Dental Ceramics: Part II – Recent Advances In Dental Ceramics

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American Journal of Materials Engineering and Technology, 2015, Vol. 3, No. 2, 19-26Available online at http://pubs.sciepub.com/materials/3/2/1 Science and Education PublishingDOI:10.12691/materials-3-2-1Dental Ceramics: Part II – Recent Advances in DentalCeramicsSrinivasa Raju Datla1, Rama Krishna Alla2,*, Venkata Ramaraju Alluri1, Jithendra Babu P1, Anusha Konakanchi31Department of Prosthodontics, Vishnu Dental College, Bhimavaram, West Godavari, Andhra Pradesh, IndiaDepartment of Dental Materials, Vishnu Dental College, Bhimavaram, West Godavari, Andhra Pradesh, India3Department of Chemistry, Sasi Merit School, Bhimavaram, West Godavari, Andhra Pradesh, India*Corresponding author: ramakrishna.a@vdc.edu.in2Received April 06; Revised April 19, 2015; Accepted April 23, 2015Abstract Over the last decade, it has been observed that there is an increasing interest in the ceramic materials indentistry. Esthetically these materials are preferred alternatives to the traditional materials in order to meet thepatients’ demands for improved esthetics. Dental ceramics are usually composed of nonmetallic, inorganic structuresprimarily containing compounds of oxygen with one or more metallic or semi-metallic elements. Ceramics are usedfor making crowns, bridges, artificial denture teeth, and implants. The use of conservative ceramic inlay preparations,veneering porcelains is increasing, along with all-ceramic complete crown preparations. The earlier ceramics arevery fragile and can not with stand the high tensile forces. Several modifications have been made in ceramics inorder to address this quandary. This article is a review of dental ceramics; divided into two parts such as part I and II.Part I reviews the composition, structure and properties of dental ceramics from the literature available in PUBMEDand other sources from the past 50 years. Part II reviews the developments in evolution of all ceramic systems overthe last decade and considers the state of the art in several extended materials and material properties.Keywords: all-ceramics, sintering, feldspar, silica, glass, firing, slip casting, zirconia, CAD-CAMCite This Article: Srinivasa Raju Datla, Rama Krishna Alla, Venkata Ramaraju Alluri, Jithendra Babu P, andAnusha Konakanchi, “Dental Ceramics: Part II – Recent Advances in Dental Ceramics.” American Journal ofMaterials Engineering and Technology, vol. 3, no. 2 (2015): 19-26. doi: 10.12691/materials-3-2-1.1. IntroductionIn dentistry, ceramics represents one of the four majorclasses of materials used for the reconstruction of decayed,damaged or missing teeth. Other three classes are metals,polymers, and composites. The word Ceramic is derivedfrom the Greek word “keramos”, which literally means‘burnt stuff’, but which has come to mean more specifically amaterial produced by burning or firing [1]. A ceramic isan earthly material usually of silicate nature and may bedefined as a combination of one or more metals with anon-metallic element usually oxygen. The AmericanCeramic Society had defined ceramics as inorganic, nonmetallic materials, which are typically crystalline in nature,and are compounds formed between metallic and nonmetallicelements such as aluminum & oxygen (alumina - Al2O3),calcium & oxygen (calcia - CaO), silicon & nitrogen(nitride- Si3N4) [2]. In general, ceramics are used forpottery, porcelain glasses, refractory material, abrasives,heat shields in space shuttle, brake discs of sports cars,and spherical heads of artificial hip joints [1,3]. Indentistry, ceramics are widely used for making artificialdenture teeth, crowns, bridges, ceramic posts, abutments,and implants and veneers over metal substructures [1,4].Ceramics are characterized by their refractory nature,hardness, chemical inertness, biocompatibility [5,6,7] andsusceptibility to brittle fracture [8,9]. Our previous articlereviewed the composition, structure and properties ofdental ceramics from the literature available in PUBMEDand other sources from the past 50 years [10]. This articleis Part II of dental ceramics which reviews thedevelopments in evolution of all ceramic systems over thelast decade and considers the state of the art in severalextended materials and material properties.Dental ceramics are usually referred to as nonmetallic,inorganic structures primarily containing compounds ofoxygen with one or more metallic or semi-metallicelements like aluminum, calcium, lithium, magnesium,phosphorus, potassium, silicon, sodium, zirconium &titanium [1,11]. The term porcelain is referred to a specificcompositional range of ceramic materials made by mixingkaolin, quartz and feldspar in proper proportioning andfired at high temperature [1,10,12]. Porcelain is essentiallya white, translucent ceramic that is fired to a glazed state [7].The first All Ceramic crown was introduced by Land in1903. All ceramic anterior restorations appear very natural.Unfortunately, the ceramics used in these restorations arebrittle and subject to fracture from high tensile stress. Allmetal restorations are strong and tough but from estheticpoint of view they are acceptable for posterior restorations.Research aimed at better understanding of materials andinnovative processing methods to develop better ceramics.Fortunately, the esthetic qualities of porcelain can becombined with strength and toughness of metal to produce

20American Journal of Materials Engineering and Technologyrestorations that have both a natural tooth-like appearanceand very good mechanical properties. Therefore, verygood aesthetics and adequate mechanical properties can beobtained in a single restoration if a strong bond is attainedbetween the porcelain veneer and a metal sub structure(Figure 1). Brittle fracture can be avoided or at leastminimized. This kind of restoration is often termed asmetal ceramic restoration or more popularly called asporcelain fused to metal [1]. On the other hand, numerousresearchers have also tried with reinforcing the ceramicswith different crystalline phases which significantlyincreased the mechanical properties. The mechanisms ofstrengthening of these crystalline reinforced ceramicmaterials were discussed in the literature [1,10] and whichis summarized in Figure 2. However, the nature, amount,particle size, distribution and refractive index ofcrystalline phases influence the mechanical and aestheticproperties of ceramics [13].Figure 1. A 6-unit Metal-Ceramic bridgeFigure 2. Methods of strengthening dental ceramics2. Porcelain Fused to Metal (PFM)These restorations contain metal substructure where aceramic veneer is applied over it. Various alloys can beused to fabricate the metal substructure including highgold alloys, low gold alloys, gold – palladium alloys andnickel – chromium alloys [1,11,12]. The composition ofthe ceramic generally corresponds to that of the glassesexcept for an increased alkaline content. Greater amountof alkali content such as soda and potash were added inorder to meet the thermal expansion to a metal coping andalso to reduce fusion temperature. The addition ofglass/metal modifier (K2O) results in the formation of highexpansion leucite crystals. Leucite (K Al2 Si2O6) ispotassium aluminum silicate with a large co-efficient ofthermal expansion (COTE) which can increases thethermal expansion of porcelain, so that it could match tothat of COTE of dental alloys [1,11,12,14,15]. Colorpigments and opacifiers control the color and translucencyof the restoration [1].The success of the PFM restoration depends on theeminence of bond between the porcelain and metalsubstructure. The major problems associated with metalceramic restorations are the failure of porcelain veneer,crazing, cracking and separation of the porcelain from theunderlying substrate. A strong bond can be achieved byeither chemical or mechanical or thermal (compression)bonding mechanisms [16]. The metal substructure isfabricated using conventional casting technique. The castmetal is subjected to several treatments in order toimprove the bond strength with porcelain. The porcelainslurry is then applied on the cast metal surface andsintered. Sintering process is usually carried out undervacuum conditions in order to reduce the porosity in thefinal veneering ceramic [1,11,12,17,18]. However, theamount of porosity mainly depends on the sinteringtemperature, time, and viscosity of the melt [19]. Cheunget al [19] suggested that the shorter sintering time athigher temperature reduce the porosity. The properlymade ceramic crown is stronger and more durable thanPJC. The success of metal-ceramic restoration depends onthe amount and size of crystalline phase and also on theskill of the technician [4]. However, there is a chance ofappearance of metal at the cervical margins, discolorationof porcelain due to the presence of some alloying elementsespecially silver (imparts green color called as greeningeffect) and also possible disadvantage of the alloy beingused [1]. In addition, the metal copings may be susceptible

American Journal of Materials Engineering and Technologyto cohesive and adhesive ceramic failure due toinsufficient framework and /or metal ceramic debonding[20,21]. Some metal ions may be released in to thegingival tissue and the gingival fluid may cause allergyand staining of gingival tissues [22,23,24,25]. Theselimitations of PFM restorations led to the development ofall-ceramic restorations with the reinforcement of certaincrystalline phases with innovative fabrication techniques.3. Recent Advances in All-ceramicRestorationsDental ceramics are composite materials [13,26]. Theterm ‘all-ceramic’ refers to any restorative materialcomposed exclusively of ceramics, such as feldspathicporcelain, glass ceramic, alumina core systems and withany combination of these materials [27]. According toKelly [28] the ceramic may be defined as a “composite”that means a composition of two or more distinct entities.All-ceramic systems use different types of crystallinephases which influence the physical, mechanical andaesthetic properties of all-ceramics. The nature, amount,particle size and coefficient of thermal expansion ofcrystalline phases influence the mechanical and opticalproperties of the materials [13,29]. In 1965 Mclean andHughes [30] reported that the strength and fracturetoughness of feldspathic glass can be significantlyimproved by the addtion of aluminium oxide particles(70 % vol). The castable glass-ceramic crown system [31]and “shrink free” [32] (Cerastore, coores Biomedical,Lake wood, Colo) ceramic systems were introduced in theName of Processing techniqueSintered porcelainsCastable glass ceramicsSlip cast ceramics211980s with superior esthetics using new innovativeprocessing techniques. Later, 100% polycrystallinesubstructure ceramics were introduced with crystalline materials are highly opaque and theiresthetic qualities are not up to the standard. So, it can beconcluded that the increase in the crystalline phase wouldincrease the mechanical properties substantially, withcompromised aesthetic properties [33]. Therefore, it isnecessary to optimize both the crystalline and glassyphases in order to attain adequate mechanical andaesthetic properties.All ceramic restorations are more aesthetic and appearvery natural. However, they are brittle and subject tofracture under high tensile stress. Numerous researchershave attempted reinforcing the ceramics with variouscrystalline materials using innovative fabricationtechniques to improve the short comings of all-ceramicrestorations. Kelly [28] classified all-cermaic restorationsas predominantly glassy materials, particle filled glassesand polycrystalline ceramics. All the three materialsconsist of feldspathic glass as the main glassy phase withvarying amounts of filler particles such as opacifiers,colorants and high-melting glass particles. In addition tothese filler particles, filled glasses and polycrystallineceramics contain leucite crystals with 17-25 mass% and40-55 mas% respectively [28,33]. Based on theirprocessing techniques all-ceramic restorations are widelyclassified into sintered porcelains, castable glass ceramics,machinable ceramics, slip-casted ceramics, heat pressedand injection molded ceramics. The detailed classificationis mentioned in Table 1.Table 1. All-ceramic restorative materials based on their processing technologyType of ceramicCrystalline PhaseBrand & ManufacturerLeucite- reinforced Feldspathic porcelainSanidineOptec HSP, Jeneric/Penetron Inc.,Alumina based porcelainAluminaHiceram, Vident, Baldwin Park, CA.Magnesia based core porcelainForsteriteVident, Baldwin Park, CA.Zirconia based porcelainMirage IIMyron International, Kansas City, KSMica based porcelainsTetrasilicic fluoromicaDICOR, Dentsply InternationalHydroxyapatite based porcelainsOxyapatiteCerapearl, Kyocera, San Diego, CA.Lithia based porcelainsLithium DisilicateVident, Baldwin Park, CASlip-Cast Glass InfiltratedAluminaIn-Ceram Alumina, Vident, Baldwin Park, CASlip-Cast Glass InfiltratedSpinelIn-Ceram Spinell, Vident, Baldwin Park, CA12 Ce-TZP-aluminaIn-Ceram Zirconia, Vident Baldwin Park, CA3Y-TZPCercon , DentsplyLeuciteAluminaIPS Empress , IvoclarIPS Empress Eris,IvoclarAlceram, Innotek Dental Corp, Lakewood, CA.DICOR MGC, Dentsply International, Inc.,York, PAVitablocs , Mark IIVident, Baldwin Park, CA.Vita-Celay, Vident, Baldwin Park, CAIn-Ceram AL, Vident, Vident, Baldwin Park,CA.Procera All Ceram, Nobel Biocare, USALeuciteIPS Empress CAD, IvoclarLithium disilicateIPS e.max CAD, IvoclarLava CAD/CAM, 3MMinnesoutaSlip-Cast Glass InfiltratedLeucite-basedHot pressed, injection-moldedLithium basedceramicsCerestoreLithium disilicateSpinelTetrasilicic fluoromicaCerec systemSanidineSanidineCelay systemMachinable ceramicsProcera systemCAD BasedLava CAD/CAM SystemAluminaY-TZPESPE,St.Paul,

22American Journal of Materials Engineering and Technology3.1. Sintered PorcelainsSintering is the consolidation process of ceramicpowder particles through heating at high temperatureswhich results in atomic motion. The sintering of porcelainpromotes physical-chemical reactions responsible for thefinal properties of the ceramic products. The amount ofporosity decreases in the last stage of sintering. Theamount of porosity is mainly influenced by the sinteringtemperature, time, and viscosity of the melt [19,34].3.1.1. Leucite-reinforced Feldspathic PorcelainConventional feldspathic porcelains were reinforcedwith approximately 45 vol% of tetragonal leucite crystals[13,35,36], which are responsible for high compressivestrength and modulus of rupture. Optec HSP is thecommercially available leucite-reinforced feldspathicporcelain. Vaidyanathan et al [37] reported that greaterleucite content of porcelain compared with conventionalfeldspathic porcelain for metal-ceramics leads to a highermodulus of rupture and compressive strength. Numerousstudies have reported that the large amount of leucite inthe material also contributes to a high thermal contractioncoefficient (25 x 10-6/ C) which is much more than thethermal contraction coefficient of the glassy matrix (8 x10-6/ C) resulting in the development of tangentialcompressive stresses in the glass around the leucitecrystals when cooled. These stresses can act as crackdeflectors and contribute to increase the resistance of theweaker glassy phase to crack propagation [1,11,12,38].Leucite reinforced ceramics are indicated to fabricateveneers due to their good optical properties [39]. The lowrefractive index of these crystals makes ceramic materialstranslucent even with high crystalline content [40] leadingto improved flexure strength (160 - 300 MPa) [41] whichalso depends on the shape and volume of the crystals. Theglass matrix is infiltrated by micron size crystals of leucite,creating a highly filled glass matrix [42].3.1.2. Alumina-based PorcelainAlumina (Al2O3) is the strongest and hardest oxideknown [43] and can be reinforced into ceramics by ,11,22,44,45,46,47]. Reinforcement of aluminaimparts high mechanical properties as it has high modulusof elasticity (MOE); 350 GPa, and high fracture toughness,3.5 to 4 MPa [1,13]. COTE is the same for both aluminaand glass matrix [1,13].3.1.3. Magnesia-based PorcelainThe flexural strength and COTE (14.5 x 10'6/ C) ofmagnesia (MgO) is very high and closely matches withthat of the body and incisal porcelains designed forbonding to metal (13.5 x 10-6/ C). The core material ismade by reacting magnesia with a silica glass within the1100-1150 C temperature range that results in theprecipitation of forsterite (Mg2SiO4) crystals [48,49]which is responsible for strengthening of ceramics. Thestrength of these porcelains is further increased by glazing[1,49].them by a mechanism called “transformation toughening”[1,10,11,13,45,51]. Zirconia has cubic structure at itsmelting point (2680 C) and on cooling from thistemperature results in crystallographic transformationssuch as tetragonal and monoclinic phase at 2370 C and1170 Crespectively[52,53,54].Thiscrystaltransformation induces internal stresses as it isaccompanied by a 3% - 5% volume expansion [53,54].Stabilizing agents such as calcia, magnesia, yittria andceria are added to partially stabilize the tetragonal phase atroom temperature and to control the volume expansion aswell [55]. This partially stabilized zirconia has high initialflexural strength and fracture toughness [55]. Applicationof tensile forces also induces crystallographic transformationfrom the tetragonal phase to monoclinic phase with anassociated 3% - 5% localized expansion [56], which isresponsible for toughening of ceramics. [1,52,55,56,57,58].Yittria stabilized zirconia (YSZ) increase fracture toughness,strength and thermal shock resistance and decreasetranslucency and fusion temperature [1,11,13,58,59]. Mostdental zirconia ceramics are opaque and copings need tobe veneered for high aesthetics [1,3,11].3.2. Glass CeramicsA glass ceramic is also called as castable glass ceramicas it is processed by using lost-wax pattern castingprocedure. The first commercially available castableceramic material for dental use is ‘Dicor’, which wasdeveloped by Dentsply international and supplied assilicon glass plate ingots containing MgF2 [1,11,13,22]. Aglass ceramic prosthesis is fabricated in a vitreous or noncrystalline state and then converted to crystalline state bycontrolled devitrification process using heat treatmentcalled as ceramming. The process of ceramming takesplace in two phases such as crystal nucleation and crystalgrowth, which results in increasing the strength andfracture toughness of glass ceramics by interrupting thecrack propagation through them under masticatory forces[1,2,13,22,60,61,62]. The significant aspect of thisceramics is the Chameleon effect in which a part of coloris picked up from adjacent tooth [2].Fabrication of glass ceramic restoration involvesinvesting the wax pattern in phosphate bonded investmentmaterial or leucite based gypsum bonded investmentmaterial followed by burnout of wax pattern. The moltenceramic material is casted with a motor driven centrifugalmachine at 1380 C. After removal of sprue, the glass isinvested again and heat-treated at 1075 C for 6 hours toproduce crystallization of glass to form a mica-ceramicsuch as “tetrasilicic fluoromica crystals”. This crystalnucleation and crystal growth process is called‘ceramming’ [1,2,13,22,60,61,62]. These crystals increasestrength, toughness, abrasion resistance, thermal resistanceand chemical durability. The final shape is achieved byapplying a thin layer of porcelain veneer of the requiredshape and fire [1]. Hydroxy apatite based porcelains arealso belong to glass ceramic category in which oxyapatitecan be transformed in to hydroxy apatite on exposed tomoisture [13].3.1.4. Zirconia-based Porcelain3.3. Slip-Cast CeramicsZirconia is a polycrystalline material reinforced intoconventional feldspathic porcelain in order to strengthenSlip-casting involves the pouring of an aqueousporcelain slip on a refractory die. The porosity of the

American Journal of Materials Engineering and Technologyrefractory die helps condensation by absorbing the waterfrom the slip by capillary action. Then it is fired at hightemperature on the refractory die. During this heattreatment, the refractory die shrinks more than thecondensed slip, and helps in easy separation. The resultingceramic is very porous and must be either infiltrated withmolten glass or fully sintered before veneering porcelaincan be applied [13,63]. Ceramics processed by slip-castingtechnique exhibit reduced porosity, and higher toughnessthan conventional feldspathic porcelains. However, thismethod may be technique sensitive in dental operatory,and also it is difficult to have accurate fit [63,64,65,66].These slip-casted ceramics contain two 3-dimensionalinterpenetrating phases such as core framework andinfiltrated glass. The core is usually made up of aluminaor zirconia or spinell and the infiltrated glass is usually23lanthanum aluminosilicate glass with sodium and calcium.Lanthanum glass has less viscosity and promotes properinfiltration [66]. Based on the type of core material, slipcasted ceramics are classified into Inceram-Alumina,Inceram-Zirconia and Inceram-Spinel. Fabrication of theseceramics involves application of core slurry on therefractory die and heated at 120 C for 2 hours followed bysintering at 1120 C for 10 hours. Then apply lanthanumglass on the framework and fire at 1100 C for 4 hours[67,68,69]. Flexural strength of inceram-zirconia is veryhigh compared to the other slip casted ceramics[4,70,71,72]. However these inceram-zirconia ceramicsare highly opaque compared to others [1,73,74,75].Properties of all the three types of slip casted ceramics aredetailed in Table 2.Table 2. Composition and Properties of glass infiltrated slip-cast ram-ZirconiaCompositionFlexural Strength (MPa)Alumina and Lanthanam GlassMgO and AluminaAlumina and Zirconia500350700TranslucencyTranslucentHighly TranslucentOpaqueIndicationsAnterior and posterior crowns, Anterior 3-unitAnterior crowns, inlays and onlays.bridges.3.4. Pressable CeramicsFabrication of the pressable ceramics involves theapplication of external pressure at elevated temperaturesto obtain sintering of the ceramic body. So, based on theprocessing technology these ceramics are also called as“hot-pressed” ceramics or "heat-pressed" ceramics. Thisfabrication technique prevents the porosity, and extensivegrain growth or secondary crystallization in ceramics withassociated increase in density and superior mechanicalproperties [4,13]. Pressable ceramics are catogerized in totwo generations including the first generation of heatpressed dental ceramics contains leucite as reinforcingcrystalline phase (IPS Empress 1) and the secondgeneration is lithium disilicate-based (IPS Empress2) [4].First generation heat-pressed ceramic such as IPSEmpress1 is dispersed with 35 to 45 vol % leucitecrystalline phase [36]. The strengthening mechanisminvolves the dispersion strengthening of leucite crystals[76,77], formation of stable tetragonal phase at processingtemperature and also involves in the development oftangential compressive stresses around the crystals uponcooling, due to the difference in thermal expansioncoefficients between leucite crystals and glassy matrix[78]. However, the strength and fracture toughness may bediminished when the micro cracks joins with each otherresults in decoupling the crystals from the glass matrix[79]. The amount of porosity is approximately about 9%[77]. Dong et al [80] suggested that that the flexuralstrength of these ceramics can be significantly improvedafter additional firings, due to additional leucitecrystallization.Second generation heat-pressed ceramic such as IPSEmpress2 contains about 65 vol % lithium disilicate as themain crystalline phase [4,77]. Numerous studies haverevealed that it forms two different phases such as lithiummetasilicate (Li2SiO3) and cristobalite (SiO2) during thecrystallization process, prior to the growth of lithiumdisilicate (Li2Si2O5) crystals [81]. The final microstructurePosterior Crown and Bridgeds.consists of highly interlocked lithium disilicate crystalsand layered crystals which contribute to strengthening.The mismatch in COTE between lithium disilicate crystalsand glassy matrix is also likely to result in tangentialcompressive stresses around the crystals, potentiallyresponsible for crack deflection and strength increase [82].Crack propagation can takes place easily in directionsparallel to crystal alignment and has high resistance tocrack propagation in the direction perpendicular to crystalalignment [77,83,84]. IPS Empress2 ceramics werecharacterised with about 1% porosity [77]. Properties oftwo types of pressable ceramics are detailed in Table 3.Table 3. Properties of pressable ceramicsPropertyFlexural Strength (MPa)0IPS Empress1IPS Empress2112 10400 40COTE (ppm / C)15 0.2510.6 0.25Pressing Temperature (0C)1150 – 1180890 – 920Veneering Temperature (0C)910800More recently, IPS e.max Press (lithium disilicateglass-ceramic ingot for the press technique) and IPS e.maxZirPress (fluorapatite glass-ceramic ingot for the press-ontechnique) ceramic systems were developed by Ivoclaurvivadent. IPS e.max Press is processed in the dentallaboratory with the known Empress pressing equipment.This equipment is distinguished for providing a highaccuracy of fit. The microstructure of IPS e.max Press ischaracterized as needle-like lithium disilicate crystals,which are embedded in a glassy matrix. The flexuralstrength of IPS e.max press is more than IPS Empress,comparatively [85].The "shrink-free" Cerestore system was introduced withinjection-molded technology. The commercial materialavailable with this system is Alceram (Innotek DentalCorp, Lakewood, CO) which contains a magnesium spinel(MgAl2O4) as the major crystalline phase [86]. Thismaterial has the excellent marginal fit of the restorations[87].

24American Journal of Materials Engineering and Technology3.5. Machinable CeramicsThe evolution of CAD-CAM (Computer aided designand Computer aided machining) technology for theproduction of machined inlays, onlays, and crowns led tothe development of a new generation of machinable ceramics.The advantages of this system include; impressions arenot needed, which saves the dentist chair time and alsoavoids cross-contamination between the patient-dentistoperational field and the dental technician [88,89,90,91].Dr. Duret was extensively worked on the development ofCAD-Cam System [92]. He has developed Sopha system,which had led to the development of CAD-CAM systemin dentistry [91]. Most popular systems available formachining all-ceramic restorations include CEREC(Siemens, Bensheim, Germany) system, Celay (MikronaTechnologie, Spreitenbach, Switzerland) system andProcera Alceram (Noble Biocare, USA) system.The CEREC system was developed by Dr. Moermann[93]. CEREC stands for “Chair side EconomicRestoration of Esthetic Ceramics” and it was the firstfully operational CAD-CAM ceramic. There are twocommercial materials available in this system. They areVita Mark II (Vident, Balsdwin Park, CA) and DicorMGC (Dentsply International, Inc., York, PA). Vita markII contains sanidine (KAlSi3O8) as a major crystallinephase. Sanidine imparts more opacity to the ceramic.Dicor MGC is a machinable glass ceramic similar to Dicor,with the exception in the fabrication technique. Thismaterial contains 70 vol% of tetrasilicic fluoromicacrystals. This higher vol% of crystalline phase is addressesits superior mechanical properties [94]. The fabrications ofceramic prosthesis involve the scanning of prepared toothstructure and digitize the information in to the computer.Design the restoration in the computer and activate themilling machine to cut the ceramic into the required shape.Adhesive resin cements are most commonly suggested forluting of these all-ceramic crowns to improve the fractureresistance [16, 95,96,97]. However, numerous studieshave shown that the overall fracture resistance of DicorMGC was independent of cement film thickness [98].The Celay system uses a copy milling technology tomanufacture ceramic inlays or onlays from resin analogs.This system is a machinable device based on pantographictracing of resin inlay or onlay fabricated directly on to theprepared tooth or die. Commercially available Celaysystem material is Vita-Celay (Vident, Baldwin Park, CA).Sanidine is the major crystalline phase in this material.Recently, In-Ceram pre-sintered slip cast alumina blocks(Vident, Baldwin Park, CA) have been developed and theycan be machined with the Celay copy-milling system. Thismaterial is mainly used to make crowns and fixed partialdentures [99].Procera All Ceram System (Nobel Biocare, USA) wasintroduced by Dr. Andersson [100], it was the first systemwhich provided outsourced fabrication using a networkconnection. The master die is scanned and the scannedimages are sent to processing center/laboratory through aninternet. In the processing center/laboratory, an oversizeddie is milled to compensate the firing shrinkage. The die ismilled by a computer controlled milling machines usingscanned images. Aluminum oxide powder is compactedon the die and coping is milled by a computer controlledmilling machines [100,101].Other systems developed using milling technique wereDCS Precident system with a laser scanner, CerconSystem with no CAD component, and CICERO(Computer integrated crown reconstruction) system. Morerecently, Lava CAD/CAM System (3M ESPE, St. Paul,Minnesouta) was introduced. It is used for fabrication ofzirconia framework for all ceramic restorations usingyttria stabilized tetragonal zirconia poly crystals whichhave greater fracture resistance than conventionalceramics. Lava system uses a laser optical syste

3. Recent Advances in All-ceramic Restorations Dental ceramics are composite ma[13,26]. The terials . term ‘all-ceramic’ refers to any restorative material composed exclusively of ceramics, such as feldspathic porcelain, glass ceramic, alumina core systems and w

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