Finish lines/cosmetic dentistry course by Indian dental academyDocument Transcript
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.
4. Beveled shoulder.
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
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
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
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
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
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
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
Functions of the bevel are as follows:
1. To seal restoration against cement leakage and subsequent bacterial
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
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:
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
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
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
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
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.
A SEMINAR ON
FINISH LINES IN FPD
Date : 04-08-07
Signature of Prof & HOD
SIBAR INSTITUTE OF DENTAL SCIENCES
Herbert.T Shillingburg JR, Sumiya Hobo: Fundamentals of
Fixed Prosthodontics; 3rd
Stephen.F Rosentiel, Martin F. Land, Junhei Fujimoto:
Contemporary Fixed Prosthodontics; 3rd
William F.P Malone, David L Koth: Tylman’s Theory and
Practice of Fixed Prosthodontics; 8th