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BDS2 Dental materials science > Amalgam > Flashcards

Amalgam Flashcards

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1
Q

why is there concern amalgam has health risks

A

role of mercury in disease causation with its systemic distribution and accumulation in the body

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2
Q

what is amalgam

A

an alloy formed by the reaction of:

  • mercury (liquid)
  • silver, tin, copper and other metals (powder)
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3
Q

when was the ‘first amalagm war’

A

1840s (american society of dental surgeons banned use of amalgam)

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4
Q

when was the second amalagm war

A

1970s-1990s

mercury fear - health and environment

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5
Q

how is amalgam classified

A

by composition and by particle shape and size

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6
Q

what are the different types of composition of amaglam

A
  • tradtional

- copper enriched

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7
Q

what are the different types of particle shape and size of amalgam

A
  • originally applied to traditional but now also to copper enriched
  • affects handling and final properties
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8
Q

what is amalgam composed of

A
  • Ag (70%)
  • Sn (25%)
  • Cu (3%)
  • Zn (most today don’t have zinc) (1%)
  • Hg (1%)
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9
Q

what is the function of silver, tin in the powder

A 
intermetallic compound (Ag3Sn)
- gamma phase, reacts with Hg liquid to form amalgam
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10
Q

what is the function of copper in the powder

A

increases strength and hardness

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11
Q

what is/ was the function of zinc in powder

A

it was there to oxidise rather than have the other metals oxidise (sacrifical lamb)

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12
Q

what is the function of Hg in powder

A

“pre-amalgamated” alloys react faster

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13
Q

what proportion of weight of amalgam is powder and liquid

A

powder: 50%
liquid: 50%

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14
Q

what is the function of Hg in liquid

A
  • triple distilled (very pure)

- reacts with other metals

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15
Q

what are the different particle types in amalgam

A

Lathe cut

  • coarse, medium, fine
  • formed by filling ingots

Sperical, spheroidal

  • range of particle sizes
  • formed by spraying molten metal into inert atmosphere
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16
Q

Describe the setting reaction of amalgam

A

silver + tin –> unreacted silver/tin + tin/mercury

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17
Q

describe the set structure of amalgam

A

gamma (silver/tin)
- good strength and corrosion resistance

gamma 1 (silver/mercury)
- good corrosion resistance
gamma 2 (tin/mercury)
- weak and poor corrosion resistance
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18
Q

what do voids do to the set structure

A

decrease strength and increase corrosion

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19
Q

what is the tensile strength of gamma

A

170MPa

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20
Q

what is the tensile strength of gamma 1

A

30MPa

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21
Q

what is the tensile strength of gamma 2

A

20MPa

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22
Q

What is the mean strength of amalgam

A

60MPa

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23
Q

how do setting dimensional changes differ between traditional and modern amalgams

A

Traditional (big changes in expansion)

  • intial contraction (solution of alloy particles in Hg)
  • expansion (gamma 1 crystallisation

Modern (expansion/contraction is <0.2% so little clinical sign)

  • small contraction
  • solid solution of Hg in Ag3Sn
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24
Q

why do we now have zinc free materials

A

It can react with saliva to get zinc oxide and hydrogen, so bubbles of hydrogen formed within amalgam

  • pressure build up causes expansion
  • downward pressure cause pulpal pain
  • upward - restoration sitting proud of surface
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25
Q

what do properties of amalgam depend upon

A
  • handling factors
  • cavity design
  • products
  • corrosion
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26
Q

What is the strength like of amalgam

A

early: fairly poor

later (>24hrs): OK

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27
Q

what is the abrasion resistance like of amalgam

A
  • high, suitable for posterior teeth

- too high for deciduous

28
Q

what factors decrease the strength of amalgam

A
  • undermixing
  • too high Hg content after condenstaion
  • too low condensation pressure
  • slow rate of packing (increments do not bond)
  • corrosion
29
Q

What is ‘creep’

A

when a material is repeatedly stressed for long periods at low stress levels (i.e. stress is below elastic limit) it may flow, resulting in permanent deformation

30
Q

why is creep a problem for amalgam

A
  • amalgam is visco-elastic
  • high in traditional materials
  • affects marginal integrity
    n. b. also marginal integrity depends on cavity design and corrosion
31
Q

what materials are affected by creep

A

amalgam
alloys
waxes
plastics

32
Q

how does creep affect amalgam

A
  • amalgam fits cavity, force is low magnitude force and is applied frequently
  • amalgam changes shape, sitting proud of tooth surface
  • amalgam vulnerable to fracture at margins (ditched margins)
33
Q

what is the biocompatibility of amalgam like

A

so far it’s safe to use but concern about mercury toxicity

34
Q

what is the thermal expansion of amalgam like

A

x3 of tooth

35
Q

what is the thermal conductivity of amalgam like

A

high, may need to use liner/ varnish in deep cavities

36
Q

does amalgam bond to tooth

A

no

37
Q

how do we get amalgam to stay in place

A

needs mechanical retention

38
Q

what are the handling properties of amalgam like

A

mixing, working and setting times are OK, varies between types

39
Q

what is the viscosity properties of amalgam like

A

packed (condensed) into cavity - user friendly

40
Q

aesthetics of amalgam

A

poor

41
Q

radiopaque?

A

yes

42
Q

anticariogenic?

A

no

43
Q

smooth surface?

A

yes, if polished well, may deteriorate over time

44
Q

setting shrinkage?

A

modern materials tend to have net overall shrinkage

45
Q

what part of the set structure is the most electronegative and why is that important

A

gamma 2
weakens material particularly at margins
corrosion

46
Q

how do we reduce corrosive properties of amalgam

A
  • copper enriched, polishing margins

- avoiding galvanic cells

47
Q

what are advantages of spherical particles

A
  • less Hg required
  • higher tensile strength
  • higher early compressive strength
  • less sensitive to condensation
  • easier to carve
48
Q

what are copper enriched alloys also called?

A

non-gamma2
higher copper
copper enriched

49
Q

what proportion of copper enriched alloys are copper

A

> 6%

50
Q

what are the different types of copper enriched alloys

A
  • dispersion modified (original type)

- single composition types (introduced to increase uptake by profession)

51
Q

how many reactions happen in dispersion modified copper enriched alloys

A

2

52
Q

what are dispersion modified copper enriched alloys made of

A

Ag-Cu spheres + conventional lathe cut alloy

53
Q

What are the two reactions that happen in dispersion modified copper enriched alloys

A
  1. gamma + Hg –> gamm + gamma 1 + gamma 2

2. gamma 2 + Ag-Cu –> Cu6Sn5 + gamma 1
takes several days

54
Q

how is Ag-Cu and Cu6Sn5 arranged in dispersion modified setting reaction

A

Ag-Cu as a sphere
Cu6Sn5 as halo around spheres

(also gamma 1 and gamma present)

55
Q

in single composition formulations (Cu enriched) what is the powder composed of

A

Ag, Sn, Cu

  • copper 12-30%
  • particle types: spherical and lathe cut
56
Q

in single composition formulations (Cu enriched) what is the setting reaction

A

Ag-Sn-Cu + Hg –> Ag-Sn-Cu + gamma 1 + Cu6Sn5

no gamma 2

57
Q

what are the benefits of copper enriched alloys

A
  • higher early strength
  • less creep
  • higher corrosion resistance
  • increased durability of margins
58
Q

out of trad-lathe, trad-spherical, Cu dispersion and Cu single, what are the % creep values

A

trad-lathe (6.3%)
trad-spherical (1.1%)
Cu dispersion (0.46%)
Cu single (0.07%)

59
Q

how do the compressive strength values change between the 4 alloys between day 1 and day 7

A

lathe (45-302 MPa)
spherical (120-370)
CU dispersion (118-387)
Cu single (272-485)

60
Q

what is the thermal expansion coefficient of amalgam compared to enamel and dentine

A

Amalgam (22-28ppm/oC)
enamel (11)
dentine (8)

61
Q

advantages of amalgam

A

strong

user friendly

62
Q

disadvantages of amalgam

A
  • corrosion
  • leakage (doesn’t bond)
  • poor aesthetics
  • mercury (pereived toxicity and environmental impact)
63
Q

which amalgam should we use?

A

Encapsulated
- Hg hygiene

Trad alloys

  • served well, lifetime in excess of 10 years
  • but av lifetime 4-5 years (?)

Copper enriched
- some clinical trials showing material is superior

GDH&S
- we use permite

64
Q

what are the properties of permite

A
  • non-gamma 2, spherical and lathe cut
  • huge compressive strength quickly (260MPa in 1hr, 500MPa in 24hrs)
  • very low microleakage compared to other brands (0.04 ml/min)
  • compressive strength(500)
  • tensile strength (60)
  • elastic modulus (30)
  • hardness (100)
65
Q

how does amalgam compare to other composites failure rate after 8 years

A

much better than all composites (lower failure rate)

BDS2 Dental materials science (10 decks)

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