Dental ceramics final

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) REFERENCES/BIBLIOGRAPHY
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INTRODUCTION
The 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 stoneware.
The originator of the first porcelain paste used for denture work was a French
apothecary, Alexis Duchateau. His early dentures were ill-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.
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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.
• 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”.
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• 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 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
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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 -1300o
C(2372o
F)
• Medium fusing-1101-1300o
c(2013-2072o
F)
• Low fusing-850-1100o
c(1562-2012o
F)
• Ultra low fusing-<850o
c(1562o
F)
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.
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• Enamel porcelain: is used in areas requiring maximum translucency, for
example- at the incisal edge.
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
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
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(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
/o
c) compared with feldspar glasses (10x10-6
/o
c). The
tendency of feldspar to form crystals of leucite in liquid glass during “Incongruent
melting” when heated at temperatures between 1150o
c and 1530o
c 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 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.
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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
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.
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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
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
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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 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.
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.
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- 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.
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.
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:-
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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.
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: -
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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.
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 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.
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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
-500
c 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.
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
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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.
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.
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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 PORCELAIN
Methods 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:-
- 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.
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- 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+
.
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, 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
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through the bulk of porcelain, producing an opacifying effect which may
not be aesthetic in most dental restorations.
METAL FREE CERAMIC SYSTEMS/ ALL CERAMIC RESTORATIONS
Porcelain 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.
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
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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.
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 (1200-1250o
c).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.
20

- 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 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 10200
c. 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
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.
21

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.
- 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 9300
c. the
second layer of duceram LFC is applied over base layer using same
technique and baked at 6600
c.
- 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
22

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:-
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 1350o
c, followed by heat-
treatment at 1075o
c for 10hrs. This heat-treatment causes microscopic plate
like crystals of crystalline material (mica) to grow within the glass matrix.
23

This crystal 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:-
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.
24

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):-
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%)
25

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.
. 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.
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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.
- 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.
b) Need for special laboratory equipment (pressing oven and die material)
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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 1150o
c 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.
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.
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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 → 8500
c 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 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.
29

- 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 1120o
c for 10hrs to produce an opaque porous core. This process
is called ‘slip casting’.
- 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 11000
c 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.
30

- 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.
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.
31

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 1170o
c.
-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.
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 (10-50µm) than those of markII (4µm). MarkII has
32

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.
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 unit is to combine the scan head for
the optical impression with the reconstructuion and fabrication module in a single
mobile workstation.
33

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 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.
34

- 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.
- 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.
35

-No metal in mouth.
2) Benefits for the dentist:-
- Economic production in the laboratory.
- Increased precision.
- 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:-
36

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→
symmetric grinding by the
instruments.
e) Radio control
communication:-
Security → 1m distance to
other electric devices.
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.
Allows cable free bi-
directional data transfer to
grinding unit.
-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.
Allows placement of cerec
3unit components as desired.
37

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.
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
38

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 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 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
39

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 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.
40

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 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.
41

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.
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.
42

Based on composition: Firstly all alloys are separated into one of two major types:
- Noble (precious)
-Base metal (non-noble /non precious)
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.
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
43

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 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.
44

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.
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.
45

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 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.
46

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.
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:
47

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 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.
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.
48

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 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.
49

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.
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.
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 second wax strip by capillary
action. The finalized metal coping is then veneered with porcelain. The advantages
of this system include:
50

• 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)
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:-
51

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 (160o
C) 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
/o
C and requires similar low expansion veneer porcelains and since the
coefficient of expansion values for porcelain used with metals averages around
13.5x10-6
/o
C , 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
/o
C. 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.
 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
52

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 11500
C 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.
 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.
53

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 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.
54

 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.
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.
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.
55

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.
CONTRAINDICATIONS:
a) When complete isolation cannot be achieved.
CONCLUSION:
Ceramics 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.
56

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