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BDS2 Dental Materials Sciences > amalgam > Flashcards

Flashcards in amalgam Deck (67)
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1
Q

amalgam - valuable restorative material or cause for concern?

A

Mercury is known to be slightly toxic to humans – does this make amalgam a health risk?
- Can’t be proven as a health risk

Lots of dental materials do not undergo appropriate clinical trials

2
Q

2 main types of amalgam

A
  • traditional

- copper enriched

3
Q

what makes amalgam

A

alloy formed by the reaction of:

  • mercury (liquid)
  • silver, tin, copper & other metals (powder)

Alloy is traditionally several metals. Amalgam is several metals and non-metal mercury (which is very dense)

4
Q

use of amalgam

A

widely used

  • England & Wales: 22 million restorations.
  • USA: 160 million restorations
5
Q

how long has amalgam been in use?

A

long history (first 659 AD China)

6
Q

2 ways to classify amalgam

A
  • composition

- particle size and shape

7
Q

how to classify amalgam on composition?

A
  • traditional
  • copper enriched

most of today’s amalgams are zinc free
(silver, tin, copper, mercury)

8
Q

constituents in amalgam powder

A
  • silver, tin
  • copper
  • zinc (sometimes)
  • mercury
9
Q

silver, tin function in amalgam powder

A
  • intermetallic compound Ag3Sn

Gamma phase, reacts with Hg liquid to form amalgam

10
Q

copper function in amalgam powder

A
  • increases strength & hardness

- ensures longevity in oral cavity

11
Q

zinc function in amalgam powder

A
  • scavenger during production - preferentially oxidises & slag formed / removed (sacrifice for other materials)
  • some zinc free
12
Q

mercury function in amalgam powder

A

(few materials)

“pre-amalgamated” alloys - react faster small in powder before adding more in liquid

13
Q

liquid constituents of amalgam

A

mercury (50% by weight)

14
Q

function of liquid mercury in amalgam

A
  • triple distilled (very pure)

- reacts with other metals

15
Q

2 particle types of amalgam

A
  • lathe cut

- spherical, spheroidal

16
Q

lathe cut amalgam particles

A
  • coarse, medium, fine (fine shaving/shard, irregular shape)

- formed by filing ingots

17
Q

spherical, spheroidal amalgam particles

A
  • range of particle sizes
  • formed by spraying molten metal into inert atmosphere form globules
  • easier to control
18
Q

setting reaction

A

Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn2Hg3

gamma –> gamma (unreacted) + gamma1 + gamma2

19
Q

what makes the amalgam matrix?

A

Ag2Hg3 + Sn2Hg3

gamma1 + gamma2

20
Q

what do gamma1 phase do?

A

BINDS/encapsulates UNREACTED AgSn and gamma2 particles

21
Q

set structure of different phases

A

gamma: good strength & corrosion resistance
- lathe cut
- intermetallic compound
- has strength and corrosion resistant

gamma1 good corrosion resistance

  • Soup which encapsulates
  • Gamma1 phase holds gamma and gamma2 phases together
  • Good corrosion resistance but not as strong as gamma

Gamma2 weak and poor corrosion resistance

  • Spherical
  • Weakest and likely to erode

voids decrease strength & increase corrosion

22
Q

gamma strength and corrosion resistance

A

good strength & corrosion resistance

  • lathe cut
  • intermetallic compound
  • has strength and corrosion resistant
23
Q

gamma 1 strength and corrosion resistance

A

good corrosion resistance

  • Soup which encapsulates
  • Gamma1 phase holds gamma and gamma2 phases together
  • Good corrosion resistance but not as strong as gamma
24
Q

gamma2 strength and corrosion resistance

A

weak and poor corrosion resistance

  • Spherical
  • Weakest and likely to erode
25
Q

tensile strength of amalgam

A

has the mean value of all the stages of amalgam components in the reaction phases

26
Q

traditional amalgam setting dimensional changes

A
  • initial contraction - solution of alloy particles in Hg

- expansion - gamma1 crystallisation (ends up expanded by 0.4%)

27
Q

modern amalgam setting dimensional changes

A
  • small contraction – closer to original dimension

expansion / contraction is <0.2%, so little clinical sign

solid solution of Hg in Ag3Sn

28
Q

why now zinc free amalgam?

A

interaction of unreacted zinc with saliva/blood -
Zn + H2O –> ZnO + H2

bubbles of H2 formed within amalgam

  • pressure build up causes expansion
  • downward pressure cause pulpal pain
  • upward - restoration sitting proud of surface
29
Q

amalgam properties are dependent on…. (2)

A
dependent on handling factors
- proportioning &amp; trituration
(Membrane that separates the powder and liquid broken on shaking)
- condensation
- carving &amp; polishing

dependent on cavity design

30
Q

mechanical strength of amalgam

A

generally consider compressive but others important

  • early (1hr) - Traditional materials, poor(ish)
  • late (> 24hrs) - OK (develops strength)
31
Q

abrasion resistance of amalgam

A

high,

  • suitable for posterior teeth
  • too high for deciduous

no undulations in surface due to poor abrasion resistance

32
Q

factors decreasing amalgam strength (5)

A

undermixing

too high Hg content after condensation

too low condensation pressure
- not enough pressure when packing into cavity

slow rate of packing
- increments do not bond

corrosion

33
Q

amalgam creep

A

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

(amalgam, alloys, waxes, plastics)

34
Q

why does amalgam creep?

A

visco-elastic

- high in traditional amalgams

35
Q

what does amalgam creep effect?

A

affects marginal integrity - does it maintain good contact with surrounding dental material

NB also marginal integrity depends on:

  • cavity design
  • corrosion
36
Q

biocompatibility of amalgam

A

Concern about mercury toxicity
- Never enough data – so far it is safe

NIH Technology Assessment Conference, 1992 - Effects and Side Effects of Dental Restorative Materials

  • “Although mercury is released from dental amalgams the quantities released are very 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 recognised that the supporting data are incomplete.”
37
Q

thermal expansion of amalgam

A

3 times of tooth tissue

- create gap - risk

38
Q

thermal conductivity of amalgam

A

high, may need to use liner / varnish in deep cavities as heat/cold can reach pulp potentially
- liner acts as an insulator to prevent heat getting to pulp

39
Q

amalgam bond to tooth/compatible with bonding systems

A

does not bond to tooth
- needs mechanical retention
(walls splay outwards)

  • some suggest using new bonding systems based on 4-META to get bond to tooth not yet widely accepted
40
Q

mixing, working and setting times of amalgam

A

OK - reasonable

- varies between types

41
Q

viscosity of amalgam

A

packed (condensed) into cavity

- “user friendly”

42
Q

aesthetics of amalgam

A

poor

- used on posterior teeth only

43
Q

is amalgam radiopaque?

A

Yes

- Able to investigate on X-rays for gaps, parts broken off etc

44
Q

is amalgam anticariogenic?

A

no

45
Q

does amalgam have a smooth suface?

A

Yes, if polished well, may deteriorate over time

- Not a plaque trap if smooth

46
Q

does amalgam have setting shrinkage?

A

modern materials tend to have net overall shrinkage

47
Q

corrosion of amalgam

A
  • gamma2 most electronegative (weakest)

weakens material particularly at margins
- corrosion products may contribute to sealing margins (!)

reduce by

  • copper enriched, polishing margins
  • avoiding galvanic cells
48
Q

advantages of spherical amalgam particles (5)

A
  • less Hg required
  • higher tensile strength
  • higher early compressive strength
  • less sensitive to condensation
  • easier to carve
49
Q

other names for copper enriched alloy amalgams

A
  • non-gamma2

- higher copper

50
Q

how much copper is in copper enriched amalgams?

A

> 6%

51
Q

2 types of copper enriched amalgams

A

Dispersion modified
- original type

Single composition types
- introduced to increase uptake by profession

52
Q

dispersion modified amalgam

A

originally Ag-Cu spheres + conventional lathe cut alloy

  • tried to see if makes stronger - as traditional has weak gamma 2 phase (corrosion likely)
  • (now some single composition dispersed alloys - spheres & lathe cut particle same composition)

originally thought spherical particles would act as strengthening agent, but increased copper content gave beneficial modifications to setting reaction
- 1. as conventional material
(gamma + Hg –> gamma + gamma1 + gamma2)
- Add silver copper reacts with gamma 2 phase
(2. gamma2 + Ag-Cu –>Cu6Sn5 + gamma1 (takes several days))

Converted to copper tin and some gamma 1

No gamma 2 and silver copper particles added have a halo of copper tin

53
Q

single composition copper enriched formulations

A

Incorporate copper into silver tin instead of silver copper particles
- So silver, tin and copper in powder added to liquid mercury

powder Ag - Sn - Cu
- copper 12-30%
particle types: spherical; lathe cut

setting reaction
- Ag-Sn-Cu + Hg –> Ag-Sn-Cu + g1 + Cu6Sn5
No gamma2 phase

54
Q

beneficial property of setting reaction for copper enriched amalgams

A

no gamma 2 phase

55
Q

benefits of copper enriched amalgams (4)

A
  • Higher early strength (first hour; greater than traditional amalgam)
  • Less creep (most common cause of amalgam failure)
  • Higher corrosion resistance
  • Increased durability of margins
56
Q

which amalgam type has the least amount of creep/

A

copper enriched single formulation

over traditional lathe and spherical and copper enriched dispersion

57
Q

constituents of copper enriched dispersion amalgam

A

Ag-Cu spheres + conventional lathe cut amalgam

58
Q

constituents of copper enriched single formulation amalgam

A

silver, tin and copper in powder added to liquid mercury

59
Q

which amalgam type is strongest?

A

copper enriched single formulation

  • Stronger at an early stage and after a week
  • better at resisting fracture in first few days of placement – practical for eating perspective
60
Q

thermal expansion coefficient of amalgam

A

2-3 times greater expansion/contraction than tooth tissues

61
Q

2 advantages of amalgam use

A
  • strong

- user friendly

62
Q

4 disadvantages of amalgam use

A
  • corrosion
  • leakage - does not bond (due to creep)
  • poor aesthetics
    mercury:
  • perceived toxicity
  • environmental impact
63
Q

advantage of encapsulated amalgam

A

Hg hygiene

64
Q

what amalgam is used in GDH?

A

permite
- Non-gamma2, spherical and lathe cut

Stronger compared to other brands

  • More likely to survive for longer time
  • Resisting more forces than other brands

lower microleakage

65
Q

amalgam compressive strength compared to enamel and composite

A

Twice compressive strength of enamel (500 Vs 250 MPa) and stronger than composite (300MPa)

66
Q

elastic modulus of amalgam compared to enamel and composite

A

More rigid than composite (100 Vs 90 KHN) but not as good as enamel (350)

67
Q

what material has the lowest posterior failure rate after 8 years)

A

amalgam (5.8%)