FINISH LINES
IN FPD
Finish
Lines
INTRODUCTION
The ultimate goal
in fixed and removable prosthodontics is the maintenance and preservation of
the remaining dentition. The execution of this goal can be achieved initially
by tooth preparations that are clinically sound and will increase the longevity
of the abutments. Likewise, proper tooth preparation and contoured restorations
that are periodontically acceptable are of major importance in maintaining
optimal periodontal health, restoration of occlusal harmony, and stability of
the restored dentition. Restoration of teeth is possible only if sufficient
space is created for the application of the appropriate thickness of material
required. Preference for the shoulder with a bevel preparation allows ample
room for the periodontal tissues and the bulk of the restorative materials
(metal crowns with acrylic resin veneers or porcelain-fused-to metal). The
indications and contraindications for each type of full coverage preparation
will be reviewed.
TYPES OF FINISH LINES
Over
the years there is often discussion about the various types of full coverage
preparations and their advantages and disadvantages. There are four types of
finishing lines for full coverage restorations:
1.
Knife edge.
2.
Chamfer.
3.
Shoulder.
4.
Beveled shoulder.
Knife-Edged Preparations:
A
knife-edge, or a feather-edge preparation that is basically designed so that as
the tooth is prepared zero cutting results at the gingival termination. The
dentist employs the rotary instrument and leans the cutting stone or bur inward
by rotating on that gingival termination
and cutting mostly at the occlusal end. It is a process of tipping the rotary
instrument occlusally. When planning the taper of this type of preparations, a number
of problems are observed, especially with a short crowned tooth or on a tooth
with a normal anatomic crown where the preparation ends at the cementoenamel
junction.
1.
When using ceramometal
restorations and aesthetic considerations are critical, because there is zero
cutting at the gingival termination and aesthetic concerns are of primary
concern and a metal collar is not to be used, then the resultant slip joint
type of crown becomes overcontoured gingivally. Concomitant with this, the
entire contour of the crown becomes greater, as without overcontouring, color
cannot be achieved in the gingival portion.
2.
The retention and resistance
form of the preparation is compromised. As the preparation becomes overtapered,
the ability of the crown to be retained on the tooth structure becomes diminished.
As an illustration, altering the taper from a perfectly parallel preparation to
one with a 6-degree taper, which is considered the ideal because it is
achievable, almost 50 per cent of the retention is lost. With alteration from a
5-degree taper to about 20 degrees, 25 per cent of the retention remains. Thus,
retention is developed on the basis of the luting strength of the cement.
Cement has a crystalline structure, so it does not fracture at one time. Each
time this cement is challenged, more fracturing of the crystals occur until,
finally, enough of the crystals are fractured to enable the restorations to
loosen. Thus, these overtapered preparations have compromised long-term
retention.
3.
Another negative aspect of
overtapered preparations is that they develop internal stress wedging. As force
is applied into the ceramometal crown with a conically shaped preparation, it
will act like a wedge. The crown exerts a force on the preparation, even if
cement is in between. All materials have flow, even though they are solid. That
flow is enough to cause wedging of the metal. The veneering material is strong
under compression but is weak under tension. The internal stress wedging tends
to expand the metal substructure, causing the porcelain veneer to craze and fracture
over a period of time.
However,
there is a place for a knife-edge preparation in the dentist’s armamentarium.
This is the type of preparation that the clinician should utilize with long
clinical crowns found with postperiodontal surgery cases. With a
postperiodontal case, the clinical crown encompasses the anatomic crown and
part of anatomic root structure. If the preparation extends to the tissue
because of old restorations, root caries, root sensitivity, and aesthetics,
very long preparations will be developed. A shoulder preparation cannot be
developed, because once the practitioner cuts past the junction of the enamel
and onto the cementum, the root may begin to taper severely. Thus, the roots
become narrower, the farther apically the tooth is prepared. In these
compromised cases, if a shoulder is cut, the resultant long, thin preparation
will fracture easily. Interestingly, a knife-edge preparation when employed
with a long clinically crowned tooth is not a overtapered as on short clinical
crowned tooth; therefore, diminished retention of a normal sized preparation is
not a concern with long preparations.
4.
Another problem with
knife-edged preparations is the resistance form. Resistance form is the ability
of a crown to withstand displacement from eccentric or lateral forces. A
lateral force is applied when the mandible goes into eccentric movements. This
is a rotational force that tends to dislodge a crown.
5.
Three factors reduce the
resistance to dislodgement from rotation.
a . The longer preparation the more resistant to dislodgment.
b. The more parallel a preparation, the more resistant to rotation
forces.
c.
The smaller diameter the crown,
the more resistant to rotation forces.
For
example, given the same length and taper, a bicuspid is more resistant to being
dislodged by rotation that a molar. The molar then becomes the liability. In
consequences, in the case of a long-span fixed partial denture extending from a
cuspid to a second molar, cementation wash out occurs on the molar. Rarely, is
it on the anterior tooth, as the molar has the larger diameter and thus the
least resistance to dislodgment. As a result the management of a large-diameter
tooth requires more parallelism and a longer preparation in order to avoid
dislodgment. In addition, grooves may have to be cut into the preparation to
augment the retention and resistance forms. A light chamfer is really a
knife-edge preparation that has a greater amount of tooth removed gingivally.
Another problem associated with knife-edge preparations is that it is quite
difficult to read a finishing line on the die. It disappears and thus there is
a considerable amount of interpretation by the technician. However, if the
beginnings of a shoulder or a light chamfer are cut on these long preparations
and the dentist marks the end of the preparation on the die, which is 1mm past
the shoulder or a light chamfer, then the technician will know where to end the
crown restoration. An indication of a shoulder or a light chamfer simplifies
the impression procedure. Basically, there is nothing wrong with knife-edge
preparations when utilized appropriately, which is usually in periodontally
compromised cases.
Summary of shoulderless preparations is follows:
1.
Little resistance to marginal
distortion during firing of porcelain.
2.
Margin not always distinct.
3.
Poor control over placement of
subgingival margin.
4.
Insufficient preparation in
cervical area.
5.
No control over reduction of
cervical tooth structure, and
6.
Employed with long clinical
crown lengths following periodontal surgery.
The Chamfer Preparation:
A
chamfer, according to Boucher is “a
marginal finish either curved or formed by a plane at an obtuse angle to the
external surface of a prepared tooth.” One advantage of a chamfer preparation
is that any round-ended instrument employed produces the same type of a cut, no
matter at what angle or height the diamond stone is held. This facilitates the
preparations of proposed abutment teeth to be created in relationship to the
soft tissue and that are not made on the same horizontal level throughout. By
following the varying soft-tissue levels. The same configuration of full
coverage preparation will be developed at all the way around the tooth, as the
rotary instrument moves from one vertical height to another. A uniform type of
geometry gingivally is established with a chamfer preparation. The geometric
design obtained with a chamfer preparation will be related not only to the
design of the tip of the instrument, because the tips do vary with different
manufacturers, but also with diameter of the chamfer cutting instrument
employed. There are three different chamfer types of prepartions:
1.
Hybrid. Insert the chamfered
stone about one third of the depth of the stone and obtain a hybrid between a
chamfer preparation and an exaggerated knife-edge type of preparation.
2.
Ski-sloped. Insert the
chamfered stone into the radius of the instrument or half the depth of the
stone; then a more ideal type of chamfer preparation is developed.
3.
Rounded shoulder. Insert the
chamfered stone into its full diameter, the resulting type of chamfer
preparation appears to approximate a rounded shoulder.
Butt Joint Preparation:
A
butt joint preparation employs a ceramometal crown with a bevel created on the mesial, distal, and lingual surfaces,
but not on the labial surface. When constructing a ceramometal crown with a
labial porcelain butt joint, there are several methods used to bake porcelain
to the butted shoulder accurately:
1. One method is the refractory die model concept of Sozio.
2.
Use of platinum foil at the
labial shoulder is another method. This is probably the most successful and
practical technique, as most laboratory technicians are comfortable using this
one and it is repeatable. Technicians are used to employing platinum foil when
constructing porcelain jacket crowns.
3.
A third technique consists of
mixing wax and porcelain together in a ratio of six parts porcelain to one part
wax by weight. This mixture is then waxed in to the butt joint shoulder area on
the die. The technicians can then lift this section off the die for firing. The
wax acts as a luting medium and burns off during firing.
During
the preparations of anterior teeth, there is a concept called a trigon. A
trigon is the labiogingival contour of the termination of the preparation, it
is distal to the middling of the center of the maxillary central and lateral
incisors and is usually in the midline of the maxillary cuspid. This results in
a slightly distal eccentric triangular tooth neck that produces a more
aesthetic result in full coverage restorations than an arcuate labiogingival
contour. The curvature from the height of the trigon to the distal aspect is of
small radius, and mesially there is a more gentle curve of a longer radius. The
desired triangular shape will then result, which is more aesthetically
pleasing. Basically, 99 per cent of the resultant aesthetics comes from the
soft tissues. If unhealthy tissues or tissues that are abnormal in contour and
form are present, an aesthetic restoration will not result. An unacceptable
result is usually not related to the ceramics it is related to diseased tissue
or tissue presenting abnormal form and contour. If the tissue is healthy with
normal contours and tone, a restoration that is slightly off hue will be acceptable, as long as it
does not have the gray-green opaque hue of a nonvital tooth and is of the same
value. Thus when the dentist is having a problem with aesthetics, it is usually
associated with the soft tissues. If the clinician prepares the tooth and soft
tissue properly, the ceramist will have a good opportunity to produce an
acceptable restoration.
BEVELING
Functions of the bevel are as follows:
1.
To seal restoration against
cement leakage and subsequent bacterial invasion.
2.
To permit finishing and
burnishing on die or tooth.
3.
To Provide circumferential rigidity.
4.
To initiate reproduction of the
contour removed in preparation and provide control of the emergence profile
during framework try-in.
The
factors considered in determination of margin placement, subgingivally, supragingivally,
or at tissue height are the concepts of aesthetics, crown length, caries rate,
existing restorations, root sensitivity, and predisposition of periodontal
disease. The important issue involved is that most of the time margins are
going to be placed subgingivally. Crispin
and Watson did a study that revealed that a majority of people do not
show the margins with normal smiling and speaking. Many patients have a phobia
about a margin showing even on a bucispid or on a molar, even though it will not
show during normal function. In this upwardly mobile society, people are
interested mainly in esthetics. They do not want to see their dental
imperfections. Indeed, the state of health is a situation in which people are
not aware of their parts. As soon as a people become aware of their parts, they
know that they have a part problem and become concerned about it. Thus, in the
same view, the best prosthesis is a prosthesis that does not show. That is why
these people use contact lenses instead of eyeglasses. When they brush their
teeth, if there is no margin showing, they feel good about themseleves, and
they forget that crowns are present. Thus, as much as the periodontist advises
not to place crown margins subgingivally, the reality of practice is that people
want subgingival margins.
Terminating
a crown margin at tissue height has the disadvantage of poor aesthetics in an
area of maximal plaque accumulation. The
other extreme is margin placement 2 to 3mm subgingival.
Subgingival margins are employed for the following:
1.
Aesthetics.
2.
Presence of subgingival caries.
3.
Presence of existing
restorations with subgingival margins.
4.
Short clinical crowns with
greatly reduced retentive capacity.
5.
High susceptibility to root
caries.
A
preferable compromise is to prepare a shoulder at tissue height and prepare the
bevel 0.5 to 1mm below the tissue, thus burying the metal collar while
minimizing the insult to the tissue. If the margin is placed too far
subgingivally, gingival inflammation results, and the restoration’s aesthetics
will be compromised. Thus, if the margin is carefully placed and finished
ideally, good long-term results are possible.
The
biologic width is the amount of space that is necessary to house the
periodontal complex, consisting of the transeptal fibres and circular fibers 2
to 3mm between the crest of bone and any restoration. If this width is not
present, inflammation will result, and the inflammation will persist until
alveolar resorption occurs to re-establish the 2 to 3mm biologic width. As a
consequence when a patient undergoes crown-lengthening procedures, not only is
tissue removed, but also bone to ensure a proper biologic width.
When
a crown is prepared on enamel, a right angle shoulder is cut. As soon as the
cementoenamel junction is passed, a shoulder that is in reality 110 to 135° is
prepared. When a bevel is placed on a 135° shoulder, the shoulder will appear
to be too far supragingivally. This is only an illusion. The gingival terminus
of the bevel placed 1mm subgingivally is still in that position and should not
be altered. The mistake that can be made is to drop the shoulder, as it is
thought to be too high and the collar will show. When the shoulder is dropped,
the bevel is lost and a new bevel must be cut. Then the operator may inadvertently
extend into the junctional epithelium and the fibrous connective tissues. Do
not drop the shoulder. When the metal casting is returned and at the time of
its try-in, a water soluble pen is used to mark the tissue height on the
casting so the width of the metal collar can be determined by machining the
casting. If this step is not carried out intraorally, the technician may leave
too wide a metal collar. To correct this, porcelain will have to be backed on
the collar resulting in poor color and overcontour. Thus the metal must be
machined properly.
Most
dentists do not make bevels; they cut collars. Collars are 80 to 90° angles and
extend beyond the shoulder. The reason that most dentists make collars is
because they get their primary retention-resistance form from the collar. The
preparations tend to be overtapered, and thus by making a collar retention and
resistance form is obtained. The true purpose of the bevel is for marginal
integrity. The retention and resistance form is obtained from the axial walls
of the preparation. In an endodontically treated tooth, in which the entire
preparation will be on post, a long bevel is desired because it is like a
barrel hoop that holds the barrel together. It becomes important because some
of the stress of retention and resistance is taken off the post and core. The
long collar binds the root together, and this is important. With a short
preparation, a long bevel is valuable for retention. However, long bevels and
collars are an aesthetic liability.
Theory and Practice of the 45° angle Bevel :
The
beveled shoulder preparation properly placed in relation to the tissue has
offered an excellent solution to almost all problems faced in ceramometal
design. The one exception is aesthetics, especially the long term effect. The
development of many techniques for butt joint porcelain fabrication with
metalceramic restorations and new generations of techniques and materials such
as Cerestore ceramics and castable ceramics points to the aesthetic deficiency
of the beveled shoulder preparation. These techniques have one common goal; the
elimination of the metal collar and its aesthetic limitations.
A
bevel is placed on a crown preparation to reduce the closing angle at the
margin to compensate for the incomplete seating of the crown. A bevel less than
60° does not substantially decrease the closing angle. It is not effective in
compensating for discrepancies of fit. Seating of cast restorations can be
improved by the use of die spacers applied to the die and by vibration during cementation.
With die spacers and this technique, a decreased closing angle of long bevel
may not be necessary.
Instrumentation
during placement of a bevel can create a trough in the tissue that will aid in
obtaining accurate and predictable impressions of the gingival margin.
When
subgingival placement of margins is needed for aesthetics, the preferred bevel
is one that would yield a crown designed to bring metal and porcelain to a
common margin termination with good fit, contour, and color. A bevel of 45° can
produce satisfactory aesthetic result and is satisfactory from a laboratory
standpoint. Not only does a porcelain margin accumulate less plaque, but margin
exposure due to recession at gingival tissue (which occurs with time) is less
objectionable from the aesthetic standpoint. Greater discrimination in
evaluation of margin adaptation is possible.
When
comparing the marginal opening of cemented porcelain fused to metal crowns of
three different casting designs; 80° bevels with metal collars. 80° bevels with
porcelain applied to the labial collars, and 45° labial bevels with metal and
porcelain to a common margin termination. There are no statistically
significant difference between the margin opening of the three groups.
Porcelain application and firing did not distort the facial margin. The 45°
bevel with porcelain to the margin has greater aesthetic potential and the same
margin adaptation as the 80° bevel with an all-metal collar.
CONCLUSION
The placement of
finish lines has a direct bearing on the ease of fabrication a restoration and
on the ultimate success of restoration. Best results can be expected from
margins that are as smooth as possible and are fully exposed to a cleansing
action. Finish lines should be duplicated by the impression, without tearing or
deforming.
Finish lines
should be placed in enamel when it is possible to do so. Subgingival finish
line restorations have been described as a major etiologic factor in
periodontitis. So proper diagnosis and treatment planning ,skill in execution
of tooth preparation with correct finish line contour help to attain basic
principles of tooth preparation like marginal integration and preservation of periodontium.
CONTENTS
Introduction
Types of finish lines
Knife edge
Chamfer
Shoulder
Bevelling
Subgingival margin finish lines
Conclusion
References
Ø Herbert.T Shillingburg JR,
Sumiya Hobo: Fundamentals of Fixed Prosthodontics; 3rd
Edition.
Ø Stephen.F Rosentiel, Martin
F. Land, Junhei Fujimoto: Contemporary Fixed
Prosthodontics; 3rd Edition.
Ø William F.P Malone, David L
Koth: Tylman’s Theory and Practice of Fixed Prosthodontics; 8th
Edition.
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