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2 - Dental Material Science > Dental Ceramics > Flashcards

Dental Ceramics Flashcards

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1
Q

What is porcelain a type of?

A

ceramic

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2
Q

What is removed from dental ceramics and what replaces it?

A

Dental Ceramics need to be translucent so Kaolin (opaque) is removed and feldspar and silica replace it

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3
Q

What. material are dental ceramics?

A

glasses

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4
Q

What components is feldpathic porcelain made of?

A
  • feldspar
  • borax
  • silica
  • metallic oxides
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5
Q

What does feldspar do?

A

 Acts as a flux
 Lowers the fusion and softening temperature of the glass
 It is the lowest fusing component and flows during firing forming a solid mass around the other components

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6
Q

What form are conventional dental ceramics?

what do they require?

A

powder
mixed with distilled water and built up into the restoration

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7
Q

How is the powder made?

A

 The powder is made by heating the constituents to a high temperature >1000oC
 Cool rapidly (Fritting)
 In water creating cracks and crazing of the ceramic mass
 Mill the Frit to a fine powder
 Add binder (often starch)

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8
Q

What happens to feldspathic ceramics when heated to 1150-1500 degrees?

A

form leucite (potassium aluminium silicate)

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9
Q

What phase is leucite formed and what does this determine?

A

 This forms around the glass phase of the ceramic.
 Gives a powder of known physical and thermal properties.
 No further chemical reaction is required during fabrication of the restoration
 The powder melts together to form the crown

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10
Q

How is the crown fabricated?

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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11
Q

What is sintering?

A

 Heating leads to SINTERING
 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
 Over time there is controlled diffusion and a solid ceramic mass is formed

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12
Q

What happens to the material during sintering?

A

contrcts by 20%

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13
Q

What are the favourable properties of conventional dental ceramics?

A
  • smooth, translucent aesthetics
  • less susceptible to staining
  • chemically stable - unaffected by ph
  • good biocompatibility
  • similar thermal properties to tooth
  • dimensional stability (except during fabrication
  • high compressive strength
  • high hardness
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14
Q

What are the negative mechanical properties of conventional feldspathic ceramics?

A
  • Tensile strength – very low
  • Flexural strength – very low
  • Fracture toughness – very low
  • Static fatigue
  • Surface micro-cracks
  • Slow crack growth
  • Brittle
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15
Q

Where should conventional feldspathic ceramics only be used?

A

only anterior crowns
low stress areas

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16
Q

How does alumina core work?

why can it only be used as a core

A

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

Aluminous porcelain is opaque and can only be used as a core material

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17
Q

How are conventional ceramics made stronger?

A

metal coping
using
alumina core
zirconia core

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18
Q

How much alumina is in a conventional aluminous core?

A

max 50% alumina

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19
Q

What covers the core?

A

All of these core types are then veneered with conventional feldspathic porcelain to produce the final crown

20
Q

What is the problem of alumina cores?

A

lack of flexural stregth
cannot be used for posterior crowns or bridgework

21
Q

When does zirconia powder sinter?

A

heated to over 1600

22
Q

Why is zirconia stabilised with yttria?

A

pure zirconia can crack on cooling

23
Q

What does more yttria mean?

A

 The more Yttria the more translucency
 More Yttria reduces the physical properties

24
Q

What type of crystal is normal zirconia at room temperature?

A

monoclinic crystal

25
Q

What type of crystal is yttria stablised zirconia?

A

tetragonal

26
Q

What happens when a crack begins in yttria stablised zirconia?

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

27
Q

How is a zirconia core fabricated

steps

A

 Impression is taken of the preparation and sent to the lab
 A model is cast and then scanned digitally
 Raw Zirconia block is selected for milling
 A presintered block is much easier to mill
 Milling for a three unit bridge will take around an hour
 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 ziconia core is used in GDH?

A

zerion LT

29
Q

What are disadvantages of zirconia cored crowns?

A

 Expensive equipment required
 Potential for veneering porcelain to debond from core
 Zirconia core is opaque ? Are aesthetics much better than metal ceramic
 Inert fitting surface, cannot etch or bond
 BUT
 Once you have the equipment, they are cheaper to make
 Cost of metal is increasing
 Fit is generally excellent

30
Q

Why do ceramics all have a surface sintered layer?

A

best aesthetics

31
Q

What materials have the same fabrication method?

steps

A

 Zirconia
 LiDiSi
 Metal
 Ceramic filled composite resin

32
Q

What is a milled crown?

A

a type of dental crown made using a computerized milling machine (CAD/CAM) to cut a restoration from a solid block of material

33
Q

What are the steps of a milled crown fabrication?

A

 Cast goes into scanner
 Scanned image of cast
 Lower cast is scanned and ‘articulated
 Select crown margin
 Adjust crown margin
 Select crown type and place on ‘model
 Select crown type and place on ‘model
 Adjust shape and size of selected crown
 Save file
 Send to milling machine
 Can be anywhere in the world (GDH mainly go to Spain)
 30 – 40 minutes you have your crown
 In GDH return from Spain takes 48hrs
 Still requires final finishing (polisj
 In GDH this is still done on a plaster model

34
Q

What is cast and pressed ceramic technique?

ceraming process

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

35
Q

How are cast crowns prepped aesthetically?

A

 The cast crown can be stained
 More often it is cut back labially and veneered with appropriate feldspathic porcelains

36
Q

What ceramic types are used for cast/pressed?

A

glass- ceramics.
 Lithium Disilicate Glass
 Leucite Reinforced Glass

37
Q

What are the 2 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%

38
Q

In casr and pressed ceramics, what prevents crack propatation and increases fracture toughness?

A

 Lithium disilicate glasses have a unique needle-like crystals
 This makes crack propagation through this material very difficult → Good Fracture toughness

39
Q

What are 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

40
Q

Why is layered porcelain bad for strength?

A

Layered crowns are more likely to chip due to stresses between core and veneer

41
Q

Sintered vs Milled?

A

 For the same material a milled crown will be stronger than a built up or pressed crown.
 The block will have been subjected to the ideal heat treatments to maximise its properties and all blocks will be consistent
 As aesthetics of ‘blocks’ of ceramic improve these will become the most commonly used crown.
 Already acceptable in posterior teeth

42
Q

What type of crown should you use for posterior teeth?

single crowns/shorter span bridge

A

single crowns/shorter span bridges

43
Q

What type of crown should you use for anterior single teeth/ short span bridgework where aesthetics are most important?

A

LiDiSi
 Can probably use as far back as first premolar

44
Q

What type of crown should you use for anterior heavier occlusion/ long span bridgework where aesthetics are most important?

A

 Zirconia cored with zirconia where occlusal contacts will meet

45
Q

What can both zirconia and lididi crowns be cemented with?

A

conventional or resin cements

46
Q

How are LiDiSi crowns bonded to the tooth?

A

 Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface (LiDSi crowns)
 This etched surface can be bonded to, using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent and resin cement

47
Q

ID

How are zirconia crowns bonded to the tooth?

A

 Zirconia cored crowns do not contain silica and are not affected by acid but can be air abraded to create retentive surface
 They are strong enough to be self supporting and can be luted with a conventional dental cement.
 Some evidence of bonding to 10-MDP containing bonding agents if resin cement is to be used

2 - Dental Material Science (17 decks)

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