Metal-Ceramic Restorations PDF

METAL-CERAMIC RESTORATIONS Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry  METAL-CERAMIC RESTORATIONS Metal-ceramic restorations consist of a cast metallic framework (or core) & at least two layers of ceramic are baked  The first layer...

METAL-CERAMIC RESTORATIONS Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry  METAL-CERAMIC RESTORATIONS Metal-ceramic restorations consist of a cast metallic framework (or core) & at least two layers of ceramic are baked  The first layer applied is the opaque layer (to mask the darkness of the oxidized metal framework & this thin opaque layer also contributes to the metal-ceramic bond) The next step is the buildup of dentin and enamel (most translucent) porcelains The dentin or enamel porcelain powder is mixed with modeling liquid (mainly distilled water) to a creamy consistency REQUIREMENTS FOR A METAL-CERAMIC SYSTEM 1. The alloy must have a high melting temperature. 2. The veneering porcelain must have a low fusing temperature 3. The porcelain must wet the alloy readily when applied as a slurry to prevent voids forming at the metal-ceramic interface. 4. A good bond between the ceramic and metal 5. Compatible coefficients of thermal expansion (CTE) of the porcelain and metal The CTE of the metal is slightly higher than that of the porcelain, placing the veneering porcelain in compression (stronger) following cooling REQUIREMENTS FOR A METAL-CERAMIC SYSTEM 6.Adequate stiffness and strength of the metal 7. High resistance to deformation at high Temperature 8. Adequate design of the restoration is critical (The preparation) surface), the position of the metal-ceramic junction should be located at least 1.5 mm from all centric occlusal contacts. METAL-CERAMIC BONDING The most common mechanical failure for metal-ceramic restorations is debonding of the porcelain from the metal. Factors controlling metal-ceramic adhesion: The formation of strong chemical bond Mechanical interlocking between the two materials Thermal residual stresses The porcelain must wet and fuse to the surface to form a uniform interface with no voids. The contact angle between the porcelain and metal is a measure of the wetting ,Low contact angles (good wetting)  The formation of an oxide layer (the key to an adequate metal-ceramic bond). Noble metal alloys, which are resistant to oxidizing, usually have other more easily oxidized elements added, such as indium (In) and tin (Sn), to form an oxide layer and improve the bond.  The oxide layer is formed during a special firing cycle prior to porcelain application. Some noble alloys containing silver (discoloration or greening) explained by ionic diffusion of silver in the porcelain. The increased area of the rough metal surface also permits the formation of a greater density of chemical bonds. If the metal and ceramic have largely different thermal expansion coefficients, the porcelain will crack and/or delaminate from the metal. Due to high thermal residual stresses between the metal and porcelain The difference of 0.5 × 10−6/° C in thermal contraction between metal and porcelain causes the metal to contract slightly more than does the ceramic during cooling.  CERAMICS FOR METAL-CERAMIC RESTORATIONS Ceramics for metal-ceramic restorations must fulfill five requirements: (1)Simulate the appearance of natural teeth (2) Fuse at relatively low temperatures (3) Thermal expansion coefficients compatible with alloys (4) Age well in the oral environment (5) Low abrasiveness. DENTAL PORCELAIN COMPOSITION AND MANUFACTURE Manufacture: Many dental porcelain manufacturers buy feldspar as powder already screened and cleaned from impurities to their specifications.  Feldspathic dental porcelain consists of a glassy (or amorphous) phase and leucite (KAlSi2O6) as a crystalline phase. The glassy phase formed during the manufacturing process has properties typical of glass, such low toughness and strength, and high translucency. EFFECT OF DESIGN ON METAL-CERAMIC RESTORATIONS Because ceramics are weak in tension and can withstand very little strain before fracturing The metal framework must be rigid to minimize deformation of the porcelain. Copings should be as thin as possible to allow space for the porcelain to mask the metal framework without over contouring the porcelain.  The labial margin (little porcelain thickness at the margin to mask the appearance of the metal coping and to resist fracture. Recommended margin designs include a 90-degree shoulder, a 120-degree shoulder, or a shoulder bevel. the shoulder depth is at least 1.2 mm  These stresses should be minimized by placing the metal-ceramic junction at least 1.5 mm from centric occlusal contacts. Thickness of the connector should be large enough to prevent deformation or fracture FAILURE AND REPAIR OF METAL- CERAMIC RESTORATIONS When metal-ceramic prosthesis fails, it is often due to adhesive failure between porcelain and metal or cohesive failure within the ceramic near the metal- ceramic interface. Ideally, the prosthesis should be retrieved, metal surfaces should be cleaned, and a new oxide layer should be formed on the exposed area of metal prior to porcelain application and firing. This cannot be achieved intra-orally, Thank You