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3 DMS > Dental Ceramics > Flashcards

Dental Ceramics Flashcards

1
Q

What is Kaolin?

A

Hydrated aluminium silicate
An opaque clay found in dental ceramics

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2
Q

What is found in dental ceramics?

A

Kaolin <5%
Quartz (silica) 12-25%
Feldspar 70-80%
Metal oxides 1%
Glass up to 15%

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3
Q

What does feldspar do?

A

Acts as a flux, lowering the temp of other glasses in the mixture, allowing fusing of the glasses

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4
Q

How are dental ceramics made?

A

Powder is made by heating constituents to >1000ºC then rapidly cooling
A binder (often starch) is added and the powder is mixed with distilled water and built up into the restoration

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5
Q

How does the powder form a crown?

A

Felspathic ceramics form leucine when heated
This forms around the glass phase of the ceramic
Gives the powder physical and thermal properties
The powder melts together to form the crown

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6
Q

How are crowns formed?

A

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
The crown is heated in a furnace to coalesce the powder into ceramic

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7
Q

What is sintering?

A

When the ceramic particles begin to fuse into a single mass
The glass phase softens and will coalesce
Over time a solid ceramic mass if formed
Material contracts by about 20%
Heating leads to sintering

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8
Q

What are the aesthetic properties of ceramics?

A

Best aesthetic properties of any restorative material
Colour stable
Very smooth surface
Retain surface well
Optical properties - reflectance, translucency, opacity, transparency, opalescence

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9
Q

Describe the chemical stability of ceramics

A

Chemically very stable
Generally unaffected by the wide pH range found in the mouth
Do not stain from food/drink
Good biocompatibility

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10
Q

Describe the thermal properties of ceramics

A

Similar to tooth substance
Coefficient of thermal expansion similar to dentine
Thermal diffusivity is low - protective of the remaining tooth

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11
Q

Describe the dimensional stability of ceramics?

A

Once fully fired the material is very stable
During fabrication shrinkage is a problem and 20% is normal for a feldspathic ceramic crown

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12
Q

Describe the mechanical properties of ceramics

A

High compressive strength, hardness
Low tensile strength, flexural strength, fracture toughness - all lead to failure during loading
Static fatigue
Surface micro-cracks
Slow crack-growth

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13
Q

Where can feldspathic crowns be used?

A

Only anterior crowns
Too brittle for use anywhere else

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14
Q

Why are alumina core crowns used?

A

Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture

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15
Q

What are the advantages of alumina core crowns?

A

High fracture toughness
Relatively cheap to make
No specialist equipment required
Less labial reduction required

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16
Q

What are the disadvantages of alumina crowns?

A

Can only be used for anteriors - not strong enough for posteriors
Alumina is opaque
More palatal reduction required
Lack of flexural strength

17
Q

Give examples of other ceramic techniques and materials

A

Zirconia
Lithium disilicate
Cores
Pressed crowns
Monolithic/milled crowns
Luting

18
Q

Describe zirconia cores

A

Zirconia dioxide is a naturally occurring material
Very hard
Different forms at different temperatures
Zirconia powder does not sinter unless heated to over 1600ºC

19
Q

What is Yttria stabilised Zirconia?

A

3-5% of the material is yttria - the more yttria the more translucent and less physical properties
If a crack begins when stress at the crack tip reaches a critical level, the crystal structure transforms into the monoclinic structure, expanding the material, closing the crack tip

20
Q

How is a zirconia core fabricated?

A

Imp taken of prep and sent to lab
Model is cast and digitally scanned
Software created bridge structure
Raw zirconia block is milled
Framework is heated to 850ºC which causes shrinkage but computed deals with this
Framework is stained to appropriate colour
Zirconia is then veneered with feldspathic porcelain to produce the final restoration

21
Q

What are the problems with zirconia cored crowns?

A

Expensive equipment required
Potential for veneering porcelain to deboned from core
Zirconia core is opaque
Inert fitting surface, cannot etch or bone

22
Q

What are the different types of milled core crowns and bridges?

A

Zirconia
Lithium disilicate
Precious metal
Non-precious metal
Titanium
Composite

23
Q

How are milled crowns fabricated?

A

Cast goes into scanner
Crown is designed on CAD machine
File is saved and sent to milling machine
Polish
Cement

24
Q

Describe how cast and pressed ceramics are made

A

The restoration is waxed up like as in a metal restoration
Invested
Cast from a heated ingot of ceramic
No sintering occurs
Once de vested and cleaned the restoration is heated to improve its crystal structure and produce crack inhibiting crystals
This process is called ceraming

25
Q

What ceramics are used in cast and pressed ceramics?

A

Glass ceramics:
Lithium disilicate glass
Leucine reinforced glass

26
Q

What are the stages of ceraming?

A

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%

27
Q

What are the advantages of the different crown types?

A

Monolithic block crowns milled from a single block of material are the strongest
Zirconia crowns are stronger than LiDiSi
LiDiSi have better translucency and so better aesthetics
Crowns with layered porcelain rather than just stained monolithic block have better aesthetics
Layered crowns are ore likely to chip due to stresses

28
Q

Compare sintered and milled crowns

A

For the same paternal a milled crown will be stronger than a build up or pressed crown
As aesthetics of blocks of ceramic improve these will become the most commonly used crown

29
Q

Which crowns should be used in posterior teeth?

A

Monolithic zirconia - can be used for single crowns and shorter span bridges

30
Q

Which crowns should be used for aesthetical anterior crowns?

A

LiDiSi - can probably use as far back as first premolar

31
Q

What crown should be used for anterior bridgework?

A

LiDiSi - short span with no parafunction

32
Q

What crown should be used for a longer span bridge or if heavier occlusion?

A

Zirconia cored with zirconia where occlusal contacts will meet

33
Q

What are luting crowns?

A

When zirconia or LiDiSi crowns are cemented with conventional or resin cements

3 DMS (8 decks)

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