Ceramic Flashcards
What feature of decorative ceramic is not ideal for a dental material
it is opaque
we want it to be translucent
How do we make ceramic translucenct
kaolin is removed and feldspar and silica replaces
What is the composition of dental ceramics
<5% kaolin 12-25% quartz (silica) 70-80% feldspar 1% metal oxides up to 15% glass
What is feldspar made of
potash feldspar (potassium alumina silicate)
soda feldspar (sodium alumina silicate)
What is the function of feldspar
it acts as a ‘flux’ which is something that promotes melting
it lowers the fusion and softening temperature of the glass
it is the lowest fusing components and flows during firing forming a solid mass around the other components
What are the components in dental ceramics
feldspar
borax
silica
metallic oxides
What are the function of metallic oxides
they convey color to the ceramic
What color does chromium give to ceramic
green
What color does cobalt give to ceramic
blue
What color does copper give to ceramic
green
What color does iron give to ceramic
brown
What color does manganese give to ceramic
lavender
What color does nickel give to ceramic
brown
How is the powder in conventional dental ceramic made
the constituents are heated in the oven resulting in a glassy mass which is cooled rapidly (fritting) in water and it creates cracks and crazing of the ceramic mass
it is then ground into a fine powder and a binder (often starch) is added
the powder is then given to the technician and is mixed with distilled water and built up into the restoration
When you make a crown from conventional ceramic what do you have to do
heat it again
What do feldspathic ceramics form when heated
when heated to 1150-1500 leucite is formed
What is leucite
potassium aluminum silicate
What does leucite form
it forms the glass phase of the ceramic and it gives a powder of known physical and thermal properties
the powder melts together to form the crown in what’s called ‘sintering’
How is the crown fabricates from conventional dental ceramic
powder is mixed with water and applied to the die with a brush
crown is built up using different porcelains for dentine and enamel but these are not tooth colored
the crown is heated in a furnace to coalesce the powder into ceramic
the heating leads to sintering and this occurs just above the glass transition temperature
during sintering the glass phase softens and will coalesce
overtime there is controlled diffusion and a solid ceramic mass is formed
What is sintering
it is when the ceramic particles begin to fuse into a single mass
during sintering the material contracts by about 20%
considerable skill is required by the technician to judge the contraction in 3D
How are the aesthetic properties of conventional dental ceramics
they have the best aesthetic properties
colour stable
very smooth surface and retain them better than other materials meaning less staining over time
What are the optical properties of conventional dental ceramics
reflectance translucency opacity transparency opalescence
How is the chemical stability of conventional dental ceramics
very stable
generally unaffected by the wide pH range found in the mouth
do not take up stain from food and drink
How is the biocompatibility of conventional dental ceramics
minimal adverse effects on biological tissues
What are the thermal properties of conventional dental ceramics
similar to tooth
coefficient of thermal expansion is similar to dentine
thermal diffusivity is low meaning it can protect the pulp from hot and cold
Why is the coefficient of thermal expansion being similar to dentine a positive thing
results in low stresses to the restoration in the mouth during use
How is the dimensional stability of dental ceramics
once fully fired the material is v 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
How is the compressive strength of conventional ceramic
high
How is the hardness of conventional ceramic
high
can lead to abrasion of opposing teeth especially if not glazed
How is the tensile strength of conventional ceramic
very low
can lead to failure during loading
How is the flexural strength of conventional ceramic
very low
can lead to failure during loading
How is the fracture toughness of ceramic
very low
can lead to failure during loading
What is static fatigue
time dependent decrease in strength even in absence of any applied load
Why does static fatigue occur in conventional ceramic
it is probably due to the hydrolysis of Si-O groups within the material, over time in an aqueous environment
What are surface micro cracks and how do they occur
can occur during manufacturing, finishing or due to occlusal wear
these are areas where fractures can initiate
What is slow crack growth
cyclic fatigue under occlusal forces in a wet environment over time
smaller loads over a long period of time can cause cracks to propagate
What effect do the mechanical properties of conventional ceramic on its use
can only be used in low stress areas
only in anterior crowns
not in all patients (depends on biting forces - class II have high biting forces anteriorly)
too brittle for use elsewhere
How are the problems with conventional ceramics overcome
want to 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 fo ceramic
What can the strong coping be made of
metal
aluminia core
zirconia core
What is alumina core
it is used as a core material in PJCs
What is the effect of using an alumina core on the flex strength
double that of feldspathic porcelain (>120Mpa) which is not high but better
How do the alumina particles help improve strength of the ceramic
they act as crack stoppers preventing cracks propagating through the material causing fracture
Why can aluminous porcelain only be used as a core material
it is opaque
What are the disadvantages of alumina core
not strong enough for posterior use
aesthetics excellent but enough room was required for aluminous core and feldspathic layers above meaning more tooth prob
possibly more palatal reduction required than a metal ceramic crown but less labial reduction is required as the prep is about a mm all the way around
relatively cheap to make
no specialist equipment is required, just a furnace
What % of alumina is in conventional aluminous cores
max of 50%
increased alumina content increases the strength resulting in new techniques called INCERAM and PROCERA
What is the alumina content of an inceram crown
the core material has an alumina content of around 85%
What is the technique used for IN-CERAM
slip casting
Describe the slip casting technique
the ceramic core is formed onto a refractory model
a fine slurry of alumina is applied to the model and heated to 1120 degrees for 10 hrs which is below the glass transition temp for alumina
partial sintering occurs and a porous core is produced
the core is infiltrated with lanthanum glass at 1100 degrees
this results in a high strength ceramic with >400 MPA flexural strength
What is in-ceram spinel
has spinel rather than alumina as its core material which provides better aesthetics as its more translucent but lower flex strength
What is in-ceram zirconia
has 33% zirconia replacing alumina in core
higher strength, poorer aesthetics
What is procera
has a pure alumina core and its 99% pure
it is even more complicated
the core is made centrally not in every lab
a fully densified alumina core is produced at around 1700
it has a high flexural strength >700 MPA
has possibly better translucency than glass infiltrated core
What happens to the different alumina core types
both of these core types are then veneered with conventional feldspathic porcelain to produce the final crown
both core types are probably suitable for single posterior but not for bridges
What is zirconia
zirconium dioxide
naturally occurring
it occurs in different forms at different temperatures
very hard
At what temperature does zirconia powder sinter
1600
What is the zirconia used in dentistry
Yttria stabilized zirconia
What can happen to pure zirconia on cooling
it can crack
How much Yttria is present in the material
<1%
What is Yttria
tetragonal crystal structure
What happens if a crack begins in the yttrium stabilized zirconia
if it 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 results in a material that’s strong enough to use as a bridge
Describe the process of fabricating a zirconia core
impression taken for the prep and sent to the lab
model is cast and scanned digitally
soft ware unit creates a bridge substructure on a virtual preparations
minimum thicknesses of connectors are determined and fabricated
raw zirconia block is elected for milling
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
its stained
the zirconia core is veneered with feldspathic porcelain to produce the final restoration
What type of zirconia block is easier to mill
pre sintered
How long does milling for a 3 unit bridge take
1 hr
What are the problems with zirconia cored crowns
expensive equipment is required
potential for veneering porcelain to debunk from the core
zirconia core is opaque meaning wise aesthetics
inert fitting surface so cannot etch or bond
Why is there potential for veneering porcelain to de-bond from the zirconia core
they have different rates of expansion and contraction so there is weakness between the core and porcelain so when subjected to forces debunking can happen
What are the benefits f the zirconia core
once you have the equipment they’re cheaper to make as the cost of metal is increasing
fit is good
What are the milled core crowns and bridges
zirconia lithium disilicate precious metal non precious metal titanium
What is the benefit of ceramics vs the milled core crowns and bridges
all have a surface sintered layer for best aesthetics
Compare sintered vs milled for strength
for the same material a milled crown will be stronger than a built up/pressed one
the block will have been subjected to the ideal heat tx to maximize its properties and all blocks will be consistent
Compare sintered vs milled for aesthetics
aesthetics of hand layered crowns are better but the aesthetics of milled ones are getting better and you can stain them to improve aesthetics
Describe the process of fabricating a milled crown
- cast 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
- adjust shape and size of selected crown
- save file and send to milling machine
- crown requires finishing
Describe the process for a purely digital process
same but scan in the mouth and design on CAD machine then mill, polish, cement
Describe the process for cast and pressed ceramics
a different technique more like casting a metal restoration
- restoration waxed up as you would for metal
- invested
3, cast from a heated ingot of ceramic (1100 degrees) - no sintering occurs because the ceramic ingot is fully condensed prior to firing
- once de-vested 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 porcelain
What are the ceramics used in the cast and pressed ceramic processes called
they’re glass ceramics
lithium dislocate glass
leucite reinforced glass
Describe what happens in CERAMING
stage 1 - crystal formation maximum number of crystal nuclei are formed
stage 2 - crystal growth to maximize the physical properties
with some materials you get 100% crystal growth
What is the crystal size in strong materials
small crystal size and high volume fraction of crystals
makes it harder fo cracks to propagate through the material
What are the lithium disilicate crystals like
unique needle like crystals making crack propagation harder and giving it a good flexural strength
not as strong as zirconia but better aesthetics than mono block zirconia
How are the silica containing ceramics cemented
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
How are the zirconia cored crowns cements
they do not contain silica so are not affected by acid
strong enough to be self supporting and can be luted with a conventional dental cement
some new zirconias contain silica to increase ability to bond but pure silica can’t be bonded to tooth
What is translucency
Reflects light and makes it change direction but also allows transmits some of the light
What is reflectance
the measure of the proportion of light or other radiation striking a surface which is reflected off it.
What is opacity
not allowing light to pass through
What is transparency
transmits light
