CAST GOLD INLAY: Clinical tips for tooth preparation, impression
making and wax pattern for exam going post graduate students
Authors1. Dr. Roopa Nadig
Dean and Head of department
Dayananda Sagar College of Dental Sciences
Bengaluru
2. Dr Srirekha A
Prof and HOD
The Oxford Dental College
Bengaluru
3. Dr. Vedavathi.B
Professor
Dayananda Sagar College of Dental Sciences
Bengaluru
As clinicians, we have the responsibility of choosing the right restoration that
fulfils the functional longevity, comfort, biocompatibility and esthetic needs of
our patients. There has been a tremendous advancement in the rotary
instruments used for tooth preparation and the materials used for making dental
impressions, restorations and adhesive cementation with guided magnification;
which has changed the outlook of indirect restorations.
The indirect restorative procedure requires meticulous care and devotion to
perfection on part of the dentist in cavity preparation and fabrication of
restoration to derive high degree of service and satisfaction to the patient and
hence this exercise has remained as part of final evaluation in university
examinations.
Inlay and Onlay: Definition
The distinction between the two designs is unclear but what generally accepted
is: Those indirect restorations that remain within the body of the tooth without
cuspal coverage (intra coronal) or at times capping a few of the cusps but not all
the cusps would be considered as an Inlay. Whereas Onlays replace tooth tissue
including all cusps.
Indications
Inlays are usually advocated when difficulty is anticipated or experienced in
obtaining an acceptable contour, contact and occlusion with a directly placed
restoration or in restorations that are subjected to high functional stresses.
Some of those clinical situations include …
1. Large proximal caries: Generally, once the cavity width exceeds 1/3rd the
intercuspal distance, a significant amount of the functional stress acts on the
restoration rather than the tooth-demanding a stronger restoration. Also,
obtaining an acceptable proximal contact relation in such cases may not be
possible especially with direct composite restoration.
2. Faulty proximal contact relationships, diastema - are often the culprits for
the initiating proximal caries. Failure to correct the contact and contours of such
teeth often results in food impaction, secondary decay and/ or repeated fractures
of the restoration. Direct restorations can seldom fulfil these criteria.
3. Repeated fracture of a directly placed restoration may also indicate the
placement of an inlay. Common causes for repeated fractures may be again due
to above cited reasons or due to excessive functional forces on account of faulty
occlusal relationships, and /or parafunctional habits.
4. Sub gingival lesions: Difficulty in access , isolation and curing precludes
the use of direct adhesive procedures. Generally cast restorations are preferred
choice in these situations.
Advantages: In all the above mentioned situations, inlay should be considered
as the restoration of choice rather than direct resin restorations as these
restorations have the advantages of being stronger and precise control of
contours and proximal contacts can be achieved.
Disadvantages: May demand more invasive preparations than direct
restorations, increased chair time/ multiple visits, cost, and technically
demanding.
This article addresses the common clinical concerns of post graduate students in
the steps in cast gold inlay exercise.
UNIVERSITY EXAM CRITERIA
Inlay Exercise- 30 marks
(i) Tooth preparation for class II inlay (gold or esthetic)-20 marks
(ii) Fabrication of indirect wax pattern-10marks. (Note: If one prefers to do
an esthetic inlay, unfortunately there is no mention of an equivalent step in the
Dental council of India regulations. Since these regulations are only guidelines,
individual universities/ colleges can perhaps take a decision in this regard.)
Cast gold alloys:
Traditional cast gold alloys have been in use for indirect restorations for over a
century with good survival rate and are considered the “gold standard” till date.
Advantages:
• Strength: Cast restorations have very good yield strength even in thin
sections.
• Ductility of the material is very good which gives excellent marginal
adaptation
• Biocompatibility: Indestructible in oral fluids. Cast gold restorations are
relatively unaffected by tarnish and corrosion.
• Abrasion resistance and low wear rate: similar to tooth enamel.
• Is capable of reproduction of precise form and minute details of cavity
and occlusal morphology
• Any manipulation like soldering can be done even after polishing.
Owing to these desirable properties, cast gold is still preferred by many
clinicians for their patients ;
 When the non-tooth colour is not a major concern. Eg: Molars having
short clinical crowns, difficult to isolate can particularly be challenging to
restore with indirect tooth coloured restorations. More so, if they are
inaccessible and are subjected to abnormal occlusal stresses particularly
in patients who clench or brux.
 Gold inlays are most suitable in sub-gingival restorations, where
difficulty in isolation precludes the use of adhesive technique.
 Also, in patients with para-functional habits, cast gold is preferred for its
strength and abrasion resistance - which prevents abnormal wear of the
opposing teeth.
Based on their ability to withstand stresses, cast gold alloys can be
classified/indicated for following restorations:
Type 1: Low strength – for castings subject to very slight stress, e.g. inlays
Type 2: Medium-strength – for castings subject to moderate stress, e.g. inlays
and onlays
Type 3: High-strength – for castings subject to very high stress, e.g. onlays
Analysis and preplan :
Patient selection: The indications for an indirect inlay restoration should preexist in the case you have selected and remember, YOU DON’T TRY TO
CREATE ONE. Although all the above cited clinical situations are acceptable,
it is preferable to select a straight forward case as one will be working under
stress coupled with few other constraints.
Preferable to avoid:
 Worn out dentition. Should have minimal or no occlusal facets.
 Teeth without opposing tooth in occlusion .
 Very young tooth with large pulp chamber with high pulp horns.
 Tooth with gingival inflammation-as it may induce bleeding during
preparation and impression making
 Deep dentinal caries in close proximity to the pulp
 Subgingival caries.
 Extensive caries with broad contacts on mesial surface of upper first
molar as it falls in the smile zone.
 Teeth with aproximal caries ( unless one of them is already restored )
Preparing the selected case for the exam:
1. Good quality radiographs – IOPA and Bitewing radiographs.
2. Study the extent of caries in enamel and dentin. proximity to the pulp and
gingival extent of caries
3. Study models- help to evaluate the occlusion and occlusal contacts
4. Do a thorough oral prophylaxis a week before exams-avoids gingival
bleeding and other related complications during tooth preparation and
impression making
5. Models and impressions for any temporaries, occlusal stent, Special tray
that is planned
6. Plan the cavity design before starting the case. Consider the caries
extension, anatomy of the tooth, occlusal and proximal relationship and
contour of the tooth that needs to be re-established or improved
7. Evaluate the occlusal contact of teeth in intercuspal position and occlusal
contact that occur during mandibular movements. The pattern of occlusal
contact will influence the cavity design. Any existing occlusal prematurities that need to be eliminated could be addressed before hand.
8. Anesthesia of the operating field is a pre requisite to good dentistry.
Anesthetizing the tooth and adjacent tissues eliminates pain, decreases
salivation as the patient is less sensitive to stimulation of oral tissues,
increases operator’s efficiency because patient is comfortable
9. Magnification of some form will certainly enhance the quality of your
work
Armamentarium: Basic operative chair side tray setup (Fig 1)
1. Burs for inlay cavity preparation
 Carbide burs- No. 271 -elongated pear shaped bur with rounded
end which result in rounded line angles and point angles
169 L-Long shank tapered fissure for fissure extension (Fig 2)
 No. 8865 -flame shaped fine diamond point – to prepare occlusal
and gingival cavosurface bevels as well as the proximal flares-the
circumferential tie features (Fig 2)
 No. 2 or 4 round bur-to remove dentinal caries on the pulpal floor
or axial wall
2. Special hand instruments- Gingival margin trimmers and enamel hatchets
and sharp spoon excavator.
3. Retraction cord-gingival tissue retraction and hemorrhage control (more
so for subgingival caries)
4. Impression trays: stock tray / special trays both are acceptable.
Fig 1-Armamentarium
Fig 2- Dimensions and configuration of No. 271, No. 169L, and No. 8862
instruments.
Basic principles of indirect restorations:
I. Conservation of tooth structure-Maximizes the strength of remaining tooth
structure, lessens post- operative sensitivity and pulpal injury.
II Retention and resistance form:
III Pulpal considerations
IV Gingival considerations
V Finish lines
VI Contours
VII Occlusion
Tooth preparation for cast gold class II inlay
Always status quo of tooth will decide the design of tooth preparation and
never the other way around…
Outline, resistance and retention form:
Preparation path: Is usually parallel to the long axis of the tooth crown. The
preparation should have a single path, opposite to the direction of the occlusal
loading. The cutting instrument is always held parallel to long axis of tooth to
develop a single draw path, so that the cavity does not have any under cuts.
Adhering to this feature will help the retention of the restoration and decreases
its micro movements during function.
Outline form: The occlusal portion should have smooth flowing curves with a
dove tail that provides retention to the restoration and prevents lateral
displacement. The proximal portion is usually box shaped.
Occlusally include all the faulty pits and fissures and proximally till the gingival
extent of the caries is completely eliminated. In the process, the occlusal
portion i.e, the facial, lingual and proximal margins may be located on the
inclined planes of the corresponding cusps, triangular ridges or marginal ridges.
Pulpal floor and axial wall depth: approximately 0.5 mm into the dentin or from
DEJ (with the initial depth of 1.5 mm from the central groove)
Facial and lingual margins of the proximal portion should be in the
corresponding embrasure- makes it self cleansing.
Gingival seat should be on sound tooth structure, preferably in enamel or dentin.
Gingival margins should be extended to include any surface defect or
concavities and to eradicate marginal undercut.
Line angles should be well defined in occlusal and gingival portion, with
rounded axio-pulpal line angle.
External outline form is slightly bigger than the internal outline form and should
have smooth flowing curves avoiding any sort of sharp line angles. Application
of taper by placement of bevels makes the outline form slightly wider for cast
restoration.
Apico-occlusal taper of the restoration:
The preparation should be as parallel as possible for maximum retention.
However, to facilitate seating of the restoration into and out of the preparation, a
slight apico-occlusal taper is preferred. Factors influencing apico-occlusal taper
include the length of preparation wall or axial surfaces, its surface area and the
need to provide for extra retention. The taper should be on an average of 2-5
degrees per wall (Fig 3). It can be increased or decreased depending on the
length of the wall. The greater the wall length, more taper is necessary, but
should not exceed 10 degrees. In shallow cavities, taper should be minimal to
enhance the resistance and retention form. The integrity of the retained marginal
ridge should be enhanced by providing sufficient taper to maintain adequate
dentin support for enamel.
Fig 3-Inlay taper and line of draw (line xy)
Circumferential tie –
It is the peripheral marginal anatomy of the preparation. It includes flares and
bevels. It is designing of the cavosurface margins to minimize microleakage.
The primary flare is the conventional and basic part of the circumferential tie
facially and lingually for an intra coronal preparation (Fig 4). For the box
preparation, flare of the proximal walls will create slightly obtuse cavosurface
angle. It makes the facial and lingual margins of the tooth preparation, more
cleansable and finishable. Is indicated when there is minimal extension of caries
and when normal contact is present.
Fig 4A,B- Tooth preparation with primary flare and dove tail retention
Retention features for a cast inlay restoration
• Occlusal dove tail that resists proximal displacement of inlay (Fig 4)
• Parrellism with minimal taper of the walls
• Frictional retention
Resistance features for a cast inlay
•
Flat Pulpal floor and gingival seat
•
Well defined line angles
•
Axio-pulpal line angle is rounded to dissipate the concentration of
stresses that would otherwise occur in this area
•
Bevels, flares
Features that provide Convenience form
• Taper
• Bevels and flaring- aid in placing the margins in an area easy to finish
Removal of any caries and block undercuts:
Caries remaining on the pulpal floor, axial, buccal or lingual wall should be
removed with spoon excavators or slow speed round steel burs. Resin modified
Glass Ionomer cement or Composite resin may be used to block the undercuts
to avoid unnecessarily extending the entire preparation (Fig 5A, B). Never ever
extend the entire wall to eliminate the undercut. Preservation of as much tooth
structure as possible in line with MID principles holds good for all types of
restorations including inlays.
Fig 5A- Removal of caries, B- blocking the undercuts .
Placing bevels:
Short bevels are placed with slender, flame shaped fine grit diamond instrument
on the occlusal and gingival margins and to apply the secondary flare on the
facial and lingual walls. This should result in 30 to 40 degree marginal metal
(Fig 6) for gold inlay that can be easily burnished. A gingival retraction cord
already in place helps to provide access and increases visibility, prevents
gingival tissue damage and subsequent hemorrhage during preparation and
impression making. Alternatively, a suitable GMT can also be used to bevel the
gingival margin.
Fig 6- Occlusal bevel is extended across the entire occlusal margin
Functions of Bevels: It creates an obtuse angle marginal tooth structure which
is bulkiest and strongest configuration of marginal tooth anatomy. Also,
enhances adaptation of cast gold restoration by taking the advantage of
burnishabilty due to its property of ductility.
Gingival bevel- Removes unsupported enamel in this region, resulting in 30
degree marginal metal that is easy to burnish. It also provides a lap sliding fit
which improves the fit of inlay at gingival seat area and minimizes
microleakage (Fig 7) by reducing the amount of luting cement exposed to the
oral cavity.
Fig 7- A, The retraction cord is inserted in the gingival sulcus and left for
several minutes. B-D, Diamond instrument preparing lingual secondary flare E,
Beveling the gingival margin. F, Properly directed gingival bevel resulting in 30
degree marginal metal. G, Failure to bevel the gingival margin results in a weak
margin formed by undermined rods and 110 degree marginal metal, an angular
design unsuitable for burnishing. H, Lap, sliding fit of prescribed bevel metal
decreases the 50 um error of seating to 20 um. I, A 50-um error of seating
produces an equal cement line of 50um along the un beveled gingival margin.
Functions and indications of secondary flare: In broad contact areas or
malposed contact area, both walls are thinned down, the primary flare ends with
an acute angled marginal tooth structure and do not bring the facial/lingual
walls to cleansable or finishable areas .A secondary flare placed peripheral to
primary flare will accomplish this without changing the 45 degree angulation.
Secondary flare at the correct angulation creates an obtuse angle of marginal
tooth structure (Fig 7, 8).
Fig 8- Secondary flare
Finishing enamel walls and margins and inspection of the completed
preparation–
Excellent adaptation of the cast restoration can occur only if the tooth has been
finished properly. Ensure that all the preparation walls must be smooth; all the
walls except the axial wall should diverge occlusally. No undercut area should
exist that could interfere with placement and withdrawal. All cavosurface
margins must be distinctly defined.
Assess your prepared tooth using a hot GP stick or impression compound
before you proceed any further. Look for the cavity dimensions, taper,
Undercuts, line and point angles, circumferential tie features (Fig 9).
Fig 9-Trial impression made using impression compound
Impression making, temporary restoration, die and indirect wax pattern
A definitive impression with elastomers is made, registering every detail of
tooth.
Armamentarium and check list:
•
Occlusal stent prepared on diagnostic cast for temporaries
•
Impression trays: stock tray / special trays both are acceptable
•
If you are using special tray- fabricated using diagnostic castremove spacer, stopper, reference marking, and vents before making
impression.
•
Pre-cut retraction cord of required dimension
•
Matrix band is burnished and attached to the retainer
•
Hollow metal sprue is selected and filled with sticky wax
Impression of the prepared tooth
The primary objective of an impression is to record the finer details of the
prepared tooth, in addition to the adjacent & opposing hard and soft tissues
accurately. Of various impression materials available, Additional Silicone is
more preferred for its elasticity, ease of withdrawal from under cuts and
reproduction of finer details.
Other impression materials are not preferred due to following reasons




Irreversible hydrocolloid/Alginate - Not strong, more distortion
Polysulfide elastomeric material - Messy, distortion, stains clothes
Condensational elastomeric material- Higher shrinkage
Polyether rubber base – Stiff
Retraction Cord Placement
The purpose is to displace gingiva away from preparation margin, to control
moisture due to gingival hemorrhage, GCF. It improves access and helps to
record finer details more accurately in the impression (Fig 10A, B). This is
more critical for an elastomeric impression which is hydrophobic and for wax
which is water repellent if one is making a direct wax pattern using inlay wax.
Fig-10 A- showing retraction cord, B- Fischers cord packing instrument
Type - Braided, non-impregnated is usually preferred
Size – 2-0, 3-0-Single /double cord technique,
Depends on the health of the gingival tissue around the prepared tooth
Length -1mm beyond the gingival width of the Preparation or entire
circumference of the tooth
Cord Packer – Force is directing apically & towards tooth. Start at line angles
Duration of application should be less than 10-15 min to avoid gingival
Ischemia
Remove the cord before making impression. Retraction cord should be slightly
wetted to prevent peel away of gingival epithelium while removal.
Techniques of making impression
It can be done as one step or two step procedure.
1. Single step/ squash technique/simultaneous putty wash technique
The Putty/Heavy body material is kneaded and loaded onto the impression
tray. Simultaneously, Low viscosity/light body material is mixed and
injected around the prepared tooth and onto putty material. The loaded tray
is then seated properly in the patient’s mouth. Allow both the putty and light
body materials to polymerize simultaneously (Fig 11 A- E Pameijer CH
1983 Quientessance Jl).
Advantages with this technique are that it is less time consuming, doesn’t
require a tray Spacer and there is no change in the tray position as seen with
2 step technique. However, the disadvantages with this technique are, it is
impossible to control thickness of light body material and many a times, the
Putty material gets exposed which doesn’t record finer details ( Donovan
TE, Chee WW. Fig 11 F).
Fig 11A-F- Steps in impression making
2. Two step putty wash technique-preferred
In this, the preliminary putty impression of the prepared tooth and the dental
arch with spacer serves as the tray material. Once the tray impression is ready,
the spacer is removed and reline impression is made using light body material
(Fig 12). The advantages with this technique are that we can achieve uniform
thickness of wash impression material without exposed the putty material,
thereby recording the finer details more accurately. However, the disadvantages
are that it involves 2 steps, needs spacer and is time consuming.
Fig 12A-D- Steps in impression making
Ways to provide space for reline material (Monzavi A& Siadat H)
Fig 13 A-D-Ways to provide space for relining
Prior to making the Putty impression, Modelling wax can be slightly softened
and adapted to the dental arch or Polythene sheet, Vacuum formed occlusal
splint (Yu-Jen Wu A. & Donovan T.E J) can also be tried. Scraping the inside
part of the impression with a scalpel blade/rotary bur (Leendert Boksman)
serves as another way (Fig 13).
Few points to remember while making additional silicone impression
 Do not wear powdered gloves while making impression as the sulphur
powder in the gloves can inhibit polymerisation of rubber base
impression material.
 Tray placement may vary intraorally in 2 step impression procedure. This
can be prevented by marking a reference marking on the tray to coincide
with a stable intraoral landmark.
 Impression held for 6-8min intraorally and be removed in a sudden snap
to minimize distortion of the impression
 Impression should be flushed thoroughly under running water to remove
patient’s saliva and other debris
 Disinfect the impression by immersing in 2% Glutaraldehyde solution for
10 minutes before pouring the cast
 Pour additional silicone impression only after 20 min to allow release of
Hydrogen gas to prevent the poured cast becoming flaking and rough.
Read the impression - use magnifying lens
Look for the thickness and the extension of the putty and light body impression
materials. Check whether there is any void, discrepancy, step formation, tear
and exposure of impression tray (Fig14A-D).
Fig 14A- Impression showing void, B-discrepency, C-non uniform thickness of
material, D-tear
Prefabricated or stock trays are usually used for dual consistency impressions.
They result in unnecessary wastage of impression material, Non uniform
thickness of the materials-thereby non uniform shrinkage and variable
distortion. Stock trays are bulkier and can cause more discomfort to both the
patient and the doctor. Since they require more material, they are costlier.
To overcome these drawbacks, Special trays made using diagnostic casts are
usually preferred.
Custom/ special trays can be fabricated using Self cure acrylic resin. Ensure
that the tray extension is 2-3 mm beyond the neck of teeth. They are prepared 6
hours prior to impression procedure to avoid any irritation to the oral mucosa
due to monomer of the acrylic resin. Tray Spacer 1.5-2mm thick for elastomeric
materials can be achieved by adapting modelling wax that provides for uniform
thickness of the impression material .Tray stops 3×3 mm windows are cut in the
modelling wax spacer adapted to the cast at canine premolar area, retromolar
region which help while tray repositioning, also prevent undue pressure applied
during impression procedure. Tray vents can be drilled using round rotary burs
in a straight hand piece that prevent the impression getting pulled-away from
tray, thereby prevent undue shrinkage of the impression (Fig 15). Remember to
apply a tray adhesive usually of Butyl rubber and allow it to dry before making
the impression.
Fig 15- A-Tray with vents, B-adapting modelling wax for spacer
Temporization of prepared tooth
Fig 16-A,B- Preparation of temporary
Never dismiss the patient without temporizing it. Between the time the tooth is
prepared and inlay is cemented a resin temporary is placed to protect the tooth.
Temporisation of the prepared tooth is essential to maintain the same occlusal
and proximal contact/contour of the prepared tooth to the adjacent and opposing
teeth and prevents fracture of weakened cusps. In addition to maintaining the
form and function of the prepared tooth. This prevents ingrowth of gingival
tissue, and prevents patient discomfort.
Indirect temporaries -although time consuming is preferred over direct
temporary restorations. They provide better contact/contour, have good
strength, there is no locking-in of the restorative material in the undercuts and
no irritation to pulp.
 An occlusal stent made using composite resin or rubber impression on the
cast/ tooth prior to preparation by filling the cavitated lesion if any, with
wax may help in establishing the occlusal anatomy easily later on.
Chemical cure temporizing resin is injected into the impression, placed
onto the cavity preparation under pressure for 3mts & removed, allowed
to set for another 5mts outside, excess removed, finished and cemented
with non- eugenol temporary cements(Tempocem)(Fig 16A,B).
 Chemical curing resins such as Protemp, Luxatemp, Telitemp can be
used for provisionals.
Direct temporaries using IRM and ZOE cement makes the procedure less
cumbersome and easy but certainly not a preferred choice as you may alter the
prepared tooth while trying to remove these cements.
STUDY CAST/DIE PREPARATION
Dental casts serve as positive replica of the entire arch and die is the positive
replica of the single prepared tooth. The material used for die should most
importantly have sufficient strength, abrasion resistance and reproduce finer
details accurately. Usually Type IV, V gypsum products are used.
The die cutting should be atleast 1 inch apical to the prepared finish line using
Di-Lock or Pindex devices (Fig 17A-C). Dies help to adjust contacts and
contours precisely.
Fig 17A- Die cutting, B- Di lock device, C- Pindex device
Die spacer usually of model paint, nail varnish should be coated 0.5-1 mm short
of prepared margins that would result in optimal luting film thickness of 25
microns.
Indirect wax pattern fabrication- Wax pattern forms the outline of the mould
during casting. It can be directly done in the patients mouth or outside in the
laboratory when it is called Indirect method. This can be done with or without
the use of matrix band retainer.
Indirect method of wax pattern fabrication is easy to fabricate, less time
consuming and there is less chances of distortion. However, if the study model
is not the exact replica of the prepared tooth, it would result in distortionsnecessitating recalling the patient again. Type II – Soft Wax is us for fabricating
pattern by Indirect method according to ANSI/ADA specification No. 4
(Anusavice).
Wax build-up techniques
It can be done in the following ways
1. Bulk technique – Wax is overfilled & carved- preferred-less
voids/discrepancy
2. Incremental build-up of wax – result in more voids, stresses, warpage,
discrepancy
METHODS TO SOFTEN WAX (Fig18A-D)
Fig 18A- uniform heating of wax, B,C- wax addition metod, D-Die dipped in
molten wax
1. Dry heat/ flaming using blue part of the flame for Bulk fill technique.
However, wax adaptation is questionable.
2. Wax addition method-flow & press method
Pieces of wax are held on the wax spatula and heated indirectly over
the flame and is flowed into the preparation. Ensure flow of wax to the
entire depth of the preparation. There are more chances of voids and
distortion seen with this technique.
3. Hot water bath
It results in inclusion of water droplets which may smear the pattern
and splatter.
4. Temperature controlled oven-dipping wax technique – most
preferred.
The prepared die is directly dipped into the molten wax, allowed to
cool down and the excess is carved away. Is less time consuming and
requires extra initial Investment.
Armamentarium required for wax pattern fabrication
Wax Spatula, Lacrons carver, P.K.T Instruments (Fig 19)
PKT 1- Large increments of wax. PKT 2- small addition of wax
PKT 3- Burnish and carve occlusal surface . PKT 4- all purpose carver
PKT 5- to refine triangular ridges and occlusal grooves .
STEPS IN INDIRECT WAX PATTERN FABRICATION USING
MATRIX BAND AND RETAINER-PREFERRED (Fig 20A-E)
Fig 20 A-E Steps in indirect pattern fabrication using matrix band
Check the height of the matrix band, burnish it, apply separating medium and
apply to the prepared tooth. Inlay wax is softened in any of the methods one has
mastered and filled into the prepared tooth in increments ensuring complete
flow of wax into the preparation. Allow the wax to cool and carve away the
excess. The occlusal anatomy is carved with a sharp Lacrons carver, matrix
band retainer removed, checked for the high points using opposing cast by
dusting talcum powder (Fig 20). Warm wet cotton pellet is used to finish the
occlusal surface. Silk thread can be passed proximally to finish the same. Wax
Sprue is usually attached to the indirect wax pattern. However, a preselected
hollow metal sprue filled with sticky wax is then attached to maximum bulk of
pattern with the use of sticky wax for its rigidity.
SPRUE forms a channel through which the molten metal flows
Types of sprues (Fig 21)
1. Wax - Low thermal conductivity, transmit minimal heat – minimal
Distortion-Preferred for Indirect wax pattern
2. Resin/ Plastic- requires 2 step burnout, leaves more carbon residues
3. Metal –rigid is preferred for direct pattern-removed before casting- may
loosen / roughen the walls of investment- prevented by coating with waxpreferred in exams for indirect wax pattern. Hollow metal sprue is filled
with sticky wax to reduce the thermal conductivity of metal, so that it
holds less heat, thereby resulting in less heat transfer and less distortion
of the wax pattern. It also provides better retention to the pattern.
Fig 21 A- Sprue-wax, B-Resin, C-Metals- hollow and solid
Sprue length
The length of the sprue should be such that it results in 3.25mm of GBI or 6mm
of PBI from the trailing end of the casting ring. Inappropriate sprue length can
result in localised shrinkage porosity, Improper venting of gases from the
investment material resulting in Back pressure porosity and incomplete castings
(Fig 22 A,B). This can be prevented by placing reservoirs 1.5 mm from the
sprue -Pattern junction, flaring the junction which provides for extra molten
metal and addition of chill set sprues/ vents which are wax rods placed farthest
from the pattern are useful.
Fig 22 A- Sprue length and its effects on casting, B-Incomplete casting
Sprue diameter
Sprue diameter dictates the flow of molten alloy. It should be the same as the
thickest portion of the pattern. Too narrow sprue may result in premature
solidification of molten alloy before reaching the mould space resulting in
localized shrinkage porosity and incomplete castings. If too large can result in
hot spot and suck back porosity. Preferred sprue diameters for various teeth are
mentioned by Vimal Sikri (Fig 23)
Fig 23-Sprue diameters
Sprue attachment
Sprue is attached to the bulkiest portion of the pattern or at the least anatomic
position at an angle of 45 degree to the pattern. (Fig 24).
Acute/ right angulation would result in turbulence of alloy within mould
resulting in Suck back porosity. This can be prevented by flaring the spruepattern junction (Asgar and Peyton 1959) by adding sticky wax that minimizes
stresses of attaching metal sprue to inlay wax pattern due to low thermal
conductivity.
Fig 24 A-C Attachment of sprue at 45 degree
Evaluate the pattern
Once the sprue is attached, evaluate the pattern for its outline, dimensions, taper
of walls, line and point angles, circumferential tie preparation feature, gingival
bevel and for any voids/discrepancies in the flow of wax. Then build the
proximal contact using low fusing inlay wax to compensate for matrix band
thickness and to prevent open contact (Fig 25 A, B).
Fig 25 A – Wax pattern- recording all details of cavity preparation, including
the occlusal and gingival bevel. B- building proximal contact
Casting defects seen due to wax distortion
Waxes tend to return back to their original shape due to the release of stresses
incorporated at various stages wax manipulation due to their property of elastic
memory (Fig 26). This can be prevented by uniform heating of the wax while
manipulation and pattern making and invest the pattern immediately within 5
min or store in a Humidor. However, indirect wax pattern can be left on the die
for not more than 45min.
Fig 26 A,B Wax returning to original shape due to release of stresses
Incomplete castings occur due to fracture of thin sections of wax before or
during Investing or due to incomplete wax elimination during burn out
procedure. This results in rounded margins and incomplete castings (Fig 27)
Fig 27- Incomplete casting- rounded margins Fig 28- Surface roughness,
irregularities and discoloration
Surface roughness, irregularities and discoloration may result from
impurities in the wax (Fig 28)
Nodules, ridges/ veins in the casting occur due to incorporation of air
bubbles/Water film to wax pattern which can be prevented by applying proper
Wetting agent before investing (Fig 29 A, B)
Fig 29 A- Nodules, ridges and veins in casting, B-Wetting agent
Investing and casting procedure
The sprued pattern is then placed in a lined investment cylinder and invested in
an investment material (gypsum bonded ) which is mixed under vacuum so as to
avoid the incorporation of air and hence porosities in the investment
mould Thereafter, once the investment has set, the wax is removed from the
mould by burning it off. For gold alloys this is achieved by placing the
investment mould in a furnace at either 450°C (slow burn out) or 700°C (fast
burn out). The mould is then placed into a casting machine and gold alloy, in
molten state, is forced into it using centrifugal force. Once cooled, the
surrounding investment is removed, the sprue is cut off the restoration and the
resultant alloy casting trimmed, polished, tried and cemented. (Fig 30).
Fig 30 - Conservative mesio-occlusal cast gold inlay, cementation and
finishing.
Conclusion: Inlays offer excellent restorations that are underused in dentistry.
While the technique requires multiple patient visits, precision and excellent
laboratory support, the resulting restorations have the potential to last longer. In
Contemporary dental practice, Ceramic and composite Inlays have become
viable esthetic restorative alternatives to gold alloys owing to escalation in
aesthetic demands. However there are sufficient evidences to suggest that well
fitted cast gold restorations have longevity superior to any other restorative
material .
Bibliography:
1. Anusavice KJ, Phillips' science of dental materials.11th ed. St
Louis:Saunders Elsevier;2003.
2. Gopikrishna V. Sturdevant's art & science of operative dentistry-e book:
second south asian edition. Elsevier Publications;2013.
3. Sikri VK. Textbook of operative dentistry: 4th ed. CBS publisher;2017
4. Summitt, JB, Robbins JW, Hilton TJ, Schwartz RS. Fundamentals of
operative dentistry- a contemporary approach. 4th ed Quintessence
Publishing Co, Inc., Illinois.2013
5. Mulic A, Svendsen G, Kopperud SE. A retrospective clinical study on
the longevity of posterior Class II cast gold inlays/onlays. J Dent.
2018;70:46-50.