Metal-Ceramic Preparation PDF

Metal-Ceramic Preparation The metal-ceramic crown is one of the most widely used fixed restorations due to: - Predictable esthetic result - Good physical properties. It consists of a complete-coverage metal substructure (or framework) that is veneered with a layer of fused porcelain to mimic...

Metal-Ceramic Preparation The metal-ceramic crown is one of the most widely used fixed restorations due to: - Predictable esthetic result - Good physical properties. It consists of a complete-coverage metal substructure (or framework) that is veneered with a layer of fused porcelain to mimic the appearance of a natural tooth. The porcelain is mechanically and chemically bonded to the metal. The extent of the veneer can vary, and it requires substantial additional tooth reduction wherever the metal substructure is to be veneered with dental porcelain. Components A) The metal substructure is fabricated in a special metal-ceramic alloy that is compatible with veneering porcelain. B) After preparatory finishing procedures, this substructure, or framework, is veneered with multiple layers of dental porcelain. a. The first ceramic layer (the opaque layer): masks the dark metal oxide it is responsible for the metal-ceramic bond is the primary source of color for the completed restoration b. The second layer (body porcelain): slightly translucent body porcelain c. Enamel Layer: forms the incisal and cuspal parts gives translucency leading to a more natural appearance contains relatively little pigmentation The shiny, lifelike appearance of the completed metal-ceramic restoration results from a surface glaze formed during an additional firing after the restoration has been shaped. Page 1 of 4 Only when a crown is sufficiently thick can the darker color of the metal substructure be masked, and the veneer duplicates the appearance of a natural tooth. The porcelain veneer must have a certain minimum thickness for esthetics. Consequently, more tooth reduction is necessary, and metal-ceramic preparation is one of the methods least conservative for tooth structures. Indications: 1- Teeth that require complete coverage with esthetic demands (e.g., the anterior teeth) 2- Long-span fixed dental prostheses 3- Extensive tooth destruction 4- Can accommodate a rest for a removable prosthesis 5- Can accommodate cast or soldered connectors 6- Endodontically treated 7- The need for superior retention and strength (cannot use partial coverage) 8- Allow recontouring of axial surfaces and correction of minor malinclinations. 9- Allow correction of the occlusal plane Contraindications: 1- Presence of active caries or untreated periodontal disease 2- In young patients with large pulp chambers because of the high risk of pulp exposure 3- When a more conservative retainer can be used, unless maximum retention and resistance form are needed, as for a long-span fixed dental prosthesis 4- Allergy Advantages: 1- Combines the strength of cast metal with the esthetics of ceramics. 2- Retentive qualities are excellent because all axial walls are included in the preparation 3- Allows correction of axial form 4- The preparation is much less demanding than for partial-coverage retainers Page 2 of 4 Disadvantages: 1- Requires significant tooth reduction 2- Increased potential for periodontal disease in case of subgingival margin 3- Inferior esthetics in comparison with all-ceramic restorations (may appear slightly grayish in comparison with the translucency that can be achieved with all-ceramic crowns) Instruments Round-ended diamonds Flat-ended, tapered diamond (for shoulder margin preparation) Finishing stones Football- or wheel-shaped diamond (for lingual reduction of anterior teeth) Preparation Veneered posterior Occlusal reduction If the occlusal surface is to be established in metal, a minimum clearance of 1.0 mm on nonfunctional cusps and a minimum clearance of 1.5 mm on functional cusps must be achieved If the occlusal surface is to be established in porcelain, clearance must be a minimum of 2 mm. Caution must be used during the occlusal preparation phase because excessive occlusal reduction will shorten the axial preparation walls and thus is a common cause of inadequacies in retention and resistance form in the completed preparation Proximal and Lingual Surfaces Smooth chamfer margin of about 0.5 mm in width Occlusal convergence of a 6 degrees taper, measured as the angle between opposing axial walls The interproximal margin must follow the soft tissue contour Page 3 of 4 Buccal reduction Reduction of 1.2-1.5 mm is necessary to produce a restoration with satisfactory appearance, (to produce sufficient space to accommodate the metal substructure and porcelain veneer) Veneered anterior Incisal Reduction 2 mm of clearance for adequate material thickness to allow translucency in the completed restoration. Proximal and Lingual Surfaces Smooth chamfer margin of about 0.5 mm in width Occlusal convergence of a 6 degrees taper, measured as the angle between opposing axial walls The interproximal margin must follow the soft tissue contour A football-shaped diamond to reduce the lingual fossa to obtain a 1 mm clearance Labial Reduction Performed in 2 planes (cervical and incisal planes). The cervical plane determines the path of insertion, it should be parallel to the long axis of the tooth The incisal plane provides the space needed for the porcelain veneer. It usually extends half to two-thirds of the facial surface, depending on the shape of the tooth Total amount of labial reduction is 1.5 mm Finishing The margin must provide distinct resistance to vertical displacement of the tip of a periodontal probe or an explorer, and it must be smooth and continuous circumferentially. All line angles should be rounded Page 4 of 4