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2. INDIAN DENTAL ACADEMY
Leader in continuing dental education
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3. INTRODUCTION
Tooth preparation is defined as the mechanical treatment of
dental disease or injury to hard tissues that restores the tooth to
original form.
Reduction of tooth structure is preceded by a mental image
of the design of the artificial crown and the anticipated occlusion.
The point at which the preparation terminates on the tooth is
called the finish line.
The classic design for the preparation must be visualized, so
modification can be instituted. Diagnosis and discipline tooth
pipes are essential to successful fixed prosthodontics.
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4. Objectives to Tooth Preparation
• Reduction of the tooth in miniature to provide retention and
support.
• Preservation of healthy tooth structure to secure resistance
form.
• provision for acceptable finish lines.
• Performing pragmatic axial tooth reduction to encourage
favorable tissue response from artificial crown, contours.
There are 3 requirements for successful restoration margins They must fit as closely as possible to the finish line of the
preparation against to minimize the width of the exposed
cement.
They mist have sufficient strength to withstand the forces of
mastication and
Whenever possible they should be located in areas where the
dentist can finish and inspect them and the patient can clean
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them.
5. A properly tapered preparation is essential for close fitting
margins. There can be no undercuts or irregularities on the axial
wall that will prevent complete seating or cause the margins to
spread as a restoration is being inserted. Roughness of the tooth
surface under margins can prevent close adaptation.
They fore all bevels should be given as smooth finish lines with
a fine instrument.
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6. FINISH LINE REQUIREMENTS
The points at which the preparation terminates on the tooth is
called the finish line. A finish line serves many functions.
i). During visual evaluation of the tooth preparation, it is a
measure of the amount of the tooth structure already removed.
It also delineates the extent of the cut in an apical direction.
ii). The finish line is one of the features that can be used to
evaluate the accuracy of the impression made for induct
procedures.
iii). On the die, a distinct finish line helps in the evaluation of the
quality of the die and aid in trimming it accurately.
iv). The correct marginal adaptation of the pattern depends on
an obvious finish line.
v). The evaluation of the restoration is also aided by a proper
finish line.
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7. vi). At the time of cementation, a sharp finish line aids in
determining whether the restoration is fully seated Several forms
of finished lines can be developed, i.e. chamfer knife edge,
feather edge, shoulder and beveled shoulder.
The more commonly used forms of gingival finished lines
are the knife-edge, the shoulder, the beveled shoulder and the
chamfer. The knife edge finish line was popular before the
development of high speed cutting.
Instruments and accurate impression materials. It is still
used on tipped teeth where the axial surface of the tooth needs
the path of insertion at an angle greater than 15˚.
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8. Four fundamental criteria for successful margins areAcceptable marginal adaptation.
ii. Tissue tolerant surfaces.
iii. Adequate contour.
iv. Sufficient strength
Under most circumstances feather edges and chisel edges are
unacceptable. Historically their main advantage was that they
facilitated the making of the impressions with rigid modeling
compound in copper bands, because there was no edge on which
a band could catch.
There are situations when knife - edge margins are a distinct
advantage, i.e. in younger patients in accessible areas of the oral
cavity. Knife edge finish lines are also employed in areas other
than gingival terminations, i.e. edge preparations and the outline
of partial veneer crowns. www.indiandentalacademy.com
10. Chisel edge
This type of finishing line is used after periodontal surgery
that results in long clinical crowns termination apically on the tooth
structure. If a shoulder and bevel were to be created, and over
prepared and probable pulpal exposure are likely.
Feather edge finish line
Feather edge or shoulderless crown preparation should be
avoided because although they are conservative of tooth
structure, they fail to provide adequate bulk at the margins. Over
contoured restorations often result from feather edge margins,
because the technicians can handle the wax pattern without
distortion, only by increase its bulk beyond the original contours.
A variation of the feather edge, the chisel edge or knife-edge
margin is formed when there is a large angle between the axial
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surfaces and the unprepared tooth.
11. Because the cut surface is more nearly parallel to the path of
insertion, the knife-edge finish line produces the best marginal
seal. For most situations, however the knife-edge finish line is not
recommended because it is difficult to follow on both tooth and
die. Although it produces a good fit the margin is weak. The most
likely result of this type of finished line is over contouring.
Shoulder finish line
The shoulder finish line is usually associated with complete
porcelain crowns or at times with porcelain fixed metal crowns
and presently injectable porcelain.
It is arduous to prepare, difficult to obtain an accurated margin
and more likely to promote
adverse pulpal involvement.
Controlled removal and pulpal avoidance are paramount during
preparation.
The gingival contour for posterior teeth also are extremely
difficult because of belated micro-leakage and accessibility on
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distal surface of molars.
12. Conversely the increased tooth reduction performed during
should preparation permits more latitude for the gingival contour
of the cast restoration.
A shoulder provides more bulk metal than the a heavy chamfer
and this helps reduce extortion during porcelain firing.
Because the shoulder margin allows room for porcelain, it is
recommended for all ceramic crowns, especially when the
porcelain margin technique I used it should form an 90 degree
angle with the prepared surface. An acute angle is likely to chip.
A 120 degree slope shoulder margin is used as an alternative
to 90 degree shoulder for the facial margin of a ceramic crown.
The slope shoulder reduces the possibility of leaving unsupported
enamel and yet leaves sufficient bulk to allow thinning of the
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metal framework to a knife-edge for good aesthetics.
13. Bevelled shoulders
Modification of the full shoulder with a level is considered a
more judicious course of treatment.
The angle of this level approaches the path of inception of the
restoration and improves marginal adaptation.
The bevel with a rounded axial angle is indicated for porcelain
fixed to metal preparation.
This preparation has a smooth evenly distributed level from
proximal to proximal on the lingual side.
A rounded axial angle allows the metal bulk to resist functional
distortion.
Under some circumstances beveled margins are more suitable
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for cast restoration, particularly if a edge or shoulder exists
possibly from dental caries, cervical or previous restoration.
14. The objective in beveling is 3 fold:
i. To allow the cast metal margin, to be bent or burnished against
the prepared tooth structure.
ii. To minimize the marginal discrepancy caused by a complete
crown than fails to seat completely, and
iii. To protect the unprepared tooth structure from chipping when
access for burnishing is limited, there is little advantage is
beveling. This applies particularly to a gingival margin, where
beveling would lead to sub-gingival extension of preparation or
placement of the margin on dentine rather than on enamel. Facial
margin of maxillary partial coverage restoration should be
beveled to protect the remaining tooth structure and to allow for
burnishing.
A beveled shoulder margin is often recommended for a facial
surface of a metal ceramic restoration where a metal collar is to
be used. The beveling removes unsupported enamel and allows
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finishing of the metal.
15. Chamfer finish line
A chamfer is an obtuse angled gingival termination.
It is a concave extra coronal finish line that possesses greater
angulations than a knife-edge with less width than a shoulder.
According to El- Ebrashi, margins with chamfers provide a gingival
area with an acceptable stress distribution and an adequate seal
and required minimal uniform tooth reduction.
The chamfer also enhances accurate die trimming for technical
fabrication of the cast restorations.
A chamfer is the preferred
cervical finish line for fixed
prosthodontics and should be utilized whenever possible because
it is easily developed and visually intact.
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17. Feather head finish line
Advantages - Conservative of tooth structure
Disadvantages- Does not provide sufficient bulk
Indications - Not recommended
Chisel edge or Knife edge
Advantages - Conservative of tooth structure
Disadvantages- Location of margin difference to control
Indications
- Occasionally on tiled teeth
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18. Chamfer
Advantages - Distinct margin
- Adequate bulk
- Easier to control
Disadvantages
Care needed top avoid unsupported lip of enamel
Indications - Cast metal restorations
- Lingual margin of metal ceramic crowns
Shoulder
Advantages - Bulk of restorative material
- Preparation produces fewer sensitive teeth
after cementation
Disadvantages - Less conservation of tooth structure
Indications - facial margin of metal ceramic crowns
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- Complete ceramic crowns
19. Shoulder with bevel
Advantages - Bulk of material
- Removes unsupported enamel
- Allows finishing of the restoration
Disadvantages - Less conservative
- Extends preparation apically
Indication - Facial margins of posterior metal ceramics crowns
with super gingival margins
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21. Location of finish lines
The placement of finish lines has a direct bearing on the case of
fabricating a restoration and upon the ultimate success of the
restoration.
The best results can be expected from margins that are as
smooth as possible and are fully exposed to a cleansing action.
Whenever possible the finish line should be placed in a area
where the margins of the restoration can be finished by the dentist
and kept clean by the patient.
In addition they must be placed so that they can be duplicated
by without tearing or deforming the impression when it is removed
past them.
Finish lines should be placed in enamel when it is possible to do
so. In the past the traditional concept has been to place margins
as far gingivally as possible. There is no longer much support for
this approach as a routine measure.
Sub-gingival restoration have been described as one of the
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major etiologic factors in periodontics.
22. It has been reported that the margin located even with the
gingival crest will produce less inflammation than one located
either above or below the level of the crust.
The super gingival position was seen as least harmful and one
level with the crest of the few gingival was intermediate in its
potential for damage. Because preparations are frequently
extended sub-gingivally to obtain that length. The placement of
finish lines can also be altered from ideal locations by caries, the
extensions of previous restoration’s or trauma.
Caution must be exercised in this regard if caries or in this
regard if caries or fracture require that the finish line be placed
near the alveolar crest, a crown lengthening procedure may be
necessary. The combined dimensions of the epithelial and
connective tissue attachment of the restoration margin in this area
will probably result in gingival inflammation, loss of alveolar crest
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height and formation of periodontal pocket.
23. The prevent this untoward occurrence, it may be necessary to
perform surgery to guarantee this biologic width by surgically
moving the alveolar crest 3 mm apical to the location of the
proposed finish line this will allow space for the connectives and
epithelial attachments and a healthy gingival sulcus. If the deep
finish line is located inter-proximally and will require extensive
removal of bone between the tooth being restored and the
adjacent tooth. It may be better to extract the tooth in question
rather than periodontally compromise its healthy neighbour.
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24. Instrumentation
Rotary instruments must be selected that allow the tooth to be
reduced according to the requirements of proper retention and
resistance form and finish line development.
Also instruments must be available for placement of welldefined and smooth boxes, grooves and pin-holes where these
are needed.
These instruments are generally available in course, medium
and fine grit with wide range of shapes and diameters.
The coarse the medium grit is used for removal of tooth
structure and the fine grit diamond instrument leaves the tooth
and finish line adequately smooth for all procedures related to
fixed prosthodontics tapered round and diamond instrument forms
not only the proper preparation taper, but also establishes a
cervical chamber when the tooth is reduced until the instrument tip
penetrates into the tooth, a distance equal to one half its diameter.
This instrument is the onewww.indiandentalacademy.comfor preparation requiring a
best suited
chamfer finish line.
25. Chamfer is formed by allowing the round end instrument to
penetrate into the tooth a depth equal to one half of its tip
diameter. The chamfer should never be prepared wider than half
the tip of the diamond, lest in unsupported lip of enamel result.
Margin placement
Whenever possible, the margin of the preparation should be
supragingival. Sub-gingival margins of cemented restorations
have been identified as a major factor in periodontal disease,
particularly when they encroach on the epithelial attachment.
Supra-gingival margins are easier to prepare accurately without
trauma soft tissues. They can usually also be situated on hard
enamel whereas sub-gingival margins are often on dentine or
cementum.
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26. Other advantages of supra-gingival margins includei) They can be easily finished.
ii) They are more easily kept clean.
iii) Impressions are more easily made with less potential for soft
tissue damage.
iv) Restorations can be easily evaluated at recall appointments.
However a sub-gingival margin is justified if an of the following
pertain to –
Dental caries cervical erosion or restoration extend
subgingival.
The proximal contact area extends to the gingival crest.
Additional retention is needed.
The margin of a metal ceramic crown is to be hidden behind
the labio-gingival crest.
Root sensitivity cannot be controlled by mores conservative
proceduce.
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Aesthetics
27. Margin length
The junction between a cemented restoration and the tooth is
always a potential site. For recurrent caries because of dissolution
of the luting agent. Where possible it should be kept as short as
possible.
Rough or irregular junctions greatly increase the length of the
margins and reduce the possibility of obtaining a good fitting
restoration.
Margin geometry
For evaluation, the following guidelines should be considered:
It should be easily to prepare without over extension.
It should be readily identified in the impression and in the die.
It should give a distinct margin to which a wax pattern can be
finished.
It should provide for sufficient bulk of the material. This will enable
the wax pattern to be handled without distortion as well as give the
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restoration strength and wear porcelain is used, aesthetics.