Tooth preparation for cast metal restoration / endodontic courses by indian dental academy

TOOTH PREPARATION
FOR CAST METAL
RESTORATIONS

INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education

www.indiandentalacademy.com

 INTRODUCTION
 DEFINITONS OF INLAY AND ONLAY
 BRIEF LOOK AT CASTING METALS
 INDICATIONS AND
CONTRAINDIACTIONS
 ADVANTAGES AND DISADVANTAGES
 INSTRUMENTATION FOR TOOTH
PREPRATION FOR CAST RESTORATION
 PRINCIPLES OF TOOTH PREPRATION
FOR CAST RESTORATIONS

 Types of margins, design features and functions
of occlusal and gingival bevels.
 Types and design features of facial and lingual
flares.
TOOTH PREPARATION FOR INLAY CAST
RESTORATION
i) Indications
ii) General shape
iii) Internal anatomy


 TOOTH PREPARATION FOR ONLAY CAST
RESTORATION
i) Indications.
ii) General shape.
iii) Internal anatomy.
TOOTH PREPRATION FOR CAST RESTORATIONS
WITH SURFACE EXTENSIONS
i) Skirt
ii) Collar
 CONCLUSION.
 REFERENCES.

INTRODUCTION
The cast metal restoration is versatile and the
procedure requires meticulous care both in
preparation and laboratory procedures.
The practice of restorative dentistry
improved permanently on cast metal restorations
when it was first introduced by Taggart for cast
gold restorations.
Today, cast metal restorations and its applications
have become basic treatment modalities in
dentistry both for strengthening the tooth
structure and maintaining the functional value.

DEFINITIONS
INLAY
It is primarily an intracoronal
cast restoration that is fabricated
outside the oral cavity and placed in
the prepared cavity.
ONLAY
An onlay is combination of
intracoronal and extra coronal cast
restoration when one or more cusps
are covered.

Brief look at casting metals :
Gold alloy is most commonly used, however due to its
expense other metals which are in use are :

Noble Noble Base Base
(Gold (Palladium metals metals
based) based) (Cobalt (Nickel
based) based)
Major Gold Palladium Cobalt Nickel
constituents
‘’ Silver Silver Chromium Chromium

‘’ Copper Copper Tungsten Iron

‘’ Gold Molybdenum

INDICATIONS FOR CAST RESTORATIONS
A) Extensive tooth involvement
Ultimate in both efficiently replacing lost tooth structure
and supporting remaining tooth structure.
B) As an adjuvant to successful periodontal therapy
i) physiologically restoring the dimensions of the
contact contour, marginal ridges and embrasures.
ii) Splinting of teeth loosened by periodontities to a
better bone supported teeth. The rigid connection of several
cast restoration assures distribution of the applied forces to
the best supported teeth and minimises force on disabled
teeth.


C) Correction for occlusion:-
If any drastic change is planned for the occlusal
table or occluding parts of a tooth, cast restorations are ideal.

D) Restoration of Endodontically treated teeth:-
After endodontically treated, the tooth become
brittle. Almost always the clinical crown portion of these teeth
will need reinforcing restoration.

E) Support for and preparatory to partial or complete dentures:-
Weather they are a clasp type, over denture type,
bar attachment. Most removable prosthesis will need cast
restorations.


F) Low incidences of plaque accumulation or decay :
patients to receive a cast restorations should
have their plaque accumulation in rigid control.

G) Esthetics :
Of all metal restorations properly fitted cast
restorations enhance esthetic appearance.

CONTRAINDICATIONS :

a) Developing and deciduous teeth:
Growth or resorption may be affected by the
traumatic nature of the procedures of the cast restorations.


b) High Plaque and caries indices :
Patients with rampant caries and
poor oral hygiene should not be given cast
restorations

c) Occlusal Disharmony :
Cast restorations should not be
used in patients with severe occlusal
interference or other defects .

d)Dissimilar Metals :
Gold-based castings are avoided
in patients already having silver restorations.


Advantages and Disadvantages of cast metal
Restorations
Better yield, compressive, tensile and Being a cemented restorations,
sheer strengths of alloy. interphase leading to leakage around
and under cast restoration will be a
common problem.
Reproduces precise form and minute Extensive tooth involvement creates
detail. hazards to vital tooth structures.
Metals that are usually used are not The cathodic effect of dental alloys
significantly affected by tarnish and towards other metals used in the same
corrosion . mouth will lead to galvanic
deterioration.
Fewer voids, no layering effects, less Lengthy procedures, requiring many
internal stress will lead to strong visits, temporary coverage between
structure. visits, expense of alloy.
Cast restorations can be finished, Some cast alloys exhibit high abrasive
polished or glazed outside the oral resistance which may lead to
cavity.
imbalance occlusion.

Instrumentation for tooth preparations for
cast restorations
A) Removal of undermined enamel and
gaining access:
undermined enamel is easily
removed with hand cutting
instruments like chisel, hatchet,
wedelstaedt.

B) Removal of caries dentin and placing
intermediary basing :
Decayed dentin is usually
excavated using spoon excavators,
cleared of the diseased tissue ,proper
intermediary base is placed and
locked in place.

c) Defining the facial and lingual marginal and axial
parameters of the extra coronal portion of the
preparation :
using #3,4, or 5 round bur or ball shaped
diamond stone, gauge out the axial surface near
contemplated circumferential tie, Several of these
gougings may be made within the parameters of
the extracororonal preparation. They will serve as
guide lines in over all axial reduction.


D. Bur no.271 : tungsten
carbide tapering fissure with
0.8mm width
E. Bur no. 169L tapering
fissure with 0.5 mm
F. Preliminary Shaping:
Tapered fissure burs
or diamond stones with or
without rounded ends, are
used to reduce the axial
surface to the depth of the
gouges, cutting strokes
should be preplanned in
starting and ending points,
their angulations and their
direction of pressure.

 G .Boxing up :
Using a tapered fissure bur, wall
proper, e.g., for cast alloy onlay, the
inner(pulpal) half to two-thirds of the
facial or lingual wall occlusally , the axial
half of the facial to two-thirds of the facial
or lingual walls proximally, and the axial
half two thirds of the gingival floor
proximally. Care is taken in this step as it
retains the resistance and retention form
and hence angulation of cutting tool is
very influential.


H. Establishing circumferential tie
constituents:

Gingival bevels are best created with
gingival margin trimmers. primary and
secondary flares are done so with chisels
or hatchets. occlusal bevels are prepared
with cone shaped aluminum oxide stone.
A feather-edge finishing line is prepared
with filamentous stone . A beveled
shoulder is prepared with a tapered
fissure bur and a GMT.


A hallow-ground bevel
can be prepared either
with a torpedo-shaped
stone or bullet shaped
stone, followed by
round bur. For the
reverse secondary flare,
the preparation can be
done using taper end
diamond stone followed
by hand instrument
hatchet or bi angle
chisel.


I) Finishing and establishing the continuity
between the circumferential tie
constituents:
Smoothing of the tie can be
done with hand instruments and 12 fluted
or 40 fluted carbide burs. Also fine grit sand
paper discs, if access allows can be used
effectively.
Establishing the continuity between the
circumferential tie constituents is best done
by wedelstaedt chisel, biangle chisel or any
other sharp chisel.


PRINCIPLES OF CAVITY PREPRATION
I) Outline form:
i) External outline form:
 The external outline for inlay should consists of
straight lines and smooth flowing curves, avoiding
any short angles.
 Enamel rods at the cavosurface margin should be
supported by dentin and supported laterally by rods
that lie within the preparation.
 The cavosurface margin is placed in sound,
unbroken tooth tissue to obtain a well fitting casting.
 The placement of bevels make the outline form
slightly wider for cast restorations.

ii) Internal outline form:
 The Pulpal floor and the axial wall
of the inlay preparation must be placed in
dentin. Care must be taken to protect the pulp.
 Pulpal floor will be usually positioned
0.5mm into dentin below the central groove.
 In shallow preparations, parallelism
enhances the resistance and retention form of
the preparation.
 line angles in both occlusal and
proximal portions of the preparations should be
well defined. The axio-pulpal line angle should
be slightly rounded.

 It is sometimes desirable to incline the cutting
instrument so that it forms either an exaggerated taper
from cavosurface to pulpal floor or a long bevel on that
area of the wall.
 This procedure protects the thin wall of enamel that
remains at the cavosurface by maintaining a supporting
edge of dentin.

II) Resistance and retention forms :
The preparation of the tooth for a cast
restoration must be so designed that will resist dislodging
forces of compression and tension.
Inlay taper:
Cavity walls must diverge from the floor of the
preparation externally forms a basic design for all cast
restorations

 Removal of wax pattern and insertion of the casting is
facilitated by the taper.
 A range between 4 to 6 degrees is used as it provides
adequate retention of the cemented casting.
 The axial length of the preparation will influence the
amount of taper. longer preparations require taper in
higher range, short preparations in the lower range.
 Pulpal and cervical floors ideally should be
perpendicular to lines of force that will influence the
restoration. Floors positioned perpendicular to these
lines of force will absorb the stress over a broad area
of the tooth.
 Well defined line angles are also important in obtaining
resistance and retention form.

 The occlusal interlock or dovetail is a major factor in
resistance and retention form.
 Specially designed features like pinholes or postholes are
placed parallel to the line of draw of the preparation and with
appropriate concern for the pulp
 Tapered grooves extending from cervical floor to the
occlusal surface, are sometimes placed in the dentin portion
of the proximal walls to form a locking key to aid in
preventing lateral dislodgement of the restoration.

III) Removing carious dentin :
Removal of deeper carious lesion frequently precedes
the establishment of resistance and retention form.


IV) Convenience form :
 This form provides accessibility and visibility
required to complete operative procedures
thoroughly.
 Opening the preparation to its approximate final
outline form establish an intact dentin enamel
junction enhances access and visibility for
removing carious dentin and old restoration.
 Extension, taper and flare of proximal walls to
permits access for disking and bevel placement,
and extension to allow proper finishing and
adaptation of the margins of the restorative
material are all examples of convenience form.


V) Finishing enamel walls and margins
 If Coarse or medium grit diamond
instruments have been used during cavity
preparation the walls and margins should be
finished with carbide finishing burs or fine
abrasive discs.
 The cervical bevel of indirect
preparation is most frequently placed with the
flame shaped extra fine finishing bur or gingival
marginal trimmers for convenience and they
provide a steeper bevel to prepare for an
effective adaptation of the metal margin.
 A bevel placed with a flame shaped
rotary instrument will establish excellent cervical
margin and will further blend together with the
buccal andwww.indiandentalacademy.com
lingual proximal finish lines.

 a bevel on the cervical margin of a box preparation
for the direct technique should be uniform about 1/4
to ½ of mesio-distal width of the cervical floor and
must include the proximal cervical cavosurface
angles. Such a bevel is placed with GMT prior to
finishing the proximal enamel walls.

VI) Cleaning and inspection of the cavity :
Upon completing the cavity preparation, the walls,
floors, margins should be cleaned with water. after
drying with cotton pledgets and a gentle stream of
warmed air, the cavity should be scrutinized carefully
for any imperfections.


Primary tooth preparations are more less the same as
mentioned above and the dissimilar features are ;
1.Path of insertion : (a) should be parallel to the long
axis of the tooth. (b) should be opposite to the
occlusal load, this helps the inlay to seat well in the
cavity without rocking or to prevent any micro
movements.

2.Taper :It is the preparation in which the walls of the
cavity in intracoronal cast restoration is diverged to
occlusal margin and in extracoronal, converged to
occlusal margin. The amount of taper should be
normally 2˚ to 5˚ for one wall, for both walls should
not exceed 10˚.The sum of taper of both walls is
called “cone angle taper” and should not exeed 10˚.

Taper depends on :

1.Length of the preparation: Longer the wall, increased
taper and shorter wall, minimal taper and parallel
the wall.

2.Surface involvement of cavity : More complex the
surface involved, less should be taper and more
parallelism.

3.Need for retention : reduce the taper to have
maximum retention.


Types of margins in a cast restoration

Bevel, Chamfer, Shoulder are commonly used
margins for cast restorations, how ever
other margins are feather edge, chisel
edge, sloped shoulder, shoulder with
bevel.


Advantages and Disadvantages
Bevel Removes Extends
unsupported preparation into
enamel, allows sulcus if used
finishing of on apical
metal margin.
chamfer Distinct margin, Care needed to
adequate bulk, avoid
easier to unsupported lip
control of enamel
Shoulder Bulk of Less
restorative conservative
material


BEVELS
Bevels are the “flexible extensions” of a cavity
preparation.
This peripheral marginal anatomy of the preparation
is called “circumferential tie”, bevels, being the
part of the circumferential tie, are one of the
major retention forms for a cast restorations. and
has following features:
a) Enamel must be supported in the sound dentin.
b) Enamel rods forming the cavosurface margin
should be continuous with sound dentin.
c) Enamel rods forming the restorative material and
angular cavosurface angles should be trimmed.

FUNCTIONS OF BEVELS
1)To increase the bulk of the material at the margins of
the preparation.
2)By increasing the bulk, burnishing of the cast
restoration is possible.
3)The cement line is hidden or marked by the bevel thus
preventing marginal leakage.
4)Discrepancy in the cavity preparation or cast
restoration is marked by the bevel.
5)It improves the resistance of the tooth structure.
6)Improves retention –reverse bevel also called as
flexible extension, i.e., any surface defect like attrition
can be involved in the preparation.

The bevels available for cast restoration

1) Partial: it involves part of
enamel
2) Short: It involves entire
enamel.
3) Long: It involves all of
enamel but one half of
the dentinal wall.
4) Full: This involves all of
the enamel and dentinal
wall.


5) Hollow ground: It is concave in
form, truly it is not a bevel,
some times given to increase
retention.

5) Inverted Bevel: It is indicated
only in metal ceramics. it is
given on the labial shoulder.


7) Counter bevel : in case of capping
the cusps. It is started from inner
dentine involves enamel with a flat
plane on enamel and is taken back.
8) Reverse bevel: bevel towards axial
wall and slopping towards it, made
of dentine fully, aids in retention
and prevents proximal
displacement.

Types and design features of facial and
lingual flares.

a) The Primary Flare:
This is the conventional and basic part of the
circumferential tie facially and lingually of the proximal box
for an intracoronal preparation.
It is very similar to the long bevel formed of an enamel
and part on the dentin, on the facial or lingual proximal wall.
Primary flares have 45 degree angulation to the
dentinal wall proper.


Functions and indications :
Brings proximal facial and lingual
walls to self cleansable and
finishable areas.
They are indicated for any facial
or lingual proximal wall of any
intracoronal cavity preparation. It
is prepared on enamel and
dentin.


The secondary flare :
It is a flat plane superimposed
peripherally to a primary
flare. usually it is prepared
on enamel , but sometimes
may involve dentin. Unlike
primary flare, secondary
flare have different
involvements, angulations
and extent depending on
their functions.


Functions of secondary flare
In a very widely extend lesion bucco-lingually ,the buccal
and lingual structure will be badly thinned, the primary
flare will end with acute angled marginal tooth structure,
here a secondary imposed flare will create the needed
obtuse angulation of the marginal tooth structure without
any sacrifice to resistance and retention form, because
the wall proper and primary flare maintained at their
proper location and angulations.


In very broad contact areas, the
primary flare will not bring facial
and/or lingual areas to self
cleansable areas, however a
secondary flare placed
peripheral to that primary flare
will accomplish this without the
change in a 45 degree
angulation and the resistance
and retention forms.


Surface defects or decalcifications, facial
or lingual to the primary flare’s facial
and lingual margin respectively, can be
involved in the preparation with the
secondary flare without to extend or
angulate the primary flare more than
indicated.


 In ovoid teeth peripheral margin
undercuts esp. apt to be present
occluso-apically on the facial and/or
lingual peripheries of the cavity walls.
Elimination of these undercuts via wall
proper or primary flare extension will
unnecessarily involve and weaken
tooth structure. How ever a secondary
flare superimposed on primary flare
will eliminate these undercuts with only
minimal sacrifice to tooth structure.


Tooth Preparation for Inlay cast
restoration :
 Indications :
 cavity width does not exceed 1/3 inter cuspal
distance.
 Strong self-resistance cusps remain.

 Indicated teeth have minimal or no occlusal facets

and if present limited to occlusal surfaces.
 The tooth occlusion or occluding surfaces are not to

be changed by restorative procedure.

 General shape :
Outline of occlusal preparation is dove
tailed. the proximal portion is usually boxed in
shape.

 Location of margins :
In occlusal portions, the margins are
located on inclined planes of corresponding
cusps , triangular cusps or marginal ridges.
The most peripheral margins of the
preparations are located away from contact with
the opposing tooth surface during centric closure
and extrinsic movements of the mandible.
All adjacent wear facets,
supplementary grooves, areas of decalcification
should be included in beveled portion of the
cavity preparation only. The margins of this
design fulfill all requirements of extension of
prevention.

 Internal anatomy :
The wall
proper-taper, should taper from
either 2-5 degrees or be parallel to
each other. Each wall should make
right angles/slightly obtuse angle
with pulpal floor. Preferably each of
wall proper should be parallel to
long axis of crown.

 The occlusal bevel : it is along bevel, almost one
third of the facial and lingual (proximal) walls. This
beveled outer plane of the walls will have
angulations of 30-45 degrees to the long axis of
the crown. This angulation should increase as the
width of the cavity increases to accommodate
more bulk of the cast alloy and to resist increased
stress near the cusps on inclined planes.

 Pulpal floor : should be flat for
most extent if this is not possible,
atleast the peripheral portions
should be flat .Generally should
be 1 to 1.5mm from DEJ. The
pulpal floor should meet all
surrounding walls at definite line
angle except its junction with the
axial wall, where it should be
rounded.

 Proximal portion: Axial wall should be flat or
slightly rounded in bucco-lingual direction, either
vertical or slightly divergent(5-10 degrees)
towards pulpal floor in the gingivo-occlusal
direction. The depth axially should be 1 to 1.5mm
from DEJ. However different depths may be
necessary according to carcinogenic pattern.

 Proximally facial and lingual walls : composed
of two planes ,axial half and proximal half.
Axial half (i.e. facial or lingual wall proper ) is
completely formed by dentin and meets axial
wall at right angle. This is main resistance and
retention form of this part.
Proximal half: Is formed of primary flare in
enamel and dentin ,45 degree angle to wall
proper.

Secondary flare: Some times it is necessary to impose
a third plane in the form of secondary flare, placed on
the enamel peripherally .This simplifies impressions and
wax patters techniques.

 The gingival floor: Proximally should be flat in the
bucco-lingual direction, making a slightly obtuse angle
with the buccal and lingual walls. The axio proximal line
direction is formed in two planes, the axial half and
proximal half. The axial half consists of gingival floor,
being perfectly flat, formed of dentin making either right
angle or slightly obtuse angle with the axial wall. The
proximal half should be beveled in the form of a long
bevel inclining gingivally. this bevel is usually angulated
on the average of 30-45 degrees.

 The junction between the occlusal bevel and the
secondary or primary flare proximally and also
the junction between the primary and secondary
flares proximally and the gingival bevel should
be rounded and smooth.

Secondary modes of retention :

1) Luting cement : fills gap between inlay and
tooth giving a physio chemical bonding.
Physical – Zinc phosphate, chemical : glass
inomer + polycorboxylate. The exposed
cement dissolves in the oral cavity and so
should not be considered as main retentive
factor.

2) Grooves : Place two
grooves, one on bucco axial
and other on lingual axial line
angle with #165 bur. It is
totally placed on dentine.
Depth of groove should be
0.3mm at the expense of
buccal and lingual walls and
never at expense of axial
walls.

3)Reverse Bevel :
It is given on
gingival seat. This bevel has
generally three planes, i) reverse
bevel plane where inclination is on
gingivoaxial plane which prevents
proximal displacement.
ii) secondly, flat plane made of
dentine.
iii) thirdly ,plane which is sloping
away from the axial wall made of
enamel and dentine, this helps in
proximal displacement.

4) Internal box:
made on the
pulpal floor, which improves the
retention by 4-5 times. this is
on the uninvolved side. it
should not have sharp line and
point angles and definite walls.
This prevents micromovement
of the inlay. Internal box should
always be reciprocated with a
reverse bevel or groove to
avoid micromovement.

5) External box : these are box
shaped preparations opening to
axial tooth surface .they can be
proximal, facial or lingual. They can
be either stepped occlusally or
gingivally.

6) Roughening of pulpal floor : at
specific areas of tooth preparation,
esp. in pulpal floor, is done for
more retentive and laterally
locking. irregularities should have
no frail or undermined enamel.
Creating different levels out of flat,
dished up, gingival or pulpal floors
could change a mechanically
negative situation into a positive
one.

 7) Precementation grooves : after casting make
grooves on the walls of the inlay and/or grooves
on the cavity wall exactly opposite. This will
house with solid mass of cement which helps in
good retention

8)Electrolytic etching of inlay :

This gives hinge like projections.

Procedure : Protect proximal and occlusal surface
with sticky wax and keep it in the electrolyte
solution of 0.5 normal nitric acid.
Inlay is kept in anode and metal with
increased EMF as cathode.(316 stainless steel )
current causes microporosities on fitting
surface which enables the luting cement to flow
into it for better retention. This process takes place
for around 10 to 15 mins.

Tooth preparations for onlay cast
restorations
It is partly intracoronal and partly extra coronal type of
restoration, which has cuspal protection as main feature.
Indications:
1)cupal protection is to be considered if the lesion
width is1/3 or ½ the intercuspal distance.
2) cuspal protection is mandatory, if the width of the
lesion exceeds ½ the intercuspal distance.
3) In the tooth preparation. if the length:width ratio of
the cusps is more than 1:1 but not exceeding 2:1, cuspal
protection to be considered.
4)if the length:width ratio is more than 2:1,cuspal
protection is mandated.

5) whenever there is a need to change the dimension, shape
and interrelationship of the occluding tooth surfaces, onlay
cast restorations are the ideal, most conservative
restorations.
6) Onlays are ideal restorations for abutment teeth for a
removal partial denture or fixed prosthesis.
7) Onlays are a ideal supporting restorations for remaining tooth
structure, combined with conservative tooth involvement.
8) Onlays are indicated when it is necessary to include wear
facets that exceed the cusp tips and triangular ridge crests
facially and/or lingually.

GENERAL SHAPE :
Onlays are dovetailed internally and follow cuspal
anatomy externally. Proximally box or cone shaped. The main
feature of the design of the tooth preparations are capping of
functional cusps and shoeing of the non-functional cusps.

Onlay Tooth Preparation Procedure

Occlusal preparation:
The initial entry is made in
the central fossa to a depth of
approx. 1.00 mm into dentin(2.5mm
in the total depth of the tooth). In
some cases this may be needed to
extend to greater depth because of
caries or pervious restoration .
The occlusal outline form
should be as conservative as the
carious lesion permits. The bur is
kept in the long axis of the intended
depth of the insertion so that the
taper of the bur provides the
desired 3 to 5 degree divergence
for each internal cavity wall.

Proximal boxes:
The boxes are created
on the proximal surface. The
facial and lingual walls should
exhibit a combined divergence
of 6 to 10 degrees from each
other as was provided in the
occlusal area of the
preparation.


The faciolingual dimension
is likely to be
determined by the
presence of a
restoration, caries
lesion. The bevels will
extend the preparations
slightly beyond the
proximal contact area so
that the margins of the
restoration will be
accessible for finishing
with a disk.


Cuspal Reduction:
A carbide bur
or diamond bur is used to
reduce the cusp. Depth cuts of
1.5 to 2 mm are made for the
centric cusp(s) and cuts 1.0 to
1.5 mm are made for the non
centric cusps.


After the depth cuts are
placed, a uniform
reduction of the cusps that
parallels the generally
anatomic contours of the
occlusal surface is made.


The cuspal heights are
reduced to the full extent
of the depth cuts.
Reduction of centric
cusps generally needs to
be greater than that for
the non centric cusps
because less occlusal
force tends to be exerted
against a non centric
cusp.


Shoulder Preparation:
A shoulder is
prepared on the external surface of
the centric cusp to provide a band
of metal to protect the tooth. The
bur is held parallel to the external
surface of the tooth and a shoulder
about 1.0mm in height and 1.0 mm
in axial depth is cut.
The finish line
should extend gingivally at atleast
1.0mm beyond any occlusal
contacts. The occlusoaxial line
angles are rounded. There must be
adequate (1.0 to1.5mm) clearance
in all eccentric mandibular
movements.


Non centric cusp :
A chamfer
or long bevel is given
instead of shoulder in non
centric cusp(s). A barrel
shaped bur can be used to
create chamfer. The bur is
positioned at an angle of
approximately 45 degrees
to the axial surface .This
provides additional
protection of the cusp.


Gingival Bevel:
A smooth and distinct bevel is
established on the gingival margins with
no.7901 finishing bur, a thin tapered
diamond, or a gingival margin trimmer.
This bevel should be
approximately 0.5mm width and at angle of
approx.45 degrees to the external surface
of the tooth.


Shoulder Bevel:
A 1.0 mm bevel is placed on the
shoulder with No.7901 or fine diamond bur. This
bevel is blend with proximal bevels. Any sharp
angles at the junction of the various bevels and
across the occlusoaxial line angles are
eliminated.
Proximal Bevels:
The proximal bevel or flare is
established with greater disk, a fine tapered
diamond. Divergence is established from the
gingival floor occlusally. The proximal bevel
should blend smoothly with the gingival bevel,
the buccal and lingual bevels.


Retention grooves :
If retention grooves are
needed, grooves are placed
in both proximal boxes. A
no.169 bur is used to bisect
the facioaxial and linguoaxial
line angles.
The grooves must
diverge toward the occlusal
aspect faciolingually and be
aligned with the internal part
of insertion.


TOOTH PREPARATION FOR CAST
RESTORATIONS WITH SURFACE
EXTENSIONS
 Modifications for basic onlay and inlay tooth
preparations and restoration involving part or all of the
axial surface(s), but short of veneer crown
preparation.

i) Skirt.

ii) Collar.


Skirt:
 It is more extensive surface extension. It is
also superimposed on the basic intra-coronal
inlay or onlay cavity preparation facially or
lingually.
 Indications:
 Skirting is required to involve defects
with more dimensions (especially depth). To
impart resistance and retention on a cast
restoration in lieu of missing or shortened
opposing facial/lingual walls.

 When the contact areas and the contour of the
proximal surfaces are to be changed in the
contemplated restorations. They will allow
sufficient cast material to be accommodated
without sacrifice of facial and lingual walls.
 Facially/lingually tilted teeth in order to
restore occlusal plane. They will allow for the
bulk, resistance and retention of the additional
occlusal cast material required in building the
occlusal table when so indicated skirts should be
prepared on side towards which the tooth is
tilted


 Every effort should be made to have the axial
reduction of skirt parallel to rest of the cavity
preparation. If skirt is to be used to change the
contact and contour of the tooth, it should be
extended far enough on facial and lingual surfaces
of teeth to create sufficient retention and avoid
marginal over hangs and over contouring. Like
wise, if the skirt is used to create a regular
occlusal plane for tilted teeth it should be extended
far enough on facial or lingual proximally, away
from direction of tilting.

 This helps in minimize effect of
displacing forces in tilted direction. And also
accommodation of enough cast material.


Collar :

 Most involving are surface wise and
depth wise, it can be one of the two
types.
 Cuspal collars- involve the facial or
lingual surfaces of one cusp in a multi-
cusped tooth.
 Tooth collars – entire facial or lingual
surface of the tooth.


 Indications :
 They help in retention and resistance when an
entire cusp is lost prior to tooth preparation or
when it is necessary to remove it due to
excessive undermining.
 They help retention in shortened tooth.
 They help in resistance and enhance support in
endodontically treated tooth.
 They are used in situations where pins are
contra-indicated for retention purposes.
 They are used for cast materials with low
castability.


 With axial depth of 1.5mm to
2 mm, collar surface extend
gingivally in a beveled
shoulder finishing line,
making it most reproducible
extension. Collars should
have less taper toward the
cavity preparations than
skirts. This improves retention
in the shortened tooth or cusp
preparations.


 The peripheral beveled portion of the
collar will have its angulation and extent
dictated by same features as those
governing the angulation and extent for
gingival bevels of inlays and onlays as
more the bevel angulation better will be
the marginal seating of cast restorations.


Conclusion:
Cast metal inlays and onlays offer excellent restorations that
may be utilized in dentistry. Even though the technique is
long and posts multiple visits for patient, the resulting
restoration is durable and long lasting. High noble alloys
are advised for patients with allergy or sensitivity to other
restorative materials. Cast metal onlays in particular, can
be designed to strengthen the restored tooth structure
than a full crown.

The high degree of satisfaction and service derived from a
properly made cast metal restoration is a reward for the
painstaking application required.


References:
Art and science of operative dentistry –
sturdevant – 4th edition.
Operative dentistry-modern theory and
practice - M.A.Marzouk.
Fundamentals of operative dentistry-
Richerd.s.Schwartz. -2nd edition
Text book of operative dentistry-vimal.k.sikri
Internet sources.

Thank you


Tooth preparation for cast metal restoration / endodontic courses by indian dental academy

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