta Stomatol Croat. 39(1), 47­51, Effect of thermal tempering on strength and 2005. crack propagation behavior of feldspathic porcelains, J Dent Res. 70(6), 1009­13, 1991. [11] MARTÍNEZ, F., et al. Cerámicas dentales: clasificación y criterios de selección, [23] ANUSAVICE, K.J., SHEN, C., and RCOE. 12(4), 253­263, 2007. LEE, R.B. Strengthening of feldspathic porcelain by ion exchange and tempering, J Dent Res. [12] WEINSTEIN, M., WEINSTEIN, L., and 71(5), 1134­8, 1992. WEINSTEIN, A., Porcelain covered­metal reinforced teeth, 1962. [24] DUNN, B., LEVY, M.N., and REISBICK, M.H. Improving the fracture [13] ANUSAVICE, K.J. Degradability of resistance of dental ceramic, J Dent Res. 56(10), dental ceramics, Adv Dent Res. 6, 82­9, 1992. 1209­13, 1977.

[14] KNORPP, E.A., Control arrangement for [25] ROSA, V., et al. Effect of ion exchange dental furnaces, especially microprocessor­ on strength and slow crack growth of a dental controlled preheating furnaces, 1991. porcelain, Dent Mater. 25(6), 736­43, 2009.

[15] ANUSAVICE, K.J., Phillips Ciencia de [26] SEHA MIRKELAM, M., et al. Effect of los materiales dentales. Undécima ed: Elsevier Tuf­Coat on Feldspathic porcelain materials, J España, 2004. Oral Rehabil. 32(1), 39­45, 2005.

[16] MEYER, J.M., O'BRIEN, W.J., and YU, [27] ANUSAVICE, K.J., GRAY, A., and C.U. Sintering of dental porcelain enamels, J SHEN, C. Influence of initial flaw size on crack Dent Res. 55(4), 696­9, 1976. growth in air­tempered porcelain, J Dent Res. 70(2), 131­6, 1991. [17] DENRY, I.L. Recent advances in ceramics for dentistry, Crit Rev Oral Biol Med. 7(2), 134­43, 1996. Arango et al 34

[28] ASAOKA, K., KUWAYAMA, N., and [39] SHERRILL, C.A. and O'BRIEN, W.J. TESK, J.A. Influence of tempering method on Transverse strength of aluminous and feldspathic residual stress in dental porcelain, J Dent Res. porcelain, J Dent Res. 53, 683­690, 1974. 71(9), 1623­7, 1992. [40] AL­SHEHRI, S. Relative fracture [29] DEHOFF, P.H., ANUSAVICE, K.J., and toughness and hardness of dental ceramics, VONTIVILLU, S.B. Analysis of tempering Saudi Dent J. 14(2), 67­72, 2002. stresses in metal­ceramic disks, J Dent Res. 75(2), 743­51, 1996. [41] CHAIYABUTR, Y., GIORDANO, R., and POBER, R. The effect of different powder [30] CESAR, P.F., et al. Correlation between particle size on mechanical properties of sintered fracture toughness and leucite content in dental alumina, resin­ and glass­infused alumina, J porcelains, J Dent. 33, 721­729, 2005. Biomed Mater Res B Appl Biomater. 88(2), 502­ 8, 2009. [31] BERTOLOTTI, R.L. Calculation of interfacial stress in porcelain­fused­to­metal [42] ALVAREZ­FERNÁNDEZ, M.A., et al. systems, J Dent Res. 59(11), 1972­7, 1980. Características generales y propiedades de las cerámicas sin metal, RCOE. 8(5), 525­546, [32] NIELSEN, J.P. and TUCCILLO, J.J. 2003. Calculation of interfacial stress in dental porcelain bonded to gold alloy substrate, J Dent [43] SORENSEN, J.A., et al. In­Ceram fixed Res. 51(4), 1043­7, 1972. partial dentures: three­year clinical trial results, J Calif Dent Assoc. 26(3), 207­14, 1998. [33] FAIRHURST, C.W., et al. Porcelain­ metal thermal compatibility, J Dent Res. 60(4), [44] TYSZBLAT, M., Process for the 815­9, 1981. preparation of a dental prosthesis by slight solid phase fritting of a metal oxide based [34] VASQUEZ, V., et al. Mechanical and infrastructure, 1988. thermal cycling effects on the flexural strength of glass ceramics fused to titanium, Dent Mater [45] ANDERSSON, M. and ODEN, A. A J. 27(1), 7­15, 2008. new all­ceramic crown. A dense­sintered, high­ purity alumina coping with porcelain, Acta [35] SHEU, T., et al. Mechanical properties Odontol Scand. 51(1), 59­64, 1993. and thermal expansion behaviour in leucite containing materials, J Mater Sci. 29, 125­128, [46] ANDERSSON, M., et al. Clinical results 1994. with titanium crowns fabricated with machine duplication and spark erosion, Acta Odontol [36] DENRY, I.L., et al. Effect of cubic Scand. 47(5), 279­86, 1989. leucite stabilization on the flexural strength of feldspathic dental porcelain, J Dent Res. 75(12), [47] OILO, G. Flexural strength and internal 1928­35, 1996. defects of some dental porcelains, Acta Odontol Scand. 46(5), 313­22, 1988. [37] PINTO, M.M., et al. Influence of pH on slow crack growth of dental porcelains, Dent [48] PELÁEZ­VARGAS, A., et al. Mater. 24(6), 814­23, 2008. Microhardness obtained with two ceramic paste preparation methods, J Dent Res. 87(Spec Iss B), [38] MCLEAN, J.W. Evolution of dental 0294, 2007. ceramics in the twentieth century, J Prosthet Dent. 85(1), 61­6, 2001. 35 Dyna 163, 2010

[49] DRUMMOND, J.L., et al. Mechanical [61] DENRY, I.L. and ROSENSTIEL, S.F. property evaluation of pressable restorative Flexural strength and fracture toughness of Dicor ceramics, Dent Mater. 16(3), 226­33, 2000. glass­ceramic after embedment modification, J Dent Res. 72(3), 572­6, 1993. [50] GONZAGA, C.C., et al. Mechanical properties and porosity of dental glass­ceramics [62] THOMPSON, J.Y., et al. Fracture hot­pressed at different temperatures, Mat Res. surface characterization of clinically failed all­ 11(3), 301­306, 2008. ceramic crowns, J Dent Res. 73(12), 1824­32, 1994. [51] ARDLIN, B.I. Transformation­ toughened zirconia for dental inlays, crowns and [63] EL­MOWAFY, O. and BROCHU, J.F. bridges: chemical stability and effect of low­ Longevity and clinical performance of IPS­ temperature aging on flexural strength and Empress ceramic restorations­­a literature surface structure, Dent Mater. 18(8), 590­5, review, J Can Dent Assoc. 68(4), 233­7, 2002. 2002. [64] PROBSTER, L., et al. In vitro evaluation [52] VLEUGELS, J., YUAN, Z.X., and VAN of a glass­ceramic restorative material, J Oral DER BIEST, O. Mechanical properties of Rehabil. 24(9), 636­45, 1997. Y2O3/Al2O3­coated Y­TZP ceramics, J Eur Ceram Soc. 22, 873­881, 2002. [65] SORENSEN, J.A., et al. IPS Empress crown system: three­year clinical trial results, J [53] CRAIG, R.G. and PEYTON, F.A. The Calif Dent Assoc. 26(2), 130­6, 1998. microharness of enamel and dentin, J Dent Res. 37, 661­668, 1958. [66] KELLY, J.R., Dispersion strengthened composite, 1990. [54] HASSAN, R., CAPUTO, A.A., and BUNSHAH, R.F. Fracture toughness of human [67] FISCHER, H. and MARX, R. enamel, J Dent Res. 60(4), 820­7, 1981. Improvement of strength parameters of a leucite­ reinforced glass ceramic by dual­ion exchange, J [55] RASMUSSEN, S.T. and PATCHIN, Dent Res. 80(1), 336­9, 2001. R.E. Fracture properties of human enamel and dentin in an aqueous environment, J Dent Res. [68] MANSOUR, Y.F., et al. Clinical 63(12), 1362­8, 1984. performance of IPS­Empress 2 ceramic crowns inserted by general dental practitioners, J [56] FUENTES, V., et al. Microhardness of Contemp Dent Pract. 9(4), 9­16, 2008. superficial and deep sound human dentin, J Biomed Mater Res A. 66(4), 850­3, 2003. [69] BRODKIN, D., PANZERA, C., and PANZERA, P., Pressable lithium disilicate glass [57] EL MOWAFY, O.M. and WATTS, D.C. ceramics, 2002. Fracture toughness of human dentin, J Dent Res. 65(5), 677­81, 1986. [70] GARVIE, R.C., HANNINK, R.H., and PASCOE, R.T. Ceramic steel?, Nature. [58] ADAIR, P.J., Glass­ceramic dental 258(5537), 703­704, 1975. products, 1982. [71] PICONI, C. and MACCAURO, G. [59] GROSSMAN, D., Tetrasilicic mica Zirconia as a ceramic biomaterial, Biomaterials. glass­ceramic method, 1973. 20(1), 1­25, 1999.

[60] STOOKEY, S.D., Method of making ceramics and products thereof, 1956. Arango et al 36

[72] GUAZZATO, M., et al. Strength, [75] ZIVKO­BABIC, J., CAREK, A., and fracture toughness and microstructure of a JAKOVAC, M. Zirconium oxide ceramics in selection of all­ceramic materials. Part II. prosthodontics, Acta Stomatol Croat. 38(1), 25­ Zirconia­based dental ceramics, Dent Mater. 28, 2005. 20(5), 449­56, 2004. [76] PICONI, C., et al. Y­TZP ceramics for [73] PILATHADKA, S., VAHALOVA, D., artificial joint replacements, Biomaterials. and VOSAHLO, T. The Zirconia: a new dental 19(16), 1489­94, 1998. ceramic material. An overview, Prague Med Rep. 108(1), 5­12, 2007. [77] SEGAL, D. Chemical synthesis of ceramic materials, J Mater Chem. 7(8), 1297­ [74] DEVILLE, S., et al. Low­temperature 1305, 1997. ageing of zirconia­toughened alumina ceramics and its implication in biomedical implants, J Eur Ceram Soc. 23, 2975­2982, 2003.