Published on March 7, 2014
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 6degree taper, which is considered the ideal because it is achievable, almost 50 per cent of the retention is lost. With alteration from a 5degree 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. 2. 3. 4. 5. 6. Little resistance to marginal distortion during firing of porcelain. Margin not always distinct. Poor control over placement of subgingival margin. Insufficient preparation in cervical area. No control over reduction of cervical tooth structure, and 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 roundended 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 by Dr.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.
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