L5 Metal ceramic crowns Flashcards
What is the strongest crown type?
All metal crown
List the 4 crown types.
- Full veneer metal crown/all metal crown
- Metal ceramic crown
- All ceramic crown
- Composite crown
How can ceramic crown strength be improved?
- Using a reinforced ceramic core system
- Using a metal substructure (MCC)
Describe reinforced ceramic core systems.
- Glass infused ceramics
-Almunious substructure (porcelain jacket crown) - Pressable material e.g. emax
- Machined cermaics
How is a metal substructure used to improve the strength of a ceramic crown?
- Metal substructure, opaque layer of ceramic on top, outer layers of porcelain on top of this
- Metal gives strength to the overlying ceramic
- Metal has a rough surface which ceramic can easily adhere to
- Chemical bond forms between the oxidised metal and the porcelain base layer
Describe the bonding of ceramic to metal in MCCs.
- Mechanical interlock: sandblasted metal surface
- Chemical bonding: oxide layer on metal can bond to ceramic
- Compression fit: difference in co-efficient of thermal expansion between metal and ceramic, ceramic shrinks onto metal, provides compression of ceramic and tension of the metal base layer to provide tight compression fit
Does the metal alloy of an MCC have a high or low melting point?
- Metal alloy has high melting point (higher than ceramic)
- Means the metal substructure doesn’t melt as the ceramic is fused on top
- High melting point achieved as palladium and platinum are added to the gold alloy substructure, or cobalt chromium is used as a base metal alloy
What are the advantages of MCCs?
- Metal is strong in thin sections so less tooth needs to be removed
- Metal margin may allow a shallower preparation if aesthetics is less important e.g. posterior tooth, low lip line
- Metal is better in parafunction
- Cheaper to produce
- Metal margins are easier to adjust than ceramic
- Metal margins can be polished more eaily than ceramic
What are the disadvantages of MCCs?
- Metal needs to be hidden so the ceramic has to be thicker, therefore more tooth is removed compared to an all ceramic crown
- Opacity requires good ceramic work, more technically challenging
- Crown failure causes metal to be exposed, very unaesthetic
- Potential metal allergy issues
How do the principles of crown preparation differ for an MCC compared to the generic crown principles?
- Increased tooth reduction, higher risk of pulpal damage (doesn’t apply to posterior teeth as aesthetics not as significant)
- Taper and prep height equally important
- Shoulder margin for metal and ceramic, chamfer for metal only
- Occlusal reduction for posterior teeth depends on if surface is made of ceramic or metal, more removal needed for ceramic
Describe the order of preparation for an anterior MCC.
1) Incisal edge reduction
2) Interproximal reduction with thin needle bur and hand instruments
3) Chamfer bur interproximally
4) 2 planes of reduction buccally
5) Margin preparation: chamfer all around, then shoulder bur just on buccal surface
6) Rugby bur for palatal reduction
7) Refine sharp edges
What are the measurements for a MCC?
- Buccal shoulder margin = 1-1.2mm
- Palatal chamfer margin = 0.5mm (half bur width)
- 1.5-2mm occlusal reduction (2mm functional cusp)
What variations are there for MCC coverage?
Occlusal coverage:
- Fully covered occlusal surface
- Partial coverage with ceramic only buccally (aesthetic)
- Metal palatal surface or ceramic palatal surface
Inform patients of risks and benefits of taking more or less tooth structure.
What is best for the patient functionally e.g. bruxism- metal for longevity.
