DEFINITION OF TOOTH PREPARATION

Transcription

1 Lec 1. د In the past, most restorative treatment was due to caries (decay), and the term cavity was used to describe a carious lesion in a tooth that had progressed to the point that part of the tooth structure had been destroyed. Thus the tooth was cavitated (a breach in the surface integrity of the tooth) and was referred to as a cavity. Likewise, when the affected tooth was repaired, the cutting or preparation of the remaining tooth structure (to best receive a restorative material) was referred to as a cavity preparation. Now many indications for treatment for teeth are not due to caries and, therefore, the preparation of the tooth is no longer referred to as cavity preparation but as tooth preparation, and the term cavity is used only as a historical reference. DEFINITION OF TOOTH PREPARATION Tooth preparation is defined as the mechanical alteration of a defective, injured, or diseased tooth to best receive a restorative material that will reestablish a healthy state for the tooth, including esthetic corrections where indicated, along with normal form and function. Included in the procedure of preparing the tooth is the removal of all defective or friable tooth structure because remaining infected or friable tooth structure may result in further caries progression, sensitivity or pain, or fracture of the tooth and or restoration. OBJECTIVES OF TOOTH PREPARATION In general terms, the objectives of tooth preparation are to : (1) remove all defects and provide necessary protection to the pulp (2) extend the restoration as conservatively as possible (3) form the tooth preparation so that under the force of mastication the tooth or the restoration or both will not fracture and the restoration will not be displaced (4) allow for the esthetic and functional placement of a restorative material. Much of the scientific foundation on which these objectives are executed was presented by Black. For many years the Black tooth preparations, with few modifications, formed the basis for most operative preparation procedures. Modifications of Black's principles of tooth preparation have resulted from the improvements in restorative materials, instruments, and techniques, and the increased knowledge and application of preventive measures for caries and periodontal disease. STAGES AND STEPS OF TOOTH PREPARATION The preparation procedure is divided into two stages (initial and final) 1-The first stage of tooth preparation is referred to as the initial tooth preparation stage. In this stage, the mechanical alterations of the tooth are extended to sound tooth structure (sound dentin or enamel supported by noncarious dentin) in all directions (facially, lingually, gjngivally, incisally or occlusally, mesially, and distally) while adhering to a specific, limited pulpal or axial depth (Fig. 1). In this way, the final dimensions of the restoration can be anticipated (minus any necessary bevels). The preparation walls are designed in the initial stage of tooth preparation to both retain the restorative material in the tooth and resist potential fracture of the tooth or restoration from masticatory forces delivered principally in the long axis of the tooth. Additional features for retaining the restorative material and protecting against fracture may be deemed necessary as part of the final stage of tooth preparation. 2-Final tooth preparation, the second stage of tooth preparation, is the completion of the tooth preparation. It includes excavating any remaining, infected carious dentin; removing old restorative material if indicated;

2 protecting the pulp; incorporating additional preparation design features that both minimize the chance of tooth or restoration fracture against oblique forces and maximize the retention of the material in the tooth; finishing preparation walls, particularly regarding the margins; and performing the final procedures of cleaning, inspecting, and, sometimes, sealing the preparation before placement of the restorative material. (Bonded restorations seal the prepared tooth during the bonding process.) NOMENCLATURE Nomenclature refers to a set of terms used in communication by persons in the same profession that enables them to better understand one another. CARIES TERMINOLOGY Dental caries is an infectious microbiologic disease that results in localized dissolution and destruction of the calcified tissues of the teeth. Moreover, caries is episodic with alternating phases of demineralization and re mineralization, and these processes may be occurring simultaneously in the same lesion. morphologic types of caries: caries spread are evident in clinical observation, namely, carious lesions originating: (1) in enamel pits and fissures, (2) on enamel smooth surfaces, or (3) on root surfaces. Caries of Pit-and-Fissure Origin. Pit-and-fissure caries can form in the regions of pits and fissures usually resulting from the imperfect coalescence of the developmental enamel lobes. When such areas are exposed to those oral conditions conducive to caries formation, caries usually develops (Fig. 2, A). As caries progresses in these areas, sometimes very little evidence is clinically noticeable until the forces of mastication fracture the increasing amount of unsupported enamel. The caries forms a small area of penetration in the enamel at the bottom of a pit or fissure and does not spread laterally to a great extent until the dentinoenamel junction (DEJ) is reached. At that time, the disintegration spreads along the junction and begins to penetratee the dentin toward the pulp via the dentinal tubules. In diagrammatic terms, pit-and- fissure caries may be represented as two cones, base to base, with the apex of the enamel cone at the point of origin and the apex of the dentin cone directed toward the pulp.

3 Perfect coalescence of the enamel developmental lobes is indicated by faultless enamel areas termed grooves and fossae. Usually these areas are not susceptible to caries because they are cleansed by the rubbing of food during mastication. However, in areas of no masticatory action in neglected mouths, caries may develop in a groove or fossa. Caries of Enamel Smooth-Surface Origin. Smooth-surface caries does not begin in an enamel defect, but rather in a smooth area of the enamel surface that is habitually unclean, and is thereby continually, or usually, covered by plaque (see Fig. 2, B and C). The disintegration in the enamel in smooth-surface caries also may be pictured as a cone, but with its base on the enamel surface and the apex at, or directed to, the DEJ. The caries again spreads at this junction in the same manner as in pit-and-fissure caries. Thus, the apex of the cone of caries in the enamel contacts the base of the cone of caries in the dentin. Grooves and Fissures; Fossae and Pits. On the enamel surface, grooves or fissures mark the location of the union of developmental enamel lobes. Where such union is complete, this "landmark" is only slightly involuted, smooth, hard, shallow, accessible to cleansing, and is termed a groove.where such union is incomplete, the landmark is sharply involuted to form a narrow, inaccessible canal of varying depths in the enamel and is termed a fissure The distinction made between a groove and a fissure also applies to an enamel surface fossa that is nondefective and a pit that is defective. TOOTH PREPARATION TERMINOLOGY A prerequisite to the comprehension of terms in either tooth preparation or classification is a knowledge of all terms of tooth description as presented in dental anatomy, including the names and positions of tooth surfaces. Simple, Compound, and Complex Tooth Preparations. A tooth preparation is termed 1-simple if a tooth preparation only one tooth surface is involved 2-compound if a tooth preparation two surfaces are involved 3-complex if a tooth preparation involving three (or more) surfaces

4 Abbreviated Descriptions of Tooth Preparations. For brevity in records and communication, the description of a tooth preparation is abbreviated by using the first letter, capitalized, of each tooth surface involved. Examples are: (1) an occlusal tooth preparation is an O; (2) a preparation involving the mesial and occlusal surfaces is an MO; and (3) a preparation involving the mesial, occlusal, and distal surfaces is an MOD. Tooth Preparation Walls Internal Wall. An internal wall is a prepared (cut) surface that does not extend to the external tooth surface (Fig. 10). Axial wall. An axial wall is an internal wall parallel with the long axis of the tooth (see Fig. 10). Pulpal wall. A pulpal wall is an internal wall that is both perpendicular to the long axis of the tooth and occlusal of the pulp (see Figs. 10). External Wall. An external wall is a prepared (cut) surface that extends to the external tooth surface, and such a wall takes the name of the tooth surface (or aspect) that the wall is toward (see Fig. 10). Floor (or Seat). A floor (or seat) is a prepared (cut) wall that is reasonably flat and perpendicular to those occlusal forces that are directed occlusogingivally (generally parallel to the long axis of the tooth). Examples are the pulpal and gingival walls (see Fig. 10). Tooth Preparation Angles. Although the junction of two or more prepared (cut) surfaces is referred to as an angle, in fact, the junction is almost always "softened" so as to present a slightly rounded configuration (the exception being a tooth preparation for gold foil), inspite of this rounding, these junctions are still referred to as angles for descriptive and communicative purposes. Line Angle. A line angle is the junction of two planal surfaces of different orientation along a line. An internal line angle is a line angle whose apex points into the tooth. An external line angle is a line angle whose apex points away from the tooth Point Angle. A point angle is the junction of three planal surfaces of different orientation.

5 Cavosurface Angle and Cavosurface Margin. The cavosurface angle is the angle of tooth structure formed by the junction of a prepared (cut) wall and the external surface of the tooth (see Fig. 2, D,). The actual junction is referred to as the cavosurface margin (see Fig. 6-2). Thus the angle formed by the lingual and incisal surfaces of an anterior tooth would be termed the linguoincisal line angle. Proximal surface: is the surface that face the adjacent tooth Marginal ridge: border the lingual surface of anterior teeth and the occlusal surfaces of posterior teeth CLASSIFICATION OF TOOTH PREPARATIONS Classification of tooth preparations according to the anatomic areas involved as well as by the associated type of treatment was presented by Black and is designated as Class I, Class II, Class III, Class IV, and Class V. Since Black's original classification, an additional class has been added, Class VI. Class I refers to pit-and-fissure lesions, whereas the reimaining classes are smoothsurface lesions. Classification was originally based on the observed frequency of carious lesions on certain aspects of the tooth. Class I Restorations. All pit-and-fissure restorations are Class I, and they are assigned to three groups, as follows. Restorations on Occlusal Surface of Premolars and Molars. The names of the walls, line angles, and point angles of an occlusal conventional tooth preparation are identified by the labels and legends in Figs. 12, 13, and 14, noting again that a preparation takes the name of the tooth surface (or aspect) that the wall is toward. Restorations on Occlusal Two Thirds of the Facial and Lingual Surfaces of Molars. The names of the walls, line angles, and point angles of these tooth preparations are the same as those depicted for the preparations for Class V restorations Restorations on Lingual Surface of Maxillary Incisors. The names of the walls, line angles, and point angles of these tooth preparations also are the same as those depicted for the preparations for Class V restorations Class II Restorations. Restorations on the proximal surfaces of posterior teeth are Class II.

6 Class III Restorations. Restorations on the proximal surfaces of anterior teeth that do not involve the incisal angle are Class III. Walls, line angles, and point angles of a representative conventional tooth preparation are identified in Figs. 18, 19, and 20. Class IV Restorations. Restorations on the proximal surfaces of anterior teeth that do involve the incisal edge are Class IV. Walls, line angles, and point angles of a representative conventional tooth preparation are identified in Figs. 21, 22, and 23. Class V Restorations. Restorations on the gingival third of the facial or lingual surfaces of all teeth (except pit-and-fissure lesions) are Class V. Walls, line angles, and point angles of a representative conventional tooth preparation on an anterior tooth are designated in Figs. 24, 25, and 26. Class VI Restorations. Restorations on the incisal edge of anterior teeth or the occlusal cusp heights of posterior teeth are Class VI. INITIAL AND FINAL STAGES OF TOOTH PREPARATION

7 Initial Tooth Preparation Stage Step 1: Outline form and initial depth Step 2: Primary resistance form Step 3: Primary retention form Step 4: Convenience form Final Tooth Preparation Stage Step 5: Removal of any remaining infected dentin and/or old restorative material, if indicated Step 6: Pulp protection, if indicated Step 7: Secondary resistance and retention forms Step 8: Procedures for finishing external walls Step 9: Final procedures: cleaning, inspecting, sealing Step 1: Outline Form and Initial Rules for establishing outline form for pit-and-fissure tooth preparation 1. Extend the preparation margin until sound tooth structure is obtained and no unsupported and/or weakened enamel remains. 2. Avoid terminating the margin on extreme eminences such as cusp heights or ridge crests. 3. If the extension from a primary groove includes one-half or more of the cusp incline, consideration should be given to capping the cusp. If the extension is two-thirds, the cusp-capping procedure is most often the proper procedure, which removes the margin from the area of masticatory stresses. 4. Extend the preparation margin to include all of the fissure that cannot be eliminated by appropriate enameloplasty. 5. Restrict the pulpal depth of the preparation to a maximum of 0.2 mm into dentin (except when (1) preparing a tooth for a gold foil restoration, in which case the initial depth is 0.5 mm into dentin or (2) when the occlusal enamel has been worn thin. To be as conservative as possible, the preparation for an occlusal surface pit-and-fissure lesion is first prepared to a depth of 1.5 mm, as measured at the central fissure. 6- When two pit-and-fissure preparations have less than 0.5 mm of sound tooth structure between them, they should be joined to eliminate a weak enamel wall between them. 7. Extend the outline form to provide suffi cient access for proper tooth preparation, restoration placement, and finishing procedures. Rules for establishing outline forms for proximal surface tooth preparations 1. Extend the preparation margins until sound tooth structure is obtained and no unsupported and/or weakened enamel remains. (Sometimes, unsupported but not friable enamel may remain in tooth preparations for bonded restorations.)

8 2. Avoid terminating the margin on extreme eminences such as cusp heights or ridge crests. 3. Extend the margins to allow sufficient access for proper manipulative procedures. 4. Restrict the axial wall pulpal depth of the proximal preparation to a maximum of 0.2 to 0.8 mm into dentin (the greater depth when the extension is onto the root surface; the lesser depth when no retention grooves will be placed). 5. Usually, gingival margins of tooth preparations are extended apically of the proximal contact to provide a minimum clearance of 0.5 mm between the gingival margin and the adjacent tooth. Otherwise, this gingival extension is to sound tooth structure and no farther. 6. Likewise, the facial and lingual margins in proximal tooth preparations usually are extended into the respective embrasures to provide specified clearance between the prepared margins and the adjacent tooth. The purpose of this clearance is to place the margins away from close contact with the adjacent tooth so that the margins can be better visualized, instrumented, and restored. Step 2: Primary Resistance Form. Primary resistance form may be defined as that shape and placement of the preparation walls that best enable both the restoration and the tooth to withstand,with-out fracture, masticatory forces delivered principally in the long axis of the tooth. The relatively flat pulpal and gingival walls prepared perpendicular to the tooth's long axis help resist forces in the long axis of the tooth and prevent tooth fracture from wedging effects. The fundamental principles involved in obtaining primary resistance form are: (1) to use the box shape with a relatively flat floor, which helps the tooth resist occlusal loading by virtue of being at right angles to those forces of mastication that are directed in the long axis of the tooth (2) to restrict the extension of the external walls (keep as small as possible) to allow strong cusp and ridge areas to remain with sufficient dentin support (3) to have a slight rounding (coving) of internal line angles to reduce stress concentrations in tooth structure (4) in extensive tooth preparations, to cap weak cusps and envelope or include enough of a weakened tooth within the restoration to prevent or resist fracture of the tooth by forces both in the long axis and obliquely(laterally) directed (5) to provide enough thickness of restorative material to prevent its fracture under load (6) to bond the material to tooth structure when appropriate. Step 3: Primary Retention Form. Primary retention form is that shape or form of the conventional preparation that resists displacement or removal of the restoration from tipping or lifting forces

9 The retention form developed during initial tooth preparation may be adequate to retain the restorative material in the tooth. Sometimes, however, additional retention features must be incorporated in the final stage of tooth preparation. Principles. 1-the material is retained in the tooth by developing external tooth walls that converge occlusally. In this way, once the amalgam is placed in the preparation and hardens, it cannot dislodge without some type of fracture occurring. In these preparations, the facial and lingual walls of the occlusal portion of the preparation, converge toward the occlusal surface. This convergence should not be overdone for fear of leaving unsupported enamel rods on. 2-Adhesive systems provide some retention by micro-mechanically bonding amalgam to tooth structure and also reducing or eliminating microleakage. However, until longevity studies demonstrate that bonding systems provide complete retention form, traditional retention features should be provided for amalgam restorations, especially for root-surface restorations. Step 4: Convenience Form. Convenience form is that shape or form of the preparation that provides for adequate observation, accessibility, and ease of operation in preparing and restoring the tooth. On occasion, obtaining this form may necessitate extension of distal, mesial, facial, or lingual walls to gain adequate access to the deeper portion of the preparation. FINAL TOOTH PREPARATION STAGE Step 5: Removal of Any Remaining Enamel Pit or Fissure, Infected Dentin, and/or Old Restorative Material, if Indicated. Removal of any remaining enamel pit or fissure, infected dentin, and/or old restorative material is the elimination of any infected carious tooth structure or faulty restorative material left in the tooth after initial tooth preparation. Step 6: Pulp Protection, if indicated. Although the placement of liners and bases is not a step in tooth preparation in the strict sense of the word, it is a step in adapting the preparation for receiving the final restorative material. Therefore The reason for using traditional liners or bases is to either protect the pulp or to aid pulpal recovery or both. Step 7: Secondary Resistance and Retention Forms. After removal of any remaining enamel pit or fissure, infected dentin, and/or old restorative material (if indicated) and pulpal protection has been provided by appropriate liners and bases, additional resistance and retention features may be deemed necessary for the preparation. Many compound and complex preparations require these additional features. When a tooth preparation includes both occlusal and proximal surfaces, each of those areas should have independent retention and resistance features.

10 Because many preparation features that improve retention form also improve resistance form, and the reverse is true, they are presented together. The secondary retention and resistance forms are of two types: (1) mechanical preparation features (Retention locks, grooves, and coves). (2) treatments of the preparation walls with etching, priming, and adhesive materials. In addition to mechanical alterations to the tooth preparation, certain alterations to the preparation walls by actions of various materials also afford increased retention, as well as resistance to fracture. Both enamel and dentin surfaces may be treated with etchants and/or primers for certain restorative procedures Step 8: Procedures for Finishing the External Walls of the Tooth Preparation Finishing the preparation walls is the further development, when indicated, of a specific cavosurface design and degree of smoothness or roughness that produces the maximum effectiveness of the restorative material being used. The objectives of finishing the prepared walls are to: (1) create the best marginal seal possible between the restorative material and the tooth structure, (2) afford a smooth marginal junction, (3) provide maximum strength of both the tooth and the restorative material at and near the margin. Step 9: Final Procedures: Cleaning, Inspecting, and Sealing. includes removing all chips and loose debris that have accumulated, drying the preparation (do not desiccate), and making a final complete inspection of the preparation for any remaining infected dentin, unsound enamel margins, or any condition that renders the preparation unacceptable to receive the restorative material. Naturally, most of the gross debris has been removed during the preparation steps The usual procedure in cleaning is to free the preparation of visible debris with warm water from the syringe and then to remove the visible moisture with a few light surges of air from the air syringe. ADDITIONAL CONCEPTS IN TOOTH PREPARATION Several new restorative techniques have been advocated for use with amalgam restorations. In assessing these and future proposals, the operator must remember the fundamental requirements for a successful amalgam preparation. These prerequisites for success are: (1) 90-degree junctions of amalgam with tooth structure (2) mechanical retention form (3) adequate thickness for the amalgam material. Amalgam Box-Only Tooth Restorations. Box-only tooth preparations for amalgam may be advocated for some posterior teeth in which a proximal surface requires restoration, but the occlusal surface is not faulty. A

11 proximal box is prepared and specific retention form is provided, but no occlusal step is included. Such restorations obviously are more conservative, in that less tooth structure is removed. Amalgam Tunnel Tooth Restorations. In an effort to be conservative of tooth structure removal, others advocate a tunnel tooth preparation. This preparation joins an occlusal lesion with a proximal lesion by means of a prepared tunnel under the involved marginal ridge. In this way, the marginal ridge remains essentially intact. Adhesive Amalgam Restorations. Other techniques advocated for amalgam restorations use adhesive systems. Some of these materials bond the amalgam material to tooth structure. Others seal the prepared tooth structure with an adhesive resin before amalgam placement. Although the proposed bonding techniques vary, the essential procedure is to prepare the tooth similar to typical amalgam preparations except that more weakened, remaining tooth structure may be retained. Next, the preparation walls are treated or covered with specific adhesive lining materials that mechanically bond to both the tooth and the amalgam.