Porcelain Fused To Metal Crown Materials

Porcelain fused to metal (PFM) crowns have long been a staple in restorative dentistry, offering a blend of strength and aesthetics. The selection of appropriate materials for PFM crowns is a critical decision that impacts the crown's longevity, appearance, biocompatibility, and overall success. This article delves into the various materials used in PFM crowns, exploring their properties, advantages, disadvantages, and clinical considerations.

Understanding PFM Crowns: A Brief Overview

PFM crowns consist of two primary components: a metal substructure and a porcelain overlay. The metal provides the necessary strength and support to withstand occlusal forces, while the porcelain offers a tooth-like appearance, mimicking the natural shade, translucency, and texture of adjacent teeth. The fusion of these two materials requires careful selection of metals and porcelains that are compatible in terms of their thermal expansion coefficients and bonding mechanisms.

Metal Substructures: Foundation of Strength

The metal substructure is the backbone of the PFM crown, providing the necessary rigidity and resistance to fracture. Several types of alloys are used in the fabrication of these substructures, each with its own set of properties and clinical indications.

High Noble Alloys

• Composition: High noble alloys contain at least 60% noble metals by weight, with gold (Au) accounting for at least 40% of the alloy. Other noble metals commonly included are platinum (Pt), palladium (Pd), and silver (Ag).
• Advantages:
  • Excellent corrosion resistance: Gold and platinum are highly resistant to corrosion in the oral environment, minimizing the risk of allergic reactions and marginal discoloration.
  • Biocompatibility: High noble alloys generally exhibit excellent biocompatibility, reducing the potential for adverse tissue reactions.
  • Precise fit: These alloys offer excellent castability and can be easily manipulated to achieve a precise fit.
  • Strong bond with porcelain: High noble alloys form a strong chemical bond with porcelain, reducing the risk of porcelain chipping or fracture.
• Disadvantages:
  • High cost: Gold and platinum are expensive, making high noble alloys the most costly option for PFM crowns.
  • Lower strength: Compared to base metal alloys, high noble alloys may have lower strength, especially in thin sections.
• Clinical Indications: High noble alloys are indicated for patients with a history of metal allergies, those requiring excellent biocompatibility, and in situations where a precise fit is paramount.

Noble Alloys

• Composition: Noble alloys contain at least 25% noble metals by weight, with the remainder consisting of base metals such as nickel (Ni), chromium (Cr), and cobalt (Co). Palladium-silver alloys are a common example of noble alloys.
• Advantages:
  • Good corrosion resistance: Noble alloys offer good corrosion resistance, though not as high as high noble alloys.
  • Moderate cost: Noble alloys are less expensive than high noble alloys, making them a more affordable option.
  • Adequate strength: These alloys provide adequate strength for most clinical situations.
• Disadvantages:
  • Potential for allergic reactions: The presence of base metals increases the risk of allergic reactions in susceptible individuals.
  • Lower bond strength: Noble alloys may exhibit lower bond strength with porcelain compared to high noble alloys.
  • Tarnishing: Silver-containing alloys may tarnish in the oral environment, leading to discoloration of the crown margins.
• Clinical Indications: Noble alloys are suitable for patients without known metal allergies and in situations where a balance between cost and performance is desired.

Base Metal Alloys

• Composition: Base metal alloys contain less than 25% noble metals by weight and are primarily composed of non-noble metals such as nickel (Ni), chromium (Cr), and cobalt (Co).
• Advantages:
  • High strength and rigidity: Base metal alloys possess superior strength and rigidity compared to noble and high noble alloys, making them suitable for long-span bridges and patients with heavy occlusal forces.
  • Low cost: Base metal alloys are the least expensive option for PFM crowns.
  • Thin margins: Their high strength allows for the creation of thin, precise margins.
• Disadvantages:
  • High potential for allergic reactions: Nickel is a known allergen, and base metal alloys have a higher potential to cause allergic reactions in sensitive individuals.
  • Poor corrosion resistance: Base metal alloys are more susceptible to corrosion in the oral environment, leading to discoloration and potential release of metal ions.
  • Difficult to cast: These alloys can be challenging to cast accurately, potentially compromising the fit of the crown.
  • Opaque: Base metal alloys are highly opaque, which can make it difficult to achieve optimal aesthetics in the final restoration.
• Clinical Indications: Base metal alloys are generally reserved for patients without known metal allergies and in situations where high strength and low cost are primary considerations. They are often used for posterior restorations where aesthetics are less critical.

Porcelain Overlays: Achieving Natural Aesthetics

The porcelain overlay is responsible for the aesthetic appearance of the PFM crown. Dental porcelain is a ceramic material specifically formulated to mimic the optical properties of natural teeth, including their shade, translucency, and surface texture.

Types of Dental Porcelain

• Feldspathic Porcelain: This is the most traditional type of dental porcelain, characterized by its high glass content. Feldspathic porcelain offers excellent aesthetics but has relatively low strength. It is typically used for layering and creating intricate color effects.
• Leucite-Reinforced Porcelain: Leucite is a crystalline phase that is added to feldspathic porcelain to increase its strength and fracture resistance. Leucite-reinforced porcelain is suitable for veneers and anterior crowns where moderate strength is required.
• Lithium Disilicate Porcelain: Lithium disilicate is a high-strength ceramic material that offers a good balance of aesthetics and durability. It is commonly used for both anterior and posterior crowns, as well as inlays and onlays. While often used as an all-ceramic material, it can also be layered onto a metal substructure.
• Zirconia-Reinforced Porcelain: Zirconia is an extremely strong ceramic material that is known for its exceptional fracture resistance. Zirconia-reinforced porcelain combines the strength of zirconia with the aesthetics of porcelain, making it suitable for high-stress applications such as posterior crowns and bridges. However, the veneering porcelain can still be susceptible to chipping.

Porcelain-Metal Bonding

The bond between the porcelain and the metal substructure is crucial for the long-term success of PFM crowns. Several factors influence the strength and durability of this bond:

• Thermal Compatibility: The coefficient of thermal expansion (CTE) of the porcelain and the metal must be closely matched to prevent stress concentrations at the interface during heating and cooling cycles.
• Oxide Layer Formation: A thin, uniform oxide layer on the metal surface is essential for creating a chemical bond with the porcelain. This oxide layer provides reactive sites for the porcelain to adhere to.
• Porcelain Composition: The composition of the porcelain must be carefully controlled to ensure proper wetting and bonding to the metal oxide layer.
• Firing Cycles: The firing cycles used during porcelain fabrication must be optimized to achieve complete sintering and a strong bond with the metal.

Clinical Considerations in Material Selection

The selection of appropriate materials for PFM crowns requires careful consideration of several clinical factors:

• Patient Allergies: A thorough medical history should be obtained to identify any known metal allergies. If a patient is allergic to nickel, for example, base metal alloys should be avoided.
• Occlusal Forces: Patients with heavy occlusal forces or bruxism require stronger metal substructures, such as base metal alloys or high noble alloys with adequate thickness.
• Aesthetic Demands: The aesthetic requirements of the patient should be considered when selecting the porcelain overlay. Feldspathic porcelain offers the best aesthetics but has lower strength, while zirconia-reinforced porcelain provides high strength with acceptable aesthetics.
• Tooth Location: The location of the tooth in the arch influences the choice of materials. Anterior teeth require greater attention to aesthetics, while posterior teeth require greater strength.
• Subgingival Margins: If the crown margins are located subgingivally, biocompatible materials such as high noble alloys should be considered to minimize the risk of gingival inflammation.
• Parafunctional Habits: Patients with parafunctional habits such as clenching or grinding require stronger materials to withstand the increased stress.
• Cost: The cost of the materials should be considered, especially for patients with limited budgets. Base metal alloys are the least expensive option, while high noble alloys are the most costly.

Fabrication Process of PFM Crowns

The fabrication of PFM crowns involves several steps, including:

1. Tooth Preparation: The tooth is prepared by removing a sufficient amount of tooth structure to create space for the metal substructure and porcelain overlay.
2. Impression: An impression of the prepared tooth is taken and sent to a dental laboratory.
3. Die Fabrication: A die is created from the impression, which is a replica of the prepared tooth.
4. Metal Substructure Fabrication: The metal substructure is fabricated using either the lost-wax technique or CAD/CAM technology.
5. Porcelain Application: The porcelain is applied to the metal substructure in layers, with each layer being fired in a porcelain furnace.
6. Finishing and Polishing: The crown is finished and polished to achieve the desired shape, contour, and surface texture.
7. Cementation: The PFM crown is cemented onto the prepared tooth using a dental cement.

Advantages of PFM Crowns

PFM crowns offer several advantages over other types of crowns:

• Strength and Durability: The metal substructure provides excellent strength and durability, making PFM crowns suitable for both anterior and posterior teeth.
• Aesthetics: The porcelain overlay provides a natural-looking appearance that blends well with adjacent teeth.
• Versatility: PFM crowns can be used to restore a wide range of dental conditions, including decayed, fractured, and discolored teeth.
• Long-Term Success: With proper care and maintenance, PFM crowns can last for many years.
• Cost-Effectiveness: PFM crowns are generally more cost-effective than all-ceramic crowns.

Disadvantages of PFM Crowns

PFM crowns also have some disadvantages:

• Potential for Porcelain Chipping: The porcelain overlay can chip or fracture, especially under heavy occlusal forces.
• Metal Margin Display: Over time, the gingival tissue may recede, exposing the metal margin of the crown.
• Allergic Reactions: Some patients may be allergic to the metals used in the substructure.
• Opacity: The metal substructure can make the crown appear opaque, especially in the incisal area.
• Greyish Hue: In some cases, the metal substructure can impart a greyish hue to the porcelain overlay.

Alternatives to PFM Crowns

Several alternatives to PFM crowns are available, including:

• All-Ceramic Crowns: All-ceramic crowns are made entirely of ceramic materials, such as lithium disilicate or zirconia. They offer excellent aesthetics and biocompatibility, but may not be as strong as PFM crowns.
• Gold Crowns: Gold crowns are made entirely of gold alloys. They are highly durable and biocompatible, but their metallic appearance may not be acceptable to all patients.
• Zirconia Crowns: Zirconia crowns are made of a very strong ceramic material called zirconia. They are highly durable and can be used for both anterior and posterior teeth.
• Porcelain Veneers: Porcelain veneers are thin shells of porcelain that are bonded to the front surface of teeth. They are used to improve the appearance of teeth that are chipped, stained, or slightly misaligned.

Conclusion

The selection of appropriate materials for porcelain fused to metal (PFM) crowns is a critical decision that impacts the crown's longevity, aesthetics, and biocompatibility. Dentists must carefully consider the patient's medical history, occlusal forces, aesthetic demands, and budget when selecting the metal substructure and porcelain overlay. While PFM crowns have been a reliable restorative option for many years, newer materials and techniques, such as all-ceramic crowns and CAD/CAM technology, are constantly evolving and offering improved aesthetics and durability. Ultimately, the best choice of material depends on the individual needs and preferences of the patient, as well as the clinical expertise of the dentist.