Dental Ceramics Flashcards
What is Kaolin?
Hydrated aluminium silicate
An opaque clay found in dental ceramics
What is found in dental ceramics?
Kaolin <5%
Quartz (silica) 12-25%
Feldspar 70-80%
Metal oxides 1%
Glass up to 15%
What does feldspar do?
Acts as a flux, lowering the temp of other glasses in the mixture, allowing fusing of the glasses
How are dental ceramics made?
Powder is made by heating constituents to >1000ºC then rapidly cooling
A binder (often starch) is added and the powder is mixed with distilled water and built up into the restoration
How does the powder form a crown?
Felspathic ceramics form leucine when heated
This forms around the glass phase of the ceramic
Gives the powder physical and thermal properties
The powder melts together to form the crown
How are crowns formed?
Ceramic powder is mixed with water and applied to the die with a brush
The crown is built up using different porcelains for dentine and enamel
The crown is heated in a furnace to coalesce the powder into ceramic
What is sintering?
When the ceramic particles begin to fuse into a single mass
The glass phase softens and will coalesce
Over time a solid ceramic mass if formed
Material contracts by about 20%
Heating leads to sintering
What are the aesthetic properties of ceramics?
Best aesthetic properties of any restorative material
Colour stable
Very smooth surface
Retain surface well
Optical properties - reflectance, translucency, opacity, transparency, opalescence
Describe the chemical stability of ceramics
Chemically very stable
Generally unaffected by the wide pH range found in the mouth
Do not stain from food/drink
Good biocompatibility
Describe the thermal properties of ceramics
Similar to tooth substance
Coefficient of thermal expansion similar to dentine
Thermal diffusivity is low - protective of the remaining tooth
Describe the dimensional stability of ceramics?
Once fully fired the material is very stable
During fabrication shrinkage is a problem and 20% is normal for a feldspathic ceramic crown
Describe the mechanical properties of ceramics
High compressive strength, hardness
Low tensile strength, flexural strength, fracture toughness - all lead to failure during loading
Static fatigue
Surface micro-cracks
Slow crack-growth
Where can feldspathic crowns be used?
Only anterior crowns
Too brittle for use anywhere else
Why are alumina core crowns used?
Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
What are the advantages of alumina core crowns?
High fracture toughness
Relatively cheap to make
No specialist equipment required
Less labial reduction required
What are the disadvantages of alumina crowns?
Can only be used for anteriors - not strong enough for posteriors
Alumina is opaque
More palatal reduction required
Lack of flexural strength
Give examples of other ceramic techniques and materials
Zirconia
Lithium disilicate
Cores
Pressed crowns
Monolithic/milled crowns
Luting
Describe zirconia cores
Zirconia dioxide is a naturally occurring material
Very hard
Different forms at different temperatures
Zirconia powder does not sinter unless heated to over 1600ºC
What is Yttria stabilised Zirconia?
3-5% of the material is yttria - the more yttria the more translucent and less physical properties
If a crack begins when stress at the crack tip reaches a critical level, the crystal structure transforms into the monoclinic structure, expanding the material, closing the crack tip
How is a zirconia core fabricated?
Imp taken of prep and sent to lab
Model is cast and digitally scanned
Software created bridge structure
Raw zirconia block is milled
Framework is heated to 850ºC which causes shrinkage but computed deals with this
Framework is stained to appropriate colour
Zirconia is then veneered with feldspathic porcelain to produce the final restoration
What are the problems with zirconia cored crowns?
Expensive equipment required
Potential for veneering porcelain to deboned from core
Zirconia core is opaque
Inert fitting surface, cannot etch or bone
What are the different types of milled core crowns and bridges?
Zirconia
Lithium disilicate
Precious metal
Non-precious metal
Titanium
Composite
How are milled crowns fabricated?
Cast goes into scanner
Crown is designed on CAD machine
File is saved and sent to milling machine
Polish
Cement
Describe how cast and pressed ceramics are made
The restoration is waxed up like as in a metal restoration
Invested
Cast from a heated ingot of ceramic
No sintering occurs
Once de vested and cleaned the restoration is heated to improve its crystal structure and produce crack inhibiting crystals
This process is called ceraming
What ceramics are used in cast and pressed ceramics?
Glass ceramics:
Lithium disilicate glass
Leucine reinforced glass
What are the stages of ceraming?
Stage 1 - crystal formation - maximum number of crystal nuclei are formed
Stage 2 - crystal growth to maximise the physical properties
Crystal phase of the ceramic can approach 100%
What are the advantages of the different crown types?
Monolithic block crowns milled from a single block of material are the strongest
Zirconia crowns are stronger than LiDiSi
LiDiSi have better translucency and so better aesthetics
Crowns with layered porcelain rather than just stained monolithic block have better aesthetics
Layered crowns are ore likely to chip due to stresses
Compare sintered and milled crowns
For the same paternal a milled crown will be stronger than a build up or pressed crown
As aesthetics of blocks of ceramic improve these will become the most commonly used crown
Which crowns should be used in posterior teeth?
Monolithic zirconia - can be used for single crowns and shorter span bridges
Which crowns should be used for aesthetical anterior crowns?
LiDiSi - can probably use as far back as first premolar
What crown should be used for anterior bridgework?
LiDiSi - short span with no parafunction
What crown should be used for a longer span bridge or if heavier occlusion?
Zirconia cored with zirconia where occlusal contacts will meet
What are luting crowns?
When zirconia or LiDiSi crowns are cemented with conventional or resin cements
