Fixed Theoretical Fall 2024 PDF

Bio-Mechanical Principles Bio-Mechanical Principles of Tooth Preparation Successful tooth preparation and subsequent restoration depend mainly on three broad categories: 1. Biologic consideration, which affect the health of the oral tissue 2. Mechanical consideration, which affect the integ...

Bio-Mechanical Principles Bio-Mechanical Principles of Tooth Preparation Successful tooth preparation and subsequent restoration depend mainly on three broad categories: 1. Biologic consideration, which affect the health of the oral tissue 2. Mechanical consideration, which affect the integrity and the durability of the restoration. 3. Esthetic consideration, which affect the appearance of the patient 29  Bio-Mechanical Principles Biologic Consideration Prevention of Damage during Tooth Preparation a. Adjacent teeth. Iatrogenic damage to an adjacent tooth is a common error in operative dentistry. Even if a damaged proximal contact area is carefully reshaped and polished, it will be more susceptible to dental caries than the original undamaged tooth surface. The technique of tooth preparation must avoid and prevent damage to the adjacent tooth surfaces. It may help to use a metal matrix band a round the adjacent tooth as protection; however, the thin band can still be perforated and the underlying enamel damaged. The preferred method to use the proximal enamel of the tooth being prepared for protection of adjacent structures. Teeth are 1.5 to 2mm wider at the contact area than at cemento-enamel junction (CEJ), and a thin tapered diamond can be passed through the interproximal contact area to leave a slight lip or fin of enamel without causing excessive tooth reduction or undesirable angulations of the rotary instrument. b. Soft tissue. Damage to the soft tissues of tongue, lips and checks can be prevented by careful retraction with aspirator tip, mouth mirror, or flanged saliva ejector. Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared. c. Pulp. Great care is also needed to prevent pulpal injuries during fixed prosthodontic procedures. Causes of injury during preparation: - Extreme temperatures. - Chemical (from different materials). - Bacterial (due to recurrent caries or microleakage). 30  Bio-Mechanical Principles All can cause an irreversible pulpitis, particularly when they occur on freshly sectioned dentinal tubules. -Temperature. Considerable heat is generated by friction between a rotary instrument and the surface being prepared. Excessive pressure, higher rotational speed, and the type, shape and the condition of the cutting instrument may all increase the heat generated. Water spray should be used accurately directed at the areas of contact between the tooth and the bur. Particular care is needed when preparing grooves or pinholes because coolant cannot reach the cutting edge of the bur. To prevent heat buildup, these retentive features should always be prepared at low rotational speed. -Chemical action. The chemical action of certain dental materials (bases, restorative resins, solvents, and luting agents) can cause pulpal damage, particularly when applied to freshly cut dentin. Cavity varnish will form an effective barrier and appears to have little effect on the retention of the cemented restoration. Chemical agents used for cleaning and degreasing tooth preparation are contraindicated because they have been shown to be pulpal irritant. -Bacterial action. Pulpal damage under restorations has been attributed to action of bacteria that either left behind or gained access to dentin because of microleakage. It is important to remove all carious dentin before placing a restoration that will serve as a foundation for fixed prosthesis. Preservation of tooth structure One of the basic tenets of restorative dentistry is to conserve as much tooth structure as possible consistent with the mechanical and esthetic principles of tooth preparation. This will reduce the harmful pulpal effects of the various procedures and material used. (Why). 31  Bio-Mechanical Principles The thickness of the remaining dentin has been shown to be inversely proportional to the pulpal response. Guidelines for conservation of tooth structure: 1. Use partial coverage rather than complete coverage restoration. 2. Preparation of teeth with minimal convergence angle (taper) between the axial walls (Fig.1). 3. Reduction of the occlusal surface should be evenly and anatomically to provide uniform thickness in the restoration (Fig.2). 4. Even and adequate reduction of the axial tooth surfaces. Sufficient tooth structure must be removed to facilitate the development of correctly axial formed contour. 5. Selection of the conservative margin compatible with the other principles of tooth preparation (Fig.3). 6. Avoidance of unnecessary apical extension of the preparation (Fig.4). 32  Bio-Mechanical Principles 1. Fig.1: excessive taper results in Fig.2: Reduction of the occlusal surface considerable loss of tooth structure should be evenly and anatomically. (shaded area). Fig. (3): a shoulder margin (2) is less Fig. (4): apical extension of the conservative than a chamfer (1). preparation can result in the need for additional tooth reduction. 33  Bio-Mechanical Principles Consideration affecting future dental health Improper tooth preparation may have an adverse effect on long-term dental health. Axial Reduction Axial reduction plays an important role in securing space for an adequate thickness of restorative material. When axial reduction is sufficient, restoration walls can have satisfactory thickness without over contouring. Margin placement The placement of the finish line has a direct bearing on the ease of fabricating a restoration and on the ultimate structure of the restoration. The best result can be expected from margins that are as smooth as possible and are fully exposed to the cleansing action. Whenever possible the finish line should be placed in area where the margin of the restoration can be finished by the dentist and kept clean by the patient. In addition finish line must be placed so that they can be duplicated by the impression, without tearing or deforming the impression when it is removed past them. Traditionally, gingival margins have been placed subgingivally. This concept originally grew from the mistaken belief that the gingival sulcus represented is caries- free. The practice of routinely placing margins subgingivally is no longer accepted. Subgingival restorations have been described as a major etiologic factor in periodontitis. The deeper the restoration margins in the gingival sulcus, the greater the inflammatory response. As a result, whenever possible, the margin of the preparation should be places supragingival. 34  Bio-Mechanical Principles Advantages of supragingival margins placement: 1. Easily prepared without trauma to the soft tissue. 2. Easily finished and kept clean. 3. Impressions are more easily made, with less potential for soft tissue damage. 4. Restoration can be easily evaluated at recall appointments. Nevertheless, there will be many situations in which subgingival margins are unavoidable: 1. Dental caries, cervical erosion, or restorations extended subgingivally and crown-lengthening procedure is not indicated. 2. The proximal contact area extends to the gingival crest. 3. Additional retention is needed by increasing the preparation stump. 4. The margin of a metal ceramic restoration is to be hidden behind the labiogingival crest. 5. Root sensitivity cannot be controlled by more conservative procedures. 6. Modification of the axial contour is indicated. A crown margin should not be placed any closer than 2.0 mm away from the alveolar crest, or bone resorption will occur. The combination of the epithelial and connective tissue attachments is normally about 2.0mm. If the margin intrudes into this biologic width, inflammation will result and bone will recede until it is once again at least 2.0mm from the crown margin. 35  Bio-Mechanical Principles Marginal Integrity The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavosurface finish line of the preparation. The configuration of the preparation finish line dictates the shape and the bulk of the restorative material in the margin of the restoration and also, can affect both the marginal adaptation and the degree of seating of the restoration. Margin adaptation. The junction between a cemented restoration and the tooth is always a potential site for recurrent caries, because of dissolution of luting agent and inherent roughness. The more accurately the restoration is adapted to the tooth, the less will be the chance of recurrent caries or periodontal disease. A well-designed preparation has a margin that is smooth and even. Rough irregular or stepped junctions greatly increased the length of the margin and substantially reduce the adaptation of the restoration. Time spent obtaining a smooth margin will make the subsequent steps of tissue displacement, impression making, die formation, waxing, and finishing much more easily and will provide the patient with a longer-lasting restoration. Margin configuration Featheredge or shoulderless crown preparations Fig. (14, A) should be avoided because, although they are conservative of tooth structure, they fail to provide adequate bulk at the margin. Over contoured restorations often result from featheredge margin because the technician can handle the wax pattern without distortion only by increasing its bulk beyond the original contour. A variation of the featheredge, the chisel edge margins formed Fig. (14, B) when there is a larger angle between the axial surfaces and the unprepared tooth 36  Bio-Mechanical Principles structure. This margin is frequently associated with an excessively tapered preparation or one in which the axial reduction is not correctly with the long axis of the tooth. A chamfer margin Fig. (14,C) is particularly a suitable for cast metal crowns and the metal portion of metal-ceramic crowns. It is distinct and readily identified, provide bulk of material, and can be placed with precision. The most suitable instrument for making a chamfer margin is a taper diamond stone with round tip; the margin formed is the exact image of the instrument. The gingival margin is prepared with diamond held precisely in the intended path of withdrawal of the restoration. Tilting it away from the tooth will create an undercut, where as angling it toward the tooth will lead to over reduction and loss of retention. The chamfer should never be prepared wider than half the tip of the diamond. A beveled margin Fig. (14, D) is suitable for cast restoration; particularly if a ledge or shoulder is already exists, possibly from dental caries, cervical erosion, or a previous restoration. The objective of beveling is (1) to allow the cast metal margin to be bent or burnished against the prepared tooth structure; (2) to minimize the marginal discrepancy caused by a complete crown that fails to seat completely; (3) to protect the unprepared tooth structure from chipping (e.g. by removing. unsupported enamel). Because a shoulder margin Fig. (14, E) allows rooms for porcelain, it is recommended for all-ceramic crowns and the facial part of metal-ceramic crowns. Especially when the porcelain margin technique is used. It should form 90-degree angle with unprepared tooth structure. The sharp 90-degree internal line angle associated with the classic variety of this finish line concentrates stress in the tooth and is conducive to coronal fracture. The radial shoulder is a modified form of shoulder finish line. The initial instrumentation of the ledge is accomplished with the same flat-end tapered diamond used for classic shoulder. a small-radius rounded internal angle is instrumented by 37  Bio-Mechanical Principles parallel end-cutting carbide finishing bur instruments. The cavosurface is 90 degrees, and the shoulder width is only slightly lessened by the rounded internal. Stress concentration is less in the tooth structure than with a classic shoulder, and support for ceramic restoration wall is good. Sloped shoulder finish line Fig. (14,F) is a modification of the classic shoulder finish line being characterized by 120-degrees as an alternative to 90-degrees. Its advantage reduces the possibility of leaving unsupported enamel and still leaving sufficient metal thickness to allow for its thinning to a knife edge for good esthetics. The shoulder with bevel Fig. (14,G) is utilized as the gingival finish line on the proximal box of inlays and onlays, and for the occlusal shoulder of onlay and mandibular three quarter crowns. This design can also be used for the facial finish line of metal-ceramic where gingival esthetic is not critical. It can be used in those situations where a shoulder is already present, either because of destruction by caries or the presence of previous restoration. To obtain optimal results with this type of finish line , finish line needs to be placed more apically Fig. (15) A B C D E F G Fig. (14): A, Featheredge. B, Chisel C, Chamfer D, Bevel E, Shoulder F, Sloped shoulder G, Shoulder with bevel 38  Finish line Designs Finish Line Advantage Disadvantage Indications Type Feather Edge Conservation of tooth Can Lead to over contouring of structure the restoration Chisel Conservation of tooth Can lead to over contoured Cases of tilted teeth structure restoration Difficult to control margin location Chamfer Distinct, well defined Care during preparation to avoid Cast metal restoration finish line formation of unsupported Lingual margin of labial Gives adequate bulk for enamel lip veneered metal ceramic the material restorations Shoulder Gives adequate bulk of Less conservative than chamfer Full veneered metal ceramic restoration finish line restoration Sharp internal line angle All ceramic crowns Shoulder with Gives adequate bulk for Less conservative to tooth Metal ceramic restorations bevel the restoration structure Previous caries or restoration Bevel removes Bevel need to be placed apical at finish line area unsupported enamel (subgingival) Beveled Removes unsupported Less conservative Ledge due to previous caries enamel Need to extend subgingival for or erosion at cervical margin Advantages of bevel optimal esthetic results 39