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Yr 3 Restorative - biomaterials and CSL > Porcelain > Flashcards

Porcelain Flashcards

1
Q

What is a ceramic?

A

Inorganic, non metallic materials

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2
Q

What are the 3 types of ceramics?

A

Crystalline, amorphous (glass) and mixed (glass-ceramics)

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3
Q

Name a specific type of ceramic material

A

Porcelain

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4
Q

What are the clinical uses of porcelain in dentistry?

A

Crowns, onlays, veneers and bridges

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5
Q

What are the main components of porcelain?

A

Kaolinite (or kaolin)
Quartz (crystallised silica)
Feldspar

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6
Q

What is kaolinite?

A

China clay (hydrated aluminosilicate)

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7
Q

What is quartz?

A

The second most abundant element in the earths crust

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8
Q

What is feldspar?

A

A mixture of potassium and sodium aluminosilicates

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9
Q

What is the composition of dental porcelain?

A

It contains little pr no kaolin, it is mainly silica and feldspar (i.e. feldspathic glass with crystalline inclusion of silica)

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10
Q

What is the composition of dental porcelain?

A

(Pic)

N.b. high fusing means high melting temp

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11
Q

Which other components may be included in porcelain?

A
  • Borax (sodium borate) - added to feldspar to reduce the melting temperature
  • Metal oxides - pigments (e.g. iron = brown, copper = green and cobalt = blue) & opalescence (e.g. zinc oxide & titanium oxide)
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12
Q

What are the physical properties of porcelain?

A
  • Excellent aesthetics

- Low thermal conductivity

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13
Q

What determines opacity/transparency of porcelain/ceramic/composite material?

A
  • Particle/crystal/grain size

- Index of refraction

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14
Q

What is the positive implication of porcelains low thermal conductivity?

A

Does not conduct heat to the pulp

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15
Q

What is the negative implication of porcelains low thermal conductivity?

A

Thermal stresses can lead to cracks or fatigue (heat accumulates in some areas)

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16
Q

Why are the aesthetics provided by porcelain so amazing?

A

= wide range of shades and levels of translucency - to mimic different natural tissues

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17
Q

What are the chemical and biological properties of porcelain?

A
  • Pretty stable (inert)
  • biocompatible
  • polished surfaces are rougher than glazed surfaces = accumulates more plaque
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18
Q

What are the mechanical properties of porcelain?

A
  • Brittle
  • Hard
  • Weak (tensile)
  • Strong (compression)
  • Low fracture toughness
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19
Q

What are the common failure modes for brittle materials?

A
  1. catastrophic failure (suddenly breaks)

2. static fatigue (slowly degrades over time, even in absence of applied load -> can propogate)

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20
Q

How does catastrophic failure occur?

A

Crack propagation (usually along inner surface of crowns)

21
Q

What reduces the strength of a porcelain (leading to catastrophic failure)?

A
  1. Surface roughness (micro cracks -> stress concentration)
  2. Internal voids
  3. Porosity
22
Q

Why does static fatigue happen?

A

SiO2 bonds slowly hydrolysed = generates OH- = elevates pH -> dissolved Na2O & K2O (feldspathic components) = accelerated by dynamic mechanical loading

23
Q

How do we improve the mechanical properties of porcelain?

A
  • Alter processing -> fabrication condition optimisation
  • produce stronger porcelains -> composition and microstructure optimisation
  • reinforce porcelain by providing a solid supportive core -> structure optimisation i.e. pfm
24
Q

How do we improve processing?

A
  • Use fine grained powder = gives more uniform microstructure and surface
  • Fire at vacuum or under pressure = reduces porosity
  • Cool slowly after removing from the furnace = reduces thermal stresses which cause cracks
  • Smoothing or further fusing = reduces surface roughness and imperfections
  • Careful tooth preparation = reduces possibility for stresses at inner surface of crown
25
Q

How do we produce stronger/tougher porcelains?

A

Need compressive stress to compensate for the stresses created on surface irregularities under force

  • > ion exchange strengthening
  • > thermal strengthening
  • > particulate strengthening/toughening
26
Q

What is ion exchange strengthening?

A

= creates compressive forces all over surface = soak the fired porcelain in a molten potassium salt

27
Q

What is thermal strengthening?

A

Can be applied by careful application of heat (during initial firing or later heat tampering)…
Molten porcelain = solidifies from outside inwards = liquid centre pulls inwards on outer edge = compressive forces

28
Q

What is particulate strengthening/toughening?

A

Alumina particles act as a “crack stopper” = improves strength and fracture toughness
= either plate like mica or needle like crystals such as lithium dislocate
= (large S.A. on interface = absorbs lots of energy along the interface)

29
Q

Which material can we use to make a strong/tough ceramic core?

A
  • Alumina
  • Alumina-reinforced feldspathic porcelain
  • Glass-infiltrated alumina
  • Spinel (magnesium aluminate)
  • Zirconia
30
Q

What are the strengths of porcelain?

A

Aesthetics, High strength (compression), chemically inert, high hardness, high elastic modulus

31
Q

What are the weaknesses of porcelain?

A

Brittle (low fracture toughness), fatigue (any defect can cause fatigue) & tooth wear (due to roughness and high hardness)

32
Q

What is the general preparation of porcelain?

A

Powder mixed with water and binder (sugar or starch = improves working properties)-> moulded, carved, compacted and dried -> fired (scintering = melts together and fuses) and cooled -> glazed or polished

33
Q

What is the name of the powder porcelain is supplied in?

A

A frit

34
Q

How are the components of porcelain mixed?

A
  • Ground
  • Mixed
  • Fused
  • Rapidly cooled
  • Ground again

= more even mix

35
Q

How does feldspar fuse the other components together?

A

It melts at the lowest temperature

36
Q

Why is the porcelain compacted onto platinum foil mould?

A

The foil is not affected by the firing temperature = porcelain shrinks inwards towards fold = assures good crown fir on tooth

37
Q

How do some companies overcome setting shrinkage of porcelain?

A

Produce partially sintered moulds i.e. for CADCAM

38
Q

How do we achieve wanted compaction?

A

Light vibration/patting

39
Q

What does compaction achieve?

A
  1. settles particles = reduces shrinkage
  2. gives uniform contraction over whole surface
  3. brings excess water to surface
40
Q

What do modern manufacturers do to make dense ‘green’ porcelain ceramics?

A

Cold isostatic pressing (CIP)

41
Q

Which method can be used t make dense ceramics directly?

A

Hot isostatic pressing (HIP)

42
Q

What is the drying and firing process?

A
  • Placed into warm atmosphere to dry ( 1000 degrees C (sometimes under vacuum)
43
Q

What happens if you put wet porcelain straight into the oven?

A

It will bloat

44
Q

How do we glaze?

A

Coat the completed restoration with low-fusing transparent glass

45
Q

What are the benefits of glazing?

A
  • improves appearance = greater translucency
  • gives impervious, smooth coating which protects against chemical attack
  • lower hardness = reduces wear
46
Q

What is CAD/CAM?

A

A computer that produces ceramics or composites in a lab or chair side
For ceramics can use fully sintered (zirconia and just put glaze on top to mach colour and strength -> tools wear a lot) or partially sintered (not filly dense, must then scienter again = shrinkage but pretty similar and predictable shrinkage = 10%)

47
Q

What are the 3 main categories of bonding of porcelains?

A
  1. cemented using luting agents
  2. fused to metal then cemented to tooth
  3. resin bonded ceramics have recently been developed
48
Q

What are the benefits of porcelain fused to metal crowns?

A

Combines properties of cast dental alloys and porcelain = good mechanical properties and aesthetics

  • materials must be compatible to form a good bona
  • alloys typically used = high-gold, low-gold, silver-palladium and nickel chromium
49
Q

What are the costs, health and safety of porcelains?

A

Materials: relatively expensive for patient £3-400
Environment: materials plentiful
Time: several visits required, less so with CAD/CAM
Longevity: adv. crown lasts 10 years, may be longer
Health and safety: uses fine powders = needs to use extractor

Yr 3 Restorative - biomaterials and CSL (16 decks)

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