• The procedure should be done slowly.
• It is time-consuming and requires special equipment.
• Silicone impression material is difficult to electroplate due to low
• Because of its hydrophilic nature, it tends to imbibe water and hence
it cannot be used in polyether impression materials.
• Polysulphide impression can be silver plated but difficult to copper
• Drawback of silver plating is that it involves the use of cyanide
solution which is highly toxic.
Die systems can be classified on the basis of their design as follows:
(i) Working cast with a separate die
(ii) Working cast with a removable die
• Dowel pin systems – straight and curved
Working cast with a separate die
In this method, either two separate impressions are made to get two
casts or a single impression is poured twice to get two separate casts.
Cast poured first from the impression is used as a die and the cast
poured second is used as a working cast. First cast is sectioned to form
a die and the other cast is used as a working cast. Wax pattern is
fabricated on the sectioned die and is then transferred and fitted onto
the working cast. This system is also called multiple pour system.
• This technique is simple and easy to use.
• No special equipment is required.
• As gingival tissues are left intact, they can be used as a guide to
• There are chances of distortion of wax pattern during transfer from
• It is difficult to transfer fragile wax pattern from die to cast.
• Seating of pattern on the cast may be difficult, as the second poured
cast is slightly larger than the first.
These systems have become more popular in recent times because of
ease of manipulating wax pattern during transfer from die to working
model. Also, manipulation of porcelain restorations is simpler.
Ideal requirements of removable die system
• Working cast with dies should be easy to mount on an articulator.
• Dies should be replaced accurately in their position originally
Various removable die systems available are:
This is of two types, namely, straight and curved.
• Dowel pins are tapered, flat-sided pins made up of
• They resist horizontal displacement.
• Dowel pin is positioned over the prepared tooth.
• There are two techniques in which the impression
can be poured with dowel pins, namely, prepour
• In this technique, the dowel pins are positioned
over the prepared tooth and are stabilized using
wire clips or bobby pin and joined by sticky wax.
• Impression is poured with die stone till the area
which covers the tooth part of the impression.
• Dowel pins should never touch the impression and
should be positioned parallel to the long axis of the
• Once the die stone sets, the bobby pins are
• A small ball of wax is placed over the tip of the
dowel pin or a plastic sleeve is placed over the
• Separating medium is applied around the dowel to
aid in easy separation of die from working model.
• Second pour is done with die stone to form the
• After setting of stone, the cast is removed from the
• The dies are sectioned with thin saw blade.
• The dowel is tapped from behind or wax is
removed and the dowel is pushed upwards to ease
its separation from the working model.
• In this technique, the impression is poured till the
• Hole is drilled once the stone sets.
• Dowel pins are cemented with cyanoacrylate into
• The remaining steps are similar to those described
• The technique followed for using curved pins is
similar to the one followed for straight pins.
• Curved dowel pins project from the base of the cast.
• After sawing, the die is removed by pressing the
curved dowel exposed with flat-ended instrument.
FIGURE 29-1 Straight dowel pins.
• It is the reverse drill press system.
• Postpour technique is used here.
• The first poured cast is placed on the worktable of the Pindex drill
• The prepared tooth is positioned below an illuminated red dot.
• The machine accurately drills parallel holes from the underside of
the cast by pressing the worktable downwards.
• The dowel pins are cemented with cyanoacrylate cement.
• Plastic sleeves are placed on the flat end of the dowel pin.
• The procedure is repeated for other prepared tooth/teeth.
• A thin sheet of utility wax is placed over the tip of the dowel pin.
• Stone is poured to make the base.
• After setting of the stone, the dies are sectioned with a saw blade.
• Dies are removed by tapping the dowel below the base of the cast.
• This system has removable die.
• It facilitates accurate placement of die pins.
• Special equipment is required.
• In this system, a special form of plastic tray is used which has
• A full arch impression is poured with die stone.
• After setting, the cast is removed and trimmed in shape so as to fit
• Then a second pour is made with the stone into the tray with the
• After setting of the stone, the tray is disassembled to free the cast.
• The die is sectioned with a saw blade till the internal grooves on the
• The die is broken with finger pressure.
• The process is repeated to separate other dies from the cast.
• It is less costly than Pindex.
• It is simple and easy to prepare.
• There is no use of dowel pins.
• It requires more space for mounting on articulator.
• It requires proper maintenance of the parts of the tray for refitting.
• It is a modification of plastic tray with internal grooves and notches.
• It is used for making working models and dies in laminate veneer
• The technique is similar to the Di-Lok system described above.
• Care is taken during sectioning of the die by saw blade.
• The saw cut is made through the interdental papilla but about 1 mm
short of the interproximal finish line.
• The die here is broken with finger pressure.
Alloy and historical perspective of
An alloy is defined as ‘a mixture of two or more metals or metalloids that
are mutually soluble in the molten state; distinguished as binary, ternary,
quaternary, etc. depending on the number of metals within the mixture’.
Historical perspective of dental casting alloys
Newer developments in dental casting alloys were influenced by the
• Price changes, especially the noble metal
History of dental casting alloys
• 1907: Introduction of lost wax technique.
• 1933: Replacement of CoCr for gold alloys in removable partial
• 1950: Introduction of resin veneers for gold alloys.
• 1959: Introduction of porcelain fused to metal technique.
• 1968: Alternatives to gold alloys such as palladium-based alloys.
• 1971: Nickel-based alloys replacing gold alloys.
• 1980s: Introduction of all-ceramic technique.
• 1999: Gold-based alloys as alternative to palladium-based alloys.
Classification of dental casting alloys and critical
evaluation of precious, semiprecious and
nonprecious alloys in prosthodontics
Classification of dental casting alloys
On the basis of total noble metal content given by American Dental
(i) High noble: Must contain =40% wt Au and =60% wt of noble metal
elements (Au, Pt, Pd, Rh, Ru, Ir, Os); also called precious alloys.
(ii) Noble: Must contain =25% wt of noble metal elements (Au, Pt, Pd,
Rh, Ru, Ir, Os); also called semiprecious alloys.
(iii) Predominantly base metal: Must contain <25% wt of noble metal
alloys; also called nonprecious alloys.
On the basis of mechanical property requirements given by ISO Draft
International Standard 1562 for casting gold alloys (2002):
(i) Type 1: Low strength – Casting which can tolerate very less stress
(e.g. inlays; minimum yield strength is 80 MPa, and minimum
percentage elongation is 18%).
(ii) Type 2: Medium strength – Casting which can tolerate moderate
stress (e.g. inlays, onlays, complete crowns; minimum yield
strength is 180 MPa and minimum percentage elongation is 10%).
(iii) Type 3: High strength – Castings which can tolerate high stresses
(e.g. onlays, thin coping, pontics, crowns and saddles; minimum
yield strength is 270 MPa and minimum percentage elongation is
(iv) Type 4: Extra-high strength – Castings which can tolerate very high
stresses (e.g. saddles, bar, clasps, certain single units and partial
denture frameworks; minimum yield strength is 360 MPa and
minimum percentage elongation is 3%).
Classification of casting metals for all metal and metal–ceramic
prosthesis and partial dentures is given in Table 29-1.
CLASSIFICATION OF CASTING METALS
Note: Alloys of all metal restorations cannot be used for metal–
ceramic restorations but alloys of metal–ceramic restorations can be
used for all metals because of the following reasons:
• Melting range could be too low to resist sag deformation at
• Coefficient of thermal contraction may not match that of porcelain.
• Alloys may not form stable oxide layer for bonding to porcelain.
Classifying casting alloys on the basis of noble metal content as
precious, semiprecious and nonprecious is often misleading. This
classification was given on the basis of cost factor. The term ‘precious’
refers to the cost of the metal and term ‘noble’ refers to the chemical
behaviour of the metal. Both gold and palladium are considered as
noble and precious; palladium is noble but not precious and it is
categorized as semiprecious alloy. Semiprecious alloys include silver–
palladium alloys and alloys containing gold percentage between 10
and 60% although mechanical properties are similar to that of gold
alloys. Nonprecious alloys are referred to as alloys which are low
costing and do not contain noble metal alloys and commonly called
base metal alloys. These have high strength, hardness and are also
tarnish resistant. But the disadvantage of base metal alloys is that
certain contents in them have questionable biocompatibility (e.g.
beryllium does have carcinogenic potential and the patient may be
Casting techniques for casting of base metal alloy
Casting technique of base metal alloy
• Base metal alloys are high-fusing alloys that experience high degree
• To achieve mould expansion, the invested pattern should be placed
• The investment ring is placed in oven at room temperature. The
temperature is brought to 815°C in 1 h.
• The investment ring is heat soaked for 2 h.
• Preheat the quartz crucible in oven.
• The quartz crucible is removed with casting tongs from the oven
and placed in the bracket of casting machine.
• The metal ingots are then placed into the crucible.
• Multiple orifice tip of gas–oxygen torch is used.
• The alloys are heated evenly by moving the torch over it.
• The ingots glow in uniformity and start to flow.
• The machine is released and the molten metal flows in the mould.
• The ring is then bench cooled.
• After cooling, the ring is cleaned and the casting is retrieved and
Casting techniques for casting titanium alloys
• Ti and its alloys are highly biocompatible on the basis of widespread
• Other advantages are its low cost, capability of bonding to ceramic
and resin cements and low thermal conductivity.
• Disadvantage is that it is difficult to cast.
• Ti has a very high melting point of 1668°C and reacts with
conventional investment and oxygen.
• Ti has low specific gravity and flows less than gold alloy when
casted in centrifugal casting machine.
• Special casting machine with arc melting capability and argon
There are three specially designed casting systems:
(i) A pressure/vacuum casting system with separate melting and
casting chamber (Castmatic, Dentaurum).
(ii) A pressure/vacuum system with one chamber for melting and
(iii) Vacuum/centrifuge casting system (Tycast, Jeneric/Penetron, and
• Introduction of newer alloys of Ti with nickel can be cast with
conventional casting methods. Also, they have good bonding with
ceramic and have lesser release of ionic nickel.
• Recently introduced computer-aided design and computer-aided
manufacturing system avoids the use of available casting methods.
Casting defects and their remedies
There can be defects in casting, if proper casting procedure is not
followed. Defected casting results in prosthesis with inferior
mechanical properties and loss of time.
Classification of casting defects
Distortion of the casting is usually due to distortion of
• Setting and hygroscopic expansion of the
investment material can produce uneven
movement on the walls of the pattern.
• Internal release of the stresses in wax pattern can
result in distortion of the casting.
• Distortion can be prevented or minimized by
proper manipulation of the wax at high
• Wax pattern should be invested as soon as possible.
(a) Investment breakdown: Too rapid heating of the
• Proper heating of the mould and alloy
(b) Air bubbles on wax pattern: This results in small
nodules on the casting (Fig. 29-3).
• Proper mixing of the investment
• Correct application of the wetting agent
• Use of vacuum investing technique
(c) Rapid heating rates: This results in fins or spines on
• Heat the ring gradually to 700°C (for 1 h minimum).
• Greater the bulk of the investment, more slowly it
(d) W-to-P ratio: Higher the ratio, rougher the casting.
• If the ratio is lesser, the investment may be thick
and cannot be properly applied to the pattern.
• Use of correct ratio and selecting investment
material of proper particle size.
(e) Prolonged heating: Prolonged heating of the
investment causes disintegration of the investment
and the walls of the mould are roughened.
(f) Pattern position: If several patterns are invested in
the same ring, care should be taken that there is at
least 3 mm spacing between the patterns.
• Casting should be completed as soon as the ring is
(g) Casting pressure: Too high or too low casting
pressure results in rougher surface of the casting.
• Adequate recommended pressure should be used
(h) Composition of the investment: The amount of silica
and quartz influences the surface texture of casting.
Porosity in dental casting is of two types, namely,
internal and external. Internal porosity occurs in the
internal surface of the casting and tends to weaken
it. External porosity occurs on the surface and can
cause discolouration or even secondary caries.
FIGURE 29-3 Air bubbles on wax pattern result in small
FIGURE 29-4 Fins or spines on casting result due to rapid
• Localized shrinkage porosity
• It is usually caused by incomplete flow of molten metal during
• It occurs normally at the sprue casting junction.
• It occurs due to freezing of sprue before the rest of the casting which
results in deficient flow of molten metal and thus causes localized
• It can also occur in the interior portion of the crown where the sprue
attaches; there can be a hot spot created by the molten metal
• This causes the local region to freeze last resulting in suck-back
• It can be prevented by reducing the temperature difference between
the mould and the molten metal.
• Using sprue of correct dimension.
• Additional sprue of small gauze can be used.
• Flaring the point of sprue attachment.
• Placement of reservoir close to the wax pattern.
These are small irregular voids in the casting due to rapid
solidification of the mould or if the casting temperature is too low.
• Most of the metals dissolve gases when they are in molten state.
• On solidification of the metal, the dissolved gases are released
• These porosities are spherical in shape and are similar to the pinhole
porosity but these are larger in size.
• It is also caused by the absorption of gases during solidification.
• Castings severely contaminated with gases are black in colour after
these are removed from the investment material.
• These large porosities can also result from gases occluded from
• It is caused by the nucleation of solid grains and gas bubbles
together such that the metal freezes at the wall of the mould.
• It can be prevented by controlling the flow of molten metal into the
• This type of porosity occurs, if air in the mould is not allowed to
• It results in the formation of large concave depression on the surface
• The entrapment of the air can also occur in dense modern
investment which results in increased mould density.
• Sufficient mould and casting temperatures.
• Sufficiently high casting pressure.
• Proper W-to-P ratio of the investment material.
• Placing the wax pattern not more than 6–8 mm away from the end
Investment materials used in fixed prosthodontics
An investment is a type of material used for forming a mould into
which a molten metal or alloy is casted. The process of forming a
Investing is defined as ‘the process of covering or enveloping, wholly or
in part, an object such as a denture, tooth, wax form, crown, etc. with a
suitable investment material before processing, soldering, or casting’. (GPT
There are three types of investment material commonly used in fixed
(ii) Phosphate-bonded investment
(iii) Silica-bonded investment
Requirements of investment material
• Investment mould should expand sufficiently in order to
compensate for alloy shrinkage on hardening.
• It should have sufficient strength to withstand the heat of burnout.
• It should be easy to manipulate.
• Mix mass should have smooth consistency.
• After casting, it should break easily.
• It should be cost-effective.
• It should accurately replicate the fine detail of the wax pattern.
• It is used for casting low-fusing alloys such as types I, II, III gold
• Based on the temperature, gypsum-bonded investment is of two
types: (i) type I – for use with high-temperature technique and (ii)
type II – for use with low-temperature technique.
• Alpha-hemihydrate (gypsum matrix) acts as a binder: 30–35%.
• Silica (quartz or cristobalite) acts as a refractory material: 60–65%.
• A rigid metal ring is lined with asbestos liner, which allows
expansion to take place in radial direction.
• This investment material should not be heated above 700°C.
• Above 700°C, it shows shrinkage and releases sulphur dioxide
which contaminates the casting.
• Alpha-hemihydrate and chemical modifiers enhance the strength of
It is used for casting high-fusing alloys (e.g. metal–ceramic alloys,
high-fusing noble metal alloys, base metal alloys).
The powder form consists of the following:
• Ammonium diacid phosphate: Acts as a binder and gives strength.
• Silica: Acts as a refractory material and provides high thermal
• Magnesium oxide: Reacts with phosphate ions.
The liquid form consists of the following:
• Aqueous suspension of colloidal silica.
• Ammonium diacid phosphate reacts with magnesium oxide to
form ammonium magnesium phosphate.
• Carbon is added to the powder to produce clean casting and aids in
divesting when gold alloys are casted but should not be used with
• This investment material has poor surface wetting property and has
chances of air bubble incorporation.
• Wax patterns of metal–ceramic fixed partial denture (FPD) should
be invested and casted as a single unit because of problems during
• It is not popular because the procedure takes more time and is
• It consists of silica which acts as a binder and is formed from
aqueous suspension of colloidal silica or ethyl silicate.
• Magnesium oxide is added to increase the strength.
• The powder is mixed with hydrolysed silicate liquid rapidly to form
• The mould is placed on a special vibrator that provides tamping
effect, i.e. the heavier particles settle at the bottom and the liquid
• This investment can be heated between 1090°C and 1180°C and can
be used for casting high-fusing base metal alloys.
Shade selection for the patient
Selection of proper shade is an important aspect in delivering an
aesthetic restoration. There are three essential factors which are
responsible for proper shade match: (i) light source, (ii) the object and
The light source used for shade matching can have definite effect on
the perception of colour. There are three light sources which are
common in dental operatory – natural light, incandescent and
fluorescent. The visible portion of electromagnetic spectrum lies
between 380 and 750 microns. Shade should be matched under more
than one type of light in order to avoid the problem of metamerism.
‘Metamerism is a phenomenon of an object which appears dif erent under
When shade is selected, characteristics of the colour are important
• Hue: Quality which distinguishes one colour from another.
• Chroma: It is the saturation or intensity of hue.
• Value: It is the relative brightness or darkness of the hue.
• Shade should be matched before tooth preparation.
• Selected shade guide should be same as porcelain used by the
• Teeth should be clean before the shade match.
• Make up, if any, should be removed.
• Patient is seated in an upright position with his mouth at the
• If possible, natural light should be used and the tooth should be
• Observations should be made quickly (within 5 s) to avoid fatigue of
• The entire shade guide is scanned quickly and worst matching
shade tabs are selected first and then eliminated.
• By this process of elimination, only a few tabs are left from which
the final shade is to be selected.
• If confusion exists between two tabs, both the tabs are placed on
• Finally, the closest shade is selected.
• In order to provide life-like restoration, the natural tooth should be
carefully observed so that the restoration can be characterized for
features such as craze lines, hypocalcification.
• It is best to draw the facial surface of the tooth in the patient chart
clearly indicating the shade, translucency areas, areas to be
characterized, etc. and this information is given to the technician.
Dentist–technician inter-relationship—
important key to success in fixed
A good communication between the clinician and technician is the key
to high-quality fixed and removable prosthodontics. This can be
achieved by close working relationship of the dentist and laboratory
technician. Clinician should have good knowledge of the laboratory
procedures and its limitation. ADA has listed guidelines for both
dentist and technician to improve inter-relationship so as to deliver
• Provide verbal or written instructions for the lab to proceed with the
fabrication or modification of the prosthesis.
• Provide the laboratory with accurate impression, working models,
interocclusal records or mountings.
• Retain a copy of the written instructions to the laboratory for a
period of time, as this may be required for medicolegal purposes.
• Follow proper infection control methods as laid down by ADA.
• Produce prosthesis as instructed by the clinician using the
• In case of doubt, clarify with the clinician.
• Provide the closest shade match as possible with the available
• Inform the clinician immediately, if there is a delay in the work.
• Inform the clinician about the material used for fabrication.
• Properly follow the infection control protocol given by ADA.
• Finish the laboratory procedure in time.
Following the guidelines by both clinician and technician and
mutual respect will ensure fabrication of high-quality prosthesis with
• Lost wax technique was first used in casting of alloys by W.H.
• Gypsum-bonded investment material is used for casting gold
• Pickling is a method of cleaning gold casting by hot acid solution
• Phosphate-bonded investment is used for casting metal–ceramic
alloys having high-melting temperature.
• The minimum fineness required for dental solder to be corrosion
• Beryllium added to base metal alloys to control oxide formation is a
• Rounded margins on the casting may be caused by wax which is
not completely eliminated during burn out procedure.
• Expansion of the investment by heat during elimination of the wax
Commonly Used Abrasives and Polishing Agents, 416
Biocompatibility of Various Dental Cements Used in Fixed
Failures in Fixed Partial Denture (FPD), 418
Factors Responsible for FPD Failures, 418
The prosthesis retrieved after casting is very rough and should
undergo a series of finishing procedures before it is placed in the
mouth. The internal and the external surfaces of the prosthesis are
finished separately and have different objectives.
• The internal surface of the prosthesis should conform and accurately
• Proper marginal fit should be inspected and the internal surface
should not bind but seat onto the prepared tooth.
• Internal surface of the casting should be inspected under
magnification for small nodules or bubbles.
• It should provide space for film thickness of the cement.
• The internal surface should be conducive to strong luting cement.
• Sandblasting is recommended on the internal surface so that it
becomes more conducive to nonadhesive cement.
• When resin cement is used, special surface treatment may be
recommended for better bonding.
• It should have a highly polished external surface because rough
surface attracts plaque accumulation.
• Before try-in, the metal surface should be given satin finish and at
the time of cementation, the external surface should have high
• Porcelain restoration should be polished and reglazed after a bisque
• Finishing and polishing are done with abrasives of various particle
sizes ranging from coarse to fine.
• When gold restorations are polished, some minute amounts of
abraded surface material are filled into the surface irregularities.
This microcrystalline surface is called the Beilby layer.
An abrasive is a very hard material with sharp cutting edges, which if
slided over the softer surface cuts a series of grooves.
Polishing agents consist of abrasives which are comparatively softer
and are reduced to very fine sizes for finishing of restoration.
Abrasives can be classified as follows:
Some commonly used abrasives and polishing agents are:
• Silicone carbide: This is one of the most commonly used abrasives in
the laboratory. It is sintered or pressed with a binder into grinding
• Diamond: This is the hardest abrasive used for tooth enamel or
porcelain. If used with ductile material, such as gold, the abrasive
particles become clogged with the softer material and the disc or the
• Aluminium oxide: This is manufactured from bauxite. Coarse-grit
aluminium oxide is abrasive and used for finishing metal–ceramic
restorations. Fine-grit aluminium oxide is used for polishing. The
stones of this grit are called poly stones.
• Emery: This is a mixture of aluminium oxide and iron oxide, called
corundum. The greater the content of alumina, finer is its grade. It is
used for finishing porcelain or gold restorations.
• Garnet: This is a red abrasive consisting of silicates of aluminium,
cobalt, manganese and iron. It is bound to paper discs with glue
and used for cutting both metal and ceramic.
• Tin oxide: It is used as fine powder for final intraoral polishing of
• Tripoli: This is a fine siliceous polishing powder and is combined
with wax binder to polish gold restorations initially.
• Rouge: This is a fine red powder consisting of iron oxide, supplied
in cake form. It is an excellent polishing agent for gold restorations.
• Sand: Sand or other forms of quartz called flint are coated on discs
and are available in various grits. It is used for finishing and
polishing of gold restorations.
• Pumice: This is a highly siliceous material used as abrasive or
polishing agent. It is used for polishing dentures and smoothening
• Kieselguhr: It is a mild abrasive and polishing agent.
• Cuttle: This is made from internal calcified shell of cuttlefish. It is
used as a fine polishing agent as paper discs.
Biocompatibility of various dental
cements used in fixed prosthodontics
The purpose of dental cement is to occupy space between the indirect
restoration and the tooth. There are various cements available to lute
the indirect restorations to the tooth. Some of the commonly used
dental cements are briefly mentioned in Table 30-1.
FIGURE 30-1 Resin cement bonds to tooth enamel by
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