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BDS2 - dental material science > amalgam > Flashcards

amalgam Flashcards

1
Q

what is dental amalgam

A

an alloy formed by the reaction of
> mercury (liquid)
and
> silver, tin, copper and other metals (powder)

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2
Q

what is an alloy

A

consists of 2 or more metals

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3
Q

what are the 2 ways to classify amalgam

A

> composition

> particle size and shape

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4
Q

classify amalgam in terms of composition

A

> traditional

> copper enriched

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5
Q

what does particle size and shape of amalgam affect

A

affects handling and final properties

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6
Q

what is the the average composition of amalgam

A
  • 70% Ag
  • 25% Sn
  • 3% Cu
  • 1% Zn (a lot of today’s amalgams don’t have zinc and are zinc free - traditional amalgams have a small %)
  • 1% Hg
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7
Q

what is included in the powder constituent of amalgam

A

> silver, tin
copper
zinc
Hg in powder

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8
Q

what is the function of silver, tin

A

intermetallic compound Ag3Sn

gamma phase - reacts with mercury (Hg) to form amalgam

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9
Q

what is the function of copper

A

increases strength and hardness

added to improve material

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10
Q

what is the function of zinc

A

scavenger during production
preferentially oxidises and slag formed / removed
some amalgams are zinc free

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11
Q

what is the function of mercury in powder

A

only a few materials have this
pre-amalgated alloys so it will react faster
[silver, tin and copper then mix with amalgam grounded down]

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12
Q

what is included in the liquid constituent of amalgam

A

mercury (Hg)

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13
Q

what is the function of mercury in liquid

A

reacts with other materials

it is triple distilled = very pure

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14
Q

name 2 particle types

A

> lathe cut

> spherical, spheroidal

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15
Q

what are lathe cut particles

A

> coarse, medium, fine
formed by filing ingots
creates lots of fragments / shards / shavings

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16
Q

what are spherical, spheroidal particles

A

> range of particle sizes
formed by spraying molten metal into inert atmosphere
easier to control
form globules on surface underneath

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17
Q

what is the setting reaction of amalgam

A

Ag3Sn + Hg = Ag3Sn + Ag2Hg3 + Sn7Hg9

gamma = gamma + gamma 1 + gamma 2

powder + liquid = unreacted particles + amalgam matrix

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18
Q

what is present in the set structure of amalgam

A

> gamma
gamma 1
gamma 2

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19
Q

what is gamma

A

intermetallic silver tin (Ag3Sn)

gives good strength and corrosion resistnace

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20
Q

what is gamma 1

A

Ag2Hg3
gives good corrosion resistance
its like a soup which encapsulates the rest

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21
Q

what is gamma 2

A

Sn7Hg9
it is weaker and has poor corrosion resistance
spherical particles

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22
Q

what do voids do

A

decrease strength

increase corrosion

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23
Q

what is the tensile strength of the different amalgam components

A

> gamma = 170
gamma 1 = 30
gamma 2 = 20
= amalgam = 60

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24
Q

what is the setting dimensional changes in traditional amalgam

A

initial contraction - solution of alloy particles in mercury

then expansion - gamma 1 crystallisation

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25
Q

what is the setting dimensional changes in modern amalgam

A

small contraction
solid solution of mercury in Ag3Sn
advantage = doesn’t expand

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

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26
Q

what happens with zinc when it interacts with saliva / blood

A

Zn + H2O = ZnO + H2

bubbles of H2 formed within amalgam
> pressure build up causes expansion
> downward pressure causes pulpal pain
> upward pressure means the restoration will sit proud of the surface
forcing material to expand = not a desirable effect hence why there is zinc free materials

27
Q

what is amalgam properties dependent on

A 
> dependent on handling factors
- proportioning and trituration 
- condensation
- carving and polishing 
> dependent on cavity design
> variations in products
> affected by corrosion
28
Q

name mechanical properties of amalgam

A

> strength
abrasion resistance
creep
biocompatibility

29
Q

what is the abrasion resistance like for amalgam

A

high
suitable for posterior teeth
too high for deciduous teeth

30
Q

what is the problem with creep

A

affects marginal integrity
changes shape
problem in tradiitonal materials
(depends on cavity design and corrosion)

31
Q

what is the biocompatibility of amalgam

A

concern over mercury toxicity

32
Q

what factors decrease strength

A

undermixing
too high mercury content after condensation
too low condensation pressure
slow rate of packing - increments don’t bond
(don’t want undulations on surface)
corrosion

33
Q

what is creep

A

When a material is repeatedly stressed for long periods at low stress levels (ie stress below the elastic limit / will not cause the tooth to fracture) it may flow, resulting in permanent deformation [will change shape]

34
Q

what does creep do to the restoration

A

causes the amalgam to change shape and sit proud of the tooth surface
protrudes around the margins
margins are exposed so when forces are applied the amalgam will fracture
resulting in ditched margins

35
Q

what is the thermal expansion of amalgam

A

3 times more than the tooth

expands and contracts more than the tooth so it can leaev gaps

36
Q

what is the thermal conductivity of amalgam

A

high

may need to use a liner / varnish (insulator) in deep cavities to protect the pulp

37
Q

is amalgam compatible with bonding systems

A

amalgam does not bond to the tooth
needs mechanical retention
cavity walls need to splay outwards

38
Q

what is the handling of amalgam like

A
  • mixing, working times and setting times are ok

- varies between types

39
Q

what is the viscosity of amalgam like

A

packed (condensed) into cavity

user friendly

40
Q

what is the aesthetics of amalgam

A

poor

dont use anteriorly

41
Q

what is the radiopacity like of amalgam

A

yes radiopaque

will show if you have left gaps in the restoration

42
Q

is amalgam anticariogenic

43
Q

does amalgam give a smooth surface

A

yes if polished well

may deteriorate over time

44
Q

what is the setting shrinkage of amalam

A

modern materials tend to have net overall shrinkage

45
Q

how does corrosion happen in amalgam

A

gamma 2 more electronegative
weakens materials (particularly at margins)
corrosion products may contribute to sealing margins
reduce by
> copper enriched polishing margins
> avoiding galvanic cells

46
Q

what are the advantages of spherical particles

A
  • Less mercury required
  • Higher tensile strength
  • Higher early compressive strength
  • Less sensitive to condensation
  • Easier to carve
47
Q

what are copper enriched alloys also called and why have they this name

A

also called non-gamma 2

high copper content (>6%)

48
Q

what are the types of copper enriched

A

> dispersion modified
- original

> single composition types
- introduced to increase uptake by profession

49
Q

what is dispersion modified copper enriched amalgam

A

> originally Ag-Cu spheres and conventional lathe cut alloy (now some single composition dispersed alloys - spheres and lathe cut particle same composition)
originally thought spherical particles would act as a strengthening agent
but increased copper content gave beneficial modifications to setting reaction

i dunno if this made sense lol love my notes x

50
Q

what is the dispersion modified setting reaction

A

as conventional material

  1. gamma + Hg = gamma + gamma 1 + gamma 2
  2. gamma 2 + Ag-Cu = Cu6Sn5 + gamma 1
    (takes several days but gets rid of gamma 2)
51
Q

what does the dispersion modified setting reaction do to the silver copper

A

adds a copper tin halo to it

Cu6Sn5 goes around the Ag-Cu

52
Q

what is the difference between single composition and dispersion modified

A

it goes straight to having no gamma 2 present instead of having the 2 separate reactions

53
Q

what is in single composition formulations

A

powder - Ag-Sn-Cu

particle types = spherical and lathe cut

54
Q

what is the setting reaction of the single composition formulation

A

Ag-Sn-Cu + Hg = Ag-Sn-Cu + gamma 1 + Cu6Sn5

55
Q

what are the benefits of copper enriched amalgam

A

> higher early strength
less creep
higher corrosion resistance (will last longer)
increased durability of margins (more difficult to break down)

56
Q

how is each amalgam material affect by creep

A

> traditional lathe cut = 6.3% = massive
traditional spherical = 1.1%
copper dispersion = 0.46%
copper single = 0.07%

57
Q

what amalgam material has the best compressive strength

A

copper single

even after 1 day placed it is double that than any of the others

58
Q

what is the thermal expansion coefficient of amalgam

A

22-28 ppm/degrees celsius

59
Q

what are the advantages of amalgam

A

• Strong
• User friendly
○ Especially copper enriched

60
Q

what are the disadvantages of amalgam

A 
• Corrosion
• Leakage
	○ Does not bond
	○ Potential to creep
• Poor aesthetics
• Mercury 
	○ Perceived toxicity 
	○ Environmental impact
61
Q

what is the advantage of encapsulated amalgam

A

mercury hygiene

62
Q

what is good about traditional amalgam

A

lasts a long time

average = 4-5 years but can last longer than 10 years

63
Q

what amalgam does the dental hospital do

A

permite
has a high compressive strength
lower microleakage

BDS2 - dental material science (13 decks)

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