MATERIALS USED IN FIXED RESTORATIONS

Materials used in fixed restorations can be classified as:

Metals

Porcelain

Resins and solders

METALS

Taggart in 1907 introduced the “lost wax” technique for casting dental

restorations. Veneering of metal substructure with porcelain became successful

in the late 1950’s

Classification of dental casting alloys

A. According to function

1. Gold casting alloys (Bureau of standards,1927).

Type I (soft). Small inlays-easily burnished, subject to slight stress.

Type II (medium) Inlays subject to moderate stress, thick three-quarter crowns,

abutments, Pontics and full crowns.

Type III (hard). Inlays subject to high stress, thin three quarter crowns, thin cast

backings, abutments pontics, full crowns, denture bases and

short span fixed partial dentures. These alloys can be age

hardened.

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Type IV (extra hard) Inlays subject to very high stress, denture base bars,

clasps, long span FPDS, full crowns can be age

hardened.

Types III and IV are generally called “Crown and bridge alloys”.

2. Metal ceramic (hard and extra hard)

Suitable for veneering with dental porcelain, copings, thin walled

crowns, short span FPDS (hard type) and long span FPDS (extra hard type)

3. Removable partial denture alloys-Base metal alloys and type IV gold alloys.

B. According to description (composition).

1. Crown and bridge alloys.

a) Gold based (noble) i) type III& IV gold (high gold)

ii) alternative crown and bridge alloys (low

gold)containing less than 60% but more than

40%Au.

b) Non gold-based

i) Sliver palladium alloys 70 –72 %Ag, 25%Pd. Pd resists the

tarnishing of Ag. Some Ag-pd alloys contain small amounts (15%)

of Cu and have properties similar to type IV gold alloys.

ii) Base metal alloys Ni-or co- based, cheaper

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2. Metal ceramic alloys

a) Noble metal alloys

i) Gold –platinum – palladium

ii) Gold –palladium –silver

iii) Gold –palladium

iv) Palladium –silver

v) High palladium

b) Base metal alloys

i) Nickel – chromium

ii) Cobalt – chromium

iii) Other systems

The silver in pd-Ag alloys can cause discoloration (yellow, green or

brown) of some porcelains. Non-greening porcelain systems have partially

over come this.

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Typical compositions of some modern noble metal dental alloys

Au% Cu% Ag% Pd% In, Sn, Fe Ga, Zn

Type I Gold 83 6 10 0.5 Balance

Type II Gold 77 7 14 1 Balance

Type III Gold 75 9 11 3.5 Balance

Type III Low gold 46 8 39 6 Balance

Type III Ag-Pd - - 70 25 Balance

Type IV Gold 69 10 12.5 3.5 (+30.opt)

Balance

Type IV Low gold 56 14 25 4 Balance

Type IV Ag-pd 15 14 45 25 Balance

Metal ceramic (White) Gold 52 - - 38 Balance

Metal ceramic Pd-Ag - - 30 60 Balance

Metal ceramic (yellow) Gold 88 - 1 6.5(+4.0pt) Balance

Metal ceramic High pd 0-6 0-15or0-

8Co

0-6.5 74-88 Balance

The physical properties and handling characteristics of Ni-Cr alloys are

improved by addition of 2%by weight of beryllium. One particular brand of Ni-

Cr-Be alloy has a low enough casting temperature to be successfully cast into a

gypsum bonded investment. Ni gives strength and Cr the passivating effect

which makes the alloy corrosion resistant. Be reduces vision temperature,

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improves casting characteristics, refines grain structure and participates in

porcelain bonding.

Metal ceramic alloys have 3 common features:

a) The potential to bond to dental porcelain

b) Coefficient of thermal expansion compatible with porcelain

c) Sufficiently high solidus temperature permitting the application of

low fusing porcelains.

Properties of modern crown and bridge alloys

Among the minor additives, zinc is added primarily as an oxygen

scavenger. In the absence of Zn, silver causes absorption of O2 during melting,

the O2 rejected during solidification causes gas porosity. Indium, tin and iron

harden the alloy. The elimination of Ag from these alloys markedly decreases

the propensity for the green stain at the margins of the metal porcelain

interface. All modern noble metal crown and bridge alloys are fine grain.

Copper is the principal hardener; in excessive amounts it reddens the yellow

alloys and reduces resistance to tarnish and corrosion.

Silver minimizes this reddening effect.

Pd hardens and whitens the alloy and reduces its cost.

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Lower gold content alloys

A 42%gold alloy containing 9% palladium was clinically found to

tarnish less than a75%gold alloy containing no palladium. This knowledge led

to the introduction of Ag-pd type III and IV alloys containing little, of any,

gold. In type IV Ag-Pd alloy, gold is added not for its nobility and colour, but

for its age hardening effect. When Cu is added to the Ag-pd alloy, the melting

range is reduced to permit the use of gypsum bonded investment and gas air

torch.

Physical properties

The upper limit of the melting range is the liquidus. When 75to 1500c is

added to the liquidus, we arrive at the casting temperature. The lower limit or

solidus can similarly be used to obtain the maximum soldering temperature.

The metal ceramic alloys should have high melting range so that the

metal is solid well above the porcelain baking temperature to minimize

distortion (sag) of the casting.

Heat treatment of noble metal alloys

Gold alloys can be subjected to hardening heat treatment or age

hardening, if the alloy contains sufficient amount of Cu. Type I, and II alloys

do not harden like type III and IV alloys. The alloys can also be softened by

softening heat treatment or solution heat treatment.

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Softening heat treatment

The casting is placed in a furnace at 700°C for 10 minutes and then

quenched in water. All intermediate phases are changed to a disordered solid

solution, and the rapid quenching prevents ordering during cooling. Tensile

strength, proportional limit and hardness are reduced by such a treatment, but

the ductility is increased. This enables the metal to be ground, shaped or

otherwise cold worked, either in or out of the mouth.

Hardening heat treatment

The dental casting is soaked or aged for 15 to 30 minutes at 200°C to

450°C. The casting is subjected to a softening heat treatment to relieve all

strain hardening before a hardening heat treatment. The proportional limit (or

yield strength) and modulus of resilience and hardness are increased which

makes the prosthesis withstand mechanical stresses without permanent

deformation. Some ductility is essential if margin and adjustment and

burnishing are to be done. But a cast prosthesis that has undergone plastic

deformation has failed in service. Ductility is decreased by age hardening.

Casting shrinkage

This occurs in three stages

1) Thermal contraction of the liquid metal between the temperature to which it

is heated and the liquidus temperature.

2) Contraction of metal from liquid to solid state; and

3) Thermal contraction of solid melt down to room temperature.

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Linear casting shrinkage of inlay casting gold alloys

Metal Casting shrinkage (%)

Gold (100%)

22-karat alloy

Type-I

Type-II

Type-III

Base metal

1.67

1.50

1.56

1.37

1.42

2.4%

Platinum, palladium and copper are effective in reducing casting

shrinkage. As thermal contraction of the alloy as it cools to room temperature

dominates casting shrinkage the higher melting alloys tend to exhibit greater

shrinkage.

General features of metal ceramic alloys

Porcelain has low tensile and shear strength but can resist compressive

stresses with reasonable success. To facilitate compressive loading, and

porcelain is fused to a cast alloy substructure which fits over the prepared

tooth, this can avoid or minimize brittle fracture. Earlier, mechanical retention

and undercuts were used to prevent detachment of the ceramic veneer. By

adding less than 1% oxide forming elements such as iron, indium and tin to the

high gold content alloy, the porcelain-metal bond strength was improved 3

times.

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Mechanical properties

The prosthesis should be rigid to avoid brittle fracture of porcelain.

Doubling the thickness of the metal substructure increases the rigidity by a

factor of 8. But occlusion and esthetics limit the extent to which the metal

thickness can be increased. Base metal alloys have a modulus of elasticity

approximately thrice that of previously used gold alloys and hence are more

suitable for long span bridges and thinner castings. Base metals are harder,

reducing occlusal wear significantly. Density of base metal alloys is 8.0

gm/cm3 compared to 18.39 gm/cm3 for comparable noble metal alloys, thus

making centrifugal casting of base metal alloys easier and precise.

Sag resistance is the ability of an alloy to resist permanent deformation

or wear induced by thermal stresses. It is particularly important in long span

bridges during porcelain firing. Base metal alloys will deform less than 0.001

inch, while a noble metal alloy will deform 0.009 inch. The higher fusion

temperature of base metal alloys also contributes to their superior sag

resistance.

To be compatible, the alloy must not interact with the ceramic so as to

visibly discolour the porcelain, and their bond should be strong. The use of

base metal alloys has increased rapidly at the expense of the high noble metal

ceramic alloys.

Working characteristics

a) Ease of casting : Alloy must be easy to melt and must rapidly fill the mold.

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b) Ease of soldering : the liquid solder must wet the alloy surface readily,

noble metal alloys render themselves well to both pre-ceramic and post-

ceramic soldering.

c) Ease of burnishing : noble metals with high-gold or high palladium contents

are burnishable. Ni-Cr alloys have lower casting accuracy and greater

surface roughening than cold alloys, but higher strength and sag resistance.

Casting investments

a) Gypsum bonded investments – for gold based crown and bridge alloys.

b) Carbon containing phosphate bonded investments – for gold based metal

ceramic alloys.

c) Non-carbon phosphate bonded investments – for non gold based alloys like

Ni-Cr or Co-Cr alloys.

Biological considerations

Inhalation of dust and fumes of Beryllium is toxic and hence exhaust

ventilation is necessary. Aspiration of Ni containing dust can be carcinogenic.

Ni can also cause contact dermatitis and hence is contraindicated in Ni-

sensitive patients.

Etching base metal alloys

Maryland bridge utilizes micromechanical retention of etched-metal

resin-bonded retainers. The etching of metal surface can be done either

electrolytically or using chemical etchants.

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Recycling of noble metal casting alloys

Noble metal alloys are significantly stable to react two or three times.

The non-volatile base metals like Zn, In, Sn and Fe may be lost during

remelting and this loss can be compensated by adding equal amounts of fresh

alloy to the scrap before melting.

Dental Ceramics

Dental porcelains are used to make denture teeth, single unit crowns,

fixed partial dentures and labial veneers. Single unit crown may be porcelain

jacket crown (PJC), a metal ceramic crown or porcelain-fused to metal

restoration (PFM), or the newer glass-ceramic crown.

General Considerations

Composition

i. Silica (SiO2) the crystalline form or quartz is used.

ii. Sodium, potassium or calcium carbonate increases the fluidity and

decreases the softening temperature. These glass modifiers are added in

varying amounts to produce three types of porcelains based on their firing

temperature.

High fusing : 2350 to 2500°F (1290 to 1370°C)

Medium fusing : 2000 to 2300°F (1095 to 1260°C)

Low fusing : 1600 to 1950°F (870 to 1065°C)

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iii. Feldspar – a natural mineral containing potash (K2O), alumina (Al2O3),

and silica (SiO2). Fledspar when fired at high temperatures can form a glass

phase that softens and flows slightly at porcelain firing temperatures. This

softened phase allows the porcelain particles to coalesce togather at high

temperature without complete melting – a process referred to as sintering.

Feldspar when heated between 1150°C and 1530°C undergoes

incongruent melting to form the crystalling mineral leucite, which is

potassium-aluminium-silicate mineral with a large coefficient of thermal

expansion. This property is utilized in the manufacturer of porcelains for fusing

to metal.

iv. Other additions : Boric oxide (B2O3) is added in small amounts to act as

a glass modifier to decrease the viscosity and lower the softening

temperature. It also forms its own glass network.

Pigmenting oxides are added to obtain various shades to simulate natural

teeth. These pigments are produced by fusing metallic oxides together with fine

glass and feldspar and then regrinding to a powder. These powders are blended

with unpigmented powdered frit to provide proper hue and shade.

Brown – Fe or Ni oxides

Green – Cu oxide

Yellowish brown – Ti oxide

Lavender – Mn oxide

Blue – Co oxide

Opacity is achieved by adding Zirconium, Titanium, Tin oxides.

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Mechanical behaviour and physical properties

Materials fail to exhibit the strengths that we expect from interatomic

bonds. This is because of the minute scratches and other defects present on

their surface. The defects have sharp notches whose tips are as narrow as the

spacing between the atoms. Due to stress concentration at the tips of the

notches the bonds at the notch tip break leading to crack propagation. As the

brittle ceramic have no mechanism for yielding to stress without fracture as do

metals, cracks propagate at low stress levels. So their tensile strengths are much

lower than their compressive strengths.

Methods of strengthening porcelain

Smoothen and reduction of surface flow is one of the reasons for glazing

dental porcelain, which produces a very large increase in their strength.

Strengthening of brittle materials can be done either by the introduction

of residual compressive stresses into the surface of the material or by the

interruption of crack propagation through the material.

1) Introduction of residual compressive stresses

Strengthening is gained by virtue of the fact that there residual stresses

must first be negated by the applied force before any tensile stresses can be

created in the object. Residual compressive stresses can be introduced by the

following techniques.

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a) Ion exchange (chemical tempering)

A sodium containing glass article is placed in a bath of molten

potassium nitrate, when some K ion in the bath exchange places with Na ions

on the glass surface. The larger K ions squeeze into the same space occupied

by the Na ions leading to a very large increase in residual compressive stress in

the glass surface.

b) Thermal tempering

Here the object is rapidly cooled (quenched) while it is in the soft

(molten) state. As the solidifying molten cone tries to shrink or pull the rigid

solidified outer skin, residual compressive stresses are created in the outer skin.

c) Thermal expansion coefficient mismatch

Here a metal housing slightly larger coefficient of thermal expansion is

used. During cooling from the firing temperature, the metal contracts slightly

more than the porcelain. This mismatch leaves the porcelain in residual

compression.

2) Interruption of crack propagation

a) Dispersion of a crystalline phase.

A tough crystalline material such as alumina (Al2O3) is added to glass in

a particulate form. The glass is toughened and strengthened because the crack

cannot penetrate the alumina particles easily. This technique has been utilized

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in the development of “alumina particles easily. This technique has been

utilized in the development of aluminous porcelains”.

This technique is also used in the cast glass crown Dicor where the glass

crown is subjected to a heat treatment that causes microscopic mica crystals to

grow in the glass, these crystals interrupt crack propagation.

For maximum reinforcing effect, the dispersed phase should have a

minimum difference in thermal expansion with the glass.

b) Transformation toughening

This involves incorporation of a crystalline material that is capable of

undergoing a change in crystal structure when placed under stress, absorbing

the energy from the crack. Partially stabilized zirconia (PSZ) is the usually

used crystalline material. The disadvantage is that it can produce an opacifying

effect.

Design of ceramic restorations

The design should avoid subjecting the porcelain to high tensile stresses.

So PJCs are contraindicted for restoring posterior teeth. Even on anterior teeth

with deep vertical overlap and moderate horizontal overlap PFM restoration is

to be preferred over PJC.

To prevent stress concentration sharp line angles on the preparation and

sudden changes in porcelain thickness should be avoided. The coping surface

in PFM should also not have sharp line angles.

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Colour of porcelain

Porcelain is an esthetic restorative material capable of matching the

adjacent tooth in translucence, colour and intensity. Complete colour matching

is difficult. The same object may show slight variation in colour when viewed

under different types of light sources this is the phenomenon of metamerism. A

shade guide is used to match the colour. Ideally colour matching is done under

the illumination of northern light from a blue sky as this light contains the most

even balance of light wavelengths. If this light source cannot be obtained,

colour matching should be done under two or more different light sources.

The opacity, of the cementing medium also affects the esthetic qualities

of a PJC. Zinc phosphate cement is opaque whereas silicophosphate and glass

ionomer cements are more translucent. Many cements are specifically tinted for

colour matching.

Fabrication of a ceramic restorations

Condensation : Porcelain is supplied as a fine powder that is mixed with

distilled water or another vehicle and condensed into the desired form. Particles

of different sizes allow dense packing. Dense packing has the benefits of lower

firing and less porosity in the fired porcelain. Condensation is achieved by

vibration, spatulation and brush techniques.

When mild vibrations used for packing, the excess water is blotted away

with a clean tissue. In the second method, a small spatula is used to apply and

smooth the wet porcelain. The smoothen action brings excess water to the

surface, where it is removed. In the brush technique, dry powder is placed with

a brush to the side opposite from an increment of wet porcelain. As the water is

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drawn toward the dry powder, the wet particles are pulled together (by

capillary action). The porcelain must never be allowed to dry out before

condensation is complete.

Firing procedures

After condensation, the restoration is placed on a fire-clay slab or tray

and inserted in the muffle of a porcelain furnace. Porcelain should not contact

the muffle walls or floor. Porcelain can embrittle the heating element if the

latter is contracted. During firing the powder particles fuse together.

The condensed porcelain is first placed in front of the muffle of a

preheated furnace (approximately 650°C) for 5 minutes to permit the water

vapour to dissipate, before the firing. During firing, the porcelain particles unite

at their points of contact and then the fused glass gradually flows to fill up the

air spaces. However, the mass is too viscous to allow the escape of air. Porosity

can be reduced by vacuum offset firing.

Glazing

Stains and glazes provide a more life-like appearance. External staining

is subject to chemical durability, problems. Internal staining is permanent and

life-like, particularly when simulated craze lines are built into it. Internal

staining and characterization have the disadvantage that the porcelain must be

completely stripped if staining is unsuitable.

Glazed porcelain is much stronger and prevents crack propagation then

adjust the occlusion, the transverse strength is halved.

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Cooling : sudden cooling from the firing temperature can fracture the glass.

Cooling a metal ceramic restoration too slowly can cause the coefficient of

thermal expansion of porcelain to increase and can actually make it more likely

to crack or craze.

Metal ceramic crown

When porcelain is bonded to an inner skin of metal, cracks can develop

only when the metal is deformed or broken. Esthetically, the PFM restorations

are slightly inferior to the PJC.

PFM utilizes cast or non cast metal copings.

Cast coping

To be fused to metal, the porcelains have to be low fusing and have a

coefficient of thermal expansion considerably higher than ordinary porcelains.

The alloys used should have higher melting ranges to prevent sag, creep or

melting during firing.

Gold alloys used for cast coping contain about 1% of base metals such

as Fe, In and Sn which form a surface oxide layer during “degassing” and this

layer is responsible for development of a bond with porcelain. The porcelain –

metal bond is primarily chemical in nature and is capable of forming even

when the metal surface is smooth i.e. when there is no opportunity for

mechanical interlocking. Both metal and ceramic must have closely matched

coefficients of thermal expansion, to minimize residual thermal stresses in the

latter.

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In PFM fabrication, the ceramic should contain greater amounts of soda

and potash to increase the thermal expansion to a level compatible with the

metal. Opaque porcelains contain large amounts of metallic oxide opacifiers to

conceal the underlying metal and to minimize the thickness of the opaque

layer. The metal and porcelain should preferably have compatible thermal

conductivity to resist thermal shock.

Because of the high melting temperature of the alloys, gypsum

investments can not be used, a phosphate bonded or silica bonded investment is

used. Thermal expansion is utilized to compensate casting shrinkage. The

casting should be carefully cleaned to ensure a strong bond to porcelain.

Degassing also burns off surface impurities. Oil from fingers can be a

contaminant. Ceramic bonded stones may be used for cleaning the surface.

Final texturing with an 25 alumina air abrasive makes porcelain bond to

mechanically receptive surface.

Opaque porcelain is condensed to a thickness of 0.2mm and fired to its

maturing temperature. This is followed by translucent porcelain and finally the

glaze.

Unlike acrylic resin veneered structures there is almost no wear by

abrasion or change in colour because of microleakage between porcelain

veneer and metal PFM requires removal of more tooth structure than for PJC.

Bonded platinum foil coping

Here tin oxide coating on platinum foil is utilized for porcelain bonding.

Here the thicker metal coping is replaced by a thin platinum foil, giving more

room for porcelain. Aluminous porcelain is used.

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Swaged gold alloy foil coping

Renaissance is a laminated gold alloy foil having a fluted shaped which

is swaged on to the die and flame –sintered to form a coping. An “interfacial

alloy”, powder is applied and fired, and the coping is then veneered with

porcelain.

Porcelain-metal bond

Chemical and mechanical bonds exist. Alloys that form adherent oxides

during degassing form good chemical bond with porcelain, whereas those

alloys with poorly adherent oxides form poor bonds. Minor elements like Sn or

In are believed to migrate to the interface where they oxidize and form covalent

or ionic bonds across the interface. Some Pd-Ag alloys form no external oxide

at all, but rather oxidize internally, these alloys need mechanical bonding.

Shear tests show that bond failure can be cohesive through the porcelain,

metal-oxide or metal, or adhesive at the metal-porcelain, metal oxide-porcelain

or metal oxide-metal interfaces, or a mixture of cohesive and adhesive shear

strength and tensile strength of porcelain are when fired in oxygen than when

vacuum fired.

Bonding using electrode position

Electrode-position of a layer pure gold onto the cast metal, followed by

a short “flashing deposition of tin, has been shown to improve the wetting of

porcelain onto the metal and to reduce porosity at the porcelain metal interface.

The electrodeposited layer also inhibits ion penetration from the metal, and acts

as a buffer zone to absorb stresses caused by differentials in the coefficients of

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thermal expansion between the metal casting and the porcelain during cooling.

The gold colour of the oxide film enhances the vitality of the porcelain when

compared with the normal dark oxides that require heavy opaque layers.

Castable Glass ceramic crown

The castable glass ceramic, or Dicor was introduced to dentistry in 1984.

Glass ceramics are composite materials of a glassy matrix phase and a crystal

phase. Dicor is comprised of SiO2, K2O, MgO, MgF2, small amounts of Al2O3

and ZrO2 and a fluoresing agent. It is technically described as Tetrasilicic

fluoromica glass-ceramic. It is formed into full crown restorations by a lost

wax casting process. After the transparent glass casting is recovered, it is

subjected to a heat treatment to induce partial devitrification (i.e. loss of glass

structure by crystallization), a process called ceramming. Ceramming causes

microscopic plate- like particles of crystalline material (mica) to grow within

the glass matrix. After ceramming, it is coated with a thin layer of porcelain to

provide esthetics. The final colour of the restoration is due, in part from the

colour picked up from the adjacent teeth (“chemeleon” effect) and in part from

the tinted cements used in luting.

Flaws (Griffith’s flaws) developing on the surface of glass are prevented

from propagating, by the mica crystals. The marginal adaptation or fit of Dicor

is better than gold crowns. The biocompatibility of glass ceramics is excellent.

The soft tissue response of glass ceramic restoration is similar to that of

unrestored control teeth because:

a. The marginal adaptation is exceptional

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b. The fluoride content of the material inhibits bacterial colonization and

c. The surface of the restoration is smooth and non porous.

Dicor has a low wear potential and low thermal conductivity that

insulates the underlying tooth from changes in temperature. The fabrication of

Dicor is simple, as the lost wax technique is used. Castable ceramics provide

life like vitality. Dicor can be used for single restoration like full veneer

restorations on anterior and posterior teeth, inlays, onlays, three-quarter

crowns, partial veneers and recently laminate veneers. It is contraindicated on

teeth with short clinical crowns.

Another castable glass ceramic developed in Japan produces

hydroxyapatite crystals in the glass matrix instead of mica crystals, on

ceramming.

Injection molded glass ceramic crown

This is a shrink free ceramic crown, marketed originally under the name

Cerestore. Conventional ceramics shrink 10 to 20% during firing. The primary

constituents in Cerestore are MgO, Al2O3, glass frit, silicone resin and kaolin.

These non shrink ceramics have good flexural strength.

The technique involves construction of a special non shrinking epoxy

die of the prepared tooth. A wax pattern of the coping is found on this die. The

die and pattern are invested in a gypsum bonded investment and the wax

removed with boiling water. The investment mold is then heated to 180°C. the

ceramic material supplied as dense pellets is heated to until the silicone retim

carrier in the ceramic is flowable and then injection molded by pressure into

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the heated mold. The green state coping is retrieved, sprue removed and any

adjustments made. It is then subjected to a very high temperature, firing cycle

to form a true glass ceramic core or coping.

Over this coping low fusing dentin and enamel porcelains are applied to

develop the external shape and esthetics. The equipment required is specialized

and expensive. The technique is time consuming and calls for extra attention to

detail.

Porcelain veneers, inlays and onlays

Here the tooth enamel or metal is etched and resin cement is used as the

cementing agent for the porcelain laminates. Ceramic veneers can be used on

stained hypoplastic teeth, and provide excellent esthetics. Cost and wear of

opposing natural teeth are the drawbacks.

Chemical stability

Topical fluorides such as APF and stannous fluoride, used for caries

control, produce hydrofluoric acid which etches glass and leads to surface

roughness of ceramic restorations. Hence, APF gels should not be used when

glazed porcelain restorations are present. If such a gel is used, the surface of the

restoration should be protected with Vaseline, cocoa butter or wax.

Resins

Resins may be indicated for an individual restoration or as a veneer over

a casting.

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Advantages

Esthetics

Low cost

Convenient repair, even intra orally

Ease of fabrication

No abrasion of opposing teeth

Disadvantages

Low proportional limit and pronounced plastic deformation distortion on

occlusal loading, hence resin should be protected with metal occlusal surface.

Microleakage and staining under veneers

Dimensional change during thermal cycling and water sorption

Surface staining and intrinsic discolouration.

Tooth brush wear

Resin veneered metal restoration unsuitable for RPD clasping.

Types of synthetic resins

Type I (acrylic)

Type II (dimethacrylate)

Type III (composite)

Acrylic resins are powder liquid systems based on methyl methacrylate

and similar to self cured acrylic resins. Dimethacrylate resins are cured at

higher temperatures and yield cross linked wear resistant resins. Microfilled

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composite resins use BIS-GMA, Urethane dimethacrylate, or 4, 8

di(methacryloxy methylene)-tricyclo-(5.2.1.02.6) decane resin matrixes. These

new resins are polymerized using light, or heat and pressure. Microfilled resins

have superior physical properties including better wear resistance than the

original unfilled resin.

Earlier resins had low strength and hardness, and high water sorption.

The accelerated loss of material exposed the metal framework, which required

repair with a direct filling resin.

The disparity in thermal expansion and lack of adhesion between resin

and metal lead to percolation of fluids at the resin-metal interface contributing

to discolouration of the resin and corrosion of non-noble alloys.

Rigidity of metal frame work is needed to prevent plastic deformation.

Processing porosity also leads to weakness of resin, opaque appearance,

potential for incubating micro-organisms and tissue irritation due to roughness.

Porcelains have largely replaced resins.

Resins are indicated where porcelains cause undue wear of opposing

teeth and restorations. The development of wear resistant, esthetic resin

material is warranted to meet clinical demands.

An acrylic resin called Pyroplast is still used for esthetic veneering of

castings. The polymer is mixed with monomer and applied in small increments

to the casting. It is cured in a special curing over at 275°F for 8 minutes. Then

the gingival and incisal colours are applied and blended, curing follows each

lamination. After processing the veneer is finished and polished.

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Composite Resins

Isosit, was the first chemically activated composite resin used for FPD

work. It is cured using pressure and temperature.

The majority of composite resin material, use visible light for

polymerization. A single paste is used. One system utilizes a diketone,

camphoroquinone, and a reducing agent N,N-dimethyl aminoethyl-

methacrylate.

Resin Retention

Mechanical retention or an intermediary coupling agent is used in

bonding, resin to metals framework. Retentive beads, loops or ladders have

been suggested. Opaque layer does not obstruct the retentive patterns

completely, the resin is also locked in.

Adhesive coupling agents are a recent introduction one system utilizes

flaming silica onto the metal. Another system of resin retention involves

electrolytically etching a microretentive surface, high bond strengths are

accomplished. These new techniques allow a more conservative preparation,

reduced cost and improved esthetics. A disadvantage is the difficulty in clinical

repairs of fractured veneers.

Clinical applications

In resin veneer areas, tooth structure is to be reduced by 1.5 to 2mm

depth. A beveled solder is prepared on the labial surfaces into the interproximal

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surfaces. It blends into a Chamfer finish line in veneer areas. The occlusal

surface of the restoration should be in metal.

Complete crowns in resin are only interim restorations. In mandibular

central and lateral incisors extensive tooth reduction can be avoided using resin

over minimum metallic framework. 1 to 1.5mm reduction is enough.

The pontic of an acid etched, resin bonded retainer Maryland bridge is

usually fabricated with dental porcelain and render itself to electrolytic etching.

A heat cured composite material for pontic is a reasonable solution, which is

costless. The tissue surface of pontic can be alloy, which produces a favourable

tissue response.

Custom laminate veneers

Here the teeth are minimally prepared to receive resin veneers 0.5 to

1mm reduction gives attractive results. The fabricated heat cured laminates are

bonded to the etched enamel surface using a composite resin.

Additional applications

Recently composite resin veneering materials have been considered for

use as implant materials and also for custom occlusal splint therapy.

Solders

Soldering is the joining together of metal parts by melting a filler

between them at a temperature below the solidus temperature of the metal

being joined and below 450°C.

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Jelenko classifies solders as:

Group I – traditional gold containing solders

Group II – others (special solders)

Pre ceramic soldering refers to soldering before porcelain application

and post ceramic soldering after porcelain application. Pre-ceramic solders are

high fusing, fusing only slightly beneath the softening point of the parent alloy.

They should flow well above the fusion temperature of the subsequently

applied porcelain. Post ceramic solders must flow well below the pyroplastic

range of porcelain.

Dental gold solders are given a fineness number to indicate the

proportion of pure gold contained in 1000 parts of alloy. A 585 fine solder

contains 58.5% Au, 14% Ag, 19% Cu, 3.50% Sn and 4.5% Zn, and has a flow

temperature of 780°C.

The main requirement of solder is that it fuses safely below the sag or

creep temperature of the casting to be soldered. Pre-ceramic soldering is

relatively difficult and structurally hazardous due to volatilization of base metal

solder constituents due to overheating. Volatilization causes pitting or

microporosity.

Porcelain does not chemically bond equally well to all solders. Solders

should also resist tarnish and corrosion, should flow easily, match the colour of

the units being joined and be strong.

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The noble metal content and Ag: Cu ratio determine the solder’s tarnish

resistance. If the composition of solder and the parent metal differ galvanic

corrosion results.

The surfaces to be soldered should be smoothed with abrasive disks and

not with rubber wheels or polishing compounds. The solder must wet or flow

freely over the metal surface. Ag increases and Cu decreases the flow Low

fineness gold solders are often more fluid. Proximal contacts are added, if

needed with a higher fineness solder since it flows less.

The strength of most solders is greater than the parent metal. Brittleness

is often seen with gold based Cu containing solders, on cooling to room

temperature.

FPDs fabricated from type III gold alloys are joined with gold based

solders and usually water quenched 4 to 5 minutes after soldering. Quenching

immediately after soldering causes warping of the FPD; not quenching leaves a

joint with little or no ductility. A brittle joint may easily fracture. Thus a

disadvantage of post-ceramic soldering is the loss of joint ductility. Since the

components are partially porcelain, quenching is not possible because porcelain

fracture will occur.

Conclusion

The development of newer alloys and porcelains with better working

properties are progressing in an encouraging manner. Researchers are hopeful

in their endeavour to minimize or totally eliminate the drawbacks that are

associated with these materials.

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References

1. Skinner’s Science of Dental Materials, 9th Ed – Ralph W. Phillips

2. Contemporary fixed Prosthodontics, 1st Ed – Stephen F. Rosenstiel, et al

3. Tylman’s Theory and Practice of fixed Prosthodontics, 8th Ed – W.F.P.

Malone et al.

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