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BDS3 - dental material science > dental ceramics > Flashcards

dental ceramics Flashcards

1
Q

what do decorative ceramics contain

A
  • kaolin
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2
Q

what is kaolin

A
  • it is a clay
  • hydrated aluminium silicate
  • opaque
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3
Q

why is kaolin removed from dental ceramics

A
  • because they need to be translucent, and kaolin is opaque
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4
Q

what is kaolin replaced by in dental ceramics

A
  • feldspar and silica
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5
Q

what is the composition of decorative ceramics

A
  • kaolin = 50+%
  • quartz (silica) = 15-25%
  • feldspar = 15-25%
  • metal oxides = <1%
  • glass = 0
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6
Q

what is the composition of dental ceramics

A
  • kaolin = <5%
  • quartz (silica) = 12-25%
  • feldspar = 70-80%
  • metal oxides = 1%
  • glass = up to 15%
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7
Q

what is potash feldspar

A
  • potassium aluminium silicate
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8
Q

what is soda feldspar

A
  • sodium aluminium silicate
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9
Q

what does feldspar fo

A
  • acts as a reflux
  • lowers the fusion and softening temperature of the glass
  • binds the other parts together once you’ve heated them
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10
Q

what colours do different metals oxides give

A
  • chromium = green
  • cobalt = blue
  • copper = green
  • iron = brown
  • manganese = lavender
  • nickel = brown
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11
Q

how are conventional dental ceramics made

A
  • supplied as powder
  • powder is made by heating the constituents to a high temperature >1000 degrees
  • cool rapidly = called fritting
  • mill the frit to a fine powder
  • add binder = starch
  • powder mixed with distilled water and built up into the restorations = something that looks like wet sand
  • powder then melts together to form the crown
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12
Q

what do feldspathic ceramics form whe heated to 1150-1500 degrees

A
  • leucite
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13
Q

what is a leucite

A
  • potassium aluminium silicate
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14
Q

how is the crown made

A
  • 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 = dentine in pink, enamel purple
  • not tooth coloured until they have bene fired
  • crown is heated in a furnace to coalesce the powder into ceramic
  • heating leads to sintering
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15
Q

what is sintering

A
  • 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
  • change from dry powdery mass to a block of ceramic
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16
Q

how much contraction occurs during sintering

A
  • about 20%

- technician makes it 20% too big to account for this

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17
Q

what are the aesthetic properties of conventional dental ceramics

A
  • best
  • colour is stable
  • very smooth surface
  • retain their surface better than other materials
  • reflectance
  • translucency
  • opacity
  • transparency
  • opalescence
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18
Q

how chemically stable are conventional dental ceramics

A
  • very stable
  • generally unaffected by the wide pH range found in the mouth
  • do not take up stain
  • good biocompatibility
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19
Q

what are the thermal properties of conventional dental ceramics

A
  • similar to tooth substance
  • coefficient of thermal expansion is similar to dentine
  • thermal diffusivity is low = protects underlying pulp
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20
Q

how dimensionally stable are conventional dental ceramics

A
  • once fully fired it is very stable

- shrinkage occurs during fabrication which is accounted for by technician

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21
Q

what is the compressive strength of conventional dental ceramics

A
  • very strong
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22
Q

what is the hardness of conventional dental ceramics

A
  • high

- can lead to abrasion of the opposing teeth is not glazed

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23
Q

what is the tensile strength of conventional dental ceramics

A
  • very low
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24
Q

what is the flexural strength of conventional dental ceramics

A
  • very low
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25
what is the fracture toughness of conventional dental ceramics
- very low | - pushing on tooth from the side could fracture
26
what is the static fatigue of conventional dental ceramics
- time dependents decrease 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
27
what are surface micro-cracks
- these can occur during manufacture, finishing or due to occlusal wear - these are areas where fractures can initiate - makes material more prone to fracture - there is the potential for the crack to propagate
28
how does slow crack growth occur
- cyclic fatigue under occlusal forces in a wet environment over time - smaller loads over a long period of time restoration will fail
29
when can conventional dental ceramics used
- only in low stress areas | - anterior crowns
30
how can we overcome the problems with conventional dental ceramics
- produce a strong, coping, resistance to fracture, and cover in conventional porcelain - cast or press a block of harder ceramic - mill a lab prepared block of ceramic
31
what are the options for a strong coping
- metal coping = porcelain fused alloys - alumina core - zirconia core
32
where are alumina core ceramics used
- core material in PJC's
33
what is the flexural strength in alumina core
- double that of feldspathic porcelain | - >120MPA
34
how do alumina cores have a higher flexural strength
- alumina particles act as crack stoppers preventing cracks from propagating though the material and causing fracture - cracks can go through or round th alumina particles, and either way, will result in higher flexural strength and fracture toughness
35
why can alumina porcelain only be used as a core
- because it is opaque
36
where are alumina core ceramics used
- anterior teeth, not strong enough for posterior use
37
what is the prep needed for alumina core ceramic
- possibly more palatal reduction required than in a MCC, but less labial - maybe 1mm all round
38
why are alumina core crowns good
- relatively cheap to make | - no specialist equipment, just a furnace
39
what happens if we increase alumina content
- increases the strength - INCERAM - PROCERA
40
what is INCERAM
- core material has an alumina content of around 85% (as opposed to 50% in normal) - complicated technique = slip casting - INCERAM-spinel has MgAL2O4 rather than alumina as its core material = better aesthetics but lower flexural strength - INCERAM-zirconia has 33% zirconia replacing alumina in core = higher strength but poor aesthetics
41
what is slip casting
- the ceramic core is formed onto a refractory model - fine slurry of alumina is applied to the model - it is heated to 1120 degrees for 10 hours - partial sintering occurs (because temperature is below glass transition temperature) - a porous core is produced - core is infiltrated with lanthanum glass at 1100 degrees - gives a high strength ceramic of >400MPA
42
what is PROCERA
- pure alumina core >99% - even more complicated process - core is made centrally not in every lab = send away for it - a fully densified alumina core is produced at around 1700 degrees - high flexural strength >700MPA - possible better translucency that glass infiltrated core
43
how is the final crown made with INCERAM and PROCErA
- crowns are then veneered with conventional feldspathic porcelain - used for single posterior crowns = not for bridgework
44
what are zirconia core crowns
- most popular - zirconia is a naturally occurring dioxide - very hard - used in jewellery as imitation diamonds
45
at what temperature does zirconia powder need to reach to sinter
- 1600 degrees
46
what zirconia do we use in dentistry
- Yttria-stabilised zirconia | - pure zirconia would crack on cooling
47
what is Yttria stabilised zirconia
- very small amounts of Yttria are present in the material <1% - it is a tetragonal crystal structure (normal zirconia is a monoclinic crystal)
48
how is Yttria stabilised zirconia strong, tough (1000 MPA) and hard
- 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 - strong enough to use as bridgework
49
how is a zirconia crown made
- impression taken of prep and sent to lab - model is cast then scanned digitally - software creates a bridge substructure on virtual preparation - minimum thickness of connectors is determined and fabricated - raw zirconia block is selected for milling = presintered block easier - cut framework is then heat treated at around 850 degrees - causes 20% shrinkage - framework also stained to appropriate colour - core is then veneered with feldspathic porcelain to produce final restoration
50
why is it easier to mill a presintered block of zirconia
- needs to be hard enough to cut, but not fully sintered as then it would be too hard to cut
51
what are some zirconia systems
- LAVA from 3M - IPS e.max Zir Cad - opalite - Zerion - Everest ZH
52
what are the problems with zirconia cored crowns
- expensive equipment needed - potential for veneering porcelain to debond from core - zirconia core is opaque - inert fitting surface - cannot etch or bond, can't create a retentive surface
53
how can zirconia core debond from the veneering porcelain
- the zirconia and the feldspathic porcelain have different rates of expansion and contraction, so can have areas of air form between them - so can get chipping of porcelain in the mouth
54
what is the fit of a zirconia cored crown
- generally excellent
55
what materials are used for milled core crowns and bridges
- zirconia - lithium disilicate - precious metal - non-precious metal - titanium - ceramics all have a sintered layer for best aesthetics
56
why will a milled crown be stronger than 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
57
what are the blocks fo ceramic like
- not one colour - darker cervical and more translucent as you move up - decide where in that colour you want the crown to be cut
58
what is better about a hand-layered crown to a milled crown
- aesthetics are better
59
how are milled crowns fabricated
- take impression and cast in the usual way - cast goes into the 3D scanner - get a scanned image of cast - lower cast also scanned so they can be articulated - select the crown margin = need to go round and draw where you want it to be - adjust the margin id need be - select crown type and place on model = machine finds the best fit - save file - send to milling machine = in Spain for GDH - final finishing done on plaster model
60
what is cast and pressed ceramics
- different technique more like casting a metal restoration - alternative to milling - restoration is waxed-up and invested - cast from a heated ingot of ceramic 1100 degrees - no sintering occurs - once devested and cleaning, restoration is heated to improve its crystal structure - process is called CERAMING - cast crown is then stained to look more real - sometimes cut back labially and veneered with feldspathic porcelain
61
what are glass ceramic
- lithium disilicate | - leucite reinforced glass
62
what are the 2 stages of ceraming
- 1 = crystal formation maximum number of crystal nuclei are formed - 2 = crystal growth to maximise physical properties
63
what is the ideal crystal you want
- strong materials have small crystal size, and high-volume fraction of crystal - want high number of crystals, but want the crystal to be small
64
what does lithium disilicate have that makes crack propagation through it hard
- needle-like crystals - gives it good flexural strength - but not as good as zirconia
65
how do you lute crowns
- any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface = etched surface can be bonded to using silane coupling agent - zirconia cored crowns do not contain silica and are not affected by acid = luted with conventional dental cement
66
how may tooth preparation have to change depending on type of restoration
- they will have to be different if doing a milled restoration instead of a cast restoration

BDS3 - dental material science (9 decks)

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