MC Fabrication

Fabrication of Metal-Ceramic Prostheses

• Fabrication of Metal-Ceramic (MC) prostheses involves two steps: fabrication of the metal substructure and firing of veneering ceramic.

Metal Substructure Fabrication

• Metal substructures are fabricated using the lost wax technique, CAD-CAM, or 3D printing.
• After fabrication, the substructure is finished, polished, and oxidized to create a surface oxide layer.

Ceramic Veneering

• Ceramic veneering can be achieved through manual condensation or vacuum pressing ceramic ingots.

Manual Porcelain Condensation

• Ceramic for veneering MC prostheses is supplied as a fine powder that is mixed with water and condensed into the desired form.
• The powder particles have a specific size distribution to achieve densely packed porcelain when properly condensed.
• Proper condensation is crucial for dense packing, which lowers firing shrinkage and reduces porosity in the fired ceramic.
• Condensation methods include vibration, spatulation, and brush techniques.

Firing Procedure

• The purpose of firing is to sinter the ceramic particles together at a specific time and temperature to create the prosthesis.
• Preheating the condensed porcelain mass before firing allows for the dissipation of remaining water, preventing steam production and potential voids or fractures.
• During firing, the porcelain particles fuse at points of contact, shrinking and densifying the structure.
• The fused glass flows to fill air spaces, but some air may remain trapped.
• Vacuum firing aids in reducing porosity in dental porcelain.
• Dental veneering ceramic is considered mature or fully sintered when it achieves 95% to 99% of its theoretical density.
• Multiple firings may be necessary for shade adjustments, characterization, or stain layer application.

Multiple Firing Considerations

• Multiple firings can increase the concentration of crystalline leucite in porcelain.
• Leucite crystals can alter the coefficient of thermal contraction of the porcelain.
• Increased leucite content from multiple firings can lead to an increase in the coefficient of thermal expansion (CTE), creating expansion mismatch between the porcelain and metal.
• Expansion mismatch can cause stresses during cooling and potentially lead to immediate or delayed crack formation in the porcelain.

Cooling

• Proper cooling from firing temperature to room temperature is crucial to prevent catastrophic fracture of the porcelain.
• Rapid cooling (thermal shock) can result in cracking or crazing due to tensile stress development.
• Cooling is particularly complex when the porcelain is bonded to a metal substrate.

MC Restoration Design & Properties

• The facial metal thickness of an MC restoration is typically 0.3 to 0.5 mm.
• The opaque porcelain veneer is approximately 0.3 mm thick, and the body porcelain is about 1 mm thick.
• Cracks in MC restorations will not propagate through the ductile metal core but can progress through the veneer or the interfacial region.
• Proper design and physical properties of the porcelain and metal can reinforce the porcelain and minimize the likelihood of brittle fracture.

Non-Cast MC Substructure Fabrication

• While most MC restorations use cast metal copings, novel non-cast approaches such as electrodeposition, milling, swaging, and burnishing are being developed for substructure fabrication.
• The fracture resistance of MC prostheses made from non-cast metals is less reliable compared to those made from traditional cast metal copings and frameworks.