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BDS3 - dental material science > dental ceramics > Flashcards

dental ceramics Flashcards

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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
Q

what is the fracture toughness of conventional dental ceramics

A
  • very low

- pushing on tooth from the side could fracture

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26
Q

what is the static fatigue of conventional dental ceramics

A
  • 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
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27
Q

what are surface micro-cracks

A
  • 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
Q

how does slow crack growth occur

A
  • cyclic fatigue under occlusal forces in a wet environment over time
  • smaller loads over a long period of time restoration will fail
29
Q

when can conventional dental ceramics used

A
  • only in low stress areas

- anterior crowns

30
Q

how can we overcome the problems with conventional dental ceramics

A
  • 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
Q

what are the options for a strong coping

A
  • metal coping = porcelain fused alloys
  • alumina core
  • zirconia core
32
Q

where are alumina core ceramics used

A
  • core material in PJC’s
33
Q

what is the flexural strength in alumina core

A
  • double that of feldspathic porcelain

- >120MPA

34
Q

how do alumina cores have a higher flexural strength

A
  • 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
Q

why can alumina porcelain only be used as a core

A
  • because it is opaque
36
Q

where are alumina core ceramics used

A
  • anterior teeth, not strong enough for posterior use
37
Q

what is the prep needed for alumina core ceramic

A
  • possibly more palatal reduction required than in a MCC, but less labial
  • maybe 1mm all round
38
Q

why are alumina core crowns good

A
  • relatively cheap to make

- no specialist equipment, just a furnace

39
Q

what happens if we increase alumina content

A
  • increases the strength
  • INCERAM
  • PROCERA
40
Q

what is INCERAM

A
  • 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
Q

what is slip casting

A
  • 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
Q

what is PROCERA

A
  • 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
Q

how is the final crown made with INCERAM and PROCErA

A
  • crowns are then veneered with conventional feldspathic porcelain
  • used for single posterior crowns = not for bridgework
44
Q

what are zirconia core crowns

A
  • most popular
  • zirconia is a naturally occurring dioxide
  • very hard
  • used in jewellery as imitation diamonds
45
Q

at what temperature does zirconia powder need to reach to sinter

A
  • 1600 degrees
46
Q

what zirconia do we use in dentistry

A
  • Yttria-stabilised zirconia

- pure zirconia would crack on cooling

47
Q

what is Yttria stabilised zirconia

A
  • very small amounts of Yttria are present in the material <1%
  • it is a tetragonal crystal structure (normal zirconia is a monoclinic crystal)
48
Q

how is Yttria stabilised zirconia strong, tough (1000 MPA) and hard

A
  • 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
Q

how is a zirconia crown made

A
  • 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
Q

why is it easier to mill a presintered block of zirconia

A
  • needs to be hard enough to cut, but not fully sintered as then it would be too hard to cut
51
Q

what are some zirconia systems

A
  • LAVA from 3M
  • IPS e.max Zir Cad
  • opalite
  • Zerion
  • Everest ZH
52
Q

what are the problems with zirconia cored crowns

A
  • 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
Q

how can zirconia core debond from the veneering porcelain

A
  • 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
Q

what is the fit of a zirconia cored crown

A
  • generally excellent
55
Q

what materials are used for milled core crowns and bridges

A
  • zirconia
  • lithium disilicate
  • precious metal
  • non-precious metal
  • titanium
  • ceramics all have a sintered layer for best aesthetics
56
Q

why will a milled crown be stronger than built up or pressed crown

A
  • the block will have been subjected to the ideal heat treatments to maximise its properties and all blocks will be consistent
57
Q

what are the blocks fo ceramic like

A
  • 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
Q

what is better about a hand-layered crown to a milled crown

A
  • aesthetics are better
59
Q

how are milled crowns fabricated

A
  • 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
Q

what is cast and pressed ceramics

A
  • 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
Q

what are glass ceramic

A
  • lithium disilicate

- leucite reinforced glass

62
Q

what are the 2 stages of ceraming

A
  • 1 = crystal formation maximum number of crystal nuclei are formed
  • 2 = crystal growth to maximise physical properties
63
Q

what is the ideal crystal you want

A
  • 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
Q

what does lithium disilicate have that makes crack propagation through it hard

A
  • needle-like crystals
  • gives it good flexural strength
  • but not as good as zirconia
65
Q

how do you lute crowns

A
  • 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
Q

how may tooth preparation have to change depending on type of restoration

A
  • 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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