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INDIAN DENTAL ACADEMY Leader in continuing dental education

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.

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 them.

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.

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.

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 15Ëš.

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.

•A. Feather edge •B. Chisel •C. Chamfer •D. Bevel •E. Shoulder •F. Sloped shoulder

•G. Beveled shoulder

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 ďƒ˜ 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. ďƒ˜ A variation of the feather edge, the chisel edge or knife-edge margin is formed when there is a large angle between the axial surfaces and the unprepared tooth.

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 ďƒ˜ The shoulder finish line is usually associated with complete porcelain crowns or at times with porcelain fixed metal crowns and presently injectable porcelain. ďƒ˜ 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. ďƒ˜ The gingival contour for posterior teeth also are extremely difficult because of belated micro-leakage and accessibility on distal surface of molars.

 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 metal framework to a knife-edge for good aesthetics.

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 porcelain fixed to metal preparation.

is indicated for

 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 for cast restoration, particularly if a edge or shoulder exists possibly from dental caries, cervical or previous restoration.

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 finishing of the metal.

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.

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

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 - Complete ceramic crowns

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

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 have been described as one of the major etiologic factors in periodontics.

 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 gingival inflammation, loss of alveolar crest height and formation of periodontal pocket.

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.

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 one best suited for preparation requiring a chamfer finish line.

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.

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. Aesthetics

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 restoration strength and wear porcelain is used, aesthetics.

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