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CSB > Materials for crown and bridgework > Flashcards

Materials for crown and bridgework Flashcards

1
Q

what are the 4 components needed for the choice of materials for indirect restorations

A
  1. Allow the realisation of patients’ cosmetic expectations, but not necessitate preparations involving excessive removal of sound tooth tissue.
  2. Facilitate optimum tissue response.
  3. Take account of:
    - The materials and tissues forming opposing and adjacent contacts.
    - Technical considerations.
  4. Be limited to those which satisfy the relevant standards
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2
Q

what are the 6 material requirements for indirect restorations

A
  1. Accuracy of fit.
  2. Strength to resist occlusal forces.
  3. Rigidity to avoid flexure and hence cement failure.
  4. Thermal expansion comparable to tooth.
  5. Should not attract plaque.
  6. Biocompatibility
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3
Q

what are the 4 options for crown and bridge materials

A
  1. metal
  2. ceramics
  3. metal-ceramics (porcelain fused to metal)
  4. indirect dental composite
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4
Q

what is a good metal material option for crowns and bridges

A

gold

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5
Q

what are the 2 types of metals used for crown and bridges

A
  1. gold and palladium alloys (precious alloys)
  2. base metal alloys (non-precious alloys)
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6
Q

name examples of gold and palladium alloys (precious alloys)

A

Type III gold alloys*
Silver-palladium
Palladium-silver-gold
Gold-silver-copper-palladium (Au>40%)
Palladium-copper
Palladium-tin

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7
Q

name examples of base metal alloys (non-precious alloys)

A

Nickel-chromium
Nickel-chromium-beryllium
Titanium

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8
Q

what 3 factors contribute in choosing a material for crowns and bridges

A
  1. cost
  2. corrosion resistance
  3. strength, stiffness, hardness, ductility
    - low-stress bearing inlay
    - posterior bridge
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9
Q

what is 1 star

A
  • medium-gold casting alloy
  • extra hard
  • yellow-gold
  • high proportions of gold with elements of silver and copper
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10
Q

what are the 4 roles of gold as a material in 1 star

A
  • colour
  • tarnish and corrosion resistance
  • malleability of an alloy
  • increases density of an alloy
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11
Q

what are the 2 roles of copper as a material in 1 star

A
  • strengthener
  • colour enhancer in Au-Ag- Cu crown and bridge alloys
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12
Q

copper rich Au-Ag-Cu alloys tend to have what colour

A

reddish colour

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13
Q

what are the 2 roles of silver as a material in 1 star

A
  • balances the reddening effect of copper
  • together with copper, used to control strength and hardness of crown and bridge alloys
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14
Q

silver rich Au-Ag-Cu alloys tend to have what colour

A

greenish

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15
Q

why may someone choose precious alloys over Au-Ag-Cu alloys

A

cheaper (cost of gold is steadily increasing)

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16
Q

what is an issue with reducing the gold content in alloys

A
  • reduces density which may make casting more unreliable
  • palladium and silver absorb oxygen when molten which can result in porous castings (esp. if buttons are re-used)
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17
Q

2 characteristics of base metal alloys

A
  • Have a higher modulus of elasticity than noble alloys i.e. they are more rigid
  • Implies less flexing of , e.g., a long span bridge
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18
Q

what are 2 disadvantages of base metal alloys

A
  • the need for very carefully controlled casting conditions
  • increasing concern BIOCOMPATIBILITY of dental alloys
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19
Q

what is the definition of biocompatibility

A

the ability of a material to perform with an appropriate host response in a specific application

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20
Q

3 biocompatibility issues

A
  • Grinding and casting fumes can result in conjunctivitis, dermatitis and bronchitis (which may not express itself for several years after exposure)
  • High levels of nickel are carcinogenic
  • More commonly, nickel is well known to cause contact dermatitis (a host response).
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21
Q

why may a patient opt for a porcelain fused to metal instead of metal

A

aesthetics and functional longevity

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22
Q

who introduced porcelain fused to metal

A

Weinstein, Katz and Weinstein (1962).

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23
Q

what % of porcelain fused to metal lasts over 7 years

A

95-97%

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24
Q

5 examples of choices for metal-ceramic alloys

A

High gold (88%Au, 1%Ag, 6%Pd, 4%Pt)

Gold-palladium (50%Au, 38%Pd, 10%Ag)

High palladium (80%Pd)

Silver-palladium (60%Ag, 30%Pd)

Nickel-chromium (70%Ni, 20%Cr, 10%Mo)

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25
Q

why is the porcelain in the alloys in PFM restorations modified

A

to more closely match thermal expansions

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26
Q

difference between alloys for PFM restorations and alloys for precious metals

A

the bond to base metal alloys is not as good as that of the precious metals.

27
Q

what is V-Delta SF

A
  • metal ceramic material
  • high proportion of gold and palladium with some indium and gallium
28
Q

what is the role of indium in PFM alloys

A
  • strengthens and hardens both gold and palladium and raises the thermal expansion of both
  • indium lowers the melting range of both gold and palladium, and contributes to the formation of the bonding oxide
29
Q

why does metal-ceramic materials (such as V-Delta SF alloy) have a higher melting range than 1 star

A

stable metal framework on which a porcelain veneer will be baked, therefore alloy needs a higher melting temperature so that it can retain its structure

30
Q

what is it that makes metal ceramic work

A

the coping distributes stresses and provides rigid support
- Inhibits propagation of cracks from small faults at the ceramic/metal interface.

31
Q

what 3 things allow a good bond to be achieved in a metal ceramic crown/bridge

A
  1. Mechanical retention (roughness).
  2. A direct chemical bond (ion diffusion).
  3. Mismatch of the coefficient of thermal expansion (CTE) (metal higher than ceramic).
    - The mismatch in CTE creates tangential compressive stresses in the ceramic.
32
Q

what are the 4 disadvantages of metal ceramic as a material

A
  1. Absence of light transmission (light doesn’t pass through metal), especially in cervical and proximal areas.
  2. Reduced depth of translucency.
  3. Presence of grey line at gingival margin.
  4. Sensitivity or allergy to alloys.
33
Q

what is an issue with nickel in terms of biocompatibility

A

nickel sensitivity causing acute inflammation

34
Q

what is an issue with non-precious metal bonded restorations in terms of biocompatibility

A

chronic discolouration

35
Q

from metal ceramic and ceramic, which has better optical properties

A

ceramic (light can pass through)

36
Q

what is the gold standard material in terms of aesthetics

A

all ceramic

37
Q

who created all ceramic

A

C H Land, 1901

38
Q

list ceramics with increasing toughness and bend strength

A

conventional porcelain
glass-ceramic
glass-infiltrated alumina
high-tech ceramics (e.g. zirconia)

39
Q

what is a fundamental issue with ceramics

A

brittleness

40
Q

what is the Griffith equation in material science

A

σf = K1c / Y √c

Where:
K1c = fracture toughness
σf = failure stress
c = critical flaw size
Y = constant

41
Q

how can you overcome issues such as when a material has a fixed strain

A

increasing the stress to failure

42
Q

how can you use the Griffith equation to increase the stress failure

A

increase fracture toughness
increase impact of critical flaw

43
Q

What reason are ceramics better than glass as a material

A

Ceramic consists of a crystalline material in a glassy matrix. If a crack starts to propagate in the glass, the crystalline domains deviate propagation, so progression of crack is hindered. => crack will stop before complete failure.

44
Q

what is aluminous porcelain

A

high strength ‘core’ porcelain containing up to 50% fused alumina crystals onto which a matched expansion veneer is baked

45
Q

what is the benefit of adding alumina to porcelain

A

acts as a crack stopper, preventing cracks propagating through the material, thus increasing strength

46
Q

what are glass ceramics

A

Fine grained polycrystalline materials which are obtained from an initially glassy phase following a heat treatment cycle

47
Q

why re large numbers of fine crystals useful in glass ceramics

A

crystals limit the propogation of flaws through the glass-ceramic which has far superior mechanical properties compared to the base glass

48
Q

what is IPS E.max
what does it consist of
what is the layering material

A

glass ceramics designed for crown and bridgework

  • 60% Lithium disilicate (main phase)
  • Lithium orthophosphate (secondary)
    Core material strength ~400 MPa

fluoroapatite

49
Q

how are lithium silicate glass ceramics produced

A
  1. glass powder melted and cooled in mould
  2. block produced used in CAD/CAM screening
  3. crystals are nucleated and then crystallised
  4. lithium silicate glass ceramic produced
50
Q

list 5 indications for lithium silicate glass ceramics

A
  • (thin) veneers
  • inlays/onlays
  • posterior crown
  • anterior crown
  • anterior bridge

superior aesthetics
med strength

51
Q

2 characteristics of lithium silicate glass ceramics

A

v good aesthetics
med strength

52
Q

describe the CAD/CAM process

A
  1. CCD camera (e.g. Cerec 3)
  2. video processing
  3. computer design
  4. computer miller
53
Q

name a alumina core ceramic material

A

techceram

54
Q

name 3 zirconia core ceramic materials

A

Lava
Cercon
IPS e.max ZIRCAD

55
Q

What phenomenon gives zirconia its strength

A

transformation toughening

56
Q

zirconia can exist in different crystal phases depending on what 2 things

A

time and pressure

57
Q

what happens to zirconia when you cool it below 1000C

A

goes for tetragonal to monoclinic (opposite above 1000C)

58
Q

what is the benefit of adding itria or magnesia to the composition of zirconia

A

tetragonal phase can be maintained at lower temperatures

So, if a crack encounters the tetragonal crystal, the constraints on the zirconia are released:
tetragonal → monoclinic
→ monoclinic occupies a greater volume in the bulk material than tetragonal
→ compressive forces
→ these counteract the tensile forces causing the cracks
→↓ suppressing crack propagation.

59
Q

which material has the best aesthetics

A

glass ceramics (lithium disilicate)

60
Q

which material has the best toughness

A

zirconia (zirconium oxide)

61
Q

what is an issue with veneered zirconia

A

delamination and bonding issues between zirconia core and porcelain

62
Q

what is the strength of lithium disilicate

A

360-400MPa

63
Q

what is the strength of veneered zirconium oxide

A

zirconia core= >900MPa
porcelain veneer= 80-120MPa

CSB (8 decks)

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