Metal-Ceramic Restorations Overview

Metal-Ceramic Restorations

• Metal-ceramic restorations are composed of a cast metallic framework (or core) and at least two layers of ceramic.
• The first layer applied is the opaque layer, masking the oxidized metal framework. This thin opaque layer contributes to the metal-ceramic bond.
• The next step involves building up dentin and enamel (most translucent) porcelains.
• Dentin or enamel porcelain powder is mixed with modeling liquid (primarily distilled water) to achieve a creamy consistency.

Requirements for a Metal-Ceramic System

• The alloy must have a high melting temperature.
• The veneering porcelain must have a low fusing temperature.
• The porcelain should readily wet the alloy when applied as a slurry to prevent voids from forming at the metal-ceramic interface.
• A strong bond between the ceramic and metal is crucial.
• The coefficients of thermal expansion (CTE) of the porcelain and metal must be compatible.
• Adequate stiffness and strength of the metal are necessary, as well as high resistance to deformation at high temperatures.
• Designing the restoration is critical, and the metal-ceramic junction should be positioned at least 1.5 mm away from the centric occlusal contacts.

Metal-Ceramic Bonding

• The most common mechanical failure in metal-ceramic restorations is debonding of the porcelain from the metal.
• Factors controlling metal-ceramic adhesion include: strong chemical bonding, mechanical interlocking between the two materials, and thermal residual stresses.
• Porcelain must wet and fuse to the surface forming a uniform interface without voids. Low contact angles indicate good wetting between porcelain and metal.

Formation of an Oxide Layer

• The formation of an oxide layer is essential for an adequate metal-ceramic bond.
• Noble metal alloys, resistant to oxidizing, often have elements like indium (In) and tin (Sn) added to create an oxide layer, improving the bond.
• The oxide layer forms during a specific firing cycle prior to porcelain application.
• Some noble alloys containing silver can lead to discoloration or greening due to ionic silver diffusion into the porcelain.
• The increased area of the rough metal surface promotes a greater density of chemical bonds.

Thermal Expansion Coefficients

• If the metal and ceramic have significantly different thermal expansion coefficients, the porcelain is prone to cracking or delamination from the metal.
• The difference in thermal contraction between metal and porcelain (approximately 0.5 × 10-6/°C) causes the metal to contract slightly more than the ceramic during cooling.

Ceramics for Metal-Ceramic Restorations

• Ceramics used in metal-ceramic restorations must meet five requirements:
  • Simulate the appearance of natural teeth.
  • Fuse at relatively low temperatures.
  • Have thermal expansion coefficients compatible with alloys.
  • Age well in the oral environment.
  • Have low abrasiveness.

Dental Porcelain Composition and Manufacture

• Many dental porcelain manufacturers purchase feldspar powder, which is screened and cleaned of impurities to meet their specifications.
• Feldspathic dental porcelain is composed of a glassy (amorphous) phase and leucite (KAISi2O6) as a crystalline phase.
• The glassy phase, formed during manufacturing, exhibits properties typical of glass, including low toughness and strength but high translucency.

Effect of Design on Metal-Ceramic Restorations

• Ceramics are weak in tension, and metal frameworks must be rigid to prevent porcelain deformation.
• Copings should be as thin as possible to allow space for the porcelain to mask the metal framework without over-contouring.
• The labial margin should have a small amount of porcelain thickness for better mask and resistance against fractures. Recommended margin designs include 90-degree shoulder, 120-degree shoulder, or a shoulder bevel with at least 1.2 mm depth.

Failure and Repair of Metal-Ceramic Restorations

• Metal-ceramic prosthesis failure often results from adhesive failure between porcelain and metal or cohesive failure within the ceramic near the metal-ceramic interface.
• Ideally, for repair, the prosthesis should be retrieved, metal surfaces cleaned, and a new oxide layer formed on the exposed metal area before porcelain application and firing. This step is not typically achievable intra-orally.