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BDS2 CLINICAL Dental Materials > Amalgam > Flashcards

Amalgam Flashcards

1
Q

What is amalgam

A

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

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2
Q

how can amalgam be classified

A

composition

particle shape and size

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3
Q

what are the two amalgams with differing compositions

A

traditional

copper enriched

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4
Q

what does the particle shape and size determine

A

handling and final properties

it was originally applied to traditional but now also to copper enriched

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5
Q

what is the composition of the powder

A

zinc
silver, tin
copper (copper enriched)
mercury (sometimes)

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6
Q

what is the function of zinc

A

Most amalgams nowadays don’t have zinc in them, they used to have a small amount in them and its purpose was to act as a scavenger as it was preferentially oxidized during the manufacturing process which prevented the other metals in the alloy being oxidized

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7
Q

what is the function of silver, tin

A

silver is the main component, present in combination with tin as the intermetallic compound Ag3Sn known commonly as the γ-phase. It reacts with mercury liquid to form amalgam

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8
Q

what is the function of copper

A

present to increase the strength and hardness of the amalgam

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9
Q

what is the function of mercury in the powder

A

mercury is sometimes added to provide a more rapid reaction in what is referred to as pre-amalgamation

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10
Q

what is in the liquid

A

mercury (50% by weight)

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11
Q

what is the function of mercury

A

it is triple distilled (very pure) and reacts with other metals. The mercury has to be very pure otherwise a surface layer of contaminants is formed that interferes with the setting reaction.

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12
Q

what do the particle types apply to

A

The alloy is used in the form of a powder and the size and shape of the particles in this powder are critical to the handling characteristics and the final properties of the restoration.

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13
Q

what are the two types of particles

A

lathe cut

spherical

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14
Q

how are lathe cut particles produced

A

by machining a solid ingot of the alloy on a lathe. The chippings that are produced are graded and those in the right size range are used in the powder to be amalgamated with mercury.

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15
Q

what are the different types of lathe cut particles

A

The alloy is available as coarse, medium or fine grained powder and each will handle slightly differently.

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16
Q

what should be done to lathe cut amalgam and why

A

The individual chippings will become highly stressed during the machining, and this makes their surface very reactive to mercury. A consequence of this is that the setting reaction is far too rapid unless heat treatment (which relieves internal stresses) is applied. The heat treatment is usually carried out by placing the powder in boiling water

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17
Q

what are spherical particles

A

the various ingredients of the allow are melted together and then sprayed into an inert atmosphere, where the droplets solidify as small, spherical pellets of various sizes. This method of manufacture has the advantage that no further machining processes are required and that the composition of the alloy can be readily altered.

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18
Q

how is the setting reaction for amalgam initiated

A

vigorous mixing of the two ingredients. This mixing causes the other layer of the alloy particles to dissolve into the mercury, forming two new phases which are solid at room temperature

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19
Q

what is the setting reaction for amalgam

A

Ag3Sn + Hg > Ag3Sn + Ag2Hg3 +Sn7Hg
y + mercury > y + y1 + y2
powder + liquid > unreacted alloy + amalgam matrix

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20
Q

what is the set structure for amalgam

A

Not all of the alloy particles dissolve in mercury but rather a considerable amount remains so that the final structure is one of a core of gamma held together by a matrix of predominantly y1 which is interspersed with y2

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21
Q

what is the function of gamma

A

provides good strength and corrosion resistance

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22
Q

what is the function of gamma 1

A

provides good corrosion resistance

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23
Q

what is the function of gamma 2

A

y2 is weak and poor corrosion resistance

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24
Q

what do voids do

A

voids decrease strength and increase corrosion

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25
Q

what has the strongest tensile strength from the amalgam components

A

gamma

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26
Q

what amalgam component has the lowest tensile strength

A

gamma 2

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27
Q

describe the setting dimensional changes in traditional amalgam

A

there is initial contraction (0.2%) as there is a solution of alloy particles in the mercury then there is expansion due to the crystallization of y1 (0.4%)

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28
Q

describe the setting dimensional changes in modern amalgam

A

there is a small contraction (0.2%) and the end result is a solid solution of mercury in Ag3Sn

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29
Q

why was zinc removed from amalgam

A

if some of the zinc is unreacted then it may react with water in saliva/blood to produce bubbles of hydrogen

30
Q

what can hydrogen bubbles forming in the amalgam result in

A
  • Pressure build up which causes expansion
  • Downward pressure causing pulpal pain
  • The material being forced to expand over the surface of the tooth
31
Q

what are the amalgam properties dependent on

A

handling factors

cavity design

32
Q

what are the different handling factors that the properties of amalgam are dependent on

A
  • Proportioning and trituration
  • Condensation – the final mercury content of amalgam is dependent on the quality of condensation technique more than anything else, with a properly condensed amalgam having a mercury content of 50%
  • Carving and polishing
33
Q

describe the compressive strength of amalgam

A

The early compressive strength of amalgam (first hour) of traditional materials is poor however after 24 hours it becomes okay. The strength depends on the proportion of y2 formed and if its proportion can be minimized then this will result in a stronger amalgam

34
Q

describe the abrasion resistance of amalgam

A

high making it suitable for posterior teeth but too high for deciduous

35
Q

describe the factors that decrease strength in amalgam

A
  • Undermixing
  • Too high a Hg content after condensation (composition of y1 and y2 is strongly dependent on the amount of mercury in the final composition – y2 is not strong)
  • Too low condensation pressure
  • Slow rate of packing meaning increments do not bond
  • Corrosion
36
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

37
Q

what does creep affect

A

marginal integrity

38
Q

what materials does creep affect

A

amalgams, alloys, waxes and plastic

39
Q

describe biocompatibility of amalgam

A

there are concerns about mercury toxicity. The NHS concluded that although mercury is released, the quantities are small and do not cause verifiable effects on human beings. While the current evidence supports the concept that existing dental restorative materials are safe, it must be recognized that the supporting data are incomplete.

40
Q

describe thermal expansion of amalgam

A

we want amalgam’s thermal expansion to match that of the tooth and it is 3 times that of the tooth. There is potential that as amalgam warms up, it expands more than dental tissue and this may make a gap for bacteria which results in a risk.

41
Q

describe thermal conductivity of amalgam

A

the thermal conductivity of amalgam is high meaning there is the potential for heat to reach the dental pulp therefore we require a liner to act as an insulator to prevent the heat from going all the way through amalgam and reaching the dental pulp

42
Q

describe bond to tooth systems of amalgam

A

amalgam does not bond to tooth and instead requires mechanical retention.

43
Q

describe aesthetics of amalgam

A

poor

44
Q

is amalgam radiopaque

A

yes

45
Q

is amalgam anticariogenic

A

no

46
Q

is amalgam a smooth surface

A

yes, if polished well, may deteriorate over time

47
Q

describe setting shrinkage of amalgam

A

modern materials tend to have a net overall shrinkage

48
Q

describe mixing, working times, setting times of amalgam

A

okay but varies between types

49
Q

describe viscosity of amalgam

A

packed (condensed) into cavity making it user friendly

50
Q

why was corrosion often cited as an advantage

A

corrosion products help produce a good marginal seal

51
Q

why is corrosion not an advantage in amalgam

A

crevice corrosion, caused by the formation of oxidation cell in the marginal gap can cause rapid deterioration in the properties of amalgam.

52
Q

what phase is the corrosion process especially associated with

A

y2 phase

53
Q

why is the corrosion process especially associated with y2 phase

A

The y2 phase is considerably more electronegative than the y and y1 phases. This means that in the presence of an electrolytic solution the y2 phase will act as an anode of the oxidation cell and will gradually dissolve.
This weakens the material particularly at the margins

54
Q

how is the corrosion process reduced

A

It is reduced by copper enriched amalgam, polishing the margins and avoiding galvanic cells

55
Q

what is the advantages of spherical particles

A
  • Less mercury is required
  • Higher tensile strength
  • Higher early compressive strength
  • Less sensitive to condensation
  • Easier to carve
56
Q

what are the names for copper enriched alloys

A

Can be called non-gamma2 amalgams or high copper amalgams

57
Q

what are the 2 types of copper enriched amalgams

A

dispersion modified

single composition

58
Q

describe dispersion modified copper amalgams

A

Originally a spherical allow (silver and copper) with a higher copper content was added to the conventional lathe cut allow
It was originally thought that spherical particles would act as a strengthening agent but increased copper content gave beneficial modifications to the setting reaction

59
Q

what is the second reaction in dispersion modified copper amalgams

A

Y2 + AgCu  Cu6Sn5 + y1 (takes several days)

60
Q

how can Cu6Sn5 be seen

A

as a halo around the spheres

61
Q

why is the second reaction in dispersion modified amalgams so beneficial

A

It is beneficial as we get rid of the gamma 2 phase

62
Q

what is simple composition amalgams

A

Powder containing silver, tin, copper
Copper content of 12-30%
Particle types are spherical or lathe cut

63
Q

what is the setting reaction for single composition

A

Ag-Sn-Cu + Hg  Ag-Sn-Cu + y1 + Cu6Sn5

64
Q

what are the benefits of copper enriched amalgams

A
  • Higher early strength
  • Less creep
  • Higher corrosion resistance
  • Increased durability of margins
65
Q

what is the thermal expansion coefficient for amalgam

A

Coefficient for enamel is 11 and for dentine is 8
For amalgam it is 22-28 so almost double that of enamel/dentine
Has a better range than composite
GIC has the best thermal expansion coefficient

66
Q

what are the advantages of amalgam

A

strong

user friendly

67
Q

what are the disadvantages of amalgam

A

corrosion
leakage - does not bond
poor aesthetic
mercury (perceived toxicity, environmental impact)

68
Q

what is the amalgam used in GDHS

A

permite

69
Q

what is permit

A

Permite is non gamma 2, spherical & lathe cut
It has a significant compressive strength 1 hour after placement of 260 MPa
It has a high compressive strength of 500 MPa which is greater than other brands
The least microleakage compared to other brands – 0.04 ml/min

70
Q

describe mechanical properties of amalgam

A

higher compressive strength and tensile strength than enamel
less rigid than enamel

71
Q

what is the failure rate for amalgam after 8 years

A

5.8 - lower than composite

BDS2 CLINICAL Dental Materials (15 decks)

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