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BCS > Dental amalgams > Flashcards

Dental amalgams Flashcards

1
Q

What are the two types of dental amalgams?

A
  • Conventional

- High copper content (non-gamma 2)

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2
Q

Conventional dental amalgam composition

A 
Alloy powder composition:
-Silver (Ag) 67-74%
-Tin (Sn) 25-27%
-Copper (Cu) 6%
-Zinc (Zn) 2%    +
Triple distilled mercury (Hg)
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3
Q

What is the gamma (γ) phase?

A

Ag3Sn

-intermetallic phase

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4
Q

What does shape and size of powder particles inflluence?

A

Handling
Setting reactions
Final properties of restoration

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5
Q

What are the main types of powder particles?

A 
Lathe cute (course-grain and fine-grain)
Spherical
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6
Q

Setting reaction for conventional amalgams

A

Ag3Sn + Hg (liquid) –> Ag3Sn + Ag2Hg3 + Sn7-8Hg

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7
Q

What happens during setting reaction for conventional dental amalgams

A
  1. Outer layer of alloy particles dissolve in Hg
  2. Gamma 1 phase forms within Hg liquid
  3. Gamma 2 phase forms where Sn reacts with Hg (less)
  4. Unreacted gamma alloy particles in matrix of gamma 1 (&2) matrix
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8
Q

How long do amalgam fillings survive?

A

Longest private ~ 10 years
Then students ~ 8
Then NHS ~ 4.5

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9
Q

Why are amalgams placed?

A 
1 caries
2 caries
Poor margin
Restoration fracture
Tooth fracture
-Almost 50% of time replacing existing restorations
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10
Q

Reasons for replacement of amalgam restorations

A
  1. Tooth fracture
  2. Recurrent caries
  3. Gross amalgam fracture
  4. Marginal breakdown
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11
Q

Tooth fracture

A

Weakened enamel
Undermined enamel
Residual caries

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12
Q

How to prevent tooth fracture

A

Minimal removal of tooth tissue

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13
Q

Undermined enamel

A

In cavity prep, providing flat walls and floors to cavity can undermine enamel
-unsupported enamel will break off and leave gap –> may lead to recurrent caries

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14
Q

Tooth fracture: residual caries

A
  • if any residual caries remains, will spread & undermine cusp –> eventually fracture
  • bacterial toxins will cause inflammation of pulp
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15
Q

Replacement due to recurrent caries %

A

Amalgam adults 72%
Amalgam children 56%
Composites 43%

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16
Q

Recurrent caries

A

Contamination
Poor matrix techniques
Poor condensation

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17
Q

Recurrent caries: contamination

A

Blood & saliva in cavity prevent amalgam from adapting to surface

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18
Q

Recurrent caries: poor matrix techniques

A

-Poorly adapted matrix band –> proximal overhangs / poor contact points with adjacent teeth
–> plaque accumulation
= recurrent caries

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19
Q

Recurrent caries: poor condensation

A

–> porosity and excess Hg –> reducecd amalgam strength

If marginal adaptation is poor, can lead to marginal leakage, recurrent caries and corrosion

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20
Q

Gross amalgam fracture

A

Shallow preparations
Non-retentive proximal boxes
Sharp internal angles

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21
Q

Gross amalgam fracture: shallow preps

A
  • Amalgams have low tensile strength (60MPa)

- If placed as thin sections, subjected to bending forces –> break

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22
Q

Gross amalgam fracture: non-retentive proximal boxes

A
  • frequent
  • partially due to low tensile strength
  • reduce risk by cutting retention grooves
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23
Q

Gross amalgam fracture: sharp internal line angles

A
  • -> stress concentrations

- increase risk of fracture of tooth & restoration

24
Q

Marginal breakdown

A
  • incorrect cavo-surface angles
  • delayed expansion
  • overfilling, underfilling, overcarving
  • creep and corrosion of amalgam
25
Q

Marginal breakdown: wrong cavo-surface angle

A
  • Primary cause of marginal breakdown

- Occurs more readily with acute angles

26
Q

Marginal breakdown: delayed expansion

A

-In Zn containing alloys
-if prep site is not kept dry, Zn can react with saliva
Zn + H2O –> ZnO + H2 -bubbles of H2 form within amalgam, which expand so amalgam expands
-pressure on pulp (pain)
-restoration sits proud

27
Q

Marginal breakdown: overfilling, underfilling, overcarving

A

Overfilling (without carving back) –> ledge –> fracture

Underfilling/ overcarving –> acute amalgam angles –> marginal breakdown

28
Q

Marginal breakdown: creep and corrosion

A

Long term failure

  • unavoidable (intrinsic to material properties)
  • only way to avoid is to change material
29
Q

Define creep

A

Slow movement with repeated force

30
Q

Define corrosion

A

Chemical reaction with environment

31
Q

Problems with amalgam restorations

A
  • lack of aesthetics
  • non-adhesive
  • lack of strength and toughness
  • susceptible to corrosion
  • biocompatibility
32
Q

How to get round problem of non-adhesive amalgams

A
  • cut retention grooves

- use adhesive e.g. Panavia (but then maybe you should use composite)

33
Q

What is amalgam strength linked to?

A

Its constituents (gamma 1 and 2 are the weak phases)

  • amalgam 60MPa
  • gamma 170 MPa
  • gamma 1 30MPa
  • gamma 2 20MPa
34
Q

How do we reduce the amount of gamma 1 and 2 to make amalgam stronger?

A

Use better condensation (packing) technique

-spherical particles require less mercury because easier to mix

35
Q

Lathe cut particles

A
  • Machined from solid ingot
  • Graded chippings
  • Highly reactive without prior heating
36
Q

Spherical alloy benefits

A
  • Reach full strength more quickly
  • Easier to condense
  • Easier to carve and polish
  • Lower mercury content
37
Q

Spherical alloy history

A

Introduced In ~1962
Prepared by atomization process
Same composition as lathe cut alloy

38
Q

How are spherical alloys prepared?

A

-By spraying molten material into inert atmosphere

39
Q

What is Admix?

A

High Cu Dispersed Phase Amalgam Production
-Combination of lathe cut and spherical
AgSn + AgCu alloy

40
Q

Admix setting reactions (modified)

A

γ + Hg + AgCu –> γ + γ1 + AgCu
γ2 + AgCu –> Cu6Sn5 + γ1
γ + Hg + AgCu –> γ + γ1 + AgSnCu
Same plus silver copper phase. Tin mercury reacts with silver copper to make copper tin plus silver mercury. Note no overall tin mercury γ2

41
Q

High Cu Dispersed phase amalgam microstructure

A

Constituents more separated

42
Q

Traditional amalgam: corrosion

A 
Galvanic cell: electrochemical difference in electrolyte --> metal anode corrodes
-γ1 slightly electro+ CATHODE
-γ2 slightly electro- ANODE
-electrolyte = saliva
Therefore no γ2 phase - no corrosion
43
Q

High Cu Dispersed phase amalgam benefits

A
  • Reach full strength more quickly
  • Easier to condense
  • Easier to carve and polish
  • Lower mercury content
  • Higher strength
  • More resistant to corrosion
44
Q

High Cu Single Phase Amalgams setting reaction

A

AgSnCu (Cu>12%) + Hg –> AgSnCu + γ1 + Cu6Sn5

45
Q

High Cu Single Phase Amalgams benefits

A

Same as Dispersed Phase

-no lathe cut particles

46
Q

Why do we use any lathe cut particles if advantages are the same with single phase amalgams?

A

All to do with handling

Takes time to change practices

47
Q

Dental amalgams: variations

A

Conventional: lathe cut (Ag3Sn), spherical (Ag3Sn)
High Cu dispersed: lathe cut Ag3Sn + spherical AgCu // lathe cut AgSnCu + spherical AgSnCu
High Cu single phase: all spherical AgSnCu // lathe cut AgSnCu

48
Q

Which amalgams have highest longevity?

A

Gamma2 free amalgams

49
Q

Biocompatibility

A

Maybe some truth but a lot of bad press has lack of evidence behind it
-mercury itself is very dangerous

50
Q

Amalgam safety

A
  • personal protection

- patient protection

51
Q

Potential Symptoms and hazards of Hg exposure

A
  • respiratory failure
  • kidney impairment
  • cognitive disturbances
  • reduced visuoperceptual and constructional skills
  • memory loss
  • hypertension
  • headaches
52
Q

Potential sources of Hg contamination

A
  • Spills
  • Leaky dispensers or capsules
  • Removing or polishing amalgams
  • Sterilising Hg-contaminated instruments
53
Q

Avoiding Hg contamination

A
  • Use pre-capsulated alloys
  • Avoid direct skin contact with dental amalgam
  • Use high volume evacuation when finishing or removing amalgams
  • Wear mask when removing amalgams
54
Q

Hg storage and disposal

A
  • Store bulk Hg in unbreakable containers
  • Store amalgam scrap under radiographic fixer soln
  • Dispose of contaminated items in sealed bag
  • Report & clean spilled Hg immediately using clean-up kit
  • Wear professional clothing only in dental surgery
55
Q

Additional precautions for Hg

A
  • provide proper ventilation
  • monitor Hg vapour levels in dental surgery
  • use proper work area design
  • use amalgamator with cover

BCS (35 decks)

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