Dental Ceramics Flashcards
What is porcelain a type of?
ceramic
What is removed from dental ceramics and what replaces it?
Dental Ceramics need to be translucent so Kaolin (opaque) is removed and feldspar and silica replace it
What. material are dental ceramics?
glasses
What components is feldpathic porcelain made of?
- feldspar
- borax
- silica
- metallic oxides
What does feldspar do?
Acts as a flux
Lowers the fusion and softening temperature of the glass
It is the lowest fusing component and flows during firing forming a solid mass around the other components
What form are conventional dental ceramics?
what do they require?
powder
mixed with distilled water and built up into the restoration
How is the powder made?
The powder is made by heating the constituents to a high temperature >1000oC
Cool rapidly (Fritting)
In water creating cracks and crazing of the ceramic mass
Mill the Frit to a fine powder
Add binder (often starch)
What happens to feldspathic ceramics when heated to 1150-1500 degrees?
form leucite (potassium aluminium silicate)
What phase is leucite formed and what does this determine?
This forms around the glass phase of the ceramic.
Gives a powder of known physical and thermal properties.
No further chemical reaction is required during fabrication of the restoration
The powder melts together to form the crown
How is the crown fabricated?
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
These are not tooth coloured
The crown is heated in a furnace to coalesce the powder into ceramic
What is sintering?
Heating leads to SINTERING
This occurs just above the glass transition temperature
It is when the ceramic particles begin to fuse into a single mass.
During sintering the glass phase softens and will coalesce
Over time there is controlled diffusion and a solid ceramic mass is formed
What happens to the material during sintering?
contrcts by 20%
What are the favourable properties of conventional dental ceramics?
- smooth, translucent aesthetics
- less susceptible to staining
- chemically stable - unaffected by ph
- good biocompatibility
- similar thermal properties to tooth
- dimensional stability (except during fabrication
- high compressive strength
- high hardness
What are the negative mechanical properties of conventional feldspathic ceramics?
- Tensile strength – very low
- Flexural strength – very low
- Fracture toughness – very low
- Static fatigue
- Surface micro-cracks
- Slow crack growth
- Brittle
Where should conventional feldspathic ceramics only be used?
only anterior crowns
low stress areas
How does alumina core work?
why can it only be used as a core
Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
Aluminous porcelain is opaque and can only be used as a core material
How are conventional ceramics made stronger?
metal coping
using
alumina core
zirconia core
How much alumina is in a conventional aluminous core?
max 50% alumina
What covers the core?
All of these core types are then veneered with conventional feldspathic porcelain to produce the final crown
What is the problem of alumina cores?
lack of flexural stregth
cannot be used for posterior crowns or bridgework
When does zirconia powder sinter?
heated to over 1600
Why is zirconia stabilised with yttria?
pure zirconia can crack on cooling
What does more yttria mean?
The more Yttria the more translucency
More Yttria reduces the physical properties
What type of crystal is normal zirconia at room temperature?
monoclinic crystal
What type of crystal is yttria stablised zirconia?
tetragonal
What happens when a crack begins in yttria stablised zirconia?
If a crack begins when the stress at the crack tip reaches a critical level the crystal structure transforms to the monoclinic structure
This causes a slight expansion of the material and closes up the crack tip
How is a zirconia core fabricated
steps
Impression is taken of the preparation and sent to the lab
A model is cast and then scanned digitally
Raw Zirconia block is selected for milling
A presintered block is much easier to mill
Milling for a three unit bridge will take around an hour
The cut framework is then heat treated at around 850oC to achieve its final physical properties
This causes a 20% shrinkage but the computer softwear deals with this during the milling process.
The framework is also stained to an appropriate colour
The Zirconia core is then veneered with feldspathic porcelain to produce the final restoration
What ziconia core is used in GDH?
zerion LT
What are disadvantages of zirconia cored crowns?
Expensive equipment required
Potential for veneering porcelain to debond from core
Zirconia core is opaque ? Are aesthetics much better than metal ceramic
Inert fitting surface, cannot etch or bond
BUT
Once you have the equipment, they are cheaper to make
Cost of metal is increasing
Fit is generally excellent
Why do ceramics all have a surface sintered layer?
best aesthetics
What materials have the same fabrication method?
steps
Zirconia
LiDiSi
Metal
Ceramic filled composite resin
What is a milled crown?
a type of dental crown made using a computerized milling machine (CAD/CAM) to cut a restoration from a solid block of material
What are the steps of a milled crown fabrication?
Cast goes into scanner
Scanned image of cast
Lower cast is scanned and ‘articulated
Select crown margin
Adjust crown margin
Select crown type and place on ‘model
Select crown type and place on ‘model
Adjust shape and size of selected crown
Save file
Send to milling machine
Can be anywhere in the world (GDH mainly go to Spain)
30 – 40 minutes you have your crown
In GDH return from Spain takes 48hrs
Still requires final finishing (polisj
In GDH this is still done on a plaster model
What is cast and pressed ceramic technique?
ceraming process
A different technique more like casting a metal restoration
The restoration is waxed-up, as you would for a metal restoration
Invested
Cast from a heated ingot of ceramic (1100oC)
No sintering occurs the ceramic ingot is already fully condensed prior to firing.
Once devested and cleaned the restoration is heated to improve its crystal structure producing crack inhibiting crystals.
This process is called CERAMING
How are cast crowns prepped aesthetically?
The cast crown can be stained
More often it is cut back labially and veneered with appropriate feldspathic porcelains
What ceramic types are used for cast/pressed?
glass- ceramics.
Lithium Disilicate Glass
Leucite Reinforced Glass
What are the 2 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%
In casr and pressed ceramics, what prevents crack propatation and increases fracture toughness?
Lithium disilicate glasses have a unique needle-like crystals
This makes crack propagation through this material very difficult → Good Fracture toughness
What are advantages of different crown types?
Monolithic block crowns, milled from a single block of material are strongest
Zirconia based crowns are stronger than LiDiSi
LiDiSi have better translucency hence better aesthetics
Crowns with layered porcelain rather than just stained monolithic block have better aesthetics
Why is layered porcelain bad for strength?
Layered crowns are more likely to chip due to stresses between core and veneer
Sintered vs Milled?
For the same material a milled crown will be stronger than a built up or pressed crown.
The block will have been subjected to the ideal heat treatments to maximise its properties and all blocks will be consistent
As aesthetics of ‘blocks’ of ceramic improve these will become the most commonly used crown.
Already acceptable in posterior teeth
What type of crown should you use for posterior teeth?
single crowns/shorter span bridge
single crowns/shorter span bridges
What type of crown should you use for anterior single teeth/ short span bridgework where aesthetics are most important?
LiDiSi
Can probably use as far back as first premolar
What type of crown should you use for anterior heavier occlusion/ long span bridgework where aesthetics are most important?
Zirconia cored with zirconia where occlusal contacts will meet
What can both zirconia and lididi crowns be cemented with?
conventional or resin cements
How are LiDiSi crowns bonded to the tooth?
Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface (LiDSi crowns)
This etched surface can be bonded to, using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent and resin cement
ID
How are zirconia crowns bonded to the tooth?
Zirconia cored crowns do not contain silica and are not affected by acid but can be air abraded to create retentive surface
They are strong enough to be self supporting and can be luted with a conventional dental cement.
Some evidence of bonding to 10-MDP containing bonding agents if resin cement is to be used
What colour are metallic oxides?
Chromium - Green
Cobalt - blue
Copper - Green
Iron - brown
Manganese - lavender
Nickel - Brown
