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CONTENTS INTRODUCTION HISTORY APPLICATION

CLASSIFICATION COMPOSITION MANUFACFURE PROCESSING PROPERTIES

METHODS OF STRENGTHENING METAL CERAMIC SYSTEMS ALL CERAMIC RESTORATION CONCLUSION



INTRODUCTION T he term ceramic is defined as any product made

essentially from a non metallic material by firing at ahigh temperature to achieve desirable properties.

The term porcelain refers to a family of ceramicmaterials composed essentially of kaolin, quartz, andfeldspar, also fired at high temperature.

Dental ceramics for ceramic-metal restorations belongto this family and are commonly referred to as dental porcelains



HISTROY Dental ceramics mere first used in dentistry in the late

1700s.

Porcelain jacket crowns were developed in the early 1900s.

They consisted of feldspathic or aluminous porcelain bakedon a thin platinum foil and can be considered the ancestorsof all-ceramic crowns. Because their low strength, however,porcelain jacket crowns were limited to anterior teeth.

In the 1960s, the poor match in thermal expansion (andcontraction) between framework alloys and veneeringceramics, which often led to failures and fractures uponcooling, stimulated the development of leucite-containingfeldspathic porcelains



The end of the twentieth century saw the introductionof several innovative systems for fabricating all-ceramic dental restorations. The first was a castableglass-ceramic system in which the restoration was castusing the lost-wax technique

Ceramics for slip-casting, heat-pressing, and

machining were developed concurrently within thepast fifteen years.



WHY USE CERAMIC? Biocompatibility

Aesthetics

Durability Relative ease for customised units.



APPLICATIONS Ceramics have three major

applications in dentistry:

(1) ceramics for metal crowns andfixed partial dentures,

(2) all-ceramic crowns, inlays,onlays, and veneers, when

esthetics is a priority, (3) ceramic denture teeth



CLASSIFICATION FUSION TEMPERATURE:

The high-fusing ceramics -1315 °to 1370°C

medium-fusing ceramics-1090 °to 1260 °C

low-fusing ceramics-870 °to 1065 °C.

Ultra-low-fusing dental ceramics -firing temperaturesbelow 870 °C

The medium- and high-fusing ceramics are used fordenture teeth. Dental ceramics for ceramic-metal orall-ceramic fixed restorations belong to the low- ormedium-fusing categories



Composition Category 1—Glassbased systems (mainly silica)

Composition Category 2—Glassbased systems (mainly silica) with fillers, usually crystalline (typically leuciteor, more recently, lithium disilicate)

Composition Category 3—Crystalline- based systems

with glass fillers (mainly alumina) Composition Category 4—Polycrystalline solids

(alumina and zirconia)



TYPE

Feldspathic /conventional

Leucite reinforced Aluminious porcelain

Glass infiltered alumina

Glass infiltered spinel

Glass ceramic



SUB STRUCTURE

Cast metal

Swaged metal Glass ceramic

CAD CAM porcelain

Sintered ceramic core



PROCESSING METHOD

Sintered

Cast machined



FIRING

Air fired

Vaccum fired



COMPOSITION Feldspar-60-80%- basic glass former

Kaolin-3-5%- binder

Quartz-15-25%- filler Alumina-8-20%- glass former

Boric oxide-2-7%- glass former &flux

Oxides of Na, K, Ca-9-15%- fluxes

Metallic pigments- color



Feldspar Chemically it is designated as potassium aluminum

silicate, with a composition of K20 . A1203 . 6Si02. Thefusion temperature of feldspar varies between 1125°and1170°C, depending on its purity.

Iron and mica are commonly found as impurities infeldspar. It is particularly important to remove theiron, because metallic oxides act as strong coloringagents in porcelain



MANUFACTURE Feldspathic dental porcelains in

recent years are made mainly withpotash feldspar (K20. A120,. 6SiO2)and small additions of quartz (SiO2).

Alkali metal carbonates are added asfluxes and the mix is heated to about1200 °C in large crucibles.

At high temperature, the feldspardecomposes to form a glassy phase

with an amorphous structure, and acrystalline (mineral) phase consistingof leucite (KA1Si,06 or K20 . A1203 .4Si02).



The mix of leucite and glassy phase is then cooled very rapidly (quenched) in water which causes the mass toshatter in small fragments. The product obtained, called a

frit Coloring pigments in small quantities are added at this

stage. The metallic pigments include titanium oxide for yellow-brown shades, manganese oxide for lavender, ironoxide for brown, cobalt oxide for blue, copper or chromiumoxides for green, and nickel oxide for brown.

In the past, uranium oxide was used to providefluorescence; however, because of the small amount of radioactivity, lanthanide oxides (such as cerium oxide) arebeing substituted for this purpose. Tin, titanium, andzirconium oxides are used as opacifiers.



After the manufacturing process is completed, feldspathic dental porcelainconsists of two phases. One is the vitreous (or glass) phase, and the other is the

crystalline (or mineral) phase.

The glass phase formed during the manufacturing process has properties typicalof glass, such as brittleness, non directional fracture pattern, translucency, andhigh surface tension in the f luid state.The crystalline phase is leucite, a potassium alumino-silicate with high thermalexpansion (>20 x C). The amount present (10% to 20%) controls the thermalexpansion coefficient of the porcelain



PROCESSING Porcelain Application and Condensation - After careful cleaning of

the metal framework, a thin layer of opaque porcelain is applied andbaked.

Dentin porcelain powder, in the shade selected for the body or dentinportion, is mixed with modeling liquid (mainly distilled water) to acreamy consistency and is applied on the opaque layer, with allowancesmade for shrinkage.

To produce minimum shrinkage and a dense, strong porcelain, it is

important to achieve a thorough condensation of the particles at thisstage.

Various means of condensation may be employed. The vibrationmethod is particularly efficient in driving the excess water towards thesurface.

Other methods of condensation include the spatulation , brushtechniques, Ultrasonic, gravitational &whipping



Drying- After the porcelain mix has been applied andcondensed, the restoration is placed in front of anopen preheated porcelain furnace to be dried.

This drying stage, which lasts between 5 and 8minutes, is a very important step; it ensures that any remaining excess water is removed from the porcelainmix. During the drying stage, the water diffuses

towards the surface and then evaporates After the mass is placed in the furnace, remaining free

and combined water is removed in various stages of heating until a temperature of 480°C is reached.



Firing/Sintering- Porcelain restorations may be firedeither by temperature control alone or by controlledtemperature and a specified time. Time and

temperature method is generally preferred because itis more likely to produce a uniform product

As the temperature is raised, the particles of porcelainfuse by sintering. Sintering is the process responsible

for the fusion of the porcelain particles to form acontinuous mass. During this densification, the volume change, the lower the viscosity and the finerthe particle size, the greater the rate of densification.



A minimum number of three firingoperations are needed in the

fabrication of a ceramic-metalrestoration: one for the opaqueportion, one for the dentin andenamel portion, and another for thestain and glaze. However, due to the

shrinkage associated with thesintering process, it is necessary tooversize the porcelain buildup.

During the second firing, the dentinand enamel portion, which is formed

approximately 13% oversize, is heatedto the biscuit (or bisque) bake. Thistemperature is 56°C below the fusingtemperature of the porcelain.

Virtually all the shrinkage takes placeduring this firing.

Biscuit stage of vitreous sintering



Stages of firing Low bisque –rigid and porous with little shrienkage

Medium bisque –complete cohesion of powderparticles and cohesion

High bisque –shrinkage is complete &mass exhibitssmooth surface



It has been calculated that air-firedporcelain contains as much as 6.3%

voids. This not only results inundesirable roughness and pits

when a porcelain crown must beground, but also exerts an even moreundesirable effect on the strengthand optical properties of theporcelain.

Pores are caused by air trappedduring the sintering process. Air

spaces become spherical under theinfluence of surface tension andexpand with increased temperature

Fracture surface of gingivalporcelain fired under normalatmospheric



Porcelains for ceramic-metal restorationsare fired under vacuum. As the porcelainfurnace door closes, the pressure islowered to 0.1 atmosphere. The

temperature is raised until the firingtemperature is reached; the vacuum isthen released and the furnace pressurereturns to 1 atmosphere.

The increase in pressure from 0.1 to 1

atmosphere helps compress and close theresidual pores. This occurs by viscousflow at the firing temperature. The resultis a dense, relatively pore-free porcelain,

Sintering under vacuum reduces the

amount of porosity to 0.56% (from 5.6%)in air-fired dental porcelains. Vacuum-firing improves the translucency anddecreases the surface roughness of feldspathic porcelains, and increasesimpact strength approximately 50%.

Fracture surface of gingivalporcelain fired under vacuum



An alternate method uses the principle of diffusion to secureimproved density in fused porcelain. A diffusible gas such ashelium may be introduced to the furnace at low pressure duringthe sintering (densification) stage.

The helium gas (instead of air) is therefore entrapped in theinterstitial spaces, and because its molecular diameter is smallerthan the porcelain lattice, it diffuses outward under the pressureof the shrinking porcelain

Human enamel exhibits a specific optical property calledopalescence: a scattering effect that makes it appears bluish

when viewed in reflected light and orange when viewed intransmitted light. Studies of pigment particle size and dispersionhave made it possible to produce partially opalescentrestorations that compare favorably with natural teeth.



Glazing- After the porcelain is cleaned and any necessary stains are applied, it is returned to the furnace for the finalglaze firing. Usually, the glazing step is very short; when

the glazing temperature is reached, a thin glassy film(glaze) is formed by viscous flow on the porcelain surface.

Overglazing is to be avoided, because it gives therestoration an unnatural shiny appearance and causes lossof contour and shade modification.

TYPES: Over glaze

Self glaze



Cooling- It is commonly accepted that the cooling stage is acritical one in the fabrication of ceramic-metal restorations andthat extreme cooling rates should be avoided.

Too-rapid cooling of the outer layers may result in surfacecrazing or cracking; this is also called thermal shock.

Very slow cooling as well as multiple firings, might induce theformation of additional leucite and increase the overallcoefficient of thermal expansion of the ceramic, and may alsoresult in surface cracking and crazing.

Slow cooling is preferred, and is accomplished by removing thefired restoration from the furnace as soon as the firing is finishedand placing it under a glass cover to protect it from air currentsand possible contamination by dirt



PROPERTIES The properties of dental ceramics depend

on their composition, microstructure, andflaw population.

Ceramics are brittle and contain at least

two populations of flaws: fabricationdefects and surface cracks. The average natural flaw size varies from 20

to 50 pm. Usually, failure of the ceramicoriginates from the most severe flaw

Dental ceramics are subjected to repeated(cyclic) loading in a humid environment(chewing), conditions that are ideal for theextension of the pre existing defects orcracks. This phenomenon, called slowcrack growth



A sharp decrease in viscosity occursaround the glass-transition temperature,Tg. Below Tg, the glass has theproperties of a solid. Above Tg, glass

flows more readily, and vitreoussintering takes place.

Feldspathic porcelains for ceramic-metalrestorations have a mean flexuralstrength of about 70 MPa

Slip-cast ceramics exhibit the highest values (378 to 604 MPa), followed bylithiumdisilicate heat-pressed ceramics(350 Mpa)



The flexural strength of feldspathic porcelain is between 62 and90 MPa, the shear strength is 110 MPa, and the diametral tensilestrength is lower at 34 MPa. The compressive strength is about172 MPa, and the Knoop hardness is 460 kg/mm2

Fracture toughness is also an important property of ceramics; itis a measure of the energy absorbed by the material as it fails.The fracture toughness of conventional feldspathic porcelains is

very similar to that of soda lime glass (0.78 MPa . m0.5). Leucite-reinforced and micabased ceramics have a fracture toughness

about double that of soda-lime glass The modulus of elasticity is 69 GPa for feldspathic porcelain,

varies from 62 to 72 GPa for machinable ceramics, and reaches110 GPa for lithium-disilicate heatpressed ceramics.



The linear firing shrinkage of feldspathic porcelains hasbeen reported to be approximately 14% for lowfusingporcelain (ceramic-metals) and 11.5% for high-fusing

porcelain (denture teeth). Overglazed porcelain has agreater percentage of shrinkage

The density of fully sintered feldspathic porcelain is around 2.45 g'cm3 and will vary with the porosity of the material.

The thermal properties of feldspathic porcelain include aconductivity of 0.0030 cal/sec/cm2 ,a diffusivity of 0.64mm2/sec, and a linear thermal coefficient of expansion of

12.0 x 10-6/ C between 25 and 500°C



The translucency of opaque, dentin (body), and enamel(incisal) porcelains differs considerably. Opaque porcelainshave very low translucency, allowing them to mask metalsubstructure surfaces. Dentin porcelain translucency

values range between 18% and 38%. Enamel porcelains havethe highest values of translucency, ranging between 45%and 50%.

Because dental enamel is fluorescent under ultravioletlight, uranium oxide had been added to produce

fluorescence with porcelain. However, because of the lowbut detectable radioactivity of uranium, newerformulations containrare earth oxides (such as ceriumoxide) to produce fluorescence



OPTICAL PROPERTIES Fluorscence

Metamerism

opalascence



METHODS OF STRENGTHENING Introduction of residual compressive strength

Interruption of crack propagation



Introduction of residual compressive strength Ion exchange - Heating the porcelain restoration,

which has been coated with a potassium salt, in a low-temperature oven. As a result, sodium ions from the

porcelain surface are exchanged for potassium ions.Since potassium ions are about 35% larger in diameterthan sodium ions, the surface layer develops a residualcompressive stress



Thermal tampering- quenching the surface of theobject while it is in a hot and molten stage

Thermal expansion coefficient mismatch- metal which is veneered with porcelain has a highercoefficient of thermal expansion . Hence on coolingmetal shrinks more resulting in a residual compressivestrength at the surface



Interruption of crack propagation

Reinforce it with a dispersed phase of a differentmaterial that is capable of hindering a crack frompropagating through the material. It can be by:

Dispersion of a crystalline phase

Tough crystalline material like alumina is added .crackcannot penetrate the alumina particles easily

Transformation toughening Partially stabilized zirconia is added that is capable of

undergoing a change in crystal structure under stress



METAL CERAMIC RESTORATION

The cast metal coping provides a substrate on which a

ceramic coating is fused. These ceramic-metalrestorations are highly popular and are used for mostof the crown and bridge restorations made today.



REQUIREMENTS FOR A CERAMIC-

METAL SYSTEM High fusing temperature of the alloy

Low fusing temperature of the ceramic.

The ceramic must wet the alloy readily . In general, thecontact angle should be 60 degrees or less.

A good bond between the ceramic and metal isessential and is achieved by the interactions of the

ceramic with metal oxides on the surface of metal andby the roughness of the metal coping.



Adequate stiffness and strength of the alloy core

High sag resistance is essential

Compatible coefficients of thermal expansion of theceramic and metal so the ceramic does not crackduring fabrication. The system is designed so the valuefor the metal is slightly higher than for the ceramic,

thus putting the ceramic in compression (where it isstronger) during cooling



Adhesion to metals Several factors have been identified as

promoting good adhesion or bondingof a porcelain enamel to a metal

Wetting Good wetting of the porcelain on the

metal is indicated by a low contactangle of a drop of the porcelain when

fired on the solid. Good wettingpromotes penetration of the glass intosurface irregularities and, therefore, agreater area of contact.



Adherent oxide

The presence of an adherent oxide on the metal surface that is wet by the porcelain provides a beneficial transition layer.Diffusion of atoms from the metal and porcelain into this oxideusually can be detected and is cited as evidence of a chemicalbond. A non adherent oxide can lead to a weak boundary failure.

Noble metals, which are resistant to oxidizing, must have other,more easily oxidized elements added, such as indium and tin, toform surface oxides. When these more easily oxidized elements

are added, bonding is improved. The common practice of "degassing" or preoxidizing the metal

coping before ceramic application creates surface oxides thatimprove bonding.



Base-metal alloys contain elements, such as nickel,chromium, and beryllium, which form oxides easily during degassing, and care must be taken to avoid too

thick an oxide layer. Mechanical retention

The presence of surface roughness on the metal oxide

surface can result in mechanical retention on amicroscopic level, especially if undercuts are present.



Types of bond failures



CERAMICS FOR METAL CERAMIC

RESTORATION The ceramics used for porcelain-fused-to metal

restorations must fulfil five requirements:

(1) they must simulate the appearance of natural teeth,(2) they must fuse at relatively low temperatures, (3)they must have thermal expansion coefficientscompatible with the metals used for ceramic-metalbonding, (4) they must withstand the oralenvironment, and (5) they must not unduly abradeopposing teeth.



These ceramics are composed of crystalline phases in anamorphous and glassy (vitreous) matrix. They compriseprimarily SiO2, A1203, Na2O, and K2O.

Opacifiers (TiO2, ZrO2, SnO,) and various heat-stablepigments are also added to the ceramic. Because of theircomposition, they can be considered a type of glass.

To match the appearance of tooth structures, smallamounts of fluorescing pigments such as rare earth oxides(CeO,) are added

Addition of potassium oxide and the formation of a high-expansion phase called leucite (KA1Si2O6). This phaseincreased the thermal expansion of the porcelain so itcould match that of dental alloys.



Chronologically the alloys developed for ceramic-metal restorations were Au-Pt-Pd, Ni-Ci, Co-Cr, Au-Pd-Ag, Pd-Ag, Au-Pd, Pd-Cu, and Ti



TYPES OF METAL CERAMIC SYSTEMCast metal

ceramic alloys

Noble metalalloy system

Base metalalloy system

Foil coping

Bonded Ptfoil coping

Swaged goldalloy foilcoping



PREPARATION OF CERAMIC METAL

RESTORATION Surface treatment of the metal coping before ceramic application isimportant for good bonding.

These treatments are used to roughen the coping surface and formsurface oxides. The surface may be roughened by blasting with a fineabrasive (25 to 50 pm alumina).

The first layer of ceramic is especially important with ceramic-metalrestorations because it must hide the metal; special opaque ceramicmust be used .

After the opaque layer has been applied and fired the dentin (or body)ceramic, which contains less of the opaque oxides (such as SnO, andZnO2,) pigments, and fluorescing oxides, is applied and fired. Finally,once the correct contour has been established an essentially transparent glaze layer is applied and fired.

The thermal expansions must be matched and the porcelain firingtemperatures must be low enough that the alloy will not sag.



Advantages

1. High strength

2. Potential for fixed partial dentures

3. Excellent fit

Disadvantages

1. Appearance of metal margins

2. Discoloration by metal 3. Difficulty producing an appearance of translucency

4. Bond failure with metals

5. Possible Disadvantages of alloy used



ALL CERAMIC RESTORATIONS Materials for all-ceramic restorations use a wide

variety of crystalline phases as reinforcing agents andcontain up to 90% by volume of crystalline phase. The

nature, amount, and particle size distribution of thecrystalline phase directly influence the mechanicaland optical properties of the material.

The jacket crown is the traditional, accepted term forall-ceramic crowns used for restoring the entireclinical crown portion of a tooth.

Several processing techniques are available forfabricating all-ceramic crowns: sintering, heat-pressing, slip-casting, and machining



HISTORY Porcelain jacket crowns have been used widely in

dentistry since Land developed the platinum foiltechnique in 1903.

They were fabricated with high-fusing feldspathicporcelains and were known for natural estheticsresulting mainly from high translucency and thespecialized laboratory skills used.

In 1965, alumina-reinforced porcelain crowns wereintroduced. These crowns are constructed of a copingor core of a ceramic material containing 40% to 50%alumina with an outer layer of translucent porcelain



A classification of porcelain crowns

according to composition



SINTERED ALL-CERAMIC

MATERIALS Two main types of all-ceramic materials are available

for the sintering technique:

alumina-based ceramic

leucite-reinforced ceramic



Alumina-Based Ceramic Aluminous core ceramic is a typical example of strengthening by dispersion of a crystalline phase. Aluminahas a high modulus of elasticity (350 Gpa) and highfracture toughness (3.5 to 4 MPa).

Its dispersion in a glassy matrix of similar thermalexpansion coefficient leads to a significant strengtheningof the core.

Advantages 1. High strength 2. Good fit Disadvantages 1. High initial cost 2. Long processing time 3. Lack of

bonding to the tooth structure



The first aluminous core porcelains contained 40% to 50% alumina by weight. The core was baked on aplatinum foil and later veneered with matched-

expansion porcelain. Aluminous core ceramic is nowbaked directly on a refractory die.



Leucite-Reinforced Feldspathic

Porcelain A feldspathic porcelain containing up to 45% by

volume tetragonal leucite is available for thefabricating all-ceramic sintered restorations.

Leucite acts as a reinforcing phase; the greater leucitecontent (compared with conventional feldspathicporcelain for ceramic-metal applications) leads tohigher flexural strength (104 MPa) and compressive

strength

The large amount of leucite in the material alsocontributes to a high thermal contraction coefficient.



Magnesia-Based Core Porcelain A high-expansion magnesia core

material has been developed that iscompatible with the same dentinporcelains used for ceramic-metalrestorations.

The flexural strength of unglazedmagnesia core ceramic is twice ashigh (131 MPa) as that of

conventional feldspathic porcelain(70 MPa), with an averagecoefficient of expansion of 14.5 x 10-6/C.



Strengthening is achieved by dispersion of the magnesia crystals in a vitreous matrix,and also by crystallization within the matrix.

Advantages

1. Adequate strength for most anterior crowns

2. Esthetics superior to PFM for a given shadeand

3. No risk in choosing alloy

Disadvantages 1. Not used for fixed partial dentures

2. Requires learning to do good shoulder preparation



CASTABLE ALL CERAMIC SYSTEMS Castable ceramic systems are used to cast crowns by

the lost wax process.

Impressions, models, and dies are made in the usual

manner. The restoration is waxed on the die, and the wax pattern of the crown is invested in a phosphate-bonded investment following the same procedure usedfor some metal crowns.



An ingot of the ceramic material isplaced in a special crucible andmelted and cast with a motor-driven centrifugal casting machineat 1,380°C The cast crown is a clearglass that must be heat treated toform a crystalline ceramic, whichis essentially a fluorine mica

silicate. The crystallizationprocedure takes several hours in aheat-treating or "ceramming"furnace, with a final temperatureof 1,075°C



HEAT PRESSED ALL CERAMIC

SYSTEM Heat-pressing relies on the application of external

pressure to sinter and shape the ceramic at hightemperature. Heat-pressing is also called high-

temperature injection molding Heat-pressing classically helps avoid large pores and

promotes a good dispersion of the crystalline phase within the glassy matrix



Leucite-Based Leucite-based ceramics areavailable for heat-pressing. Leucite(KA1Si206 or K20 . A1203 . 4Si0,) isused as a reinforcing phase inamounts varying from 35% to 55%.

Ceramic ingots are pressedbetween 1150°and 1180°C (under apressure of 0.3 to 0.4 MPa) into

the refractory mold made by thelost-wax technique. Thistemperature is held for about 20minutes in a specially designedautomatic press furnace.



The final microstructure of these heat-pressedceramics consists of leucite crystals, 1 to 5 micrometersin size and dispersed in a glassy matrix .

Two techniques are available: a staining technique or alayering technique involving the application of veneering porcelain.



Lithium Di silicate-Based These materials contain lithium disilicate

(Li2Si205) as a major crystalline phase.They are heat-pressed in the 890 °to 920 °

C temperature range, using the sameequipment as for the leucite-basedceramics.

The heat pressed restoration is laterlayered with glasses of matching thermal

expansion. The final microstructureconsists of about 60% elongated lithiumdisilicate crystals (0.5 to 5 micrometerslong) dispersed in a glassy matrix



Advantages

good esthetics for the leucite-reinforced materials

high strength (but higher opacity) for the lithiumdisilicate-based materials

ability to use the well known lost-wax technique.

Processing times are short and margin accuracy is

within an acceptable range



SLIP-CAST ALL-CERAMIC

MATERIALS Slip-casting involves thecondensation of an aqueousporcelain slip on a refractory die.The porosity of the refractory die

helps condensation by absorbing the water from the slip by capillary action.

The piece is then fired at hightemperature on the refractory die.

The fired porous core is later glass-infiltrated, a unique process in which molten glass is drawn into thepores by capillary action at hightemperature



Alumina-Based An alumina-based slip is applied to a gypsum refractory diedesigned to shrink during firing. The alumina content of the slip is more than 90%, with a particle size between 0.5and 3.5 μm.

After firing for 4 hours at 1100• C, the porous aluminacoping is shaped and infiltrated with a lanthanum-containing glass during a second firing at 1150 • C for 4hours.

After removal of the excess glass, the restoration is

veneered using matched-expansion veneer porcelain. This processing technique is unique in dentistry and leads

to a high-strength material due to the presence of densely packed alumina particles and the reduction of porosity



Spinel- and Zirconia-Based Two modified ceramic compositions for this technique

have been recently introduced. One contains amagnesium spinel (MgAl,O,) as the major crystalline

phase with traces of alpha-alumina, which improvesthe translucency of the final restoration. The secondmaterial contains tetragonal zirconia and alumina.

The spinel-based material has a lower modulus of

rupture than the alumina based material, whereas thezirconia-based material has a reported flexuralstrength neighboring 600 MPa.



The main advantage of slip-cast ceramics is their highstrength

Disadvantages include high opacity (with the

exception of the spinel-based materials) and longprocessing times

AC A A C A C



Machinable ceramics can be milledto form inlays,onlays, and veneersusing special equipment.

One system uses CAD/CAM

(computer assisteddesign/computer assistedmachining) technology to producerestorations in one office visit. Afterthe tooth is prepared, the

preparation is optically scannedand the image is computerized. Therestoration is designed with the aidof a computer.

MACHINABLE ALL-CERAMICMATERIALS



The restoration is then machined from ceramic blocksby a computer-controlled milling machine. Themilling process takes only a few minutes.

Advantage

Convenient

Disadvantage

very expensive marginal accuracy is poor, with values of 100 to 150 pm



A more recent system involves an industrialCAD/CAM process to produce crowns. The die ismechanically scanned by the technician and the data

is sent to a workstation where an enlarged die is milledusing a computer-controlled milling machine. Thisenlargement is necessary to compensate for thesintering shrinkage.



Another system for machining ceramics is to forminlays, onlays, and veneers using copy milling.

In this system, a hard resin pattern is made on a

traditional stone die. This handmade pattern is thencopied and machined from a ceramic block using apantographic device similar in principle to those usedfor duplicating house keys.

Marginal accuracy is a concern and there are highequipment costs.



Machinable ceramics potassium feldspar as a major crystalline phase

dispersed in a glassy matrix

Mica-based glass-ceramics are also available with this

system. Their microstructure consists of mica crystals(50% to 70% by volume) dispersed in a glassy matrix.

Pre-sintered slip-cast alumina blocks can be machinedusing the copy-milling system



PORCELAIN DENTURE TEETH Porcelain has been used for denture teeth since 1790

They have been used for complete dentures. Theanterior teeth have one or two gold-covered pins to

provide retention to the denture base. The posteriorteeth have diatoric holes located centrally in theunderside of the teeth for retention to the denturebase.



The ceramic composition of porcelain denture teethbelongs to the triaxial porcelain compositions (quartz,clay, kaolin).

The teeth are made in split molds, fired under vacuumand slowly cooled to prevent crazing



Plastic Teeth Porcelain Teeth High resilience Very brittle Tough Friable Soft-low abrasion resistance Hard-high abrasion resistance Insoluble in mouth fluids-some dimensional Inert in mouth fluids-no dimensional

change• Low heat-distortion temperature High heat-distortion temperature

Bond to denture base plastic Poor bond-to-denture base plastic Natural appearance Natural appearance Natural feel-silent Possible clicking sound in use Easy to grind and polish Grinding removes surface glaze

Crazing and blanching-if non-cross linked Occasional cracking



Advantages 1. Excellent biocompatibility 2. Natural appearance 3. High resistance to wear and distortion Disadvantages 1. Brittle 2. No bond to acrylic denture bases; requires mechanical

attachments 3. Produces clicking sound on contact 4. Cannot be polished easily after grinding 5. Higher density increases weight of teeth 6. Mismatch in coefficient of thermal expansion produces

stresses in acrylic denture base

Th ff f l i l d hi bl i h



The effect of multicolored machinable ceramics on the

esthetics of all-ceramic crowns (J Prosthet Dent

2002;88:44-9.) More esthetic restorations were achieved with single-

shade block systems than with multishade block systems. Individually stained restorations received the

most consistently high scores for esthetics and color match. Given the individual characteristics of adjacentteeth, individual staining is recommended

Shear bond strength of resin cements to both



Shear bond strength of resin cements to both

ceramic and dentin (J Prosthet Dent 2002;

88:277-84. The results of this in vitro study suggest that when the

tested ceramic restoration is cemented with a resincement system, the ceramic should be etched with

hydrofluoric acid, and silane should be applied prior tocementation. The results also suggest that an auto- or light-polymerizing dentin bonding agent should beconsidered instead of a dual-polymerizing agent



Influence of tab and disk design on shade

matching of dental porcelain Within the limitations of this study, there was no

significant difference in shade-matching accuracy between the 2 shapes, although the order of design

matching resulted in a difference in shade-matchingability. When tabs were matched first and diskssecond, improved matching was evident on the secondtest. The reverse was not true; no learning was

demonstrated when the tabs were matched after thedisks. (J Prosthet Dent 2002;88:591-7.)



Wetting characteristic of ceramic to water and

adhesive resin (J Prosthet Dent 2002;88:616-21.) surface roughening of the 3 ceramics tested, either by

etching alone or a combination of airborne particleabrasion and etching was advantageous for increasing

the surface area for bonding of chipped ceramicmaterials with resin-based composite. These processescreated numerous desirable small and uniformirregularities within the ceramic surfaces

Bond degradation behavior of self adhesive



cement and conventional resin cements

bonded to silanized ceramic Although G-CEM has higher water sorption and

solubility than conventional resin cements, it may be apractical alternative to conventional resin cement for

long-term bonding to silanized ceramic due to itsdifferent behavior with respect to bond degradation

(J Prosthet Dent 2011;105:177-184)

The effect of slurry preparation methods



on biaxial flexural strength of dental

porcelain Ceramic slurry preparation may be a relevant factor in

the performance of dental ceramics. A small variationin the relative humidity of greenstate discs after de-

molding, results in a decrease in Weibull modulus andcharacteristic strength for the one-step mixingmethod, which could be related to the presence of increased amounts of amorphous phase.

(J Prosthet Dent 2011;105:308-314)

Effect of zirconia surface treatments on



the shear bond strength of veneering

ceramic This study demonstrated that application of a liner

increased the possibility of interfacial failure of veneering ceramic at the zirconia core, and that

airborne-particle abrasion may be more useful forincreasing bond strength than liner application.

(J Prosthet Dent 2011;105:315-322)



Spectroradiometric and spectrophotometric

translucency of ceramic materials Translucency differences should be considered when

selecting a ceramic material for clinical use. This study found that the slip-cast ceramic was the most opaque,

followed by zirconia, with feldspathic and heat pressedthe most translucent

(J Prosthet Dent 2010;104:239-246

The effect of acidic agents on surface



ion leaching and surface characteristics

of dental porcelains For restoration of the affected teeth in patients who

consume acidic food and drinks, leaching of ions fromporcelains should be considered.

In terms of the porcelains evaluated, fluorapatiteporcelain may be the most suitable for restorations inthese patients.

(J Prosthet Dent 2010;103:148-162)

In vitro staining effects of stannous



fluoride and sodium fluoride on

ceramic material To minimize the deteriorating effects of long-term

fluoride treatment on dental porcelain, polishing of the ceramic surface before fluoride application is

recommended. In addition, daily application of 0.4%SnF2 could be alternated with courses of 1.1% NaF, which appears to be less damaging

(J Prosthet Dent 2010;103:163-169)



CONCLUSION Ceramics are probably the best material available formatching the esthetics of a complex human tooth.They are used for ceramic-metal crowns, fixed partial

dentures, all-ceramic restorations, and to fabricatedenture teeth. However, ceramics are brittle and fragilein tension, and the quality of the final product is very technique sensitive, in terms of strength and esthetics

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