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.

A SEMINAR ON

FINISH LINES IN FPD

Presented byDr.G.Manmohan,P.G student.Date : 04-08-07 Signature of Prof & HOD

SIBAR INSTITUTE OF DENTAL SCIENCES Guntur-522509

CONTENTS

Introduction

Types of finish lines

Knife edge

Chamfer

Shoulder

Bevelling

Subgingival margin finish lines

Conclusion

References

Herbert.T Shillingburg JR, Sumiya Hobo: Fundamentals of Fixed Prosthodontics; 3 r d Edition.

Stephen.F Rosentiel , Martin F. Land, Junhei Fujimoto: Contemporary Fixed Prosthodontics; 3 r d Edition.

William F.P Malone, David L Koth: Tylman’s Theory and Practice of Fixed Prosthodontics; 8 t h Edition.