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2 - Dental Material Science > Dental Ceramics > Flashcards

Dental Ceramics Flashcards

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

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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

stablises tetragonal phase

23
Q

What does more yttria mean?

A

as yttria increases, zirconia becomes more translucent (good for aesthetics) but less tough and strong (less suitable for high-stress areas).

change from tetragonal > cubic

24
Q

What type of crystal is normal zirconia at room temperature?

A

monoclinic crystal

25
What type of crystal is yttria stablised zirconia?
tetragonal
26
What happens when a crack begins in yttria stablised zirconia?
 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
How is a zirconia core fabricated | steps
 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
What ziconia core is used in GDH?
zerion LT
29
What are disadvantages of zirconia cored crowns?
 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
Why do ceramics all have a surface sintered layer?
best aesthetics
31
What materials have the same fabrication method? | steps
 Zirconia  LiDiSi  Metal  Ceramic filled composite resin
32
What is a milled crown?
a type of dental crown made using a computerized milling machine (CAD/CAM) to cut a restoration from a solid block of material
33
What are the steps of a milled crown fabrication?
 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
What is cast and pressed ceramic technique? | ceraming process
Ceraming is a technique similar to metal casting where a ceramic restoration is waxed-up, invested, and cast from a pre-condensed ceramic ingot at ~1100°C. No sintering is needed. After casting and cleaning, the restoration is heat-treated to enhance crystal structure and improve crack resistance.
35
How are cast crowns prepped aesthetically?
 The cast crown can be stained  More often it is cut back labially and veneered with appropriate feldspathic porcelains
36
What ceramic types are used for cast/pressed?
glass- ceramics.  Lithium Disilicate Glass  Leucite Reinforced Glass
37
What are the 2 stages of ceraming?
 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
In casr and pressed ceramics, what prevents crack propatation and increases fracture toughness?
 Lithium disilicate glasses have a unique needle-like crystals  This makes crack propagation through this material very difficult → Good Fracture toughness
39
What are advantages of different crown types?
 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
Why is layered porcelain bad for strength?
Layered crowns are more likely to chip due to stresses between core and veneer
41
Sintered vs Milled?
 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
What type of crown should you use for posterior teeth? | single crowns/shorter span bridge
single crowns/shorter span bridges
43
What type of crown should you use for anterior single teeth/ short span bridgework where aesthetics are most important?
LiDiSi  Can probably use as far back as first premolar
44
What type of crown should you use for anterior heavier occlusion/ long span bridgework where aesthetics are most important?
 Zirconia cored with zirconia where occlusal contacts will meet
45
What can both zirconia and lididi crowns be cemented with?
conventional or resin cements
46
How are LiDiSi crowns bonded to the tooth?
 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
# ID How are zirconia crowns bonded to the tooth?
 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
48
What colour are metallic oxides?
Chromium - Green Cobalt - blue Copper - Green Iron - brown Manganese - lavender Nickel - Brown

2 - Dental Material Science (17 decks)

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