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DMS > CERAMICS > Flashcards

CERAMICS Flashcards

1
Q

What are the steps in the fabrication of a crown

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

These are not tooth coloured

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

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2
Q

What does heating of the crown lead to

A

Sintering

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3
Q

What is sintering

A

occurs just above the glass transition temperature

It is when the ceramic particles begin to fuse into a single mass

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4
Q

What happens during sintering

A

the glass phase softens and will coalesce

the material contracts by about 20%

Over time there is controlled diffusion and a solid ceramic mass is formed

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5
Q

What are the aesthetics like of conventional feldspathic ceramics

A

Ceramics have the best aesthetic properties of any dental restorative material

Colour Stable

Very smooth surface
Retain their surface better than other materials, less staining long term

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6
Q

What are the optical properties of conventional feldspathic ceramics

A

Reflectance
Translucency
Opacity
Transparency
Opalescence

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7
Q

What is the chemical stability like of conventional feldspathic ceramics

A

Chemically very stable

Generally unaffected by the wide pH range found in the mouth

Do not take up stain from food/drink

Good BIOCOMPATIBILITY

minimal adverse effects on biological tissues

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8
Q

What are the thermal properties like of conventional feldspathic ceramics

A

Similar to tooth substance

Coefficient of thermal expansion is similar to dentine (Resulting in low stresses to the restoration in the mouth during use)

Thermal diffusivity is low which is protective of the remaining tooth.

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9
Q

What is the dimensional stability like of conventional feldspathic ceramics

A

Once fully fired the material is very 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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10
Q

what are the mech. properties like of conventional feldspathic ceramics

A

High compressive strength

High hardness (can lead to abrasion of opposing teeth especially if not glazed)

Tensile strength\Flexural strength\Fracture toughness – very low (all can lead to failure during loading)

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11
Q

What is static fatigue and how does it happen

A

Time dependant decrease in 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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12
Q

Can conventional ceramics suffer from surface micro cracks

A

Yes,

Can occur during manufacture, finishing or due to occlusal wear. These are areas where fractures can initiate

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13
Q

Why do conventional ceramics get slow crack growth

A

Cyclic fatigue under occlusal forces in a wet environment over time

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14
Q

Where can conventional feldspathic be used and why

A

Only anterior crowns as too brittle for use elsewhere

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15
Q

what has been used since the 1960’s to reinforce feldspathic core ceramics

A

Alumina core

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16
Q

What is the core material of PJC

A

Alumina core

17
Q

How do Alumina particles work

A

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

18
Q

Aluminous porvelain is….

A

opaque and can only be used as a core material

19
Q

What are the advantages and disadvantages of alumina cored crowns

A

Problems with all alumina cored crowns remains lack of flexural strength

Aesthetics are excellent but probably not suitable for anything other than single crowns

Much more successful anteriorly

Did not replace metal ceramic for bridgework or posterior crowns

20
Q

what is the most popular ceramic core material

A

Zirconia core

21
Q

What is zirconia

A

Zirconium Dioxide, is a naturally occurring mineral

22
Q

what does zirconia need to be heated to

A

> 1600

23
Q

What is the type of zirconia used in dentsitry and why

A

Yttria-stabilised zirconia as Pure zirconia can crack on cooling

24
Q

How much Yttria is present in Yttria-stabilised zirconia and what does it do

A

3 – 5%

The more Yttria the more translucency

More Yttria reduces the physical properties

25
Q

what is the structure of Yttria and what does this structure do

A

Atetragonal crystal structure

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

26
Q

Yttria Stabilisation of Zirconia makes the material very

A

Hard

Strong (1000MPA flexural strength)

Tough

Strong enough to be used as bridge framework

27
Q

How is a Zirconia core fabricated

A

Impression is taken of the preparation and sent to the lab

A model is cast and then scanned digitally

Software Unit Creates a bridge substructure on virtual preparations

Minimum thicknesses of connectors are determined and fabricated

Raw Zirconia block is selected for milling (A presintered block is much easier to mill)

The cut framework is then heat treated at around 850oC to achieve its final physical properties

This causes a 20% shrinkage but the computer softwear deals with this during the milling process.

The framework is also stained to an appropriate colour

The Zirconia core is then veneered with feldspathic porcelain to produce the final restoration

28
Q

What are the problmes of a Zirconia core

A

Expensive equipment required

Potential for veneering porcelain to debond from core

Zirconia core is opaque

Inert fitting surface, cannot etch or bond

29
Q

How are cast and pressed ceramics made

A

A different technique more like casting a metal restoration

The restoration is waxed-up, as you would for a metal restoration

Invested

Cast from a heated ingot of ceramic (1100oC)

No sintering occurs the ceramic ingot is already fully condensed prior to firing.

Once devested and cleaned the restoration is heated to improve its crystal structure producing crack inhibiting crystals.

This process is called CERAMING

30
Q

What are the type of ceramics used in cast and presssed ceramics

A

called glass-ceramics.

Lithium Disilicate Glass(LiDiSi)

Leucite Reinforced Glass

31
Q

What is ceraming

A

Two stage process

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%

32
Q

List advantages of different crown types

A

Monolithic block crowns, milled from a single block of material are strongest

Zirconia based crowns are stronger than LiDiSi

LiDiSi have better translucency hence better aesthetics

Crowns with layered porcelain rather than just stained monolithic block have better aesthetics

Layered crowns are more likely to chip

33
Q

How do you descide what to use

A

Posterior teeth-Monolithic Zirconia, can be used for single crowns and shorter span bridges

Anterior teeth single crowns where aesthetics are the most important factor-LiDiSi, can probably use as far back as first premolar

Anterior bridgework- LiSiDi, for short span

Anterior bridgework with a longer span or heavier occlusion- Zirconia cored

34
Q

How are Zirconia and LiDiSi crowns luted

A

cemented with conventional or resin cements

Both kinds of crowns have intrinsic strength and do not rely upon being bonded to tooth substance to prevent fracture

35
Q

what crowns can be etched

A

Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface

36
Q

How are etched crowns bonded to

A

using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent and resin cement

DMS (1 decks)

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