Porcelain Veneer Design and Application

Metal Ceramic Restorations Construction

• Metal ceramic restorations consist of a metal coping and ceramic fused to the coping.
• Veneering porcelain is used to cover the metal coping.
• The veneering porcelain is further divided into opaque porcelain and dentin or body porcelain, and enamel or incisal porcelain.

Metal Coping Design

• Metal thickness: Minimum porcelain thickness needed, smooth convex metal surface texture avoiding sharp angles to eliminate stress concentration. Even distribution of occlusal forces is ensured, and the wetting of the metal is facilitated.
• Metal-ceramic junction: A definite 90-degree angle, smooth transition between metal and ceramic for better strength. Noble alloys thickness 0.3-0.5mm and base metal alloys 0.2-0.3mm.
• Occlusal and proximal contacts: Occlusal contacts should be placed in metal whenever possible; the metal-ceramic interface should be at least 2.5 mm away from all centric occlusal contacts.
• Extension area: The area to be veneered with porcelain must be correctly extended for aesthetics and proper occlusion.
• Facial margin: Appropriate facial margin designs (A-F) are crucial to ensure aesthetics and function.

Alloy Requirements

• Coefficient of thermal expansion: The metal alloy's coefficient of thermal expansion should closely match that of porcelain (variations of 0.5-1 x 10^-6 °C). A slight thermal expansion difference between the metal coping and veneering porcelain results in compressive stresses rather than tension when cooled.
• Melting range: The metal's melting range should be significantly higher than the fusing temperature of the porcelain (at least 170-280°C) to prevent deformation during firing. This high sag resistance is necessary at elevated temperatures during porcelain firing cycles.
• Mechanical properties: The metal alloy should exhibit enough stiffness and rigidity (high modulus of elasticity) to form a strong bond with porcelain. Crucial that there are no elements that cause porcelain discoloration. (Examples of elements to avoid are silver and copper).

Basic Principles of Metal Substructure Design

• No sharp or acute angles or pits on the metal surface; use rounded contours for better stress distribution.
• The metal-ceramic junction should have a well-defined 90-degree angle and be as smooth as possible.
• Sufficient metal support for the porcelain veneer to prevent fracture due to porcelain brittleness and low flexure strength.

Porcelain Wrap Around Effect

• Increased porcelain resistance against splitting.
• Improved aesthetics and translucency due better light diffusion through the porcelain.

Proximal Contacts

• Design of proximal contacts should allow transmission of light through the porcelain enamel for enhanced natural aesthetics.

Porcelain Veneer Area Extension

• Esthetic considerations are crucial for extension.
• Occlusal considerations are critical for extension.

Facial Margin Design

• Conventional facial margin: Narrow metal collar.
• Shoulder with bevel finish line: Metal collar that creates an aesthetically pleasing contour, and a bevel finish is vital for strength and prevent marginal issues.

Subgingival Placement

• 60% of subgingival margins become visible within a two-year period.

Porcelain Application

• Opaque porcelain, body porcelain, and incisal porcelain are applied progressively on the metal coping.
• Porcelain surface finish is necessary for better appearance and function.

Porcelain Build Up

• Mixing: Porcelain powder is mixed with a binder (water or glycerin-based liquid) to form a paste.
• Application: Manual (vibrating, spatulation, brush or capillary attraction), ultrasonic methods of condensation are used to create a high quality restoration.

Armamentarium

• Necessary tools/materials for applying porcelain to the restoration. (Example: mixing spatulas, brushes, and powders)

Role of Opaque Porcelain

• Masks the color of the underlying metal coping.
• Ensures a strong metal-ceramic bond.
• Sets the initial shade of the crown.

Opaque Porcelain Application

• First application: The opaque porcelain is used to coat the entire metal coping, creating a solid base for the subsequent layers.
• Second application: The second layer of opaque porcelain is used to mask the underlying metal.
• Thickness: 0.3mm

Porcelain Firing

• Preheating and drying phase.
• First firing: Heat below the fusing temperature of dentin porcelain.
• Second firing (correction bake): Heat enamel porcelain to a temperature slightly lower than dentin porcelain.

Sintering

• Consolidation process via the formation of glass bridges, ensuring a continuous material structure.
• Volumetric restoration is made oversized to account for shrinkage that results from firing.

Effect of Vacuum and Slow Firing

• Reducing porosity during porcelain firing.
• Employing vacuum reduces the remaining bubbles, thus enhancing the final product.

Porcelain Firing

• Heating is done at lower temperature for the initial firing thus avoiding unwanted issues like crazing or cracking.
• Gradual cooling enhances overall product quality.

Contouring and Finishing

• Shaping and refining the porcelain restoration to match natural tooth contours.
• Fine tuning and polishing to obtain desired smooth/glossy surfaces.

Porcelain Surface Finish

• Staining: application of coloring pigments to match natural teeth.
• Natural or auto-glaze: creates a glossy appearance.
• Over-glaze: Low-fusing clear porcelain applied for a final glaze.
• Polishing: enhances the smoothness and gloss of the restoration

Glazing

• Aim of glazing: Producing a glossy surface, improved aesthetics, better plaque control, enhanced strength, and minimizing wear.
• Auto-glazing: The porcelain glazes itself under air when held at its fusing temperature.
• Applied Glaze: Low-fusing clear porcelain is painted on top of the restoration.

Cemented Restoration

• A final restoration method after all phases associated with metal-ceramic bonding are complete.

Metal Ceramic Bonding

• Mechanical, chemical, and compressive fit are all aspects of bonding involved in this process.

Mechanical Retention

• Micromechanical interlocking (with abrasives/air abrasion)
• Increase wettability to provide better surface area for chemical bonding.

Chemical Bonding

• Controlled oxide layer formation for better adhesion between the metal substructure and opaque porcelain.

Compressive Fit

• Slight difference in thermal expansion coefficient between metal coping and veneering porcelain causes porcelain to draw towards the metal when cooled.

Porcelain Alloy Bonding

• Opaque porcelain facilitates chemical bonding; compression and tension in thermal contraction.

Possible Modes of Failure

• Various reasons for potential failure during the alloy-porcelain application process.

Problem Areas

• Common issues in metal-ceramic restorations like improper margin fit, open interproximal contacts, porcelain fracture or chipping, porosity/undercuts in the porcelain build-up.

Problem Areas (cont.)

• Common issues in metal-ceramic restorations, inadequate/incorrect layering, improper condensation, rapid firing, incomplete drying, form/fit issues post firing.

Problem Areas (cont.)

• Potential problems associated with crown form loss from glaze application, incorrect firing temperatures, improper holding time, and rough margins.