Dental Ceramics Flashcards
Is dental ceramic porcelain?
All porcelain is ceramic but not all ceramic is porcelain
Decorative ceramics contain Kaolin, Why does dental ceramic not contain this?
- Because Kaolin is opaque and dental ceramics need to be translucent so Kaolin is removed and feldspar and silica replace it
What are the components of dental ceramic? (5)
- Kaolin (<5%)
- Quartz (silica) (12-25%)
- Feldspar (70-80%)
- Metal oxides (1%)
- Glass (up to 15%)
What is in Feldspathic porcelain? (4)
- Feldspar
- Borax
- Silica
- Metallic oxides
What kinds of Feldspar are in Feldpathic porcelain and what is the purpose of them? (5)
- Potash feldspar (potassium alumina silicate)
- Soda feldspar (sodium alumina silicate)
- Acts as a reflux (it is the lowest melting of the various constituents)
- 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
Metal oxides convey colour to the ceramic. What colour does chromium convey?
- Green
Metal oxides convey colour to the ceramic. What colour does cobalt convey?
Blue
Metal oxides convey colour to the ceramic. What colour does copper convey?
Green
Metal oxides convey colour to the ceramic. What colour does iron convey?
Brown
Metal oxides convey colour to the ceramic. What colour does manganese convey?
Lavender
Metal oxides convey colour to the ceramic. What colour does nickel convey?
Brown
What are conventional dental ceramics supplied as?
Supplied as powder
How is the conventional dental ceramic powder made?
- The powder is made by heating the constituents to a high temperature > 1000 degrees
How do we make the powder of conventional dental ceramics? (5)
- The powder us made by heating the constituents to a high temperature >1000 degrees
- Cool rapidly (fritting) (in water created cracks and crazing of the ceramic mass)
- Mill the frit to a fine powder
- Add binder (often starch)
- The powder is mixed with distilled water and built up into the restoration
Conventional dental ceramics come in different shades and come in shades like dentine and shades like enamel. However, All the dentine ones come in the same colour, irrespective of the shade they are. What colour is this?
- Pink
Conventional dental ceramics come in different shades and come in shades like dentine and shades like enamel. However, All the enamel ones come in the same colour, irrespective of the shade they are. What colour is this?
- Blue
What happens when we come to make the crown out of a conventional dental ceramic? (6)
- Feldspathic ceramics form leucite when heated to 1150-1500 degrees
- Leucite is potassium aluminium silicate
- This forms around the glass phase of the ceramic
- Gives a powder of known physical and chemical properties
- No further chemical reaction is required during fabrication of the restoration
- The powder melts together to form the crown
What is sintering?
- Sintering is a heat treatment process in which a large quantity of loose aggregate material is subjected to a sufficiently high temperature and pressure to cause the loose material to become a compact solid piece. The amount of heat and pressure administered during the sintering process is slightly less than the material’s melting point.
How do we fabricate a crown using conventional dental ceramics? (4)
- 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 in heated in a furnace to coalesce the powder into ceramic
When during the fabrication of a crown using conventional dental ceramics does sintering occur?
- 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
During sintering of conventional dental ceramics, how much does the material contract?
- Contracts by about 20%
- So considerable skill is required by a technician to judge the contraction in 3D
Describe the basic process of sintering? (3)
- Start off with dry particles together
- You heat everything up and as you heat things up they begin to come together
- They begin to coalesce and as they do that the whole size of the thing shrinks down
What are the properties of conventional dental ceramics that we want to be concerned with? (6)
- Aesthetics
- Chemical stability
- Biocompatibility
- Thermal properties
- Dimensional stability
- Mechanical properties
Do dental ceramics have good aesthetic properties?
- Ceramics have the best aesthetic properties of any dental restorative material
What makes dental ceramics aesthetics so good?
- Colour stable
- Very smooth surface
- Retain their surface better than other materials -> less staining long term
What are the optical properties of ceramics? (5)
- Reflectance (will reflect light in a natural manner)
- Translucency (they allow some of the light to pass through)
- Opacity
- Transparency (light passes through and is unchanged)
- Opalescence (get a different colour from the material in reflected and transmitted light)
Are conventional dental ceramics chemically stable?
- Chemically very stable
- Generally unaffected by the side pH range found in the mouth
- Do not take up stain from food/drink
Are conventional dental ceramics biocompatible?
- They have good biocompatibility and minimal adverse effects on biological tissues
What are the thermal properties of conventional dental ceramics like?
- Similar to tooth substance
- Coefficient of thermal expansion is similar to dentine (results in low stresses to the restoration in the mouth during use)
- Thermal diffusivity is low (protects the pulp)
What is the dimensional stability of conventional dental ceramics like?
- Once fully fired the material is very stable
- During fabrication shrinkage is a problem and must be accommodated for by the technician
- Shrinkage of 20% during firing is normal for a conventional feldspathic ceramic crown
What is the compressive strength of conventional dental ceramics like?
- High compressive strength
What is the hardness of conventional dental ceramics like?
High hardness - can leas to abrasion of opposing teeth especially if not glazed
What is the tensile strength of conventional dental ceramics like?
- Very low (low strength when pushing from side to side)
What is the flexural strength of conventional dental ceramics like?
Very low
What is the fracture toughness of conventional dental ceramics like?
Very low - all lead to failure during loading
Static fatigue may happen in conventional dental ceramics. Explain this?
- Time dependant decrease in strength even in the absence of any applied load. probably due to hydrolysis of Si-O groups within the material, over time in an aqueous environment
Surface micro-cracks may occur in conventional dental ceramics. Explain this?
Can occur during manufacture, finishing or due to occlusal wear. These are areas where fractures can initiate
Slow crack growth may happen in conventional dental ceramics. Explain this?
- Cyclic fatigue under occlusal forces in a wet environment over time
Due to all of the problems just discussed this means that conventional ceramics can only be used in low stress areas. Where would this be?
- Only anterior crowns
- Not in all patients
- Too brittle to use elsewhere
The aesthetics of conventional ceramics are good but they need to be stronger. There are several ways to do this. What are these? (3)
- Produce a strong coping, resistant to fracture, and cover in conventional porcelain
- Cast or press a block of harder ceramic
- Mill a laboratory prepared block of ceramic
Using a metal coming is one of the most obvious ways to strengthen conventional ceramic. What is this and what are the alternatives? (3)
- End up with a tooth that looks like porcelain but has a metal core
Can use alumina or zirconia as a core too
What are alumina cores used in?
- Used as a core material in Porcelain jacket crowns
What is the flex strength of an alumina core?
- Flex strength double that of feldspathic porcelain >120Mpa
- So strength not high but it is better
How does alumina core reduce the risk of fracture?
- Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
What are the aesthetics like of alumina core?
- Aluminous porcelain is opaque and can only be used as a core material
- Cannot make a whole crown out of it because would end up with a creamy yellow tooth - which is not good for aesthetics
How do alumina particles work within alumina cores?
- Essentially you have a crack begins to propagate through your fedspathic porcelain and it then hits an alumina particle
- It then has a choice - it can go through the alumina particle - which will take more energy
- Or it can go around the alumina particle - which also takes more energy
And either of these results in a higher flexural strength and a higher fracture toughness of the material
Are alumina cores strong enough for posterior crown use?
No
Aesthetics of alumina core could be excellent but enough room was required for aluminous core and feldspathic layers above so quite a lot of tooth surface has to be removed and this leads to…?
- Possibly more palatal reduction required than in a metal ceramic crowns, but less labial reduction required (so end up with a prep that is about 1mm all the way round)
Are alumina cores cheap or expensive to make?
- Relatively cheap to make
- No specialist equipment required, just a furnace
How much alumina is in conventional aluminous cores?
- A maximum of 50% alumina
An increase alumina content in alumina cores increases the strength. There are 2 new techniques, what are these?
- INCERAM
- PROCERAM
How much alumina is in IN-CERAM?
- Core materials has an alumina content of around 85%
IN-CERAM has a complicated technique called slip casting, what is this? (8)
- The ceramic core is formed onto a refractory model
- A fine slurry of alumina is applied to the model
- It is heated to 1120 degrees for 10 hrs
- This is below the glass transition temperature for alumina
- Partial sintering occurs (so partially melts and partially comes together)
- A porous core is produced
- Core is infiltrated with lanthanum glass at 1100 degrees (the porosities are filled with this gas)
- High strength ceramic >400 MPA flexural strength
IN-CERAM-Spinel has spinel (AmAl2O4) rather than alumina as its core material. How does this change the properties?
- Better aesthetics but lower flex strength
IN-CERAM-Zirconia has 33% zirconia rather than alumina as its core material. How does this change the properties?
- Higher strength but poorer aesthetics
How much alumina is in PROCERA?
- Pure alumina core >99% pure
What is the process of PROCERA like?
- Very complicated
- Core is made centrally, not in every lab
A fully densified alumina core is produced at what temperature?
Around 1700 degrees
What is the flexural strength of PROCERA like?
High flexural strength >700MPA
What is the translucency like in PROCERA compared to glass infiltrated core?
- Possibly better
Both types of alumina core are then veneered with conventional feldspathic porcelain to produce the final crown. Where can this be placed in the mouth?
- Both core types are probably suitable for single posterior crowns
- Not often used as a bridge material
What is probably the most popular ceramic core material?
- Zirconia core
Give one feature of Zirconia?
- It is very hard
What made the use of Zirconia as a core material possible?
- CAD-CAM
At what temp does zirconia powder sinter?
- Zirconia powder does not sinter unless heated to over 1600 degrees
What is the zirconia used in dentistry?
- Yttria-stabalised zirconia (this is much less likely to crack)
- Pure zirconia can crack on cooling
How much Yttria is used to stabilise zirconia?
- Very small amounts of Yttria is present in the material to stabilise it = >1%
What is normal zirconia at room temp?
- It is a monoclinic crystal
What is Yttria?
A tetragonal crystal structure (we get this structure if we put Yttria into normal zirconia)
How does Yttria make zirconia really strong?
- If a crack begins when the stress at the crack tip reaches a critical level the crystal structure transforms to the monoclinic structure from tetragonal structure
- This causes a slight expansion of the material and closes up the crack tip (this makes it really strong)
Yttria stabilisation of zirconia makes a material which is very? (3)
- Hard
- Strong (1000MPA flexural strength)
- Tough
- Strong enough to use as a bridge framework
What is the process of fabricating a zirconia core?
- Impression is taken of the preparation and sent to the lab
- A model is cast and then scanned digitally
- Software unit created a bridge substructure on virtual preparations
- Minimum thickness of connectors are determined and fabricated
- Raw zirconia block is selected for milling
- A presintered block is much easier to mill (the zirconia is presintered to a certain extent - not fully sintered - so that it is hard enough to cut but is not as hard as completely sintered zirconia would be)
- Milling for a three unit bridge will take around an hour
- The cut framework is then heat treated at around 850 degrees to achieve its final physical properties
- This causes a 20% shrinkage but the computer software deals with this during the milling process
- The framework is also stained to an appropriate colour
- The zirconia is then veneered with feldspathic porcelain to produce the final restoration
What are the problems with zirconia cored crowns? (4)
- Expensive equipment required
- Potential for veneering porcelain to de-bond from core
- Zirconia core is opaque - aesthetics aren’t as great as you might hope
- Inert fitting surface, cannot etch or bond
What are the positives of zirconia cored crowns? (2)
- Once you have the equipment they are cheaper to make (cost of metal is increasing)
- Fit is generally excellent
You can mill cores but you can also mill crowns and bridges directly from a block of ceramic. We can do this in several materials. Give examples of these? (5)
- Zirconia
- Lithium disilicate
- Precious metal
- Non-precious metal
- Titanium
Ceramics all have a surface sintered layer for best aesthetics (so it has the best physical properties and the best aesthetics)
Look at the slide on sintered vs milled
Too hard to make into a question
Look at images of fabrication of a milled crown
Too hard to put into flashcards
How would we produce cast and pressed ceramics (process called ceraming)? (7)
- 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 (1100 degrees)
- 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
Cast and pressed crowns will come out as one colour. How do we make it tooth like? (2)
- The cast crown can be stained
- More often it is cut back labially and veneered with appropriate fedspathic porcelains
The ceramics used in cast and pressed ceramics are called glass-ceramics. Give 2 examples of these?
- Lithium Disilicate glass
- Leucite reinforced glass
What is ceraming? (4)
- Two stage process
- 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%
Do strong materials have large or small crystals?
- Strong materials have small crystal size and high volume fraction of crystals (makes it much more difficult for cracks to propagate through the material)
- Lithium disilicate glasses have a unique needle-like crystals
- This makes crack propagation through this material very difficult -> good flexural strength
What is a luting crown?
- Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface
- This etched surface can be bonded to, using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent
Are zirconia covered crowns luting crowns?
- Zirconia covered crowns do not contain silica and are not affected by acid
- They are strong enough to be self supporting and can be luted with a conventional dental cement
