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BDS3 CLINICAL DMS > Ceramic > Flashcards

Ceramic Flashcards

1
Q

What feature of decorative ceramic is not ideal for a dental material

A

it is opaque

we want it to be translucent

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2
Q

How do we make ceramic translucenct

A

kaolin is removed and feldspar and silica replaces

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3
Q

What is the composition of dental ceramics

A 
<5% kaolin
12-25% quartz (silica)
70-80% feldspar
1% metal oxides 
up to 15% glass
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4
Q

What is feldspar made of

A

potash feldspar (potassium alumina silicate)

soda feldspar (sodium alumina silicate)

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5
Q

What is the function of feldspar

A

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

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6
Q

What are the components in dental ceramics

A

feldspar
borax
silica
metallic oxides

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7
Q

What are the function of metallic oxides

A

they convey color to the ceramic

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8
Q

What color does chromium give to ceramic

A

green

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9
Q

What color does cobalt give to ceramic

A

blue

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10
Q

What color does copper give to ceramic

A

green

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11
Q

What color does iron give to ceramic

A

brown

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12
Q

What color does manganese give to ceramic

A

lavender

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13
Q

What color does nickel give to ceramic

A

brown

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14
Q

How is the powder in conventional dental ceramic made

A

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

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15
Q

When you make a crown from conventional ceramic what do you have to do

A

heat it again

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16
Q

What do feldspathic ceramics form when heated

A

when heated to 1150-1500 leucite is formed

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17
Q

What is leucite

A

potassium aluminum silicate

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18
Q

What does leucite form

A

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’

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19
Q

How is the crown fabricates from conventional dental ceramic

A

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

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20
Q

What is sintering

A

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

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21
Q

How are the aesthetic properties of conventional dental ceramics

A

they have the best aesthetic properties
colour stable
very smooth surface and retain them better than other materials meaning less staining over time

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22
Q

What are the optical properties of conventional dental ceramics

A 
reflectance 
translucency
opacity
transparency
opalescence
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23
Q

How is the chemical stability of conventional dental ceramics

A

very stable
generally unaffected by the wide pH range found in the mouth
do not take up stain from food and drink

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24
Q

How is the biocompatibility of conventional dental ceramics

A

minimal adverse effects on biological tissues

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25
Q

What are the thermal properties of conventional dental ceramics

A

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

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26
Q

Why is the coefficient of thermal expansion being similar to dentine a positive thing

A

results in low stresses to the restoration in the mouth during use

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27
Q

How is the dimensional stability of dental ceramics

A

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

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28
Q

How is the compressive strength of conventional ceramic

A

high

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29
Q

How is the hardness of conventional ceramic

A

high

can lead to abrasion of opposing teeth especially if not glazed

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30
Q

How is the tensile strength of conventional ceramic

A

very low

can lead to failure during loading

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31
Q

How is the flexural strength of conventional ceramic

A

very low

can lead to failure during loading

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32
Q

How is the fracture toughness of ceramic

A

very low

can lead to failure during loading

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33
Q

What is static fatigue

A

time dependent decrease in strength even in absence of any applied load

34
Q

Why does static fatigue occur in conventional ceramic

A

it is probably due to the hydrolysis of Si-O groups within the material, over time in an aqueous environment

35
Q

What are surface micro cracks and how do they occur

A

can occur during manufacturing, finishing or due to occlusal wear

these are areas where fractures can initiate

36
Q

What is slow crack growth

A

cyclic fatigue under occlusal forces in a wet environment over time

smaller loads over a long period of time can cause cracks to propagate

37
Q

What effect do the mechanical properties of conventional ceramic on its use

A

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

38
Q

How are the problems with conventional ceramics overcome

A

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

39
Q

What can the strong coping be made of

A

metal
aluminia core
zirconia core

40
Q

What is alumina core

A

it is used as a core material in PJCs

41
Q

What is the effect of using an alumina core on the flex strength

A

double that of feldspathic porcelain (>120Mpa) which is not high but better

42
Q

How do the alumina particles help improve strength of the ceramic

A

they act as crack stoppers preventing cracks propagating through the material causing fracture

43
Q

Why can aluminous porcelain only be used as a core material

A

it is opaque

44
Q

What are the disadvantages of alumina core

A

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

45
Q

What % of alumina is in conventional aluminous cores

A

max of 50%

increased alumina content increases the strength resulting in new techniques called INCERAM and PROCERA

46
Q

What is the alumina content of an inceram crown

A

the core material has an alumina content of around 85%

47
Q

What is the technique used for IN-CERAM

A

slip casting

48
Q

Describe the slip casting technique

A

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

49
Q

What is in-ceram spinel

A

has spinel rather than alumina as its core material which provides better aesthetics as its more translucent but lower flex strength

50
Q

What is in-ceram zirconia

A

has 33% zirconia replacing alumina in core

higher strength, poorer aesthetics

51
Q

What is procera

A

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

52
Q

What happens to the different alumina core types

A

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

53
Q

What is zirconia

A

zirconium dioxide
naturally occurring
it occurs in different forms at different temperatures
very hard

54
Q

At what temperature does zirconia powder sinter

A

1600

55
Q

What is the zirconia used in dentistry

A

Yttria stabilized zirconia

56
Q

What can happen to pure zirconia on cooling

A

it can crack

57
Q

How much Yttria is present in the material

A

<1%

58
Q

What is Yttria

A

tetragonal crystal structure

59
Q

What happens if a crack begins in the yttrium stabilized zirconia

A

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

60
Q

Describe the process of fabricating a zirconia core

A

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

61
Q

What type of zirconia block is easier to mill

A

pre sintered

62
Q

How long does milling for a 3 unit bridge take

A

1 hr

63
Q

What are the problems with zirconia cored crowns

A

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

64
Q

Why is there potential for veneering porcelain to de-bond from the zirconia core

A

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

65
Q

What are the benefits f the zirconia core

A

once you have the equipment they’re cheaper to make as the cost of metal is increasing

fit is good

66
Q

What are the milled core crowns and bridges

A 
zirconia
lithium disilicate
precious metal
non precious metal 
titanium
67
Q

What is the benefit of ceramics vs the milled core crowns and bridges

A

all have a surface sintered layer for best aesthetics

68
Q

Compare sintered vs milled for strength

A

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

69
Q

Compare sintered vs milled for aesthetics

A

aesthetics of hand layered crowns are better but the aesthetics of milled ones are getting better and you can stain them to improve aesthetics

70
Q

Describe the process of fabricating a milled crown

A
  1. cast into scanner
  2. scanned image of cast
  3. lower cast is scanned and articulated
  4. select crown margin
  5. adjust crown margin
  6. select crown type and place on model
  7. adjust shape and size of selected crown
  8. save file and send to milling machine
  9. crown requires finishing
71
Q

Describe the process for a purely digital process

A

same but scan in the mouth and design on CAD machine then mill, polish, cement

72
Q

Describe the process for cast and pressed ceramics

A

a different technique more like casting a metal restoration

  1. restoration waxed up as you would for metal
  2. invested
    3, cast from a heated ingot of ceramic (1100 degrees)
  3. no sintering occurs because the ceramic ingot is fully condensed prior to firing
  4. once de-vested and cleaned, the restoration is heated to improve its crystal structure producing crack inhibiting crystals

This process is called CERAMING

  1. the cast crown can be stained
    more often it is cut back labially and veneered with appropriate feldspathic porcelain
73
Q

What are the ceramics used in the cast and pressed ceramic processes called

A

they’re glass ceramics

lithium dislocate glass
leucite reinforced glass

74
Q

Describe what happens in CERAMING

A

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

75
Q

What is the crystal size in strong materials

A

small crystal size and high volume fraction of crystals

makes it harder fo cracks to propagate through the material

76
Q

What are the lithium disilicate crystals like

A

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

77
Q

How are the silica containing ceramics cemented

A

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

78
Q

How are the zirconia cored crowns cements

A

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

79
Q

What is translucency

A

Reflects light and makes it change direction but also allows transmits some of the light

80
Q

What is reflectance

A

the measure of the proportion of light or other radiation striking a surface which is reflected off it.

81
Q

What is opacity

A

not allowing light to pass through

82
Q

What is transparency

A

transmits light

BDS3 CLINICAL DMS (9 decks)

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