CONTENTS

1) INTRODUCTION

2) TERMINOLOGIES

3) HISTORICAL PERSPECTIVE

4) CLASSIFICATION

5) COMPOSITION

6) PROPERTIES OF DENTAL PORCELAIN

7) CONDENSATION OF DENTAL PORCELAIN

8) METHODS OF STRENGTHENING

9) AESTHETICS IN PORCELAIN

10) ALL CERAMIC SYSTEMS

11) METAL CERAMIC SYSTEMS

12) PORCELAIN INLAYS

13) PORCELAIN LAMINATE VENEERS

14) ADVANCES IN DENTAL CERAMICS

15) CONCLUSION

16) REVIEW OF LITERATURE

17) REFERANCES/BIBLIOGRAPHY

1

INTRODUCTIONThe search for excellence in restorative dentistry is a never ending

endeavor.Esthetics in contemporary dentistry is partly defined by

patient’s desire for naturality and beauty .The development of ceramic

materials has helped the dentist to translate the patient’s wishes to

reality by providing the ideal restoration.

The word ‘ceramic’ is derived from the Greek word ‘keramos’ which

means ‘pottery’ or ‘burnt stuff’. A ceramic is therefore an earthy

material usually of a silicate nature and may be defined as “a

combination of one or more metals with a non- metallic element,

usually oxygen” (Gilman, 1967).

All porcelains and glass ceramics are ceramics, but not all ceramics are

porcelains or glass ceramics.

HISTORICAL PERSPECTIVE The earliest evidence of fabrication of ceramic articles dates back to

23,000yrs B.C.Historically, three types of ceramic materials were

developed:

1) Earthenware: is relatively porous, fired at low temperatures.

2) Stoneware: appeared in china in about 100 B.C, fired at higher

temperature than earthenware, has higher strength and is

impervious to water.

3) Porcelain: white translucent stoneware, obtained by fluxing

white china clay with “chine stone”, was developed in china in

1000 A.D., stronger than earthenware and stone ware.

The originator of the first porcelain paste used for denture work was a

French apothecary, Alexis Duchateau. His early dentures were ill-2

fitting because of the uncontrolled firing shrinkage. It was not until he

teamed up with the Parisian dentist, Nicolas Dubois de Chement, that

the two were able to construct complete dentures from a material they

referred to as ‘Mineral paste”. The first single porcelain teeth were

launched in 1808 by an Italian dentist,Giuseppangelo Fonzi and were

called “terrometallic teeth”. The idea of fusing porcelain to a thin

platinum foil is credited to Charles.H.Land.

TERMINOLOGIES COMMONLY USED:

ALUMINOUS PORCELAIN: A ceramic composed of a glass

matrix phase and 35% vol or more of AL2O3.

CAD-CAM CERAMIC: A machinable ceramic material

formulated for the production of inlays and crowns through the

use of a computer-aided design, computer aided machining

process.

CASTABLE DENTAL CERAMIC: A dental ceramic specially

formulated to be cast using a lost wax process/technique..

COPY MILLING: A process of machining a structure using a

device that traces the surface of a master metal, ceramic or

polymer pattern and transfers the traced spatial positions to a

cutting station where a blank is cut or ground in a manner

similar to a key-cutting procedure.

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DEVITRIFICATION: Occurs when there is excessive disruption

of the glass forming SiO4 tetrahedra in dental porcelain resulting

in the crystallization of glass or “devitrification”. Is often

associated with high expansion glasses due to increased

introduction of alkali’s (soda-Na2O) to increase the thermal

expansion. Increase in sodium and potassium ions can cause too

much disruption of the SiO4 tetrhedra and subject to

devitrification, which appears as cloudiness in the porcelain,

accentuated by repeated firings. Once devitrified, it is

increasingly difficult to form a glaze surface on the porcelain.

The regular or aluminous porcelain is less susceptible to

devitrification due to their higher silica to alkali ratio.

Consequently since the soda (Na2O) content is less, there

thermal expansion is also lower.

FELDSPATHIC PORCELAIN: A ceramic composed of a glass

matrix phase and one or more crystal phases of which the more

important phase is leucite, which is used to create high

expansion porcelain that is thermally compatible with gold-

based, palladium-based and nickel-based alloys. Hence this class

of dental ceramics are also called “leucite porcelain”.

FRITTING: The process of blending, melting and quenching the

glass components is termed “fritting”. The term “frit” is used to

describe the finer glass product. The raw mineral powders are

mixed together in a refractory crucible and heated to a

temperature well above their ultimate maturing temperature. The

oxides melt together producing gases, which are allowed to

escape and the melt, is then quenched in water. The red-hot glass

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striking the cold water immediately breaks up into fragments

and this is termed the “frit”.

GLASS CERAMIC: A solid consisting of a glassy matrix and

one or more crystal phases produced by the controlled

nucleation and growth of crystals in the glass.

INCONGRUENT MELTING: Is the process by which one

material melts to form a liquid plus a different crystalline

material.

SINTERING: A process of heating closely packed particles to

achieve interparticle bonding and sufficient diffusion to decrease

the surface area or increase the density of the structure.

VITRIFICATION: Is the development of a liquid phase by

reaction or melting, which on cooling provides the glassy phase.

The structure is termed “vitreous”.

CLASSIFICATION

According to type: -

Feldspathic porcelain

Leucite-reinforced porcelain

Aluminous porcelain

Glass infiltrated alumina

Glass infiltrated spinel

Glass ceramic

5

According to use:

Denture teeth

Metal ceramic restorations

Veneers

Inlays/Onlays

Crowns and anterior bridges

According to method of processing:

Sintering

Casting

Machining

According to firing temperature:

High fusing -1300oC(2372oF)

Medium fusing-1101-1300oc(2013-2072oF)

Low fusing-850-1100oc(1562-2012oF)

Ultra low fusing-<850oc(1562oF)

The medium fusing and high fusing types are used for the

production of denture teeth. The low fusing and ultra low fusing

porcelains are used for crown and bridge construction.

According to application:

Core porcelain: is the basis of porcelain jacket crown, must have

good mechanical properties.

Dentin or Body porcelain: more translucent than core porcelain,

largely governs the shape and color of restoration.

Enamel porcelain: is used in areas requiring maximum

translucency, for example- at the incisal edge.

6

According to Phillips’ science of dental materials:

Silicate ceramics - Dental porcelains with SiO2 (amorphous glass

phase) as the main component and additions of crystalline Al2O3,

MgO, ZrO2 and other oxides are included in this category.

Oxide ceramics - contains a principal crystalline phase of Al2O3

MgO, ThO2 or ZrO2 with either no or a small amount of glass

phase.

Non-oxide ceramics - are impractical for use in dentistry due to

their high processing temperatures and complex processing

methods. Eg: Borides, Carbides, Nitrides

Glass ceramics- Dicor, Dicor MGC

COMPOSITION

The main ingredients are:

Feldspar

Silica (Quartz or Flint)

Kaolin (clay)

Coloring pigments

Opacifiers

Stains

Glass modifiers

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FELDSPAR:

Is a precursor of common clay. Feldspar makes up the bulk of

dental porcelains and gives a translucent quality. Potassium feldspar

and sodium feldspar are naturally occurring materials composed of

potash (K2O), soda (Na2O), alumina (Al2O3) and silica (SiO2).

Potash feldspar is used in most dental porcelains, due to its

increased resistance to pyroplastic flow and increased viscosity,

while sodium feldspar lacks the true translucent quality associated

with potash feldspar. When potassium feldspar is mixed with

various metal oxides and fired at high temperatures, it can form

leucite and a glass phase that will soften and flow slightly. The

softening of this glass phase during porcelain firing, allows the

porcelain powder particles to coalesce together. This is called

“liquid- phase sintering”.

Another important property of feldspar is its tendency to form the

crystalline mineral leucite (potassium aluminium silicate) with a

large co-efficient of thermal expansion (20-25x10-6/oc) compared

with feldspar glasses (10x10-6/oc). The tendency of feldspar to form

crystals of leucite in liquid glass during “Incongruent melting”

when heated at temperatures between 1150oc and 1530oc is used to

advantage for metal bonding in the manufacture of dental

porcelains.

SILICA:

used in dental porcelain as a strengthner.

Silica is a high fusing material, remains unchanged at temperature

normally used in firing porcelain and this contributes stability to the

8

mass during heating by providing a framework around which the other

ingredients can flow. It prevents the crown from slumping during the

liquid phase. The high melting temperature of fused silica is attributed

to the three-dimensional network of covalent bonds between silica

tetrahedra, which are the basic structural units of the glass network.

The stability of the glass is dependant on the silicon -oxygen lattice

and decreased reduction of covalent bonds, if otherwise, results in

problems of hydrolytic stability and devitrification.

KAOLIN :(Al2O3.2SiO2.2H2O)

It serves as a binder. Kaolin gives porcelain its property of opaqueness.

When mixed with water, it forms a sticky mass, allowing the unfired

porcelain to be easily worked and molded. When subjected to high

heat during firing, it adheres to the framework of quartz particles and

shrinks considerably.

COLORING PIGMENTS:-

The coloring pigments added to the porcelain mixture are called “color

frits”. These powders are added in small quantities to obtain the

delicate shades necessary to imitate the natural teeth.

They are prepared by grinding together metallic oxides with fine glass

and feldspar, fusing the mixture in a furnace and regrinding to a

powder. These powders are blended with the unpigmented powdered

frit to provide the proper hue & chroma.

The color pigments used in dental porcelain consists of the following:-

1) Titanium oxide → Yellow - Brown Shade

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2) Indium → Yellow / Ivory

Indium or praseodymium (Lemon) is the most stable pigment for

producing an ivory shade. Vanadium - Zirconium or Tin oxide

plus chromium can be used, but are not so stable.

3) Iron oxide / Nickel oxide → Brown

4) Cobalt salt → Blue – particularly useful for producing some of

the enamel shades.

5) Copper or Chromium Oxide → Green

6) Chromium - tin or chrome alumina → pink

7) Iron oxide (black) or platinum grey → Grey

8) Manganese oxide → Lavender

OPACIFYING AGENTS:-

The addition of concentrated color frits to dental porcelain is

insufficient to produce a life- like tooth effect since the translucency of

the porcelain is too high.

Dental porcelain materials having varying degrees of translucency can

be manufactured by the addition of opaque materials.

An opacifying agent generally consists of a metal oxide (between 8%

& 15%) ground to a very fine particle size (<5µm) to prevent a

speckled appearance in porcelain.

commonest oxides used are:-

a) Zirconium Oxide (Refractive Index – 2.2)

b) Cerium Oxide

c) Titanium Oxide (RI – 2.52)

d) Tin oxide (RI – 2.0)

e) Zircon Oxide

10

STAINS AND COLOR MODIFIERS:-

Stains are generally low fusing colored porcelains used as

surface colorants or to provide/ imitate markings like enamel check

lines, decalcification spots, fluoresced areas etc. Color modifiers are

less concentrated than stains, used to obtain gingival effects or

highlight body colors, and are best used at the same temperature as

dental porcelain.

Stains in finely powdered form are mixed with water/ glycerin or any

other special liquid & the wet mix is applied with a brush either on the

surface of porcelain before glazing or built into the porcelain (internal

staining).

GLASS MODIFIERS:-

Potassium, sodium and calcium oxide are the most commonly used

glass modifiers and act as fluxes by interrupting the integrity of sio4

network.

The sintering temperature of crystalline silica is too high for use in

veneering esthetic layers onto dental casting alloys. At such

temperatures, the alloys would melt. In addition, the thermal

contraction co-efficient of crystalline silica is too low for these alloys.

Bonds between the silica tetrahedra can be broken by the addition of

alkali metal ions such as sodium, potassium and calcium. These ions

are associated with the oxygen atoms at the corners of the tetrahedra

and interrupt the oxygen - silicon bonds. As a result, the three-

dimensional silica network contains many linear chains of silica

11

tetrahedra that are able to move more easily at lower temperatures than

the atoms that are locked into the three-dimensional structure of silica

tetrahedra. This ease of movement is responsible for the increased

fluidity (decreased viscosity), lower softening temperatures and

increased thermal expansion conferred by glass modifiers.

Too high a modifier concentration, reduces the chemical

durability of the glass (Resistance to attack by H2o, acids and alkali).

In addition, if too many tetrahedra are disrupted, the glass may

crystallize (devitrify)

during porcelain firing operations. Hence, a balance between a suitable

melting range and good chemical durability must be maintained.

GLAZES AND ADD-ON PORCELAIN:-

The main purpose of glaze is to seal the open pores in the surface of

fired porcelain. Dental glazes consist of colorless low-fusing porcelain,

which can be applied to the surface of a fixed crown to produce a

glossy surface.

A glaze should normally mature at temperature below that of the

restoration, and the thermal expansion of the glaze should be

fractionally lower than the ceramic body to which it is applied. In this

way, the glaze surface is placed under compression and crazing or

peeling of the surface is avoided.

Glazing can be of two types: - Self-glazing (auto glazing) and add-on

glazing.

In self glazing procedure, an external glaze layer is not applied, but the

completed restoration itself is subjected to glazing.

In add-on glazing, an external glaze layer is applied on the surface.

12

Add-on porcelains are generally similar to glaze porcelains except for

the addition of opacifiers and coloring pigments. The glazing

temperature is reduced by fined grinding of the powder. Add-on

porcelains are used for simple corrections of tooth contour or contact

points.

Disadvantage of glazes:-

- Glazes are difficult to apply evenly and are often used to seal

off a poorly baked restoration.

- Detailed surface characterization is almost impossible to obtain

when separate glazes are used as they tend to produce too high a

gloss or a rough surface.

Properties:-

1) Compressive strength → 50,000 psi

2) Tensile strength → 5,000 psi

3) Shear strength →16,000 psi

4) Elastic modulus → 10X106 psi

5) Knoop hardness number → 460

6) Linear co-efficient of Thermal expansion →12X10-6/0C

7) Specific gravity →2.2-2.3

8) Thermal conductivity →0.00500C/cm

9) Linear shrinkage → high fusing- 11.5%

Low fusing- 14.0%

10) Refractive index → 1.52-1.54

13

CONDENSATION OF DENTAL PORCELAIN

Porcelain powder is built to shape using a liquid binder to hold the

particles together. The process of packing the particles together and of

removing the liquid binder is known as condensation.

- Distilled water is the most commonly used liquid binder; additions

of glycerine, propylene glycol or alcohol have also been tried.

- Propylene glycol is used in the alumina core build up. Alcohol or

formaldehyde based liquids are used for opaque or core build up,

paint on liquid for stain application.

- Starch can also be incorporated in the powder but is generally

confined for use in the coarser air-fired powders.

- The main driving force involved in condensing dental porcelain, is

surface tension. The withdrawal of water from the porcelain powder

will cause the powder particles to pack more closely.

- Therefore during build-up, the porcelain should be kept moist. High

room temperatures and dry atmospheres are to be avoided since

porcelain powder can rapidly dry out and further placement of even

wet porcelain powder on a dry surface will not allow the

undersurface to be condensed properly due to air spaces being

created in the powder bed which resists further ingress of water.

14

METHODS OF CONDENSATION:-

1) Wet brush technique/ Brush additive technique :

The consistency of the mix has a marked effect on the handling

properties of porcelain. The mix should be creamy and capable of

being transferred in small increments. The reasons for favoring the

wet brush technique are:-

a) A wet brush maintains the moisture content in the

porcelain. Metal spatulas causes more rapid drying out.

b) The brush can be used to introduce enamel colors, effect

masses or stains without changing instruments.

c) Greater control over application of small increments of

porcelain.

d) Blending of enamel veneers can be achieved with greater

delicacy.

Metal instruments such as spatulas or Le cron carver can be used

but many favor the use of high quality sable hair brushes as it is

possible to rapidly transfer small increments of wet porcelain to the

metal substructure using the fine point of a sable brush.

2) Brush application method:-

In this method, dry powder is sprinkled over the wet porcelain

surface. It enhances the risk of porcelain drying out and also the

control of powder is very difficult and time consuming.

Hence this method is not recommended.

15

2) Vibration:-

Done with the serrated handle of a porcelain instrument which is

lightly passed over the model or die pin. The particles of porcelain

powder become suspended in the excess liquid and are so oriented

that when the liquid is blotted away, the surface tension of the

liquid pulls these particles into closer proximity; the irregularity of

one particle thus fits into spaces between others. The more varied

the particle size, the greater the number of times vibration can be

employed with a visible amount of liquid removal.

4) Spatulation;-

Makes use of a porcelain carver, the wet porcelain is rubbed and

patted, bringing the liquid to the surface to be absorbed.

But there is danger of dislodging the porcelain particles during

manipulation which could cause invisible cracks, and also the sand

paper like effect that the porcelain particles have on the metal, could

remove traces of metal from the instrument and incorporate them

into the porcelain resulting in discoloration in the final porcelain

product.

5) Whipping: -

Sable hair brush is used with a light dusting action / whipping

motion, producing a gentle vibration. This method works best with

porcelain of fine grain but excessive manipulation could allow these

fine particles to float away with liquid during blotting.

16

6) Mechanical :-

A brush or spatula is attached to a vibrator and is used to agitate the

porcelain particles gently, packing them together while forcing the

liquid to surface. This device is used to transport the porcelain to the

casting or matrix while condensing the particles with maximum

efficiency.

7) Ultrasonic vibration :-

The built up restoration is placed on the vibrator. The low amplitude

(very little agitation) along with a high rate of vibrations per second

pulls the liquid to the surface with almost no disturbance to the

porcelain contour. This is a final condensing procedure used only after

the porcelain has been well condensed and contoured.

SINTERING/FIRING PROCEDURE OF DENTAL

PORCELAIN:-

Different media can employed for firing procedures:-

1) AIR FIRING PROCEDURE /AIR FIRING PORCELAIN:-

When the porcelain enters the hot zone of furnace, each grain will be

contacting its neighbor. The grains of porcelain will ‘lense’ at their

contact points and weld together once the softening point of glass is

reached.

Surface tension will cause some of this porosity to be swept out via the

grain boundaries, but if air firing techniques are used, a point is

reached in all vitrified products where flow of the ceramic / glass 17

grains around the air spaces traps the air remaining in the ceramic

body. With rising temperature, the spaces containing entrapped air

become sphere- shaped due to surface tension. Higher temperatures

increases pressure of entrapped air and the bubbles enlarge to reach

pressure equilibrium with the outside atmosphere. Cooling decreases

pressure in the bubbles and their size decreases, to reach equilibrium.

The surface of air-fired porcelain is generally free of bubbles as the

interstitial air nearest the actual surface cannot be trapped and escapes

easily, but internally air bubbles remain entrapped and cannot escape

due to the rapid firing techniques employed.

A slow maturation period should be employed to allow the maximum

amount of entrapped air bubbles to escape. During this heating, the

porcelain must not be raised to its full maturing temperature, but kept

300-500c below the maximum firing temperature recommended by the

manufacturer, such a temperature will mature the porcelain without

causing loss of color and high densities will be achieved.

2) VACUUM FIRING PROCEDURE:-

Vacuum firing porcelain was introduced primarily to give improved

esthetics in enamel porcelains, also for ease of handling and production

of dense surfaces.

Densification of porcelain by vacuum firing has been described by

vines et al (1958):-

Most of the air is removed from interstitial spaces before sealing of the

surface occurs and enables porcelain to shrink into a dense, pore-free

mass without any hindrance.

18

The little air that remains entrapped becomes sphere shaped under the

influence of surface tension and increased furnace temperature. When

air at normal atmospheric pressure is allowed to enter furnace muffle,

it exerts a strong compression effect on surface skin and hydraulically

compresses the low pressure internal bubbles resulting in a relatively

dense, pore-free porcelain.

Due to rapid action of vacuum sintering, the firing schedule for these

porcelains can be reduced in comparison with the longer period

recommended for air-fired porcelain.

Factors to be considered while firing porcelain in vacuum are:-

1) Porcelain powder must be dried slowly to eliminate all water vapor

and vacuum must be applied before placement of porcelain in hot zone

of furnace to reduce internal pores before the surface skin seals off the

interior too rapidly.

2) Vacuum firing should not be prolonged after porcelain maturation

and sealing of surface skin, which otherwise causes surface blistering

due to the rise of residual air bubbles to surface through molten

porcelain. Firing at too high a temperature can cause bloating or

swelling.

3) The vacuum should be broken whilst the work is still in the hot zone

of the furnace. This allows the dense surface skin of porcelain to

hydraulically compress the low pressure internal air bubbles.

Vacuum firing also has its limitations. If large bubbles are trapped in

the porcelain by poor condensation techniques, these bubbles cannot

be reduced in size to any significant degree.

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3) DIFFUSIBLE GAS FIRING PROCEDURE:-

High densities in dental porcelain can be produced by substitution of

a diffusible gas for the ordinary furnace atmosphere.

Air is driven out of the porcelain bed and replaced by diffusible gas

like helium, hydrogen or steam. With these gases, the interstitial spaces

do not enlarge under the influence of increasing temperature, but

decreases in size or disappear as these gases diffuse outward through

the porcelain or actually dissolve in porcelain ( vines et al; 1958).

STAGES OF MATURITY

The common terminology used for describing the surface

appearance of un-glazed porcelain is ‘bisque’.

a) Low bisque:- The surface of porcelain is porous and will easily

absorb a water soluble die. Grains of porcelain begin to soften and

‘lense’ at their contact points. Shrinkage is minimal and the fired

body is extremely weak and friable.

b) Medium bisque: - Surface of porcelain still porous. Flow of glass

grains increased and any entrapped furnace atmosphere that has not

escaped via grain boundaries will be trapped and become sphere

shaped. Definite shrinking takes place.

c) High bisque: - Surface of porcelain completely sealed and presents

a smoother surface. In non-feldspathic porcelain, a slight shine

appears at this stage. The fired body is strong and any corrections

by grinding can be made at this stage prior to final glazing.

20

Additions of porcelain can be made at any of the bisque firing

stages, but is usually done at either medium or high bisque stage.

METHODS OF STRENGTHENING DENTAL

PORCELAINMethods used to overcome the deficiencies of ceramics like brittleness,

low fracture toughness and low tensile strength, fall into two general

categories:-

A) Strengthening the material per se.

B) Methods of designing components to minimize stress

concentrations and tensile stresses.

Surface flaws are of particular importance in determining the strength

of ceramics as the maximum tensile stresses created by bending forces

in the oral environment occur at the surface of a restoration or

prosthesis. The removal or reduction in the size and number of surface

flaws can produce a large increase in strength.

A) STRENGTHENING OF THE BRITTLE MATERIAL→

occurs through following mechanisms:

1) Development of residual compressive stresses within the

surface of the material.

2) Interruption of crack propagation through the material.

1) Development of residual compressive stresses:-

21

- Introduction of residual compressive stresses within the surface

of the object is one of the most widely used methods of

strengthening glasses and ceramics.

- The residual stresses must first be negated by developing tensile

stresses before any net tensile stress develop.

Residual stresses can be introduced by following methods:-

1a) Ion exchange/ chemical tempering: Is a process involving the

exchange of larger potassium ions for the smaller sodium ions, a

common constituent of a variety of glasses.

- When sodium – containing glass article is placed in a bath of

molten potassium nitrate, potassium ions in the bath exchange

places with some of the sodium ions in the surface of glass

articles.

- Potassium ions are 35% larger than sodium ions and the

squeezing of k+ ions into the place formerly occupied by sodium

ion creates large residual compressive stresses (700 mpa) in the

surface of glasses which produces pronounced strengthening

effect.

- This process is best used on the internal surface of a crown,

veneer or inlay as it is protected from grinding and exposure to

acids.

- All ceramics are not amenable to ion-exchange. Porcelains

highly enriched with potash feldspar (alumina core materials,

dicor glass ceramic, conventional feldspathic porcelains) cannot

be sufficiently ion exchanged with K+.

22

1b) Thermal tempering: - is the most common method employed.

- It creates residual surface compressive stresses by rapid

cooling/quenching of the surface of the object while it is in

molten/softened state.

- This rapid cooling produces a skin of rigid glass surrounding a

soft molten core and as the molten core solidifies, it tends to

shrink, but the outer skin remains rigid, the pull of the

solidifying molten core due to shrinkage creates residual tensile

stresses in the core and residual compressive stresses within the

outer surface.

- Hot glass-phase ceramics are quenched in silicone oil or other

special liquids to uniformly cool the surface.

1c) Thermal compatibility:-

- The porcelain should be under slight compression in the final

restoration which is accomplished by selecting an alloy that

contracts slightly more than porcelain on cooling to room

temperature.

- The fabrication of glass or ceramic articles involves forming or

processing at high temperature and the process of cooling to

room temperature affords the opportunity to take advantage of

potential mismatches in co-efficient of thermal contraction of

adjacent material in the ceramic structure.

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- In metal ceramic restorations, the metal and porcelain should be

selected with a slight mismatch in their thermal contraction co-

efficients (Metal thermal expansion co-efficient is larger), so

that the metal contracts slightly more than porcelain on cooling

from firing temperature to room temperature. This mismatch

leaves the porcelain in residual compression and provides

additional strength for the restoration.

2) Interruption of Crack propagation through the material:

Another method to reinforce ceramics is by addition of different

material that is capable of hindering a crack from propagating through

the material.

Two different methods are:-

a) Dispersion of a crystalline phase :-

- Disruption of crack propagation by use of a dispersed crystalline

phase requires a close match between the thermal contraction

co-efficient of the crystalline material and surrounding glass

matrix.

- In aluminous porcelains, a tough crystalline material such as

alumina in particulate form is added to glass. The glass is

toughened and strengthened as the crack cannot penetrate the

alumina particles as easily as it can through glass.

- In Dicor glass ceramics, the cast glass is subjected to heat

treatment, that causes micron-sized mica crystals to grow in the

glass and when restorations are subjected to high tensile stresses,

24

these microscopic crystals will disrupt crack propagation, thus

strengthening the crown.

b) Transformation toughening:-

This technique involves the incorporation of a crystalline

material, capable of undergoing a change in crystal structure

when placed under stress.

- Partially stabilized zirconia (PSZ) is the crystalline material

usually used and the energy required for transformation of PSZ

is taken from the energy that allows crack to propagate.

- Draw back of PSZ: - Its index of refraction is higher than that of

the surrounding glass matrix and the PSZ particles scatter light

as it passes through the bulk of porcelain, producing an

opacifying effect which may not be aesthetic in most dental

restorations.

25

METAL FREE CERAMIC SYSTEMS/ ALL CERAMIC

RESTORATIONSPorcelain is the most natural appearing synthetic replacement material

for the missing tooth structure, but due to its low tensile strength and

brittleness, it has to be fused to a metal substrate to increase its

resistance to fracture. Before 1980’s, the choice of dental ceramic was

restricted to porcelain powders, which could be sintered onto metal

substructures to form relatively strong metal-bonded crowns and

bridges.

But this metal base can affect the esthetics of porcelain by decreasing

the light transmission through the porcelain and by creating metal ion

discolorations.

In addition, some patients have allergic reaction/sensitivity to various

metals. These drawbacks have prompted the development of new all-

ceramic systems that do not require metal, yet have the high strength

and precision fit close to ceramometal systems.

CLASSIFICATION OF ALL-CERAMIC SYSTEMS

1) Conventional powder- slurry ceramics:-

* Hi ceram – Alumina reinforced porcelain.

* Optec HSP – Leucite reinforced porcelain.

* Duceram LFC – Hydrothermal low fusing ceramic.

26

2) Non-cored systems based on sintering of dental ceramics:-

* Mairage

* Fortess

3) Computer - aided design/computer-aided milling (CAD/CAM):-

* Cerec

4) CAD/CAM with aluminium oxide coping:-

* Procera

5) Castable ceramics:-

* Dicor

6) Infiltrated ceramics:-

* In-Ceram

7) Pressable ceramics :-

* IPS empress

* Optec pressable ceramic

8) Machinable ceramics:-

* Cerec vitablocks

* Celay blocks

* Dicor MGC

The newer types of all –ceramic restorations have lower incidence of

clinical fracture for three important reasons:

1) The newer all-ceramic restorations are made up of stronger

materials and involve better fabricating techniques.

2) Most of the all-ceramic restorations can be etched and bonded to

the underlying tooth structure with newer dentin adhesives.

27

3) Greater tooth reduction than what was previously used for PJC’s

is carried out, that provides the lab technicians with enough

room to create thicker and stronger restorations.

I) CONVENTIONAL POWDER-SLURRY CERAMICS:-Sintered ceramics are normally based on potassium (K2O.Al2O3.6SiO2)

and /or sodium feldspar and quartz which are produced by melting a

number of minerals at high temperature (1200-1250oc).After cooling,

the mass is ground to powder of various shades and

translucencies .The technician adds water to the powder to produce a

slurry, which is built up in layers on a model to form a restoration and

heated whereby the surface of the powder particles melt and the

particles sinter together.

OPTEC HSP:-

- Leucite reinforced porcelain

- It was developed by Jeneric Inc./USA, sometime before it was

established in the German market by Keppeler and Wohr.

- Optec ceramic is a feldspathic composition glass filled with

crystalline leucite that is condensed and sintered like aluminous

porcelain and traditional feldspathic porcelain.

- The leucite concentration in optec was reported as 50.6%wt and

was appreciably greater than IPS Empress ceramic (23.6% or

41.3wt %).The manufacturer disperses the leucite crystals in a

glassy matrix by controlling their nucleation and crystal growth

during the initial production of the porcelain powder. Due to its 28

increased strength, optec HSPdoes not re quire a core when used

to fabricate all-ceramic restorations, as is necessary with

aluminous PJC’S

- The leucite and glass components are fused together during the

baking process at 10200c. The build - up, condensation and

contouring of the crown is accomplished using the powder slurry

technique on a special semi-permeable die material.

- The leucite porcelains can be used for both the body and incisal

portions as the esthetics provided by leucite crystals does not

necessitate the use of translucent porcelain. Surface stains or

pigments can be applied to give the desired shade and

translucency.

- To reduce mismatch between co-efficient of thermal expansion,

potassium ions in leucite have been exchanged for rubidium or

cesium ions.

- Sandblasting is generally recommended to achieve bonding with

resin cement.

Uses:-

- Inlays

- Onlays

- Crowns for low-stress areas

- Veneers

29

Advantages:-

a) It has a moderately opaque core compared with a metal or

aluminous core as it is more translucent than alumina core

crowns.

b) Good flexural strength – 146Mpa.

c) Does not require special processing equipment beyond what is

used for ceramo-metal restorations.

d) Lack of metal or opaque substructure.

e) Can be etched to allow optimum bonding to dentin or enamel.

f) Restorations fit accurately.

Disadvantages:-

a) Increased leucite content contributes to the relatively high

invitro wear of opposing teeth (as reported in recent study).

b) Potential marginal inaccuracy caused by porcelain sintering

shrinkage.

c) Potential to fracture in posterior teeth.

d) Requires a special die material.

DUCERAM LFC:-

Referred to as ‘hydrothermal low-fusing ceramic’. It is composed of an

amorphous glass containing hydroxyl ions.

- Properties claimed by the manufacturer for this non-crystalline

material are greater density, higher flexural strength, greater

fracture resistance and lower hardness than feldspathic

porcelain.

30

- Due to absence of leucite crystals, the hardness of the material

and its ability to abrade the opposing natural tooth structure is

reduced.

- Higher flexural strength results from an ion-exchange

mechanism of hydroxyl ions and also promotes healing of

surface micro cracks.

- The restoration from duceram LFC is made in 2 layers. The base

layer is duceram metal ceramic (a leucite-containing porcelain),

which is placed on a refractory die using powder- slurry

technique and baked at 9300c. the second layer of duceram LFC

is applied over base layer using same technique and baked at

6600c.

- The material is supplied in a variety of shades and can be

surface-characterized with compatible stains and modifiers.

Uses:-

- Ceramic inlays, veneers

- Full contour crowns

Advantages:-

a) Good flexural strength-110Mpa

b) Greater density

c) Greater fracture resistance

d) Hardness is close to that of natural tooth owing to absence of

leucite(decreases abrasion of natural tooth).

Disadvantages:-

31

a) Needs a special die material.

II) CASTABLE CERAMIC SYSTEMS;-

- Differs from sintered ceramics in that they are supplied as solid

ceramics ingots, which are used for fabrication of cores or full

contour restorations using the lost wax and centrifugal casting

technique.

- Generally one shade of material is available which is covered by

conventional feldspathic porcelain or is stained to obtain proper

shading and characterization of the final restoration.

a) DICOR:-

The use of glass ceramics in dentistry was first proposed by Mac

Culloch in 1968.

The first description of DICOR castable ceramic was given by Adair

and Grossman in 1948 and was marketed by Dentsply Int.

- It is a glass-ceramic material composed of SiO2, K2O, Mgo,

MgF2, minor amounts of Al203 and ZrO2 incorporated for

durability and a fluorescing agent for esthetics.

- The fluoride acts as a nucleating agent (source of Fl ions)

required in the crystalline phase and it also improves the fluidity

of molten glass.

- A full contour transparent glass crowns is cast at 1350oc,

followed by heat-treatment at 1075oc for 10hrs. This heat-

treatment causes microscopic plate like crystals of crystalline

material (mica) to grow within the glass matrix. This crystal 32

nucleation and crystals growth process is called

‘CERAMMING’. The crystals function in 2 ways:-

a) They create a relatively opaque material out of the initially

transparent crown.

b) They significantly increase the fracture resistance and strength

of the ceramic as they interrupt the propagation of cracks in the

material when forces are applied intraorally.

- The cerammed casting is achromatic and shade is developed by

adding external colorants .This ceramic was originally intended

to be shaded with a thin surface layer (50-100µm) of colorant

glass.

- There has been evidence that the stain layer might be lost during

occlusal adjustments, during routine dental prophylaxis or

through the use of acidulated Fl gels.

- Dentsply ( Trubyte division) has introduced ‘DICOR

PLUS’→ is a shaded feldspathic porcelain veneer applied to the

dicor substrate for fabrication of ‘ WILLI’S GLASS

CROWNS’ ( crowns of veneered Dicor ceramic).

- Marginal openings of 30-60 µm has been reported for dicor

restoration, comparable to those of metal ceramic crown.

- The internal fitting surface of the restoration is etched with 10%

ammonium-bi-fluoride to improve their bonding strength to the

composite resin and the tooth.

Advantages:-

33

a) The fit of cast glass restoration supersedes that of conventional

porcelain. This decreases the amount of resin luting agent at the

margins, thus decreasing the potential for ditching.

b) Surface hardness and occlusal wear is similar to enamel.

c) Greater flexural strength than conventional porcelain.

d) It produces good esthetics due to the ‘chameleon’ effect, in

which the restoration acquires a part of the color from adjacent

teeth and fillings as well as the underlying luting cement.

e) Ease of adjustment.

f) Inherent resistance to plaque accumulation (about seven times

less than on the natural tooth surfaces).

Disadvantages:-

a) Chances of losing low- fusing feldspathic shading porcelains,

applied for good color matching. Since the colorant is a surface

stain, any grinding on the restoration leaves an unaesthetic

opaque white area.

b) Special investment and casting equipment is required.

c) The whole process from the casting through the staining of the

cerammed restoration is technique sensitive.

Uses:-

- Inlays, onlays

- Full contour restorations or cores.

b) CASTABLE APATITE CERAMIC (CERA PEARL):-

34

The formation of a hydroxyapatite ceramic through reaction of glass

ceramics with moisture was descrided by S.Hobo and T.Iwata.

- Castable apatite ceramic was first developed by Hobo and

Bioceram group as CaO-P2O5-MgO-SiO2 glass ceramic (crystal

similar to hydroxyapatite of enamel).

- Cerapearl is composed of CaO, P2O5, MgO, SiO2 and traces of

other elements.

CaO (45%) and P2O5 (15%) are the main ingredients in glass

formation and are essential for hydroxylapatite crystal formation

MgO (5%) along with CaO decreases the viscosity of the

compound when melted and SiO2 (34%) in combination with

P2O5 forms the matrix with SiO2 regulating the thermal

properties.

- The lost wax technique is employed to produce the initial stage

of restoration and a reheating phase to develop a crystalline

microstructure.

- Its casting once obtained has an amorphous structure, but when

subjected to ceramming, forms crystalline oxylapatite (Ca 10

(PO4)6O). The unstable oxylapatite, when exposed to moisture

forms stable crystalline hydroxylapatite.

- Compared to normal enamel, crystals of cerapearl show an

irregular arrangement which probably accounts for its superior

mechanical properties. The similarity in hardness to enamel

prevents wear of opposing enamel.

- The young’s modulus, tensile and compressive strength of cera

pearl are appreciably higher than conventional porcelain and

most restorative materials.

35

. cerapearl is a rugged material and hard to bend, and distortion

with reasonably strong occlusal forces would be slight.

- Cerapearl crowns should be thicker than metal crown because of

poor flexural characteristics. The required tooth reduction for

crown is 2mm on occlusal or incisal surfaces, 1.5mm on axial

surfaces and 1.2mm on the margin. The finish line may be a

heavy chamfer or shoulder. All sharp edges should be rounded

so that stress concentration is reduced.

- Since hydroxyapatite is the main constituent of cerapearl it is

possible to fabricate a laminate veneer with excellent

biocompatibility. It is also possible to acid-etch the surface of

veneer for bonding treatment due to its similarity between its

constituents and those of natural enamel.

When this laminate veneer is bonded to enamel using light cured

composite resin with xylane coupling adhesive, a strong bond is

produced. The shade of laminate veneer is created internally by

the composite resin used for bonding.

- Cera pearl is indicated for crowns, laminate veneers and inlays

but is yet commercially unavailable and currently in a research

phase.

III) PRESSABLE CERAMICS:-This system was first described by Wohlwend et al in 1989 and

become available commercially as IPS Empress and Optec OPC with

the former being more popular of the two.

36

- The ceramic is available in the form of ingots and is primarily a

precerammed glass reinforced with leucite that prevents crack

propagation without significantly diminishing its translucency.

a) IPS EMPRESS (Injection-molded glass ceramic):-

The ceramic is primarily a glass filled with crystalline leucite [23.6%

wt coloured , 41.3% wt opaque ceramic].

- IPS empress is a pre-cerammed glass-ceramic that is heated in a

cylinder form and injected under pressure and high temperature

into a mold over a 45 minute period to produce the ceramic

substructure. In this way, the only dimensional change occurs

during cooling and can be controlled with an investment having

appropriate expansion.

- This crown form can be either stained and glazed or built-up

using a conventional layering technique.

- Contains a higher concentration of leucite crystals that increases

the resistance to crack propagation.

Advantages:-

a) Lack of metal or opaque ceramic core.

b) Moderate flexural strength.

c) Excellent fit of restoration.

d) Excellent esthetics.

Disadvantages:-

a) Potential to fracture in posterior areas.

37

b) Need for special laboratory equipment (pressing oven and die

material)

Uses:-

- Single anterior crowns, Inlays and veneers.

b) OPTEC OPC:- Leucite reinforced hot pressed ceramic

- Optec OPC is also a type of feldspathic porcelain with increased

leucite content.

- Is supplied in the form of ingots. The restoration is waxed, invested

and placed in a specialized mold with an aluminium plunger and the

ceramic ingot is placed under the plunger.

- The entire assembly is heated to 1150oc and plunger is released

which presses the molten ceramic into the mold.

- The pressed ceramic is then baked and the pressed from can be

made to full contour or as a substrate on which conventional

feldspathic porcelain can be built up.

Advantages:-

1) Strong, translucent, dense restoration.

2) Good flexural strength.

3) Can be etched and bonded to natural tooth.

Disadvantages:-

1) Causes abrasion of natural tooth due to increased leucite content.

38

2) Special oven, die material and molding procedure required to

fabricate restoration.

c) EMPRESS 2 (E-2):-

This system consists of two components:-

1) First component is a compressed core material of lithium-di-

silicate glass ceramic and lithium orthophosphate crystals with a

flexural strength of 350Mpa. The scope of use of empress-2 for

smaller bridges and posterior teeth has widened compared to

empress-1 due to its increased flexural strength.

2) The second component is a new layer/ laminated material

comprised of fluorapatite-glass crystals, which ensures a natural-

dental balance of opaqueness and translucence. A higher volume

of crystalline phase results in increased strength of IPS Empress-

2 compared to the original IPS Empress.

The pressed ceramic material is translucent such that the color of

the underlying tooth structure is transmitted through it. The

shade of the prepared tooth is determined with a specially

formulated shade guide Firing cycle → 8500c for 2mins in

vacuum.

IV) INFILTRATED CERAMICS:- This system utilized alumina as the core-material. It is supplied as a

two-component system- a powder (aluminum oxide/spinel) which is

fabricated into a porous substrate, and a low viscosity glass which is

infiltrated at high temperature into the porous substrate. The infiltrated 39

ceramic is then veneered using the conventional feldspathic porcelain

techinique.

a)INCERAM:-

- This system was developed by Sadoun (Paris) and was presented

in 1989 for the first time by Vita company.

- It is the successor system of Hi-ceram, differing from this

system by having sufficiently lower grain size of aluminum

oxide and thereby greater density.

- In ceram ceramic consists of 2 three - dimensional

interpenetrating phases: alumina (aluminum oxide) and glass.

- The core is initially extremely porous and is composed of either

aluminium oxide or spinel (Al2O3 and MgO), which is

subsequently infiltrated with molten glass.

- The spinel cores are more translucent than aluminium oxide

cores due to the crystalline nature of spinel, which provides

isotropic optical properties and partly due to its lower refractive

index compared with alumina.

- But the spinel-based core ceramic (In-ceram spinel) was not as

strong as the alumina-based material.

The core is made from fine grained particles that are mixed with water

to from a suspension referred to as a ‘slip’, is painted on a gypsum die

(absorbent refractory die). The die draws water from the slurry under

capillary pressure thereby depositing a layer of solid alumina on the

surface, which is subsequently sintered/baked at 1120oc for 10hrs to

produce an opaque porous core. This process is called ‘slip casting’.

40

- At this stage, careful handling is a must as the material is very

fragile. Also during baking, the slip (aluminous core) undergoes

a shrinkage, which must be compensated for by the expansion in

die stone. In the second phase, glass infiltration material (low

viscosity glass) of appropriate shade is applied onto the core and

fired at 11000c for 3-5hrs. The molten glass infiltrates into the

porous alumina core by capillary action resulting in a dense

composite structure, increasing the strength of the core to about

20 times its original strength.

- The aluminium oxide or spinel crystals limit crack propagation

and the glass infiltration reduces porosity and both these factors

explain why inceram is currently one of the strongest ceramic

materials in the market.

- The final infiltrated core has about 85% of the crystalline

component which confers a 3 times increase in its flexural

strength (450Mpa) compared to any other dental ceramic.

- The core is then veneered with dentin and enamel conventional

feldspathic porcelains/ vitadur N (vident) aluminous veneering

porcelain using conventional powder-slurry technique to create

the proper shade and contour.

- A compositional analysis has revealed alumina to be 99.56 wt%

and the infiltration glass to be lanthanum alumino silicate with

small amounts of sodium and calcium.

- Lanthanum decreases the viscosity of the glass to assist

infiltration and increases its index of refraction to improve

translucency.

41

- The core of aluminium oxide/spinel is so dense that traditional

internal surface etching to improve the bond to tooth structure is

not possible.

- Sadoun and Asmussen (1994) describe the creation of a

roughened surface by the application of fine grained silica to the

inceram core. The reduction in fitting accuracy was claimed to

be of the order of 10µm and could be accommodated by the use

of die spacer.

- Kern and Thompson (1994) have also applied silica coatings to

enhance bonding by a thermal process tribochemical process.

- A further development in this system is the ‘Inceram spinel’,

which uses spinel instead of alumina. Spinel is a composition

containing aluminium and magnesium oxide.

- Due to the lower refractive index of spinel compared to alumina,

the translucency of ceramic is improved but has a comparatively

lower flexural strength. The fabrication process is similar to

Inceram.

Uses:-

- Single anterior and posterior crowns.

- Anterior 3 - unit bridges.

- Implant supported bridges (recently).

Advantages:-

a) Lack of metal substructure.

b) It has extremely high flexural strength - 450Mpa strongest all

ceramic dental restorations presently available.

42

c) Excellent fit, as little shrinkage occurs due to sufficient time at

optimum temperature, which causes bonding between particles at small

areas.

Disadvantages:-

Opacity of the material and hence can be used only as a core.

a) Unsuitable for conventional acid - etching.

b) Special die material and high temperature oven is required.

c) Wear of opposing occluding enamel or dentin occurs if the In ceram

restoration is a part of heavy incisal guidance or canine rise.

b) TECH CERAM:-

This system (Tech Ceram Ltd, Shipley, Uk) for the production of high

strength, all ceramic restoration has evolved following an 8 year

development by a British team.

-A thin (0.1 – 1mm) alumina core base layer is produced using a

thermal spray technique, resulting in a density of 80-90%.

-Optimum strength and translucency are achieved by a sintering

process at 1170oc.

-The range of base layer thickness makes this technique versatile and

appropriate to a range of restoration types.

-Subsequent reproduction of esthetics is achieved by incremental

application of a range of specially developed porcelain in the

traditional manner.

-The inner fitting surface is rough and according to manufacturers, it

does not require etching or silane bonding. GlC or dual cure resin-

composite luting agents are recommended.43

V) MACHINABLE CERAMICS;-

These products are supplied as ceramic ingots in various shades and

are used either in computer aided design - computer aided

manufacturing (CAD-CAM) procedures or in copy- milling

techniques.

Two classes of ceramics are available for machining fabrication of

individual ceramic restoration and veneers:-

a) Two fine – scale feldspathic porcelains (Vita MarkI& II and

clay)

b) Two glass ceramics (Dicor MGC light and Dicor MGC dark;

Dentsply).

The difference between the microstructure of mark I and mark II is

determined by the particle size of the feldspar. The particles of mark I

have larger particle size and a broader size range (10-50m) than those

of markII (4m). MarkII has significantly higher strength achieved by

the higher homogeneity of the microstructure, optimization of the

composition and increased densification.

The basics for the feldspathic cerec vitablocs are the natural mineral

feldspar. The advantage of feldspar compared with other ceramic raw

materials are broad softening temperature intervals, the wide spread

occurance and natural purity of the materials.44

Dicor MGC ( Dentsply L.D caulk division):-

This is a machinable glass ceramic composed of fluorosilicate mica

crystals in a glass matrix. It has greater flexural strength than cast dicor

(Rosenblum and Schulman, 1997). They have been shown invitro to be

softer than conventional feldspathic porcelain and produces less

abrasive wear of the opposing tooth structure than cerec mark I but

causes more wear than cerec markII.

CELAY:-

This material can be used for CAD-CAM produced restorations or

used in the copy milling techniques. It is a fine-grained feldspathic

porcelain.

In the celay copy milling technique, a resin composite restoration is

made on a master die, the restoration is then traced with a contact

digitizer that transfers the shape to the celay milling device.

CAD-CAM:- Computer aided design – computer aided

manufacturing. This original system is replaced by a much-improved

CAD-CIM - computer aided design – computer integrated

manufacturing.

Cerec system (developed in Zurich, Switzerland) is an application-

oriented synthesis of 3-D imaging, computer aided design and

numerically controlled machining. The basic philosophy of the cerec 45

unit is to combine the scan head for the optical impression with the

reconstructuion and fabrication module in a single mobile

workstation.

CEREC 1:- The original cerec system introduced a revolutionary method of

fabricating ceramic restoration to the dental profession. The main

change or revolution in the hardware of cerec machine, was the

introduction of an electrically driven milling machine with a more

efficient cutting disc. The accuracy of cut was improved by the finer

grit and by the increased rigidity of the disc during cutting due to

increased disc thickness. The final software release for cerec [ cerec

operating system ( COS ) 2.11], allowed 3D shaping of the fitting

surface of the restoration against the cavity floor, improving the level

of fit and allowing a wider range of shapes to be accommodated. The

main limitation was, not being able to cut concave surfaces and also

extending veneers into areas of missing tooth structure proved

problematic. The main characteristic of the current generation

machinable cerecVita porcelain include ease of machinability

combined with low wear characteristic of the restoration together with

minimal wear of the opposing tooth.

The strongly translucent nature of ceramic allowed the use of light

activated composite material as luting agent. These materials have greater density and filler loading than the

traditional composites and therefore suffer less wear at the exposed

marginal interface. These modern ceramics also offer long term color

46

stability and can be polished to a high state of micro smoothness so

that a glazing layer is not required.

-The porous free nature of the entire block ensures that wear on the

surface of the ceramic does not break through into an abrasive mass

that can cause premature or accelerated wear on antagonist enamel

surface.

CEREC 2:- The cerec 2, a computer-aided design and integrated milling machine

was introduced in the United States in 1996. The introduction of this

unit has addressed virtually all of the limitations of cerec I.

- It designs and fabricates porcelain inlays, onlays, crowns and

veneers and allows immediate one visit esthetic restorations.

- A white glass free powder containing titanium oxide is placed

on the tooth and a CCD sensor makes an infra red three-

dimensional scan of the preparation in about 0.1 seconds at a

resolution of 25µm.

- A self-contained microprocessor displays the digitized image

and the dentist designs the restoration.

- Pre-fabricated /preformed porcelain ingots available in several

shades (17- vita, Vivadent ; Pro CAD, Ivoclar ) are used by a

digitally controlled six-axis micromilling machine to fabricate

the prosthesis. Milling time is approximately 10mins for a

simple restoration.

- After fitting, the porcelain is custom stained, glazed, acid etched,

silanated and cemented with standard adhesive composite resin

luting agents.47

- The new-camera provides more data with greater accuracy

resolving down to 25µm.The data set describing the tooth was

increased from 4million voxels to 32million voxels.

- The new technology employs two 32- bit processors and the user

interface were improved by increasing the resolution of the

monitor and adding a color screen.

- Full shaping of the internal and external surfaces of restoration

including detailed formation of occlusal pits and fissures is now

possible.

- Beyond full crowns, these is the possibility of making bridges by

CAD-CAM, but the longevity of ceramic materials in this

application remains questionable.

Benefits of Cerec 2 system:-

1) Benefits for the patients:-

-Esthetic and cosmetic restoration

-Best material properties in dental ceramics.

-Biocompatible.

-Cost-effective.

-Quick turn around time (1 day laboratory time)

-Perfect occlusion.

-High marginal integrity.

-No metal in mouth.

2) Benefits for the dentist:-

- Economic production in the laboratory.

- Increased precision.

48

- Better interproximal integrity.

- No polishing needed.

- Contacts optimized in the Laboratory.

CEREC 3:-The cerec technique was developed in 1984 to answer the practical

need for long-lasting dental restorations with sound marginal seals.

- The cerec 3 system (Sirona) simplifies and accelerates the

fabrication of ceramic inlays, onlays, veneers and quarter, half

and complete crowns for anterior teeth and posterior teeth.

- The system has several technical improvements over cerec 2,

including the three-dimensional cerec 3 intraoral camera,

manipulation of the picture and the grinding unit.

- Proper occlusion is established accurately and quickly as the

software simplifies occlusal and functional registration.

- The separate grinding device working true to morphologic detail

and with fine surface quality is connected to the optical unit by

radio control. Equipped with a laser scanner it can also be used

for indirect application through a standard personal computer.

- The cerec 3 system is network and multimedia ready and in

combination with an intraoral color video camera or a digital

radiography unit, can be used for patient education and for user

training.

Technical Characteristics:-

49

New technology Property Advantages

a) cerec camera:-

Principle: Active

double Triangulation i.e

→ recording Of the

cavity from 2 different

triangulation angles

provides immediate

depth scale of > 20mm

Eliminates adjusting

procedure.

Saves time, especially in

the two important

techniques, function and

correlation.

b)Image processing:-

Rapid ‘twin grab board’

for the sirocam cerec

intraoral measuring

camera.

Provides vertically

oriented optical

impression on the

oblong

format monitor.

Eliminates relearning for

previous users and also

waiting time.

c) Computer:

Medically approved

personal computer with

shock protected hard

disk.

d) Double grinding

unit:-

1.6mm cylindrical

diamond or 1.2mm and

1.6mm c.d- 45o taper

angle on top →

grinding instruments.

Control of grinding→

Incorporates greater

versatility and

efficiency of the

windows system,

network is ready.

Provides an occlusal

design that is true to

detail.

Allows tension free

grinding, reducing

stress on ceramic and

grinding instruments.

-Eliminates waiting time

in design.

-Performs grinding and

construction

simultaneously.

-Saves time in finishing

of occlusion.

-Places less stress on

ceramics and protects

the grinding instruments.

50

symmetric grinding by

the instruments.

e) Radio control

communication:-

Security → 1m distance

to other electric

devices.

Allows cable free bi-

directional data transfer

to grinding unit.

Allows placement of

cerec 3unit components

as desired.

For the practioner experienced in direct (Chair side) applications, the

completely optimized and accelerated design with simultaneous

control of projection and section views, the elimination of adjustment

and rapid superimposition of occlusal registrations are beneficial.

The cere-3 system thus is a diagnostic, restorative, training and

documentation center for the dental practice.

ADVANTAGES OF CAD/CAM:-

a) Negligible porosity levels in the CAD/CAM core ceramics.

b)The freedom from making an impression.

c) Reduced assistant time associated with impression procedures.

d) Need for only single patient appointment.

e) Good patient acceptance.

DISADVANTAGES:-

a) Need for costly equipment.

b) Lack of computer – controlled processing support for occlusal

adjustment.

51

c) Technique sensitive nature of surface imaging required for the

prepared teeth.

PROCERA:- The Procera systems were produced by Nobel Biocare, Goteberg,

Sweden and was first described in 1993.The original Procera system

was designed to fabricate crowns and fixed partial dentures by

combining a titanium sub-structure with a low fusing veneering

porcelain .The machine has since been modified to use a densely

sintered high-purity alumina coping combined with a compatible

veneering porcelain to create all ceramic restorations.

Clinical procedure:

The Procera system uses a conventional impression and stone model.

The die is properly ‘ditched/guttered’ below the finish line to clearly

define the extent of the preparation and then is placed on the rotating

platform of the Procera scanner.

A sapphire stylus forms the tip of the scanner probe that contacts the

surface of the die as it rotates around a vertical axis , and ‘reads’ the

shape into a computer in a manner similar to a key-copying machine.

Extremely light pressure of approximately 20g maintains the probe in

contact with the die as it rotates .As the platform rotates, one-data

point is collected at every degree around 360- degree circumference of

the die .Following each complete rotation, the stylus is elevated by

200µm and another circle of recordings are taken until the entire

preparation has been digitized. The average preparation will require

around 50,000 data points for accurate digitization and is extremely 52

accurate with maximum shape-related errors of ± 10µm.The

emergence angle of the coping from the tooth is selected and the relief

space for the luting agent is automatically established by a computer

algorithim. The digital information is sent to a Procera sandivik dental

laboratory (Stockholm, Sweden) via a communication link. To account

for sintering shrinkage, a model 20% larger than the original tooth is

fabricated. A high strength aluminium oxide coping (600µm) is

manufactured by compacting the material against the enlarged model

and then milling the outer shape. The ceramic coping is returned to the

local dental laboratory via express mail and an all- ceram veneering

porcelain is added by the local laboratory technician. The restoration is

custom stained and glazed and then returned to the dentist.

Restorations produced with this system have been shown to have

precise fit with marginal openings of less than 70µm and produces

minimal wear of the opposing dentition, provided the material is

suitably polished. Single crowns produced with this novel ceramic

material can be expected to give acceptable clinical service for atleast

5 years.

PROCERA ALLTITAN:-Its low thermal conductivity, low density, high strength,

biocompatibility make titanium a desirable restorative material. The

external contours of the individual titanium cores for Procera all titan

bridges are milled and graphite electrodes create the fitting surface by

a spark erosion process. Individual components of the bridge are

53

welded by laser before the structure is finally veneered with special

porcelain .In this way, the application of CAD-CAM technology,

coupled with traditional spark erosion is said to enable the production

of accurately fitting titanium-based restoration, while eliminating the

need for costly and technically difficult casting procedures.

CELAY SYSTEM:- The celay technique developed by Dr Stefan .T.Eidenberg at the

University of Zurich, is a variation in the direct - indirect restoration

concept, but without the need for a laboratory technician. A direct

process is used instead of a conventional impression in which a

moldable precision imprint material is modeled directly inside the

mouth in the cavity preparation, where it is adjusted for occlusion,

contact relations and marginal integrity. The material then undergoes a

light-hardening /curing process before it is removed from the tooth, to

serve as a prototype model to be copied and reproduced in ceramic on

a unique milling system developed by Claude of Microna technology.

The milling centre has two distinct aspects. In one half, the model to be

copied is centered in a holder, where it is manually scanned .A second

part of the milling machine contains a rotary turbine with various

cutting tools. The directly formed pattern in the vise is manually

scanned with a sensor. This sensor is directly connected to the milling

aspect. Any form scanned is thus simultaneously reproduced in all

three dimensions in a block of ceramic by the rotary turbine .The gross

54

form is developed with a diamond disk and refined with a diamond

point .The system can also be used as a purely indirect process, in

which an impression is made and a die developed in the laboratory .A

resin composite restoration is made on a master die and is then traced

with a contact digitizer that transfers the shape to the celay milling

device. The composite resin imprint prototype material is placed in the

milling unit in a bipoint metallic vise and the surface is scanned

similarly manually and reproduced in ceramic on the milling unit,

which carves out an exact replica of the plastic prototype in ceramic.

Additional characterization or colorization of the inlay if required can

be accomplished in the laboratory by refining the inlay prior to final

finishing.

ADVANTAGES:

a. A precisely fitting ceramic restoration can be developed in

one patient session.

b. It can be developed without the need for laboratory

technician.

c. The restoration is developed in factory fired high grade

porcelain.

d. The processing time required is very short .A small inlay

can be milled in 3 mins, a mesioocclusodistal inlay in less

than 8 mins and a complete onlay in 12-13 mins.

55

METAL CERAMIC SYSTEMS /TECHNOLOGY

Metal ceramic systems combine the strength and accuracy of cast

metal with the esthetics of porcelain. The six basic principle features

which distinguish a metal ceramic alloy from both crown and bridge

and removable partial denture alloys are:-

a. A metal ceramic alloy (MCA) must be able to produce

surface oxides for chemical bonding with dental

porcelains.

b. A MCA should be formulated so that its co-efficient of

thermal expansion is slightly greater than that of the

porcelain veneer to maintain the metal- porcelain

attachment.

c. The alloy must have a melting range considerably higher

than the fusing range of the dental porcelain fired on it.

This temperature separation is needed so that porcelain

56

build-up can be sintered to a proper level of maturity

without the fear of distortion of the metal substructure.

d. The alloy must have high temperature strength or sag

resistance → that is the ability to withstand exposure to

high temperatures without undergoing dimensional

change.

e. Processing should not be too technically demanding.

f. A casting alloy should be biocompatible.

ADVANTAGES OF METAL CERAMICS:-

a. High strength values due to prevention of crack

propagation from the internal surfaces of crowns due to

metal reinforcement.

b. Improved fit on individual crowns provided by cast metal

collar.

DISADVANTAGES:

a. Difficult to obtain good esthetics due to increased opacity

of metal substructure.

b. Porcelains used in metal ceramic techniques are more

liable to devitrification which can produce cloudiness.

c. Preparation for metal ceramic requires significant tooth

reduction to provide sufficient space for the materials.

d. More difficult to create depth of translucency due to

‘mirror’ effect of the dense opaque masking porcelain.

57

INDICATIONS:

a. Extensive tooth destruction as a result of caries, trauma,

or existing previous restorations that precludes the use of

a more conservative restoration.

b. To recontour axial surfaces or correct minor

malinclinations .

c. Teeth requiring fixed splinting or being used as bridge

abutments.

CONTRAINDICATIONS:

a. Patients with active caries or untreated periodontal disease.

b. In young patients with large pulp chambers due to high risk of

pulp exposure caused by substantial tooth reduction.

c. Teeth where enamel wear is high and there is insufficient bulk of

tooth structure to allow room for metal and porcelain.

d. Anterior teeth where esthetics is of prime importance →high

shades of very translucent teeth.

CLASSIFICATION OF METAL CERAMIC ALLOYS:- BY

NAYLOR,1986.

Based on composition: Firstly all alloys are separated into one of two

major types: - Noble (precious)

-Base metal (non-noble /non precious)

58

Next the alloys are arranged by system, after which each system is

further subdivided into constituent groups if present.

SYSTEM GROUP

A) NOBLE METAL ALLOYS:

1) Gold-platinum-palladium

2) Gold-palladium-silver High silver

Low silver

3) Gold-palladium

4) Palladium-silver

5) High palladium

B) BASE METAL ALLOYS:

1) Nickel-chromium Beryllium

Beryllium free

2) Cobalt-chromium

3) Other systems

Nature of metal ceramic bond:-Four mechanisms have been

proposed to explain the bond between the ceramic veneer and metal

substructure.

1) Vanderwaals forces.

2) Mechanical retention/entrapment

3) Compressive forces.

4) Direct chemical bonding.

59

Chemical form of attachment is the predominant and most

important mechanism.

1) VANDERWAAL’S FORCES:-

Vanderwaals forces are secondary forces generated by a physical

attraction between charged particles rather than by an actual sharing

or exchange of electrons as in primary (chemical) bonding. These

forces are generally weak as only a minimal attraction exists

between the electrons and nuclei of atoms in one molecule with

those of adjacent molecule. The better the wetting of the metal

surface, greater the vanderwaals forces .Porcelain adhesion to metal

can be diminished or enhanced by alterations in the surface

character of the porcelain bearing surface on the substructure. A

rough contaminated metal surface will inhibit wetting and reduce

the vanderwaals bond strength, where as a slightly textured surface

created by finishing with uncontaminated aluminium oxide (Al2O3)

abrasives followed by air abrasion with 50µm aluminium oxide,

reportedly will promote wetting by the liquid porcelain.

Vanderwaals forces are only minor contributions to the overall

bonding a attachment process.

2) MECHANICAL RETENTION OR ENTRAPMENT :-

It creates attachment by interlocking the ceramic with

microabrasion in the surface of the metal coping which are

produced by finishing the metal with non-contaminating stones or

discs and air abrasion thus enhancing the wettability , providing

mechanical interlocking and increasing the surface area for

60

chemical bonding .Despite its presence , contribution of mechanical

retention to bonding are relatively limited.

3) COMPRESSIVE FORCES:-

Dental porcelain is strongest under compression and weakest under

tension .Hence if the coefficient of thermal expansion of the metal

substrate is greater than that of the porcelain fired over it, the

porcelain is placed under compression (McLean, 1980) .When

cooling a restoration with full porcelain veneer, the metal contracts

faster than porcelain, but it is resisted by the porcelain’s lower

coefficient of thermal expansion. This difference in contraction

rates creates tensile forces on metal and corresponding compressive

forces on porcelain. Without the wraparound effect created in full

porcelain restoration, there is less likelihood of the ‘compression

bonding’ developing completely .As a consequence, a partially

veneered restoration may not encompass sufficient porcelain

bearing surface to exert significant compressive bonding forces.

4) CHEMICAL BONDING:-

Two mechanisms may exist in the chemical bonding theory.

According to one hypothesis, the oxide layer is permanently bonded

to the metal substructure on one side while the dental porcelain

remains on the other .The oxide layer is sandwiched between the

metal substructure and opaque porcelain .This though is

undesirable, as a thick oxide layer might exist that would weaken

the attachment of metal to porcelain.

61

The second and more likely theory suggests that the surface oxides

dissolve or are dissolved by the opaque layer. The porcelain is then

brought into atomic contact with the metal surface for enhanced

wetting and direct chemical bonding by sharing of electrons

between metal and porcelain .Both covalent and ionic bonds are

thought to form but only a monomolecular layer of oxide is

believed to be required for chemical bonding to occur.

Chemical bonding is generally accepted as the primary mechanism

in the porcelain metal attachment process .There are several ways

of providing a surface oxide on the metal for porcelain bonding:-

a. By introducing traces of base metals into precious metal

alloys, which on heating produces thin oxide films.

Eg: gold-platinum alloys for ceramic bonding.

b. By direct oxide production via the constituents of the

alloy.

Eg: Nickel-chromium, cobalt-chromium alloys.

c. By surface coating of metals with oxidizable metal films

such as indium or tin.

Eg: Electrodeposition of tin on platinum.

Electrodeposition of metals:-

Electrodeposition provided a strong bond when the tin coating

was in the range of 0.2-2µm.The relevant facts emerging from this

work in relation to the bonding of porcelain to metal were:-

1) A tin coating on platinum of 0.2-2µm gave optimum bond strengths

for aluminous core porcelain of around 50N/mm2 .In this range; failure

was always cohesive in the core porcelain.

62

2) Below 0.2µm of tin, the strength falls and the incidence of

interfacial failure increases at zero coating thickness.

3) Above 2µm thickness, a reduction in strength occurs .Failure is

cohesive but porcelain is weakened.

OXIDATION OR DEGASSING PROCESS:-

It refers to a procedure recommended to clean the metal of organic

debris and remove entrapped surface gases such as hydrogen .High

temperature processing like this is vital for the removal of volatile

contaminants that might not otherwise be eliminated either by metal

finishing, steam cleaning or air abrading. This process also allows

specific oxides to form on the metal surface which is responsible for

the porcelain furnace to form a mature, stable oxide layer for the

porcelain metal attachment.

Majority of manufacturers consider this process a necessary step in the

fabrication of the metal ceramic restorations although some

manufacturers do not recommend the oxidation / degassing step.

Instead they advocate minimizing the number of firings to which the

casting is subjected to .Oxidation heat softens certain types of alloys

through molecular rearrangement, which results in marginal distortion

and decrease in bond strength.

Post oxidation treatment:-

The first oxides formed may be most desirable to form bond, but to

reduce oxide layer or other contaminants, manufacturers suggest air

abrading or acid treating the casting.

Acid treatment (chemical method):Hydrofluoric , hydrochloric and

dilute sulfuric acids are used .The potential hazards of these acids

63

require that they be stored in clearly marked resalable plastic bottles.

Use of a rubber tipped instruments to place oxidized castings into the

acid is advocated .The container is then placed in the ultrasonic unit for

the recommended time .After removal the casting is rinsed thoroughly

under water .For final cleaning it is dipped in distilled water and

cleaned in ultrasonic unit for 10-15mins.

Non-acid treatment : Castings can be air abraded with pure , 50µm

aluminium oxide that is non-recycled as, if reused can cause metal

contamination .It is then steam cleaned or ultrasonically cleaned in

distilled water for 10-15 minutes.

TYPES OF METAL CERAMIC BOND FAILURE:-Classification of ceramo metal failures has been given by O’Brien W J

(1977) according to the interfaces formed at fracture.

a) Metal - porcelain: Interfacial fracture occurs leaving a clean surface

of metal and is generally seen when the metal surface is totally

depleted of oxide prior to firing porcelain or when no oxides are

available .It may also occur due to contaminated or porous metal

surfaces.

b) Metal oxide-porcelain: It is a common type of failure in the base

metal alloy system .The porcelain fractures at the metal oxide surface,

leaving the oxide firmly attached to the metal.

c) Metal - metal oxide: It also commonly occurs in base metal alloy

systems due to over production of chromium and nickel oxide at the

interface .In this interfacial fracture, the metal oxide breaks away from

the metal substrate and is left attached to the porcelain.

64

d)Metal oxide-metal oxide: Fracture occurs through the metal oxide at

the interface, resulting from an over production of oxide causing a

sandwich effect between metal and porcelain.

e) Cohesive within metal: Unlikely type of fracture for the individual

metal ceramic crown .It occurs in cases where the joint area in bridges

breaks.

f)Cohesive within porcelain :In this fracture , tensile failure occurs

within the porcelain when the bond strength exceeds the strength of

porcelain .Most commonly occurs in the high gold content alloys as an

ideal situation is created when the oxide film is only a few molecules

thick and forms a solid solution with porcelain .Prolonged or repeated

firing of base metal alloy and ceramic crown can cause excessive

dissolution of the oxide layer I the glassy porcelain resulting in tensile

failure which may not be revealed at the bond interface due to

differential thermal expansion.

FORMATION OF METAL COPING WITHOUT CASTING

PROCESS:

PLATINUM BONDED ALUMINA CROWN:-

Ceramics suffer from static fatigue believed to be due to stress-

dependant chemical reaction between water vapour and the surface

faults in the porcelain crown which cause flaws to grow to critical

dimensions, allowing spontaneous crack propagation .One method of

65

alleviating this problem was found to be the bonding of porcelain to

foil.

A method was devised whereby the surface of the platinum foil was

coated with upto 2µm of tin and oxidized in a furnace to provide a

continuous film of tin-oxide for porcelain bonding.

The bonded alumina crown consists of an aluminous porcelain crown

bonded to a 0.025mm thick platinum coping .The bonded platinum foil

acts as an inner skin on the fit surface, reducing subsurface porosity

and micro cracks in the porcelain and increasing the strength of the

unit .In order to provide a porcelain butt fit and eliminate the dark

shadow produced by a metal collar, the crown is baked onto an inner

platinum foil matrix(unplated)which is removed after baking and

provides space for the cement.

TWIN FOIL TECHNIQUE:- (By McLean et al, 1976)

This technique involves the laying down of two platinum foils in close

apposition to each other. The inner foil of 0.025mm platinum provides

a matrix for the baking of the porcelain and the outer foil which forms

the inner skin to the crown is tin-plated and oxidized to achieve strong

chemical bond with aluminous core porcelain. The outer or ‘bonded

foil’ remains in position on the internal fit surface of the crown and

will eliminate surface microcracks in the porcelain.

The bonded alumina crown was developed with the following

objectives:-

a) Reduction of metal and labor costs in construction.

b) Provision of a porcelain butt fit on the labial/buccal surface of the

crown, eliminating the dark shadow of a metal collar.

66

c) Reduction of stresses at the porcelain metal interface during

cementation procedures.

d) Improvement in strength of aluminous porcelain crown by reducing

internal microcracks and subsurface porosity.

The shrinkage of porcelain makes it difficult to achieve an accurate fit

of the core porcelain in one bake .Hence it is important to allow for

shrinkage and prevent the fired porcelain from lifting the platinum

skirt and spoiling the fit.

Two ways have been advocated:-

a) The cervical contact technique.

b) The cervical ditching technique.

The cervical contact technique relies on the application of a layer of

porcelain around the shoulder area to shrink first. The second bake will

then shrink towards the cervical porcelain and maintains the fit. The

cervical contact technique does not always work since the bulk of the

core porcelain still has to shrink during the second bake.

It is for this reason that the cervical ditching technique is strongly

recommended, where the porcelain is removed from the shoulder area

after the initial build-up is complete, such that a thinnest ditch possible

is made and just expose the cervical platinum at the shoulder.

Removal of platinum foil: It is facilitated by soaking the crown in

water. A fine pointed tweezer is used to lift the skirt away from the

edge. Peel the platinum away from the entire circumference without

damaging the fine porcelain edge (internally towards the incisal edge).

Cementation: Subsequent to tin plating and oxidation of the platinum

foil, a tin-oxide layer will be present on the fit surface of the crown.

Hence hydrogen and metal ion bridges can be formed between the

67

polar oxide layer and the poly anions in carboxylate or glass ionomer

cement .On cementation of the crown or inlay, strong physico-

chemical bonding between the platinum, cement and tooth can be

obtained, thus reducing the risk of microleakage.

BONDING TO GOLD FOIL:-

In 1979, Rojers reported a method of bonding porcelains to

electroformed, pure gold copings.UMK68 porcelain was used .More

recently, the ceraplatinum or Renaissance crown has been

marketed .This uses a formed laminated gold-palladium alloy coping,

approximately 0.05mm thick which is swaged to the die. The coping is

umbrella shaped and the corrugations allow for some expansion and

reburnishing, according to the size of the tooth preparation.

All gold foil techniques require the use of metal bonding porcelains,

since aluminous core porcelains with their high temperatures would

cause the gold to melt.

INDICATIONS FOR THE USE OF THE BONDED ALUMINA

CROWN:-

a) Porcelain veneer crowning of adolescent teeth where minimal

tooth preparation is necessary.

b) Anterior teeth, when metal reinforcement is essential.

c) Complete porcelain cantilever bridges on anterior

teeth(replacing lateral incisors)

d) In heavily worn teeth, thin or short teeth where minimal occlusal

clearance present (not less than 0.8mm), porcelain crowning of

all anterior teeth is indicated.

68

e) Repair of fractured metal- ceramic bridges, when removal of

bridge or splint is undesirable.

f) Coping jacket crowns on unit built bridge -work.

CONTRAINDICATIONS:-

a) In periodontally involved teeth, where preparations extend

deeply into root- face and no shoulder preparations are possible.

b) Posterior teeth where large areas of tooth are missing and

uneven bulk of porcelain is inevitable.

c) If lingual shoulder preparations are impossible particularly in

molar region.

Spun quartz and miracle pad are the materials used in lieu of

platinum foil or cilex to place raw porcelain in the furnace for firing

.They can be used several times and will last almost

indefinitely .Miracle pad is a more compact substance and easy to

manipulate . It also prevents porcelain from sticking to the tray.

CAPTEK SYSTEM:An alternative methodology for the elimination of the casting process

from metal - bonded crowns and bridges has been developed by Davis

Schottlander and Davis, UK.

This technique involves the adaptation of a wax strip impregnated with

gold-platinum-palladium powdered alloy to a refractory die .Firing

produces a rigid porous layer which is then infilled with gold from a

69

second wax strip by capillary action. The finalized metal coping is then

veneered with porcelain. The advantages of this system include:

Improved marginal fit(due to use of capillary cast

rather than lost wax technique)

Enhanced esthetics.

Biocompatibility (since 88% of the alloy is non-

oxidizing)

70

71

RECENT ADVANCES IN CERAMICS

SHRINK –FREE CERAMICS:

The application of an all ceramic crown employing a unique shrink-

free alumina substrate with specially formlulated porcelain veneers

(cerestore system) offers a viable alternative to both the metal

ceramic crown and traditional PJC.

The development of the advanced alumina ceramic substrate allows

the construction of highly durable all ceramic restorations with

exceptional fit.

COMPOSITION:

The microstructure of the core material consists of a multiphased

component or mixed-oxide system of aluminates.

Aluminium oxide is the primary component with alpha-aluminium

oxide (corundum) being the dominant phase in the microstructure.

Calcium and beryllium aids in sintering process and contributes to

high corrosion resistance due to absence of alkalis (Li,Na).Alpha

aluminium oxide and magnesium aluminate spinel are the major

crystalline phases of the core material.

Direct molding /Transfer/Injection molding technique:-

The shrink free- ceramic can be formed directly on the master die,

producing extreme accuracy of fit .The molding procedure is done

on the master die made from a special epoxy resin die

material(cerestore epoxy), which is heat stable and undergoes

permanent controlled expansions during curing.

The ceramic substrate which is supplied as a dense pellet of the

compacted shrink free formulation is heated until it is flowable 72

(160oC) and then transferred by pressure into a suitable mold

directly on the master die. After it sets, the green substrate is

removed from die and sintered/controlled firing is carried out

resulting in zero shrinkage of the ceramic.

MAGNESIA CORE CERAMIC:-

The co-efficient of thermal expansion of the alumina core material

is 8X10-6 /oC and requires similar low expansion veneer porcelains

and since the coefficient of expansion values for porcelain used

with metals averages around 13.5x10-6/oC , extensive cracking

results upon bonding these materials owing to thermal stresses.

The magnesia core material has a modulus of rupture strength of

19,000 psi after firing and a coefficient of thermal expansion value

of 14.5x10-6/oC. The required strength is achieved by dispersion

strengthening by the magnesia crystals in a vitreous matrix and also

by crystallization within the matrix. The strength can be doubled by

application of a glaze which may either penetrate into surface pores

to fill in the subsurface porosity or react with the core material to

produce crystallization which places the surface layer in

compression.

The magnesia core crown is fabricated by means of a modified

platinum foil matrix technique developed by Lazar, McPhee and

O’Brien, which gives an excellent fit compared to the conventional

technique.

73

HYBRID CERAMICS:-

These ceramics opened simplified ways of application .Hahn (1995,

1997) proposed a new ceramic material that is a hybrid between

organic and inorganic components. A precursor material consisting

of 50%vol polyvinyl siloxane, 30% active filler (titanium 1µm),

15% inert filler (aluminum oxide, 15µm) and 5% titanium boride

has been formulated. This mixture can be handled like composite

and cured .The precursors are stable and remain so during the heat

treatment→6 hrs at 11500C in N2 atmosphere followed by a few

minutes in O2 for surface treatment .Since the resulting ceramic is

yellow, only copings are made. They can be veneered with

feldspathic ceramic such as Vitadur.

YTZP-CERAMIC-yttriumstabilized tetragonal polycrystals

ceramic:-

Zirconium oxide (Zro2) is currently the strongest white shaded

ceramic available .This type of ceramic powders provides high

performance .In case of stress, the tetragonal crystal phase is

transformed into the hexagonal crystal phase, stress is absorbed and

no crack formation occurs.

This material cannot be processed by simple technologies. The

machining tool must be very strong and abrasion resistant since

Zro2 ceramic is an extremely abrasive product.

74

ORMOCER:-

Ormocer is an acronym for “organically modified ceramics”. These

compounds are also known in literature as “Ormosils” (organically

modified silicates).

Ormocers chemically are methacrylate substituted

alkosilanes/organic- inorganic copolymers. The ormocer matrix

consists of ceramic polysiloxane(silicon- oxygen chains).The alkyl

silyl groups of the silane allow the formation of an inorganic Si-O-

Si network by hydrolysis and condensation polymerization

reactions to give cross- linked structures and their properties may

be modified by filler particle substitution. This chemical reaction

increases the molecular weight upto values between 2000 and

20,000.In comparison, conventional BisGMA only exhibits a

molecular weight of approximately 500.Due to larger initial

molecule polymerization, shrinkage can be reduced considerably

compared to conventional hybrid composites.

This new material can be used for filling in the anterior and

posterior areas, serves as an optimum and upto date replacement for

amalgam, composites and compomers. It has a biocompatible

polysiloxane net with low shrinkage even prior to light curing.

Traditional composites and compomers polymerize organic

monomers (methacrylate) only during light curing resulting in high

shrinkage .In addition, residue monomers may remain and are

released leading to allergic reactions, as with the polymerizations

lamps used, only 60-70% of the free monomers can be converted

throughout the lifetime of the restoration. Silicon oxide, a filler

serves as a basic substance for ormocer. It is modified organically

75

by adding polymerisable side chains in the form of methacrylate

group. Through bonding of the methacrylate molecules to the

carrier medium, the molecules can no longer be eluted during

incomplete polymerization. To achieve x-ray opacity, higher than

that of enamel, silicon can partly be replaced by heavier elements

such as zirconium. Incorporaion of special glass fillers results in a

paste like composite material which should be easy to use.

A material based on the ormocer concept has recently become

available-“DEFINITE”, Degussa dental, Hanau, Germany. The

manufacturers claimed low shrinkage, high abrasion resistance,

condensability, timeless esthetics, biocompatibility and protection

against caries (No long term clinical trial results are yet available.)It

is supplied in 12 tooth shades that are based on the VITA shade

guide.

Properties of DEFINITE:-

a) ‘DEFINITE’ is an aqueous extract and is biocompatible.

b) Polymerization shrinkage of Definite is only 1.88%.

c) Finer structure of ormocer provides Definite with an excellent

abrasion resistance. Consequently Definite can be used in the

posterior area that is exposed to masticatory load and ensures

outstanding long term stability of the filling in this area.

d) Definite permanently releases fluoride, calcium and phosphate

ions that protect the adjoining cavity margin.

Physical properties as given by WALTER:-

a) Bending strength (3-point bending test)-100-160mpa.

b) Modulus of elasticity -10-17GPa.

c) Co –efficient of thermal expansion -17-25x10-6k-1

76

d) Water uptake -<1.2%

e) Solubility in water-not detectable.

f) Shrinkage-1.7-2.5vol%

CEROMER:-

Ceromer is an acronym for “ceramic optimized polymer”.

This restorative material is biocompatible, metal free which exhibits

the strength and potential wear resistance of metal supported

restorations and can be effectively adjusted and polished chair-side.

Chemistry:

The new ceromer system “TETRIC CERAM “and “TETRIC

FLOW” (Vivadent) offer two consistencies to accommodate

clinical demands. Tetric ceram belongs to a new generation of

ceromer materials with an enhanced filler composition, containing

80% filler –Ytterbium trifluoride with addition of barium alumino

flurosilicate glass particles to provide high fluoride release and

improve radioopacity. Spherical ceramic particles and pyrolitic

silica complete the composition.

This unique combination of fine silanised filler particles contribute

to the wear resistance of Tetric ceram and the particle size varies

from 0.04µm-3µm.A spherical rheological modifier was

incorporated into the ceromer, which consists of silicate platelet

agglomerates. During the application, the platelets deaggregrate and

disperse in the composite increment, allowing the material to be

easily contoured and modeled without significant slumping.

77

In 1996, Ivoclar launched Targis, which contains approximately 77

wt%filler (57vol %) and 23wt% organic resin. The filler part is

trimodal and consists of barium glass with a mean particle size of

1µm; spheroidal silica filler with a mean particle size of 25µm as

well as colloidal silica. The resin matrix consists of conventional

monomers. Superior properties of Targis are claimed to result from

an “Optimized chemical process” and an “Optimized curing

process”. The final curing takes place in a Targis power light curing

unit at approximately 97oc for 25 minutes. An organic matrix fills

the spaces between the particles and reinforces the homogenous,

three-dimensional inorganic structure in a restoration which

exhibits natural aesthetics, wear compatibility and improved wear

abrasion due to enamel like hardness of ceromer and its high

flexural strength.

INDICATIONS:

a) Represents alternatives to conventional crown and bridge

remedies for the treatment of single and multiple unit anterior

and posterior restorations in which supragingival preparation

design can improve soft tissue compatibility.

b) Benefit of adhesive bonding to short clinical crown preparation.

c) Posterior bridges with single pontics between the abutment teeth

are primary indication for TARGIS system.

d) Indicated for inlays, onlays, single and multiple unit restorations,

implant superstructures and bridges with metal framework.

e) Also indicated in cases in which centric holding cusps are

weakened or undermined.

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CONTRAINDICATIONS:

a) When complete isolation cannot be achieved.

b) Targis system is contraindicated for cases in which the

preparation margins are subgingival or in which more than one

pontic exists between two abutments.

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CONCLUSIONCeramics have a great past in dentistry .Their unsurpassed aesthetic

and biocompatible qualities have positioned them in the high -end

segment of restorative dentistry and provide the stimulus to seek to

overcome their limitations and continue laboratory and clinical

research .The future of ceramics is even greater since they offer great

potential for improvement , especially in manufacturing

technology .However the co-operation between the excellent dentist

and the highly - skilled dental technician is unavoidable.

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