Metal Ceramic Restoration Lecture Notes PDF

METAL CERAMIC RESTORATIONS Construction Dr. Hanaa Nassar Lecturer of fixed prosthodontics  Metal Ceramic Restoration Ceramic fused Metal Coping to the coping  Ve...

METAL CERAMIC RESTORATIONS Construction Dr. Hanaa Nassar Lecturer of fixed prosthodontics  Metal Ceramic Restoration Ceramic fused Metal Coping to the coping  Veneering Porcelain Dentin or Body Enamel or Opaque Porcelain Porcelain incisal Porcelain I. Metal Framework 1. Alloy requirements a. Coefficient of thermal expansion b. Melting range c. Mechanical properties of the alloy Alloy requirements a. Coefficient of thermal expansion of the alloy should closely match with that of porcelain (13-14 x 10-6 ° C) so that the difference will not be greater than (0.5-1 x 10-6 °C). ❖ This slight mismatch with a higher value for the metal will put the ceramic in a state of beneficial compressive stresses rather than tension on cooling.  Alloy requirements b. Melting range of the alloy should be higher than the fusing temperature of porcelain by at least 170-280oC so it can withstand porcelain firing without deformation. (High sag resistance) (elevated temperature dimensional changes during porcelain firing cycles). Alloy requirements c. Mechanical properties: ❖ The alloy should be of enough stiffness and rigidity (high modulus of elasticity). ❖ Capable of forming a strong bond with porcelain at the interface. ❖ No elements that causes discoloration of the porcelain veneer. (Silver and copper)  2. Metal coping design a. Thickness of the metal b. Placement of the occlusal and proximal contacts c. Extension of the area to be veneered with porcelain. d. Design of the facial margin  a. Thickness of the metal ❖Minimum porcelain thickness ❖Metal surface texture (Convex – smooth) →No sharp or acute angles: Eliminate areas of stress concentration Distribute occlusal forces evenly Facilitates wetting of metal  a. Thickness of the metal Metal ceramic junction (Definite 90° angle – smooth) Coping rigidity and strength (Noble alloys 0.3– 0.5mm and Base metal alloys 0.2-0.3mm)  Basic principles of metal substructure design 1) No sharp or acute angles. Convex surfaces and rounded contours of metal surfaces should be created. 2) The junction between metal and ceramic as definite (90o angle) and as smooth as possible. 3) Adequate support to porcelain veneer where lack of metal support results in porcelain fracture due to its brittleness and low flexure a strength.  Rounded angles The junction between metal and ceramic should be Definite (90⁰angle) and as smooth as possible. Minimal thickness of porcelain = 0.7mm Optimum thickness = 1mm Maximum = 1.5mm The metal substructure should compensate deficiencies in the form of the prepared tooth b. Contacts Occlusal contacts. Proximal contacts. Occlusal contacts No occlusal contact on porcelain-metal interface contact at porcelain metal junction leads to porcelain fractures. The metal-ceramic interface must be at least 2.5 mm away from all centric occlusal contacts to avoid metal flow and subsequent ceramic fracture. Occlusal contacts against natural opposing teeth should be wherever possible in metal. Porcelain wrap around effect Increase porcelain resistance against splitting Better translucency and esthetics Thin metal can flex and lead to porcelain fracture Proper metal thickness Occlusion in Metal Occlusion in Porcelain Wrap around of porcelain  Proximal contacts Should allow light transmission through the porcelain enamel in the proximal area. c. Extension of the veneered area Esthetics Occlusion Facial Margin Design Facial Margin Design Conventional facial margin: Narrow metal collar Shoulder with bevel finish line (Metal collar ) Subgingival placement 60% of subgingival margins become visible within a 2-year period Rounded shoulder finish line  Deep chamfer finish line Note: the minimal amount of exposed metal at the margin  Shoulder finish line With labial porcelain margin  Collarless Metal Ceramic Crown 360°porcelain margin Excellent light transmission in the gingival area Collarless Metal Ceramic Crown Shoulder finish line Collarless Metal Ceramic Crown ❖ Advantages: Esthetics Easy plaque removal ❖ Disadvantages: Difficult fabrication Marginal adaptation slightly inferior to cast metal Careless handling can lead to fracture of unsupported margin  Methods of fabrication of porcelain labial margin: Platinum foil technique. Direct lift or cyanoacrylate resin technique. Porcelain wax technique. Platinum foil technique Direct lift technique Porcelain wax technique  Shoulder Porcelain Contains Aluminous porcelain: Fuses at temperatures 30⁰-80°C than dentin or enamel porcelain. This allows repeated firing of the crown build up without distortion of the completed margin. Makes the margin more resistant to fracture as it is stronger in flexure. Dental ceramics High fusing : 1290◦ C to 1370◦ C (denture teeth) Medium fusing : 1090◦ C to 1260◦ C (porcelain jacket crown) Low fusing : 870◦ C to 1070◦ C (metal ceramic restorations ) Ultra-low fusing: less than 870 ◦ C (porcelain labial margin, correction) Porcelain composition 1. Feldspar ( 75% - 85% by wt.) 2. Silica ( 12% - 22% by wt.) 3. Kaolin or clay (3% -5% by wt.) 4. Coloring pigments(