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TOOTH PREPARATION FOR CAST RESTORATIONS
Seminar ByDr. S. HIMAGIRI
Postgraduate Student
DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICSSRI RAMACHANDRA DENTAL COLLEGE AND HOSPITAL
CHENNAI
1
CONTENTS
Page No.
Introduction 1
Definition 1
Indications 2
Contra indications 2
Advantages 3
Disadvantage 4
Principles of cavity preparation for cast restoration 4
Preparation features of the circumferential tie 7
Types and design features of occlusal gingival bevel 8
Circumferential tie constituents for extra coronal preparation 11
Types and design features of facial and lingual flares 13
Mechanical problems and preparation design solutions 15
Design of cavity and tooth preparations for cast restorations 20
Modifications for class V cast metals 27
Tooth preparation for only cast restorations 28
Modifications for class – IV and class III 33
Modification for class V cast materials 33
Tooth preparation for cast restoration with surface extension 34
References 39
2
INTRODUCTION
Dr. D. Philbrook in 1897 was credited with casting the first
restoration, which was without any evidence.
Taggart in 1907 was credited for improving cast restoration with lost
wax procedure, giving details of the casting procedure.
DEFINITION (Vimal K. Sikri)
An inlay is primarily an intra coronal cast restoration that is designed
mainly to restore occlusal and proximal surfaces of posterior teeth
without involving the cusps and rarely the proximal surface anterior
teeth.
Onlay is a combination of intra coronal and extra coronal cast
restoration when one or more cusps are covered.
Full veneer crown is an extra coronal cast restoration where all cusps
are covered.
ACCORDING TO STURDEVANT
Inlay
Class II inlay involves the occlusal and proximal surface(s) of
posterior teeth and May cap one or more but not all of the cusps.
Onlay
The class II onlay involves the proximal surface(s) of a posterior
tooth and caps all of the cusps.
3
INDICATIONS:
Extensive tooth involvement; restorations are efficient in replacing
lost tooth structure and also for supporting remaining tooth surface.
Fractured amalgam restorations.
Adjunct to periodontal therapy to correct tooth anomalies, which
predisposes to periodontal problems.
Restoration of endodontically treated teeth.
Retainers for fixed prosthesis.
Sub gingival lesions: properly finishing and polished gold alloys are
more compatible with the periodontium.
Patients with low incidence of plaque accumulation.
Fracture lines: fracture lines in the enamel, especially in teeth having
extensive restorations, should be recognized as cleavages planes for
possible future fracture of the tooth.
Esthetics: all metallic restorations (except gold) properly fitted cast
restorations are most pleasant esthetically.
CONTRA INDICATIONS:
Physiologically, young dentition with large pulp chambers and
incompletely mineralized dentin are contra indications:
Developing and deciduous teeth
4
High plaque / caries indices patient with rampant caries and poor
oral hygiene should not be given cast restoration.
Should not be used in-patient with severing occlusal interference or
other defects in the stomatognathic system.
Dissimilar metals: gold-based castings are avoided in patients
already having silver restorations.
ADVANTAGES:
Yield strength, compressive strength, tensile strength and shear
strength of alloys used for cast restorations are for greater than those
of any materials used intra orally.
Cast alloys are five times the ultimate strength of amalgam.
In impart resistance to the tooth rather than depending on tooth
structure to produce resistance form to the restoration.
Are capable of reproducing precise form and minute detail .
As the cast inlay contains one or more noble metals, they are not
significantly affecting by tarnish and corrosion process.
Restorations can be finished and polished outside the oral cavity there
by producing surface with maximum biological acceptance.
5
DISADVANTAGE:
Being a cemented restoration, several inter phases will be created at
the tooth cement casting junction, this leakage is pronounced
gingivally than the other parts of the restoration.
Restorations necessitate extensive tooth involvement in the
preparation, which creates possible hazards for the vital dental tissues.
The cathode nature of cast dental alloys towards amalgam may lead to
galvanic deterioration of amalgam. It these two restorations are
placed adjacent to or opposite to each other.
The procedure is lengthy and requiring more than one visit.
Much more expensive than other restorative materials.
Some cast alloys have a very high abrasive resistance caused wear of
opposing natural tooth.
PRINCIPLES OF CAVITY PREPARATION FOR CAST
RESTORATION
Cast alloys and ceramics can restore teeth via intra and extra coronal
preparations.
Intra coronal is mortise shaped having definite walls and floors
joined at line angle and point angle.
Extra coronal preparations are created by occlusal and axial surface
reduction, in many casts ending gingivally with no definite flat floor.
6
CAST PREPARATIONS SHOULD HAVE THE FOLLOWING
FEATURES:
Preparation path:
The preparation will have a single path, an opposite to the direction of
the occlusal loading.
Path is usually parallel to the long axis of the tooth, and it will help
restoration between and decreases its micro movements during
function.
APICO-OCCLUSAL TAPER OF A PREPARATION
Preparation should be without any under cuts.
Maximum retention is achieved by placing opposing walls parallel to
each other.
Since exact parallelism can create technical problem in processing and
in getting final materials into and out of the preparation.
Slight divergences of opposing walls are essential to facilitate cast,
fabrication with minimum errors.
7
A taper should be an average of 2 to 5 from the path of the
preparation; it can be decreased or increased according to the
following factors:
1. LENGTH OF THE PREPARATION
The greater the depth of preparation, the more taper will be but it
should not exceed 10.
2. DIMENSIONS AND DETAILS OF SURFACE INVOLVEMENT
The greater the surface involvement is and the more detailed that the
internal anatomy is, the greater will be the frictional component
between the preparation and the materials contacting it.
To diminish friction, the taper is increased but not exceed 10.
3. THE NEED FOR RETENTION
The greater the need for retention is, the more will be the need to
approach exact parallelism.
If carcinogenic and anatomical conditions dictate two different types
for opposing walls, it is preferable to create two planes for each
involved wall, i.e., inner planes parallel to each other and outer planes,
satisfying the needs compelling the different tapers. The inner plane
assures the single insertion of path of preparation.
8
PREPARATION FEATURES OF THE CIRCUMFERENTIAL TIE
Peripheral marginal anatomy of the preparation is called
“circumferential tie” and should have the following features,
advocated by NOY:
It the margin ends on enamel
Enamel must supports by dentin
Rods should be continuous with dentin
Rods should be covered with restorative material
Circumferential tie in inlays will be in the form of bevel. It’s directed
away form the cavity preparation.
TYPES AND DESIGN FEATURES OF OCCLUSAL GINGIVAL
BEVEL:
9
Partial bevel:
Involves part of the enamel not exceeding 2/3rd of its dimension.
Used to trim weak enamel rods from margin peripheries.
Short bevel
Includes the entire enamel wall employs for class I alloy.
Long bevel
Includes all of the enamel and upto ½ of the dentinal wall.
Used for class I, II and III alloys.
It preserves the internal boxed-up resistance and retention fracture
of the preparation.
10
Full bevel
Includes all of the enamel and dentinal wall
Deprives the preparation of its internal resistance and retention.
Its use should be avoided unless impossible to use any other form
of bevel.
Counter bevel:
When capping cusps to protect and support them, this type of bevel
is used, opposite to an axial wall on the facial or lingual surface of
the tooth and it will have a gingival inclination facially or
lingually.
HOLLOW GROUND BEVEL (CONCAVE BEVEL)
11
Bevel portion of circumferential tie must have a specific angulation
relative to the remaining portion of the wall.
This allows more space for cast material bulk, to improve materials
castability, retention and better resistance to stresses.
FUNCTION OF OCCLUSAL AND GINGIVAL BEVEL:
Bevels are the flexible extension of a cavity preparation, allowing the
inclusion of surface defects, supplementary grooves, or other areas on
the tooth surface.
Bevels create obtuse angled marginal tooth structure, which is the
bulkiest and the strongest configuration of any marginal tooth
anatomy and produce acute angled marginal cast alloy this
configuration will be the most amenable to burnishing for that alloy.
Bevels are major retention forms for cast restorations.
Reduce the error factor to three or more folds at the margins.
Some bevels like hallow ground and counter bevel, are used for the
resistance form of the tooth-restoration complex by encompassing
cusps.
12
CIRCUMFERENTIAL TIE CONSTITUENTS FOR EXTRCORONAL
PREPARATION:
a). The chamfer finishing line; it is the most universal design for class I,
II and III cast metals. It involves bulk and definite termination for
preparation marginally, with little tooth involvement (0.5mm maximal
depth).
Disadvantage: Its only disadvantage is the limited burnishability of the
marginal cast alloy and liability of transitional continuation of a
circumferential tie and adjacent bevel tie. It is most practical type of
finishing line for sub gingival extra coronal preparation. It is
contraindicated for class IV, V cast material due to poor castability.
b). The knife-edge finishing line: Is circumferential tie with least tooth
structure involvement. It should only be used to accommodate a very
castable-burnishAble type of alloy (Gold alloy). It should be located on
assessable areas of the tooth surfaces for proper finishing. It is most
indicated when minimal axial depth is required.
Disadvantage: possibility of indefinite termination for casting.
13
There is a chance of the margin not being covered with a casting
made of certain alloys due to lack of bulk space to accommodate less
wetting alloy. There is possibility of fracturing the alloy part of the
circumferential tie during burnishing finishing polishing. It is
contraindicated for class III, IV, V cast material
c). The beveled shoulder finishing line: It is involve with most of tooth.
It is exactly a gingival floor of an intra coronal preparation but on a
smaller scale
1. It is indicated when a definite gingival floor, with all its
components is needed for resistance – retention purposes.
2. Also when maximum bulk of the cast is needed marginally for
material that are limited in their castability or are difficult to
burnish. It is ideal design for sub gingivally located margin
because maximum predictability of the casting termination
gingivally.
It can be used for any cast material its bevel portion could be
hollow ground, as this is most suitable for class IV and V cast
material.
d). The hollow ground (concave) bevel: Is actually an exaggerated
chamfer of a concave beveled shoulder. Its tooth involvement is greater
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than a chamfer and less than a beveled shoulder. It is mechanically
comparable to beveled shoulder and superior to a chamfer care must be
taken to ensure there is no residual friable enamel or thinned tooth
structure at the periphery of this finishing design. It is ideal finishing line
for class IV and V cast material.
Mechanical problems for cast restoration and preparation design
solution in general cast restoration is used for compound and complex
tooth involvement. In addition to the principal retention form previous
described (parallelism, dovetail, surface area frictional retention,
circumferential tie, masticatory load, directed to seat the restoration),
there are numerous axillary’s mean for cast restoration.
TYPES AND DESIGN FEATURES OF FACIAL AND LINGUAL
FLARES:
Flares are the flat or concave peripheral portions of the facial and
lingual walls.
There are two types of flares:
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1. Primary flare
2. Secondary flare
PRIMARY FLARE:
It is the conventional and basic part of circumferential tie facially and
lingually for an intra coronal preparation. It is very similar to long
bevel formed of enamel and dentin on the facial or lingual wall,
primary flares always have a specific angulation, i.e., 45 to the inner
dentinal wall proper.
Main function of flare is to bring the facial and lingual margins of the
cavity preparations to cleansable – finishable areas.
SECONDARY FLARE:
Flat – plane super imposed peripherally to a primary flare
Sometimes it is prepared in a hollow ground form to accommodate
materials with low castability.
Secondary flares may have different angulations; involvement and
extent depend on their function.
Indicated in very widely extended lesions bucco-lingually, results in
primary flare ends with an acute angle marginal tooth structure.
A secondary flare with correct angulation can create the needs obtuse
angulation of marginal tooth structure.
In very broad contact areas or malposed contact area.
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MECHANICAL PROBLEMS AND PREPARATION DESIGN
SOLUTIONS:
There are numerous auxillary means of retension for cast
restorations are present.
a. Luting cements
Action is primarily mechanical, locking the cast to tooth structure
by filling the space between them, wetting the details of both the casting
and tooth preparation and filling in the vacancies or irregularities.
b. Grooves
Should be located completely in dentin
Prepare at the expense of the dentinal portion of the facial or
lingual walls or gingival floors proximally.
It helps to prevent lateral displacement of mesial / distal, facial and
lingual parts of restorations.
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It also improves seating of the restoration and minimize marginal
discrepancies.
Grooves are Prepared with tapered fissure bur and they should not
exceed 2mm in depth.
Reverse bevel
Placed at the expense of the gingival floor, creating an internal
dentinal plane inclining gingivally – axially.
It provides locking the restoration and preventing proximal displacement
of restoration.
Internal box
Prepared in dentin with four vertical surrounding walls joining a floor
at definite line and point angles, it can impart four to twelve times the
retention of an external box of same dimension.
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It is advisable to place internal boxed at the very peripheries of a
cavity preparation.
Should have minimum 2mm in three dimensions.
Slot
Internal cavity within a floor of the preparation having a
continuous surrounding wall and floor, junction between the floor and the
surrounding walls is very rounded.
Pins
Pins can be cemented and threaded, parallel and non-parallel,
vertical and horizontal.
Collor
This is a surface extension completely surrounding a cusp or a
surface of a tooth.
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Skirt
It involves a part of the axial angles of tooth.
Post
They are used as retention mode for as core foundation to be
covered with cast restoration.
Reciprocal retention
In a cavity or tooth preparation to accommodate a cemented type of
restoration, every retention mode must have an opposite retention mode
to completely immobilize the restoration placing retention modes at every
end of the preparation or parts of the preparation is called reciprocal
retention, a basic and design preparation for cast restoration.
Capping the occluso-proximal facial or lingual corner of the
preparation:
This is done for dual purpose of protecting thinned corners, due to
over preparation or wide preparation in an ovoid tooth, and adding to the
restoration retention by locking it, over a facial or lingual corner, a bevel
extension facially or lingually a knife edge or chamfer finishing line.
This procedure is not indicated for class IV and V material.
20
Pre-cementation grooving of the casting and the adjacent tooth surface
or walls
To add to the retaining capability of the luting agent, after the
casting is ready for cementation an inverted truncated cone groove is cut
on one or more of the sides of casting and opposite it on the preparation a
similar groove is cut. If they are not opposite one another, the retaining
effect will be minimized. A sufficient amount of cement should be mixed
and flown into the grooves when cementing the casting. It has been
proven that this technique will enhance the retaining effect of the cement
several fold, and it is most effective for class V.
Electrolytic etching of tooth surface of the casting
The internal surface of class IV cast material restoration can be
subjected to electrolytic etching. The margins and external surface of the
restoration are covered with sticky way, and the restoration is used as an
anodic electrode in an electrolytic cell composed of 0.5N nitric acid and a
cathode of another non noble alloy higher in the electromotive force table
a voltage current is passed into cell, leading selective etching of non
noble alloy. Etching process takes 10-15 minutes, to create irregularities.
The other preparation feature that will help solve mechanical
problems of cast restoration.
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All line and point angle should be definite, but not angular.
Roundness for class V material.
Axial wall should slant toward pulpal floor, together with rounding
axio pulpal line angle can reduce stress.
Maximum reduction should be at the occluding surface, especially
the parts of tooth surface that are in contact during static and dynamic
relation of the mandible average of 1mm should be cleared for metallic
casting in inclined place of cusp. This reduction is 1.5 mm for ceramic
restorations.
DESIGN OF CAVITY AND TOOTH PREPARATIONS FOR CAST
RESTORATIONS
General Shape:
The outline of the occlusal portion of the preparation is the dove-
tailed. The proximal portion is usually boxed in shape.
22
Location of margins
In the occlusal portion the facial, lingual, and, sometimes, proximal
margins are located on the inclined planes of the corresponding cusps,
triangular ridges or the marginal ridges (crossing ridges). This is
designed so that the bucco-lingual width of the cavity preparation
(distance measured between the buccal and lingual wall proper),
especially at the isthmus portion, does not exceed one-third the
intercuspal distance. The most peripheral margins of the preparation are
located away form contact with the opposing tooth surfaces during centric
closure and excursive movements of the mandible. All adjacent wear
facet, supplementary grooves and areas of decalcifications, or any defect
in the adjacent parts of the occlusal surface, should be included in the
beveled portion of the cavity preparation only.
In the proximal portion the facial and lingual margins are each in
the corresponding embrasure. This is designed so that the full length of
an explorer can be passed freely in the occluso-gingival direction, and so
that all undermined enamel, surface defects, and peripheral marginal
undercuts are eliminated.
The wall proper, constituting about the pulpal two-thirds of the
facial or lingual (proximal) walls, is formed completely of dentin. These
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walls should taper from each other on the average of 2-5, or be parallel
to each other, if necessary. Each wall should make a right angle or
slightly obtuse angle with the pulpal floor.
The occlusal bevel, which is a long bevel, constituting almost one-
third of the facial and lingual (proximal) walls, this beveled outer pane of
the walls will have an average angulation of 30-45 to the long axis of the
crown.
This angulation should increase as the width of the cavity
preparation increases, in order to accommodate more bulk of cast alloy,
and to be able to resist increased stresses near the cusps on the inclined
planes. Also, this increased angulation is necessary to bevel enamel rods,
which are inclined toward the cusps.
The angulation of the bevel should decrease with increased
steepness of the cusps. Sometimes, bevels are not needed at all in very
steep cusps.
The “bevel” part of the facial and lingual, and, sometimes, the
proximal walls of the inlay cavity preparation will usually be half that of
the cavity “wall proper”. This bevel is extended to include wear facets
24
and occlusal defects or decalcifications, if they are confined to the
occlusal surface. They are also extended to include supplementary
groves and to move the margin away from occlusal contacts.
In the inlay cavity preparation, the pulpal floor should be flat over
most of its extent. If this is not possible, at least the peripheral portions
should be flat.
The conventional pulpal depth of the inlay cavity preparation is a
little more than that for amalgam in order to create more length for
surrounding walls. Generally speaking, this depth should be 1-1. 5 mm
from the dentinoenamel junction. The pulpal floor should meet all
surrounding walls in a definite line angle; expect its junction with the
axial wall, where the joint should be very rounded.
In the proximal portion of the inlay cavity preparation, the axial
wall should be either flat or slightly rounded in the bucco-lingual
direction, and either vertical or slightly divergent (5-10) towards the
pulpal floor in the gingivo-occlusal direction. Divergence here is
important only insofar as it imparts some taper on the preparation,
facilitating the procedural steps for the restoration.
25
The axial wall should, meet the pulpal floor in an extremely
rounded junction as in amalgam. This prevents stress concentration in the
tooth and the casting. Furthermore, the depth, axially, should ideally be
1-1.5mm from the dentinoenamel junction. However, different depths
may be necessary according to the cariogenic pattern of the dentinal
lesion proximally.
Proximally, facial and lingual walls are comprised of two planes.
In the axial half (i.e., the facial or lingual “wall proper”) it is formed
completely of dentin ad meets the axial wall at a right angle relationship.
This is the main resistance and retention feature of that part of the
cavity preparation. The proximal half of the facial and lingual walls is
formed of a primary flare, comprised of enamel and dentin with an
unchanged 45 angle to the “wall proper.
Sometimes it is necessary to impose a third plane in the form of a
secondary flare, placed on enamel peripherally. This serves to simplify
impressions and wax patter manipulations ad for the other reasons
previously mentioned. Secondary flares should not be used if a direct
wax pattern technique is to be used. The secondary flare can have
variable angulation and extent to achieve its objectives.
26
The gingival floor, proximally should be flat in the bucco-lingual
direction, making a slightly obtuse angle with the buccal and lingual
walls.
In the axio-proximal direction, it is formed of two planes. The
axial half consists of gingival wall (floor) proper, being perfectly flat,
formed of dentin, and making either a right angle or a 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 to the wall proper. However, this angulation can
be increased by an increase in the gingival extent and length of the
surrounding walls. This will serve to minimize marginal discrepancy.
Although this bevel is usually equal in extent to the “wall proper”,
it can be increased to include notches or surface defects, or decreased if
the margin ends on cementum rather than enamel.
The extent of the bevel may also be decreased if the cavity is
increased in length occluso-apically. This creates more room for the flat
“wall proper”.
27
Te junction between the occlusal bevel and the secondary or
primary flares proximally, and also, the junction between the primary or
secondary flares proximally and the gingival bevel should be very
rounded and smooth.
Modifications for Class IV and some Class III materials
Although the general shape, location of margins, and most of the
internal anatomy of preparations for cast alloys in the Class IV (and
sometimes Class III) category are similar to those to be described for
Class I and II alloys, certain specific modifications must be enumerated.
Although the preparation will still contain internal boxed portions
(buccal, lingual, and proximal “wall proper”) occlusally and proximally,
the internal line and point angles should be more rounded.
Surrounding walls should be more parallel to one another. All
circumferential tie constituents (primary or secondary flares, occlusal or
gingival bevels) should be hollow-ground to improve the capability of
these alloys to replicate marginal details during casting.
28
Tooth preparation should be deeper axially and pulpally to
compensate for the loss of retention that results from the relatively poor
castability of these alloys.
Modifications For Class V Cast Metals
The internal anatomy of cavity preparations for these alloys will
include the following additional changes.
Definitely flat pulpal and axial walls meet surrounding walls in a very
rounded line angles.
To improve retention, the preparation should be relatively deeper and
with minimal or no taper.
The gingival floor, if its margins end on cementum (dentin) or at the
occlusal or middle third of the anatomical crown may be made a flat,
one-planed floor completely formed of dentin (after cementum
removal), or enamel and dentin terminating in a 90 cavosurface
margin. Although this might reduce the negating effect of bevels on
internal discrepancies shown marginally, exactness of the fit of cast
29
ceramics would counteract this problem. This could simply be done
by preparing these surrounding walls in a purely wall proper
configuration with no bevel or flare components. This simplified
cavity preparation could be done provided that.
No undermined enamel is left marginally.
The margins are placed in finish able, cleansable areas
Joint angles are prepared extremely rounded and the
cavosurface ones are made right angles.
Tooth preparation for only cast restorations:
Onlays are the most and universally used cast restoration for
individual teeth. It is partly intra coronal and partly extra-coronal type of
a restoration, which has cuspal protection as the main feature.
Indications:
1. Cuspal protection is to be considered if the width of the lesion is
1/3 to ½ of inter cuspal distance.
2. In the cast restoration cuspal protection is mandatory, if the width
of the lesion is exceeds ½ of inter cuspal distance.
3. In tooth preparation the length width ratio of cuspal is more than
1:1, but not exceeding 2:1, cuspal protection is to be considered.
30
4. If length; width ratio is more than 2:1 cuspal protection is
mandated.
5. When need to change the dimension, shape and interrelationship of
the occluding tooth surface the onlay cast restoration are ideal.
6. Onlay is ideal restoration for abutment teeth for a R.P.D (or) fixed
prosthesis.
7. They are ideal supporting restorations for remaining tooth
structure.
8. Onlays are necessary to include wear facets that exceeded the cusp
tips and triangular ridge crests.
General shape:
1. Onlays are dovetailed internally and follow cuspal anatomy
externally.
2. Proximally they appear as box (or) cone shaped.
3. The main feature is to capping of the functional and the shoeing f
non-functional cusps.
Location of margin:
Occluso-facio-lingual portion.
1. On the functional side cusps are capped for the additional retention
and protection.
31
2. They must be located for enough gingivally away from contact
with the opposing tooth surface.
3. Normally this will involve ¼ to 1/3 the facial (or) lingual surfaces.
4. Gingivally, margins include all facial (or) lingual grooves and
should be parallel to the contour of cusp tips and crests of adjacent
ridges.
5. On non-functional side, the facial (or) lingual margins located just
gingival to the tip and ridge crests of the cusps and away from
occlusal contact.
Proximal portion:
1. Secondary flares are used in all situations
Internal anatomy:
In occluso-facio-lingulal – portion:
1. The pulpal floor is deeper, and the facial (or) lingual wall will be
formed by four planes.
a. Wall proper : is an a intra coronal portion of the wall which
constitutes atleast half of the vertical height of the total wall.
It is completely in dentin and slightly tapered from the
opposing wall proper by 2 to 5, making a definite angle with
the pulpal floor.
32
2. The table is the transitional area between the intra-coronal and
extra-coronal parts of the preparation. It is partly in dentin and in
enamel. It is relived form opposing cusps by at least 1.5mm in
both static and functional contacts. At any location the table
should be flat, following cuspal direction in mesio-distal direction.
The table is one of the major resistance forms of the entire
structure.
Counter Bevel
The counter bevel is formed in enamel and Dentin, and it should be
relieved from opposing cuspal element at least 1mm in both static and
functional occlusal contact.
The hollow ground bevels are on the capped side will have four
different hollow ground bevels corresponding inclined planes of the cusp.
The angulation is from 30-70 between the bevel and long axis of
the teeth.
The angle varies according to:
The amount indicated for facial or lingual surface
The amount of needed retention.
33
The type of cast alloy, the less the castability of the alloy the greater
will be the angle.
The shoe
It performs same functions as table. In some situations it end in
facially or lingually with acute angle leaving frail enamel. This should
incline away facially or lingually –gingivally and follows cuspal
anatomy. The peripheral partial bevel is indicated with difficult
continuation between a shoe and a primary (or) secondary flare
proximally.
Proximal portion
The internal anatomy of tooth preparation for on lay proximally is
very similar to proximal part in cavity preparation for inlays. For onlays,
however the secondary flare with its flexible angulation is feature of
cavity preparation.
34
Modifications for class – IV and class III
1. The occlusal reduction must be greater, in order to accommodate
bulkier cast material.
2. All circumferential tie constituents must be hollow ground
3. All cusps must be capped rather than shoed, as a means of cuspal
protection.
4. The preparations should not feature any small-complicated internal
(or) external details.
5. The concavity of hollow-ground bevels should include enamel and
dentin.
Modification for class v cast materials:
1. Cuspal protection must take the form of capping rather than
shoeing.
2. In most cases, capped cusps consist of 3 distinct planes an hollow
ground bevel extending to pulpal floor to the table.
The counter bevel should be more acute than that described for
class I and class II alloys.
3. There is more occlusal reduction for the table and counter bevel to
accommodate sufficient bulk of cast ceramic.
4. The gingival, buccal, and lingual walls proximally should be
similar to inlays.
35
5. The preparation should be deeper than that for class – I and II cast
alloys due to absence of “boxed up” internal portions of
restorations.
6. No tapering should be exhibited by any wall
7. In cases of extreme occlusal involvement bucco-lingually this
modification may be accomplished.
8. The junctions between the walls and pulpal floor and table are
very rounded.
9. The facial (or) lingual margins ends in the middle or occlusal 1/3
of that surface.
Tooth preparation for cast Restoration with surface extension.
These are modification for basic onlay and inlay tooth preparations
and restoration involving part (or) all the axial surfaces.
There are three types of surface extensions that may be imposed on
any type of tooth.
Reverse secondary flare:
This is a surface extension of the basic intra coronal inlay (or)
onlay.
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Indications:
Surface extensions are required to include facial (or) lingual defects
beyond the axial angle of tooth.
They are used to eradicate peripheral undercuts.
To add retentive capability of the restoration proximally.
This type is contraindicated in class – IV and V restoration.
Reverse secondary flares can be added to cavity preparation in lieu of
a secondary flare, directly over primary flare.
It can also be placed in a cavity preparation upon a secondary flare.
The reverse secondary flare can fulfill the form of partial bevel. It
involves enamel only, with the main cavity preparation.
It ends on the facial (or) lingual surface with a knife-edge finishing
line and its extent should not exceed the height of contour of facial
(or) lingual surface in mesio-distal direction nor should it included the
tip of cups.
Skirt
This is more extensive surface extension than reverse secondary
flare, also super imposed on the basic intra-coronal inlay or onlay cavity
preparation facially (or) lingually.
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Indications
Skirting is to involve defects with more dimensions than those that can
be involved in a reverse secondary flare.
It is required to impart resistance and retention on a cast restoration.
It is necessary when contact areas and contour of proximal surfaces
are to be changed in the contemplated restorations.
Skirts are essential facially and lingually for tilted Teeth in order to
restore the occlusal plane and are prepared at the side forwards, which
the tooth is tilted.
Features:
Skirts include facial and lingual surfaces at axial angles to a depth 0.5-
1 mm in class –I and II alloys, and a depth of 1.5 – 2 mm for class III,
IV and V cast materials.
The maximum depth of skirts should be at the junction of the surface
extension with the cavity preparation.
For class – I, II and III skirt ends at mesially (or) distally in a chamfer
line. For class IV and V the skirt ends in a hollow ground bevel.
It is sometimes preferable to terminate the skirt mesially or distally in
a vertical groove, and is used to accommodate bulky material.
The depths should be 1-2mm for classes I and II and 2mm for cast
ceramics.
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The groove should be completely rounded in mesio-distal direction for
cast ceramics and surrounding walls for cast alloys.
Every effort should be made to make axial reduction of the skirt
parallel to the rest of cavity preparation and some times it is even
tapered to remainder of preparation.
For class I, II, III and IV cast alloys, intervening facial or lingual wall
proximally between cavity preparation proper and the skirt extension.
Should have boxed portion and a primary flare.
If the skirt to be used to change the contact and contour of the tooth, it
should be extended far enough on the facial and lingual surface of a
tooth create sufficient retention for the cast material.
Collar
This type of surface extension is the most involving surfacewise and
depthwise. And it may be one of two types.
Cuspal collars involve facial (or) lingual surfaces of one cusp only in
multi-cuspal tooth.
Tooth collars, which involve the entire facial or lingual surface of the
tooth.
Either type can surround the cuspal elements or apical to an already
lost cuspal element.
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Indications:
They help in retention and resistance when an entire cusp is lost prior
to tooth preparation.
They help retention in shortened teeth.
They help resistance and to enhance support for the tooth that is
endodontically treated.
They are used in situations where pins are contra indicated.
They are used in teeth with large foundations replacing cuspal
elements.
They are used in a cast alloy rest to be veneered by fused porcelain.
Features:
With axial depth of 1.5 mm- 2mm collar extensions and gingivally in
a beveled shoulder finishing line.
For class IV cast material the beveled portion of the shoulder should
be hollow ground.
For cast ceramic materials the bevel is rounded and no bevel required.
In cast alloy there should be definite line angle gingivally.
Collars should have less tapering toward the cavity preparation and
this improves retention in these shortened teeth.
The shoulder portion of collar should be perpendicular to the long axis
of the crown.
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REFERENCES
1. Gerald T. Charbeneau. Principles and practice of operative
dentistry. 3rd Edition. Varghese Publishing. 351-356.
2. William H. Gilmore. Operative Dentistry. 4th Edition. B.I.
Publication Pvt. Ltd. 260-268.
3. Lioyd Baum. Textbook of Operative Dentistry. 3rd Edition. W.B.
Saunders Company. 470-484.
4. Theodore M. Robertson. Herald O. Heyman. Sturdevants art
and science of operative dentistry. IV Edition. Mosby Company.
801-826.
5. Vimal K.Sikri. Textbook of operative dentistry. CBS Publishers.
243-256.
6. M.A. Marzouk., A.L. Simonton., R.D. Gross. Operative dentistry
modern theory and practice. All India Publishers and Distributors.
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