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1. 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.
1

2. 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
2

3. 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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4. 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 0-15
or
0-
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,
4

5. 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.
5

6. 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 1500
c 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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7. 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.
7

8. 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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9. 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.
9

10. 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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11. 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)
11

12. 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.
12

13. 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.
13

14. 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
14

15. 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.
15

16. 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
16

17. 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.
17

18. 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.
18

19. 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.
19

20. 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
20

21. 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
21

22. 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
22

23. 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.
23

24. 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
24

25. 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.
25

26. 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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27. 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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28. 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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29. 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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30. 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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