Dental Ceramics Flashcards
Kaolin
in dental ceramics
all porcelain is
ceramic
but not all ceramic is porcelain
Kaolin is
clay
clay a.k.a
kaolin
hydrated aluminium silcate
opacity in clay
important for appearance of the final product (e.g. mug)
opacity in dental ceramics
need to be translucent
so Kaolin is removed and feldspar and silica replace it
composition of decorative ceramic
Kaolin 50+%
Quatz (silica) 15-25%
feldspar 15-25%
metal oxides <1%
glass 0
composition of dental ceramics
kaolin <5%
quartz (silica) 12-25%
feldspar 70-80%
metal oxides 1%
glass up to 15%
detal ceramics are classed as
glasses
feldpathic ‘porcelain’ PJC
feldspar types
potash feldspar (pottasium alumina silicate)
soda feldspar (sodium alumium silicate)
feldspar
acts as flus
lowers the fusion and softening temperature of glass
it is the lowest fusing component and flows during firing forming a solid mass around the other components (binds)
4 key components of dental ceramics (glasses)
- feldspar
- borax
- silica
- metallic oxides
what do metal oxides convey to the ceramic
colour
colour chromium conveys
green
colour cobalt conveys
blue
colour copper conveys
green
colour iron conveys
brown
colour maganese conveys
lavendar
colour nickel conveys
brown
conventional dental ceramics supplied as
powder
how is dental ceramic powder made
- heating 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
dental ceramic powder is mixed with
distilled water and built up into the restoration (wet sand)
what do feldspathic ceramics form when heated to 1150oC-1500oC?
leucite
leucite
potassium aluminum silicate
leucite forms
around the glass phase of ceramic
reason for leucite in dental ceramics
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
how are dental ceramic powders made into restorations?
- The powder is mixed with distilled water and built up into the restoration (wet sand)
- Feldspathic ceramics form leucite when heated to 1150-1500oC
- Leucite is potassium aluminium silicate
- 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
fabrication of a crown
- 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
- 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.
- Heating leads to SINTERING
- During sintering the glass phase softens and will coalesce
- Over time there is controlled diffusion and a solid ceramic mass is formed
- During sintering the material contracts by about 20%
- Considerable skill required by technician to judge the contraction in 3D
sintering
heating leads to sintering (crown in furnance to coalesce the powder)
- This occurs just above the glass transition temperature
It is when the ceramic particles begin to fuse into a single mass.
6 properties of conventional dental ceramics
- Aesthetics
- Chemical Stability
- Biocompatibility
- Thermal Properties
- Dimensional Stability
- Mechanical Properties
aesthetics of conventional dental ceramics
- Ceramics have the best aesthetic properties of any dental restorative material
- Colour Stable
- Very smooth surface
- Retain their surface better than other materials less staining long term
- Optical properties
- Reflectance
- Translucency
- Opacity
- Transparency
- Opalescence
chemical stability of conventional dental ceramics
- Chemically very stable
- Generally unaffected by the wide pH range found in the mouth
- Do not take up stain from food/drink
- Good BIOCOMPATIBILITY minimal adverse effects on biological tissues
thermal properties of conventional dental ceramics
- 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
- Protective of the remaining tooth.
dimensional stability of conventional dental ceramics
- 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
mechanical properties of conventional dental ceramics
- High compressive strength
- High hardness
- Can lead to abrasion of opposing teeth especially if not glazed
- Tensile strength – very low
- Flexural strength – very low
- Fracture toughness – very low
- All lead to failure during loading
- Static Fatigue
- 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. (small principle)
- Surface micro-cracks
- Can occur during manufacture, finishing or due to occlusal wear.
- These are areas where fractures can initiate – potential to get bigger and fail
- Slow crack growth
- Cyclic fatigue under occlusal forces in a wet environment over time
- All of these problems mean that conventional feldspathic ceramics can only be used in low stress areas.
- Only anterior crowns.
- Not in all patients
- Too brittle for use elsewhere
overcoming the problems with conventional ceramics
(improving the strength of the ceramic)
- Aesthetics are good but they need to be stronger
- 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
strong coping types
- Metal coping
- See DMS Porcelain-fused-alloys
- Alumina core
- Zirconia core
alumina core history
- Alumina reinforced feldspathic core ceramics since 1960s
- This type of core was used as the first choice crown for anterior teeth for decades
- It is not strong enough for posterior use
- Aesthetics could be excellent but enough room was required for aluminous core and feldspathic layers above.
- Possibly more palatal reduction required than in a metal ceramic crown, but less labial reduction required.
alumina core
properties
Flex strength double that of feldspathic porcelain >120Mpa
- 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
1st choice for anterior teeth
not strong enough for posterior use
alumina core
manufacturer
Aesthetics could be excellent but enough room was required for aluminous core and feldspathic layers above.
- Possibly more palatal reduction required than in a metal ceramic crown, but less labial reduction required.
Relatively cheap to make
- No specialist equipment required, just a furnace
Conventional Aluminous cores are a maximum of 50% alumina
- Increased alumina content increases the strength leading to new techniques
- INCERAM
- PROCERA
alumina content of aluminous cores
50%
increased alumina content increases the strength leading to new techniques
- INCERAM
- PROCERA
new techniques with increased alumina content in aluminous cores
INCERAM
PROCERA
uses of alumina cores
PJC
anterior teeth 1st choice
not strong enough for posterior
In-ceram
- Core material has an alumina content of around 85%
- Complicated technique (Slip Casting)
In-Ceram-Spinel has Spinel (MgAl2O4) rather than alumina as its core material
- Better aesthetics but lower flex strength
In-Ceram-Zirconia has 33% Zirconia replacing Alumina in core
- Higher strength but poorer aesthetics
in-ceram split casting
The ceramic core is formed onto a refractory model
A fine slurry of alumina is applied to the model
It is heated to 1120oC for 10 hrs
This is below the glass transition temperature for alumina
Partial sintering occurs
A porous core is produced
Core is infiltrated with lanthanum glass at 1100oC
HIGH STRENGTH CERAMIC >400 MPA flexural strength
in-ceram-spinel
- has Spinel (MgAl2O4) rather than alumina as its core material
- Better aesthetics but lower flex strength
in-ceram-zironica
- has 33% Zirconia replacing Alumina in core
- Higher strength but poorer aesthetics
in-ceram core material alumina content
85%
procera
- Pure Alumina core >99% pure
- Even more complicated
- Core is made centrally not in every lab
- A fully densified alumina core is produced at around 1700oC
- Core is made centrally not in every lab
- High flexural strength > 700MPA
- Possibly better translucency than glass infiltrated core.
alumina core types
in-ceram
procera
alumina core types veneered
with conventional feldspathic porcelain to produce the final crown
alumina core (in-ceram and procera) uses
single posterior crowns
not used often as bridge material
zirconica core
- Probably the most popular ceramic core material.
Zirconia (Zirconium Dioxide) is a naturally occurring mineral
- It occurs in different forms at different temperatures
Very hard
- Zirconia powder does not sinter unless heated to over 1600oC
- Pure zirconia can crack on cooling
zironica core uses
used extensively in jewellery - imitation diamonds
Until CAD-CAM was introduced the use of Zirconia as a core material would not have been possible
- The Zirconia used in dentistry is Yttria-stabilised zirconia
Yttria stabilisation of Zironica
used in dentistry
Very small amounts of Yttria is present in the material <1%
- Normal Zirconia is a monoclinic crystal at room temperature
- Yttria is a tetragonal crystal structure
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
This ability leads to a material which is very
- Hard
- Strong (1000MPA flexural strength)
- Tough
- Strong enough to use as a Bridge Framework
Yttria stabilisation of zirconica
properties
Hard
Strong (1000MPA flexural strength)
Tough
Strong enough to use as a Bridge Framework
Yttria
Yttria is a tetragonal crystal structure
- Normal Zirconia is a monoclinic crystal at room temperature
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
fabrication of a zirconia core
- Impression is taken of the preparation and sent to the lab
- A model is cast and then scanned digitally
- Software Unit Creates a bridge substructure on virtual preparations
- Minimum thicknesses of connectors are determined and fabricated
- 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
zirconia systems
- Most manufacturers have their own Zirconia system
- LAVA from 3M is probably the best known
- IPS e.max Zir CAD
- Opalite
- Zerion
zirconica cored crowns
problems (4)
- 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
benefits of zirconia cored crowns
Once you have the equipment they are cheaper to make
- Cost of metal is increasing
Fit is generally excellent
5 milled core crowns and bridges
- Zirconia
- Lithium Disilicate
- Precious metal
- Non-precious metal
- Titanium
sintered layer on ceramics
for better aesthetics
Sintered Vs Milled crowns
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 it’s properties and all blocks will be consistent
As aesthetics of ‘blocks’ of ceramic improve these will become the most commonly used crown. (darker cervically, more translucent at top – decide where in the block going to cut the crown; hand layered crowns better than milled but can dye milled to improve)
- Probably already acceptable in posterior teeth
fabrication of milled crowns
- Cast goes into scanner
- Scanned image of cast
- Lower cast is scanned and ‘articulated’
- Select crown margin
- adjust crown margins
- 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
- Can be anywhere in the world
- 30 – 40 minutes you have your crown In GDH return from Spain takes 48hrs
- Still requires final finishing
- In GDH this is still done on a plaster model
benefits of a truly digital workflow
can dispense with models and impressions.
- Scan in the mouth
- Design on CAD machine
- Mill
- Polish
- Cement
chairside scanning
Trios scanner in GDH
- Used mainly by postgraduates
- Digital Dentistry SSM
- Limited but increasing undergraduate use
Dentsply Sirona CAD-CAM
cast and presented ceramics
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
- The cast crown can be stained
- More often it is cut back labially and veneered with appropriate feldspathic porcelains
ceramics used in cast and present ceramics (2)
called glass-ceramics
- Lithium Disilicate Glass
- Leucite Reinforced Glass
ceraming
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%
strong material crystal size and volume
Strong materials have small crystal size and high volume fraction of crystals
crystals in lithium disilicate
Lithium disilicate glasses have a unique needle-like crystals
- This makes crack propagation through this material very difficult so good flexural strength
- possibility to use this material for bridgework ?
- can look better than block zirconia but may not be as strong
- possibility to use this material for bridgework ?
luting crowns
- 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
- Zirconia cored 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 – doesn’t need to rely on support of underlying tooth
etching ceramics
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
Zirconia cored crowns do not contain silica and are not affected by acid
- doesn’t need to rely on support of underlying tooth
sintering
produced by or subjected to sintering (the process of coalescing a powdered material into a solid or porous mass by means of heating without liquefaction).
coalesce
come together to form one mass or whole.
milled
reduced to fine particles by grinding in a mill.
