Chapter 1 - Preparation Design for Crowns and Retainers PDF

Faculty of Public Health Department of Dental Laboratory Technology FPH - DDLT CHAPTER 1 P R E P A R AT I O N D E S I G N F O R C R O W N S & R E TA I N E R S Maya Nohra, MPH...

Faculty of Public Health Department of Dental Laboratory Technology FPH - DDLT CHAPTER 1 P R E P A R AT I O N D E S I G N F O R C R O W N S & R E TA I N E R S Maya Nohra, MPH A. Professor Head of Department TERMINOLOGY Preparation is the selected form given to a natural tooth when it is reduced by instrumentation to receive a prosthesis (e.g. artificial crown or retainer for a fixed prosthesis) Natural tooth prep by dentist to receive restoration  TERMINOLOGY Retention is the inherent property of a restoration to maintain its position without displacement under stress. Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation. Maintain its position in the long axis of tooth  TERMINOLOGY Resistance form is the shape given to a preparation to enable the restoration and remaining tooth structure to withstand masticator stress. Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces. Prevent mvmt of restor by forces directed by oblique PREPARATION DESIGN Preparation design must include the following Principals (Schillingburg): 1. Preservation of Tooth Structure 2. Retention and Resistance Form 3. Structural Durability 4. Marginal Integrity 5. Preservation of the Periodontium  PREPARATION DESIGN Kel ma Hafaz El Hakim al preparation tooth Kel ma el design ken ahla PREPARATION DESIGN GENERAL CONSIDERATIONS To prevent dislodgment, the preparation must possess a configuration that ensures retention and resistance for the casting. Retention is the quality that prevents removal in the long axis of the tooth restoration. Resistance is the quality that prevents dislodgment due to apically or obliquely directed forces. RETENTION AND RESISTANCE  RETENTION Retention is a function of: 1. height 2. diameter 3. convergence of the preparation 4. texture of the prepared surface 5. and the use of intracoronal retentive devices How retention obtained Height/ margin smooth&well countered/diameter surface area/ tapper/texture of prep surface  CONVERGENCE (TAPER) The most critical retentive factor is the convergence of the preparation Axial walls of a preparation must taper slightly to permit the restoration to seat  TAPER Two opposing surfaces, each with a 3 degree inclination (diamond held parallel to the intended path of insertion). 6 degree taper overall.  ‫كلما كانت الجدران املتعارضة للتحضير‬ TAPER.‫ كلما زاد االحتفاظ‬،‫أكثر توازيا تقريبا‬ The more nearly parallel the opposing walls of a preparation, the greater should be the retention.  TAPER Why dentist do taper on prep tooth Walls are tapered to: visualize preparation walls prevent undercuts compensate fabrication inaccuracies permit complete seating during cementation RETENTION AND RESISTANCE If a preparation is compromised by lack of sufficient tooth structure, resistance and retention can be increased by addition of intracoronal retentive devices such as grooves or box forms. To counteract the twisting forces applied in the horizontal plane, groove walls must be placed perpendicular to the direction of dislodgment forces.  RETENTION AND RESISTANCE What can create use of cement Cement creates mechanical interlocks between inner surface of restoration and axial wall of preparation RETENTION AND RESISTANCE Structurally, the restoration should possess enough bulk and therefore rigidity to withstand functional and non functional stressing. STANDARD PREPARATIONS RETENTION AND RESISTANCE Occlusogingival length: factor of retention and resistance Greater the surface area of preparation (longer preparation), the greater its retention. Preparations on Large teeth are more retentive than preparations on small teeth RETENTION AND RESISTANCE The shorter the wall the more important its inclination. Overshortened preparation will result in stress concentration in the labiogingival area, which can cause the classic half-moon fracture in this area RETENTION AND RESISTANCE Shorter preparations must be tapered as little as possible to increase resistance. Over tapering short preparation will result in a restoration that is very easy to be removed and having less retention. RETENTION AND RESISTANCE Retention and resistance form are closely interrelated and inseparable qualities, but of the two, resistance form is the most critical in preventing dislodgment of the restoration. STRUCTURE DURABILITY Restoration must contain bulk of material that is adequate to withstand the forces of occlusion. This bulk must be confined to the space created by the tooth preparation. Only this way the occlusion will be harmonious and axial contours normal. STRUCTURE DURABILITY Adequate reduction of the tooth is a must to prevent overcontour of the final restoration. The design of the preparation should incorporate the following design features: Occlusal reduction Functional cusp bevel Axial reduction OCCLUSAL REDUCTION Sufficient occlusal reduction to provide thickness and rigidity to the casting. Material of restoration withstand occlusal forces and create harmonious occlusion. Why the occ reduction is necessary for casting OCCLUSAL REDUCTION The basic inclined plane pattern of the occlusal surface duplicated to produce adequate clearance without over shortening the preparation. A flat occlusal surface may over shorten the preparation. OCCLUSAL REDUCTION What are problems —>adequate occ Inadequate occlusal reduction: makes the restoration weaker. red will not provide adequate space to allow good functional morphology under the anatomical grooves. Will allow the restoration to be easily perforated by finishing procedures or by wear in the mouth. FUNCTIONAL CUSP BEVEL An integral part of occlusal reduction is the functional cusp bevel. A wide bevel placed on the functional cusp provides space for an adequate bulk of metal in an area of heavy occlusal contact.  FUNCTIONAL CUSP BEVEL Lack of functional cusp bevel may produce several problems : Can cause a thin area or perforation. May result in over contouring and poor occlusion Over inclination of the buccal surface will destroy excessive tooth structure reducing retention. AXIAL REDUCTION Sufficient axial reduction is done to establish: retention, resistance form, proper axial contour and a space for adequate thickness of restorative material. Normal axial contours prevents: periodontal problems thin walls and weak restoration overcontoured restoration STRUCTURE DURABILITY Proper marginal design based on esthetic and functional requirements. MARGINAL INTEGRITY Margins of restoration must be closely adapted to finish line of preparation Whenever possible, cavosurface bevels should be established, since this allows even closer marginal adaptation. MARGINAL INTEGRITY Finish line configurations: 1. knife edge and Slice 2. Shoulder 3. Shoulder with a bevel 4. Chamfer 5. Heavy Chamfer KNIFE EDGE AND SLICE KNIFE EDGE AND SLICE Less destruction of tooth structure Margin not well defined Thin margin that fits finish line difficult to wax and cast Overcontoured restorations Preservation of tooth structure! SHOULDER SHOULDER Recommended for all-ceramic restorations only Resistance to occlusal forces and minimize stress that might lead to porcelain fracture Adequate bulk of material at the margin Require much tooth structure destruction Radial shoulder: rounded internal angle SHOULDER WITH A BEVEL Bevel to a shoulder creates acute edge of metal at the margin VALID CONTOUR CHAMFER CHAMFER Cast metal restorations preferred gingival finish line for full veneer metal restoration Acute margin Sufficient thickness and strength Rounded concavity, Lower stress concentration within cement film  HEAVY CHAMFER Heavy chamfer-large radius internal angle 90 degree surface: ceramic restorations Chamfer with a bevel: bevel added for use with a metal restoration HEAVY CHAMFER MARGIN DESIGN The major factor to be considered when designing a margin is the amount of tooth destruction allowed in order to give the esthetics demand of the restoration Balance between conserving tooth structure on the one hand , achieving an aesthetic and strong crown on the other. MARGIN PLACEMENT Whenever possible the finish line should be placed in an area where the margins can be finished by the dentist easily and kept clean by the patient. In addition, finish lines must be placed so that they can be duplicated by the impression without tearing or deformation. MARGIN PLACEMENT Finish line should be placed in enamel whenever possible. Supra-gingival versus sub-gingival margin. Many situations in which sub-gingival margins are unavoidable: Caries To increase retention Esthetics Extension of previous restoration Trauma PRESERVATION OF THE PERIODONTIUM The placement of finish lines has a direct bearing on the ease of fabrication and 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 cleansing action.  CLASSIFICATION OF FIXED PROSTHODONTICS RESTORATIONS 1. Full veneer crown: 2. Partial veneer crown 3. Laminate veneer 4. Inlay/Onlay/Pinlay 5. Fixed partial denture  FULL VENEER CROWN / FULL METAL CROWN Cemented extracoronal restoration. It covers the outer surface of the clinical crown.  PARTIAL VENEER CROWN Extracoronal metal restoration. it covers only part of the clinical crown. It is referred to as a partial-coverage restoration. Partial veneer crowns generally include all tooth surfaces except the buccal or labial wall in the preparation. It covers the lingual, proximal, and occlusal or incisal surfaces of the involved tooth. The facial surface is left untouched. LAMINATE VENEER - EXTRACORONAL Porcelain veneers or dental porcelain laminates are wafer thin, custom made shells of tooth-colored materials designed to cover the front surface of teeth to improve your appearance. These shells are bonded to the front of the teeth changing their color, shape, size of length  INLAY/ONLAY/PINLAY Cemented intracoronal cast restorations that gain their retention through parallel internal box forms prepared within the coronal portion FIXED PARTIAL DENTURE A fixed bridge is a restoration replacing one or more missing teeth which gains its support from the remaining natural teeth to which it is cemented. Restoration RETAINER that is cemented to the prepared abutment tooth Extracoronal restoration that is cemented to the prepared abutment tooth CROWN SELECTION Four main categories of crown selection: 1. Metal 2. Porcelain Fused to Metal “PFM” 3. All Ceramic 4. Composite ALL-METAL DENTAL CROWNS. FULL GOLD CROWNS Made entirely out of metal or more precisely, a precious alloy (gold) or non precious alloy (silver in color). Full metal crowns in general are very strong and can be expected to withstand even the heaviest biting and chewing forces well. disadvantage of metal dental crowns is their appearance. Gold color does not look natural, particularly on front teeth. PORCELAIN FUSED TO METAL Tooth-colored crown. A metal alloy is used to create a thin thimble- like cap that fits over the tooth. Porcelain is then fused to this metal substructure to form the overall shape of the crown and give it a white, natural-looking appearance. ALL CERAMIC CROWN All-ceramic crowns are fabricated entirely out of a glass-like compound such as porcelain. The advantage of all- ceramics, as compared to their porcelain-fused-to- metal ("PFM") counterparts, is their (typically) more lifelike appearance. ALL CERAMIC DENTAL CROWN ALL CERAMIC CROWN This characteristic can be attributed to the fact that a greater percentage of the overall thickness of this type of crown can be made using relatively more-translucent porcelain. (The type of porcelain that most closely mimics the light-handling characteristics of tooth enamel and therefore gives a very lifelike look.) ALL CERAMIC CROWN In comparison, PFM crowns have a metal substructure that must be masked using a layer of opaque (chalky-white) porcelain. This reduces the extent of the crown's total thickness that can be composed of translucent materials and therefore makes it more of a challenge to create as natural a look. COMPOSITE CROWN Composite resin is a filling material designed for aesthetic dental restorations. Formulated to resemble the color of your natural tooth, composite resin is often used for filling dental cavities of the teeth. COMPOSITE CROWN Composite resin consists of glass or quartz filler added to a resin medium, which produces a tooth- colored filling. The invention of composite resin offers a substitute to the amalgam dental fillings we've grown so accustomed to. This plastic and glass mixture contains no metal and can be shaped to resemble a real tooth. Thank you