What types of metal are used in porcelain fused to metal (PFM) crowns?

Types of Metal Used in Porcelain Fused to Metal Crowns

Porcelain fused to metal (PFM) crowns primarily utilize base metal alloys—specifically nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys—as well as noble metal alloys including gold-based alloys (Au-Pt-Pd, Au-Pd-Ag, Au-Pd) and palladium-based alloys (Pd-Ag, Pd-Cu, Pd-Co). 1, 2, 3

Base Metal Alloys

Nickel-Chromium (Ni-Cr) Alloys

• Ni-Cr alloys are widely used for metal-ceramic restorations due to their inexpensive price, hardness, durability, resistance to deformation, and favorable mechanical properties. 1
• These alloys allow for thin metal thickness in the porcelain area while maintaining structural integrity. 1
• A critical limitation is the allergenic potential of the nickel component, which can cause hypersensitivity reactions in susceptible patients. 4, 1

Cobalt-Chromium (Co-Cr) Alloys

• Co-Cr alloys offer comparable performance to Ni-Cr alloys but without the allergenic nickel component, making them the preferred base metal choice for patients with metal sensitivities. 1, 2
• The ASTM F1537 Co-Cr-Mo alloy (58.9-69.5 wt% Co, 27.0-30.0 wt% Cr, 5.0-7.0 wt% Mo) has received FDA approval for dental applications. 4
• Co-Cr alloys demonstrate high strength, superior wear and fatigue resistance, and notable biocompatibility due to a passivating chromium oxide layer. 4
• Some Co-Cr formulations may contain up to 1 wt% residual Ni, which still raises hypersensitivity concerns. 4

Noble Metal Alloys

Gold-Based Alloys

• Gold-based alloys (Au-Pt-Pd, Au-Pd-Ag, Au-Pd classes) have clearly established clinical integrity and acceptability over extended periods, making them the gold standard despite higher cost. 3
• These alloys demonstrate excellent long-term clinical data, superior physical properties, and strong esthetic potential. 3
• The primary limitation is relatively low sag resistance in high gold-low silver content alloys and potential thermal incompatibility with some commercial porcelain products. 3

Palladium-Based Alloys

• Palladium-based alloys (Pd-Ag, Pd-Cu, Pd-Co) are less costly than gold-based alloys but require extra precautions to minimize porcelain discoloration. 3
• Palladium-silver alloys specifically require careful handling to prevent discoloration issues. 3
• Palladium-cobalt alloys demonstrate the least susceptibility to high-temperature creep compared to all noble metal classes, but insufficient clinical data exists to advocate their general use. 3
• Copper and cobalt in these alloys can act as colorants in glasses, potentially causing porcelain discoloration. 3

Material Selection Considerations

Mechanical Properties

• Base metal alloys (Ni-Cr and Co-Cr) offer superior yield strength and modulus of elasticity (rigidity) compared to noble metals, allowing for thinner copings and better rigidity for long-span fixed partial dentures. 5
• Gross differences in castability and porcelain bonding behavior exist between alloys with nominally similar compositions. 2

Alloy Modifications

• The addition of beryllium to base metal alloys increases fluidity, improves casting fit, controls surface oxidation, and enhances bonding strength. 5
• Newer stainless steel alloys with higher nitrogen content demonstrate improved resistance to fretting corrosion. 6

Biocompatibility Screening

• Pre-implantation screening via skin patch tests or lymphocyte transformation tests is recommended for patients with a history of intolerance to jewelry, belt buckles, watches, or prior metal implants. 4
• Approximately 10-15% of the population may exhibit allergy to one or more metals commonly used in dental applications. 4

Clinical Performance

• A 3-year clinical study demonstrated comparable survival rates between metal-ceramic crowns (97.6%) and zirconia crowns (95.2%), with no significant differences in success or ceramic fracture rates. 7
• Metal-ceramic crowns using high-noble alloy with low-fusing porcelain showed 90.9% event-free survival at 3 years. 7

Critical Pitfalls to Avoid

• Never delegate alloy selection to the dental laboratory technician—dentists have legal and ethical responsibility for material selection. 3
• Document the specific alloy name on all laboratory prescriptions and in patient charts for legal protection and longevity tracking. 3
• Perform preliminary compatibility tests on new or untried alloy-porcelain combinations prior to fabricating actual restorations, as no universally accepted test exists. 2
• Consider alternative materials such as titanium alloys in patients with known metal hypersensitivity to minimize fretting corrosion risk. 6