PFM crowns Flashcards
What are the general features of PFM crowns?
Consists of a complete-coverage cast metal substructure with a mechanically and chemically bonded ceramic veneer.
Range of designs are available depending on material coverage.
It is one of the most widely used crowns.
What are the indications for PFM crowns?
Aesthetic improvement
Teeth that have been exclusively restored due to caries or trauma
Retention and strength
Protection of the existing tooth especially teeth that have been root treated
Re-contour axial surfaces or correct minor tooth misalignment
In higher stress situations
As a retainer for a FPD or a rest for an RPD
What are the contraindications to using PFM crowns?
Patients with active and unmanaged caries and periodontal disease
Young patients whose pulp chambers are large
When more conservative options are possible
What are the advantages and to using PFM crowns?
Advantages:
Good combination of aesthetics and durability
Excellent retention
Superior fit than all ceramic crown
What are the disadvantages and to using PFM crowns?
Considerable tooth reduction is needed (where the metal is covered with ceramic)
Aesthetic margins should be subgingival
Not as aesthetic as all ceramic crowns
Less flexibility in shade alteration
What are the requirements for alloy in PFM crowns?
Allow casting in thin section (0.3mm)
Porcelain-metal compatibility which consists of:
Thermal match between metal and pocelain: The metal should have higher melting temperature than porcelain for thermal stability while porcelain firing.
Colour preservation (Some components of the alloy affect porcelain colour)
Relative expansion (If the porcelain shrinks slightly less than metal it will be placed under compression)
High alloy rigidity is required to prevent flexing.
What do palladium and platinum do to alloys?
They are generally added to serve as hardening elements in alloys with high gold content
What are copper, iron and tin used for in alloys?
They are used as hardening agents at much lower concentrations than palladium and platinum
What is the function of indium, tin, and iron in alloys?
They promote the bonding of porcelain by the formation of stable adherent oxides
How does metal bond to ceramic in PFM?
Mechanical: Ceramic locking on cooling
Chemical: Fusion of ceramic to metal oxide layer
What are the component layers of PFM?
Metal 0.3mm
Opaque 0.2mm
Ceramic 0.5/1.0mm
What are the features of a metal collar and ceramic shoulder and when is each more ideal?
Metal collar is:
Conservative
More accurate fit
Unaesthetic
Subgingival placement
More suitable posteriorly
Ceramic shoulder is:
Suitable for anterior teeth and equi-gingival labial margin is possible.
What are the tools needed for a PFM prep?
- Silicone putty
- Round-end tapered bur
- Large grit
- Fine grit
- Football-shaped diamond
- Flat-end tapered bur
- Finishing stones
- Explorer and periodontal probe
- Hatchet and chisel
