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BDS3 DMS > Dental Ceramics > Flashcards

Dental Ceramics Flashcards

1
Q

Why is kaolin not found in dental ceramics?

A

Kaolin is opaque so as dental ceramics need to be translucent it is replaced with feldspar and silica

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2
Q

What are the components of dental ceramics?

A
  • Feldspar (70-80%)
  • Quartz/Silica (12-25%)
  • Glass (<15%)
  • Kaolin (<5%)
  • Metal Oxides (1%)
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3
Q

What is the function of Feldspar?

A
  • acts as a flux
    -lowers softening temperature to allow fusion
    -flows during fusing to form a solid mass around other components
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4
Q

What is the function of metal oxides in dental ceramics?

A

To aid colouration

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5
Q

Give some examples of metal oxides and the colours they create

A

chromium - green
cobalt - blue
copper - green
iron - brown
manganese - lavender
nickel - brown

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6
Q

How are conventional dental ceramics supplied

A

as a powder

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7
Q

Describe how conventional dental ceramic powder is made

A
  • constituents heated to high temperature >1000 degrees
  • rapidly cooled in water to allow cracks and crazing of the ceramic mass (fritting)
  • frit milled into a fine powder
  • binder added (usually starch)
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8
Q

How is conventional dental ceramic powder used?

A

The powder is mixed with distilled water and build up on the stone model

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9
Q

What colours are the conventional dental ceramic powders and what are they used for?

A

Pink - dentine
Blue - enamel

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10
Q

What is leucite?

A
  • potassium aluminate silicate
  • formed wien feldspathic ceramics are heated to 1150-1500 degrees
    -forms a glass phase around the ceramic
  • gives the powder known physical and thermal properties
  • powder melts together to form solid ceramic and no further chemical reaction is required
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11
Q

What is done once a restoration is build up on a model in ceramic powder?

A

Restoration is heated in a furnace to coalesce the power into ceramic

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12
Q

What difference is seen before and after a restoration has been in the furnace?

A

Once a restoration is removed from the furnace it is around 20% smaller. This must be considered when building up the restoration and is due to shrinking on heating.

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13
Q

What is sintering?

A
  • occurs as a result of heating
  • process of ceramic particles fusing into a single mass
  • occurs just above the glass transition temperature
  • glass phase softens and coalesces
  • controlled diffusion over time and a solid ceramic mass is formed
  • material contracts by around 20%
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14
Q

What are the properties of conventional dental ceramics?

A
  • best aesthetic qualities of any dental material
  • very chemically stable
  • highly biocompatible
  • good thermal properties
  • dimensionally stable
  • mechanical properties are variable
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15
Q

Discuss the aesthetics of conventional dental ceramics

A
  • best aesthetics of any dental material
  • colour stable, does not stain
  • very smooth surface
  • surface retained better than other materials (less staining etc.)
  • optical properties such as reflectance, translucency, opacity, transparency and opalescence mimic that of enamel
  • opacity and translucency can be varied which is not possible with other materials
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16
Q

Discuss the chemical stability of conventional dental ceramics

A
  • very chemically stable
  • unaffected by the wide pH range in the mouth
  • does not stain from food and drink
    -good biocompatibility with minimal adverse effects on biological tissues
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17
Q

Discuss the thermal properties of conventional dental ceramics

A
  • similar to that of tooth substance
  • coefficient of thermal expansion is similar to dentine which results in low stresses to the restoration during use
  • thermal diffusivity is low which protects the remaining tooth
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18
Q

Discuss the dimensional stability of conventional dental ceramics

A
  • very stable once fully fired
  • shrinkage during firing can be challenging so technicians must consider a 20% decrease in size
  • no creep, bending or stretching experienced
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19
Q

Discuss the mechanical properties of conventional dental ceramics

A
  • high compressive strength
  • high hardness
  • very low tensile strength
  • very low flexural strength
  • very low fracture toughness
  • static fatigue
  • surface micro-cracks
  • slow crack growth
  • the properties described as very low lead to failure during loading
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20
Q

What is the disadvantage of the high hardness of conventional dental ceramics?

A

Although the high hardness stops the restoration from corroding, there can be an imbalance between the ceramic restoration and the opposing teeth which can lead to abrasion of tooth tissue especially if the ceramic is not glazed.

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21
Q

What is static fatigue in conventional dental ceramics?

A

The time dependent decrease in strength, even in the absence of any applied load. Thought to be due to hydrolysis of Is-O groups in the material in an aqueous environment

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22
Q

What are surface micro-cracks in conventional dental ceramics?

A

Can occur during manufacture, finishing or due to occlusal areas. Fractures can initiate from these areas

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23
Q

What is slow crack growth in conventional dental ceramics?

A

Cyclic fatigue under occlusal forces in a wet environment over time

24
Q

How do the mechanical properties of feldspathic ceramics impact on their use?

A

Can only be used in low stress areas therefore only for anterior crowns. They are too Brittle to be used anywhere else.
They are not suitable for all patients such as those with a bruxism habit.

25
Q

How can problems with conventional dental ceramics be overcome?

A
  • production of a strong coping that is resistant to fracture to cover with ceramic
  • cast or press a block of harder ceramic
    mill a laboratory prepared block of ceramic
26
Q

What materials can be used for copings?

A
  • metal coping
  • alumina core
  • zirconia core
27
Q

What are the advantages of an alumina core in feldspathic ceramics?

A
  • flex strength is double that of feldspathic porcelain (>120MPa)
  • alumina particles act as crack stoppers to prevent cracks propagating and causing fracture
  • less labial reduction required than for MCCs
  • relatively cheap to manufacture
  • no specialist equipment required other than a furnace
27
Q

What are the disadvantages of an alumina?

A
  • aluminous porcelain is opaque so can only be used as a core material
  • it is not strong enough to be used on posterior teeth
  • more palatal reduction required than MCC to have enough room for aluminous core and feldspathic layers for good aesthetics
  • lack of flexural strength
28
Q

What percentage of alumina cores are alumina?

A
  • conventional aluminous cores contain a maximum of 50% alumina (furnace limits quantity)
  • new techniques have been made to increase the alumina content and therefore the strength
29
Q

What techniques were used for increasing the alumina content in aluminous cores?

A
  • INCERAM
  • PROCERA
  • both now superseded
30
Q

What newer techniques and materials are available as alternatives to conventional dental ceramics and aluminous cores?

A
  • zircon
  • lithium disilicate
  • cores
  • pressed crowns
  • monolithic/milled crowns
31
Q

What is zirconia?

A
  • Zirconia Dioxide
  • naturally occurring mineral
  • has different crystalline forms at different temperatures
  • very hard
32
Q

What temperature does zirconia sinter at?

A

1600 degrees

33
Q

What type of zirconia is used in dentistry?

A

Yttria-stabilised zirconia

34
Q

Describe yttria stabilisation

A
  • small quantities of yttria contained in material (3-5%)
  • yttria increases translucency and reduced the physical properties
  • changes the zirconia crystal structure from monoclinic (room temperature) to tetragonal
  • if a crack forms and stress reaches a critical level, structure reverts back to monoclinic structure which causes slight expansion and closes the crack tip
35
Q

What are the advantages of yttria stabilisation?

A
  • material is hard and tough
  • flexural strength is increased (1000MPa) so material is strong
  • material is strong enough to be used as a bridge framework
36
Q

How are zirconia crowns fabricated?

A
  • impression of preparation taken and sent to lab
  • model cast and scanned digitally
  • restoration structure created on virtual preparations
  • minimum thickness determined and fabricated
  • raw zirconia block selected for milling (pre-sintered is easier to mill)
  • cut restoration heat treated at 850 degrees to achieve final physical propertied
  • 20% shrinkage experienced but software compensates for this during design
  • framework stained (if zirconia is not available in different shades)
  • zirconia core veneered with feldspathic porcelain to produce final restoration
37
Q

Name the zirconia systems that manufacturers use

A
  • LAVA (from 3M)
  • IPS e.max Zir CAD
  • Opalite
  • Zeroing
  • Everest ZH
38
Q

What are the disadvantages of zirconia cored crowns?

A
  • expensive equipment required
  • veneered porcelain can debond from core
  • aesthetics not better than MCCs
  • cannot etch or bone due to inert fitting surface
39
Q

What are the advantages of zirconia cored crowns?

A
  • once equipment is purchase they are cheaper to make (especially as price of metal is increasing)
  • fit is generally excellent
40
Q

What materials can milled core crowns and bridges be made from?

A
  • zirconia
  • lithium disilicate (E-max)
  • precious metal
  • non-precious metal
  • titanium
  • composite
41
Q

How are milled crowns fabricated?

A
  • cast placed into scanner
  • images of casts articulated
  • crown margin selected and adjusted
  • crown type selected and placed on model
  • shape and size of crown adjusted
  • file sent to milling machine
  • milling of crown (30-40 minutes)
  • final finishing such as fine polishing carried out by technician
42
Q

Describe the process of ceraming

A
  • similar to that of casting a metal restoration
  • restoration waxed up and invested
  • cast from a heated ingot of ceramic (1100 degrees)
  • no sintering as ingot was fully condemned prior to firing
  • restoration devested and cleaned then heated again to improve its crystal structure and produce crack inhibiting crystals
  • cast stained or cut back labially to be veneered with feldspathic porcelains
43
Q

What ceramics are used in ceraming?

A

glass ceramics are used
- lithium disilicate glass
- leucite reinforced glass

44
Q

What are the two stages of ceraming?

A

Stage 1
- crystal formation
- maximum number of crystal nuclei formed

Stage 2
- crystal growth
- maximises physical properties

The crystal phase of ceramic can approach 100%.

45
Q

How do crystals impact in the strength of materials?

A

strong materials have small crystal size and high volume fraction of crystals

46
Q

Describe lithium disilicate glass structure

A
  • crystals have a unique needle-like shape
  • crack propagation is difficult, good fracture toughness
  • good flexural strength 350MPa
47
Q

Discuss the advantages of different crown types

A
  • Monolithic block crowns are the strongest
  • Zirconia based crowns are strong than LiDiSi
    -LiDiSi have better translucency, therefore better aesthetics
  • crowns layered with porcelain rather than stained monolithic block have better aesthetics
  • layered crowns are more likely to chip due to stress between the core and porcelain layers
48
Q

Compare the properties of sintered and milled restorations

A
  • milled crowns are stronger than build up or pressed crowns
  • aesthetics of milled restorations are poorer but currently acceptable for posterior teeth
49
Q

What type of material can be used for posterior teeth?

A

monolithic zirconia
- single crowns
- short span bridges

50
Q

What material should be used for anterior teeth where aesthetics are vital?

A

LiDiSi
- ideal for single crowns
- use as far back as first premolar

51
Q

What material should be used for anterior bridgework?

A

LiDiSi
- short span bridgework
- no parafunction

52
Q

What material should be used for longer span bridges or heavier occlusion?

A

Zirconia cored with Zirconia at occlusal contacts

53
Q

What can be used to cement Zirconia and LiDiSi crowns and why?

A

Resin Cements
- crowns have intrinsic strength
- do not rely upon bonded tooth structure to prevent fracture
- do not perform better than MCC if cemented with conventional dental cement
- e.g. glass ionomer

54
Q

How are silica containing crowns luted?

A
  • silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface
  • etched surface can be bonded to using a silane coupling
  • silane coupling agent can b bonded to the tooth using a bonding agent and resin cement
55
Q

How are zirconia crowns luted?

A
  • as they do not contain silica they are not affected by acid
  • can be abraded to create a retentive surface
  • sandblasting is used to abrade the fitting surface
  • strong enough and self supporting so can be luted with a conventional dental cement.

BDS3 DMS (10 decks)

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