Types and Causes of Failures in Metal-Ceramic Restorations

Biological Failures: Loss of abutment teeth. Caries. Deterioration of supporting tissues. Fracture in the tooth. Poor marginal adaptation. Poor oral hygiene. Aesthetic Failures: Color mismatch. Different surface characterization from natural teeth. Deformity in shape. Inappropriate contour. Placement contrary to the natural tooth arrangement. Biophysical Failures: Incompatibilities between metal and porcelain:...
Woman with ceramic braces on teeth at the dental office
Types and Causes of Failures in Metal-Ceramic Restorations
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Biological Failures:

  • Loss of abutment teeth.
  • Caries.
  • Deterioration of supporting tissues.
  • Fracture in the tooth.
  • Poor marginal adaptation.

Poor oral hygiene.

Aesthetic Failures:

  • Color mismatch.
  • Different surface characterization from natural teeth.
  • Deformity in shape.
  • Inappropriate contour.
  • Placement contrary to the natural tooth arrangement.

Biophysical Failures:

Incompatibilities between metal and porcelain:

  • Chemical bonding incompatibilities.
  • Incompatibilities in thermal expansion coefficients.
  • Incompatibilities in elastic modulus.
  • Material structure incompatibilities.

Biomechanical Failures:

  • Faulty design.
  • Metal and porcelain not having ideal thickness.
  • Microcracks and fractures within the porcelain.
  • Irregular, sharp-edged, undercut metal substructure.
  • Characteristics of porcelain:
  • Resistant to compressive stresses.
  • Weak against shear stresses.
  • Brittle and fragile.
  • Fragility: Emerges due to concentration of forces at microcracks and surface irregularities, surpassing the strength of the porcelain.

These failure types in metal-ceramic restorations highlight the complexity and the need for precision in their design, material selection, and maintenance. Biological and aesthetic failures often result from external factors and patient habits, while biophysical and biomechanical failures arise from issues in material compatibility and structural design. The understanding of these failure types is crucial for the proper management and repair of metal-ceramic restorations.

Causes of Fractures and Types of Failures in Metal-Ceramic Restorations

Causes of Fractures:

  • Insufficient tooth reduction – inadequate interocclusal space.
  • Inappropriate design of the metal substructure.
  • Sharp edges of the metal structure.
  • Porosities in the casting.
  • Rough areas resulting from surface grinding.
  • Excessively thin metal structure.
  • Improper manipulation, condensation, and firing techniques.
  • Poor metal-ceramic bonding, thick oxide formation.
  • Incompatibility in thermal expansions of metal and porcelain.
  • Shrinkage of porcelain.
  • Excessive thickness of the porcelain layer.
  • *Thickness of porcelain can lead to compressive stress in thin layers and tension stress in thick layers due to differences in thermal expansion.

Recommended Thickness for Metal and Porcelain:

  • Metal thickness: 0.3-0.5 mm.
  • Porcelain thickness: upper limit of 1.5-2 mm.

Six Types of Failures Defined by O’Brien:

  1. Between metal and porcelain.
  2. Between metal oxide and porcelain.
  3. Cohesive failure in porcelain.
  4. Metal to metal oxide.
  5. Metal oxide to metal oxide.
  6. Cohesive failure in metal.

Identification of Failures:

  • Cohesive failure occurs within porcelain or metal.
  • Adhesive failure occurs at the interface between porcelain and metal.

Repair of the Fracture:

  • Before attempting repair, the cause of the fracture must be accurately evaluated.
  • By examining which layers are involved in the fracture, it can be inferred that the failure could be due to microdefects within the porcelain, thick oxide formation, insufficient metal-porcelain bonding, or metal and porcelain not being prepared to the necessary thickness.
  • Adhesive failure, where the metal surface is fully exposed due to the disruption of chemical balances, represents the most challenging repair situation.

Understanding these factors is crucial for the effective repair and prevention of failures in metal-ceramic restorations. The approach to repair will depend on accurately diagnosing the type of failure and addressing the underlying causes.

Porcelain Repair Methods:

Removal and Laboratory Repair of Porcelain Restorations:

  • If the fractured porcelain restoration (crown or bridge) can be easily removed from the mouth, the damaged crown or bridge can be repaired in the laboratory by adding porcelain and firing.
  • Intraoral repair methods are preferred if a clinically acceptable restoration exists in multi-unit bridges and if there is a risk of damaging the bridge during removal.

Considerations for Repair Method:

  • The size of the crack or fracture, whether the metal surface is exposed, and the condition of the broken piece are important.
  • Small fractures can be corrected with a bur and polished with rubber wheels. If there is significant material loss, the missing part is completed using different methods.

Intraoral Repair Methods:

Bonding the Broken Piece Back with Composite Resins:

  • If the fracture is a separation within the porcelain body and the piece fits back into its original place:
  • Acid-etch both porcelain surfaces with 5-10% HFA for 2-4 mins, rinse, and dry.
  • Apply silane coupling agent to the porcelain surfaces and dry with air.
  • Apply bonding agent of composite resin to the surface, cure with light.
  • Apply a thin layer of composite resin to the surface, place the broken piece in its original position, remove excess material, cure with light, and polish.If the fracture exposes the metal surface:
  • Create notches on the exposed metal surface and sandblast.
  • Apply the other steps as mentioned above.

Direct Intraoral Repair with Composite Resins:

  • If there’s no large broken porcelain piece and the material loss is small, it can be repaired with light-curing composite resins of appropriate color.
  • Perform acid-etch-silan-bonding application.
  • Shape the composite material of the suitable color on top, cure with light, and polish.

If the metal surface is exposed, create notches in the metal, use opaque material to mask the color.

Indirect Repair with Pin-Retained Metal-Ceramic Castings:

  • Used when the body porcelain of a metal-ceramic bridge is heavily damaged and the metal surface is completely exposed.
  • Create small pinholes for retention on the metal surface, place matching thin plastic bristles.
  • Take impressions, get a model and prepare a thin metal facet on the model.
  • Fire porcelain onto this metal substructure, creating a pin-supported piece.
    Cement this piece onto the broken area.


Indirect Repair Over the Broken Unit with Cast Metal-Ceramic Crown:

  • This technique is typically applied to body fractures.
  • Prepare the broken unit like a tooth for a crown.
  • Take an impression and prepare a telescopic-style second metal-ceramic unit on the model.
  • Cement this crown onto the broken area to complete the repair.
  1. Repair with Indirectly Prepared Porcelain Material:

  • Similar to the 4th method, this approach is carried out.
  • Ceramic superstructures made of castable ceramic or aluminous porcelain are prepared in the laboratory to complete the broken part.
  • The broken unit is bonded with dual cure composite using acid etch and silane coupling.

In all these methods, the approach to repair porcelain restorations depends on accurately assessing the type and extent of the damage. The appropriate choice of technique and meticulous execution are crucial for the long-term success of the repair.

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