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BDS2; Dental materials > DML4: Amalgams > Flashcards

DML4: Amalgams Flashcards

1
Q

Define an amalgam

A

When mercury is mixed with another metal (commonly mercury and a silver tin alloy)

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2
Q

What is conventional/traditional amalgam

A

Low copper amalgam ~ 0 - 6%

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3
Q

What type of amalgam is commonly used today

A

High copper amalgam ~ 12 - 30%

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4
Q

Describe the variable components of high copper amalgam

A
  1. Mixture of two alloys (admix or dispersed phase)
  2. A single ternary alloy (not containing zinc)
  3. Quaternary alloy
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5
Q

What is the function of silver in amalgam

A
  • increase strength
  • increase tarnish and corrosion resistance
  • increases expansion
  • decreases flow and creep
  • decreases setting time
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6
Q

What is the function of tin in amalgam

A
  • increases setting time
  • increases flow and creep
  • decreases strength
  • decreases expansion
  • decreases corrosion resistance
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7
Q

What is the function of copper in amalgam

A
  • increases strength and hardness
  • increases tarnish and corrosion resistance
  • increases expansion
  • decreases flow and creep
  • decreases setting time
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8
Q

What are the main advantages of copper in amalgam

A

increased strength and decreased creep

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9
Q

What is the function of zinc in amalgam

A
  • scavenger

- decreases oxidation of other metals

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10
Q

What is the function of mercury in amalgam

A

Activated the reaction; without this there will only be an alloy powder

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11
Q

What is the function of palladium in amalgam

A
  • increases tarnish and corrosion resistance

- increases strength

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12
Q

What is the function of indium in amalgam

A
  • increases strength
  • increases expansion
  • increases setting time
  • decreases flow and creep
  • decreases surface tension and so the amount of mercury needed is less
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13
Q

Describe for lathe-cut particles are achieved

A
  1. Lathe-cut particles are aged by annealing at 100 degrees C to relieve internal strains
  2. The surface is treated with acid to increase reactivity by removing the oxides formed on the surface

Lathe-cut particles have a rectangular shape and can be

  • micro-cut
  • fine-cut
  • coarse-cut
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14
Q

How are spherical particles achieved

A
  1. Atomising molten alloy in an inert atmosphere

2. Acid washing to increase reactivity

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15
Q

Why are spherical particles more advantageous in amalgams than lathe-cut

A
  1. Require less mercury as there is a smaller surface area
  2. Require less mixing time
  3. Require lower condensation pressures (rounded particles move over more easily)
  4. Harden more rapidly
  5. Smooth surface for overall restoration
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16
Q

Outline the setting reaction for dental amalgams

A
  1. Mercury initially dissolves the surface of the alloy particles
  2. New mercury containing alloys formed
  3. Need to condense prior to crystallisation of new alloys
  4. Reaction is never complete
  5. Set amalgam contains unreacted Ag3Sn in a matriculates of the newly formed alloy giving a cored structure
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17
Q

What happens in conventional low copper amalgams upon setting

A

Mercury dissolves Ag and Sn from alloy and a new intermetallic compound is formed

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18
Q

What is the strongest phase in amalgam

A

Gamma Ag3Sn

  • this corrodes the second least
  • forms 30% of the set amalgam volume
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19
Q

What is the second strongest phase in amalgam

A

Gamma 1 = Ag2Hg3

  • matrix for unreacted alloy
  • this corrodes the least
  • 60% of volume
20
Q

What is the weakest and softest phase in amalgam

A

Gamma 2 = Sn7-8Hg

  • corrodes fastest, forming voids within amalgam
  • 10% of volume (this decreases over time due to erosion)
21
Q

Outline the setting reaction for admix high-copper amalgams

A
  1. Hg dissolves Ag from Ag-Cu spherical eutectic particles
  2. Hg dissolves Ag and Sn from Ag3Sn particles
  3. Gamma 2 reacts with Ag-Cu to form Cu6Sn5
  4. This happens around unconsumed Ag-Cu particles
22
Q

Outline the setting reaction in single phase high-copper amalgams

A
  • Ag and Sn dissolve in Hg
  • Gamma 1 crystals grow to bind gamma particles together
  • Cu6Sn5 forms within gamma 1 phase

This is a more direct setting reaction compared to admix reactions (there is no gamma 2 phase so the set amalgam is stronger and corrodes less)

23
Q

Outline the dimensional changes occurring in amalgams

A

Net shrinkage

  1. Initial shrinkage from dissolution of the alloy
  2. Expansion from the growth of crystalline phases and formation of intermediate compounds
  3. The higher the mercury content, the higher the expansion
  4. Remains stable after 6-8 hours
24
Q

When is the dimensional shrinkage greater

A
  1. Smaller particle size as more dissolution
  2. Higher Hg/alloy ratio as more dissolution
  3. Trituration time longer and more rapid as more dissolution
  4. Condensation pressure higher as lower mercury and less alloy formation
25
Q

What is trituration and how is this carried out

A

The mixing of dental amalgams; done by a mechanical mixer

26
Q

Describe the appearance of an under-triturated amalgam

A
  • dry
  • dull
  • crumbly
27
Q

Describe the appearance of a properly triturated amalgam

A
  • shiny mix

- separated from capsule in a single mass

28
Q

Describe the appearance of an over triturated amalgam

A
  • shiny
  • hot
  • wet
  • sticks to capsule
29
Q

Why is an amalgam condensed and what does this mean

A

This is the process of incrementally packing the amalgam into the cavity

  • this removes excess mercury
  • it prevents voids
  • gives optimum marginal adaption
30
Q

What is the difference in condensation for lathe-cut and spherical particle containing amalgams

A 
Lathe-cut = smaller condenser, higher pressure
Spherical = larger condenser, lower pressure
31
Q

Why is the amalgam carved and burnished to finish the restoration

A

Carving = brings Hg to the surface (spherical gives a better surface finish)

Burnishing = removal of residual mercury to give better margins

32
Q

When can delayed expansion of an amalgam occur

A

If a zinc containing alloy becomes contaminated with water which causes hydrogen production - this causes a build up in pressure which can cause the restoration to rise up from the cavity

33
Q

Why do amalgam restorations tarnish

A

This surface discolouration occurs because of black silver sulphide formation which can be polished off - this doesn’t cause long term problems and has no clinical effects

34
Q

Why does galvanic corrosion of amalgams occur

A

This happens because one of the metals is more electronegative and so there will be corrosion of one metal and the other will be unaffected

35
Q

Why does crevice corrosion of amalgams occur

A

Due to pace build up which increases electronegativity with respect to a clean surface

36
Q

Why does stress corrosion of amalgams occur

A

This happens due to sustained tensile force within a corrosive environment

37
Q

What are the disadvantages of amalgam restorations

A
  1. Non-adhesive; mechanical retention relies on cavity design
  2. Conducts heat and electricity
  3. Corrodes and tarnishes
  4. Poor aesthetics
  5. Amalgam is viscoelastic - creep
  6. As it is brittle, amalgam is weak in thin sections so more tooth tissue has to be removed
38
Q

Why do amalgam restorations have marginal leakage

A
  1. Because the coefficient of thermal expansion is higher than dentine so when the material expands and contracts it is different to that of dentine and so produces a gap
  2. Because low copper amalgams corrode and can result in poor sealing of the margins
39
Q

What is the benefit of amalgams tarnishing over time

A

The production of metal sulphides when the amalgam tarnishes provides a marginal seal over time

40
Q

Why are amalgam bonding agents used

A

These contain phosphonate esters and improve the retention and reduce the marginal leakage to reduce postoperative sensitivity

41
Q

How does marginal breakdown occur in amalgam restorations

A
  1. Creep can lead to flow over the margins and so a thin layer of amalgam seeps over
  2. The thin amalgam layer will fracture
  3. This results in marginal deterioration (ditching)
  4. Over filling can have the same effects
42
Q

What are lichenoid lesions associated with

A

Old and corroded amalgams

43
Q

How can an amalgam tattoo occur

A

This is a greyish black pigmentation that happens when

  • removing an old amalgam filling
  • when removing a tooth with an amalgam restoration particles can enter the surgical wound
  • amalgam dust in oral fluids; in abrasion areas
44
Q

What is a gallium alloy and what are its strengths and weaknesses

A

Developed to replace mercury in amalgams

Strengths

  • setting expansion was low
  • good compressive strength
  • low creep
  • rapid solidification
  • biocompatibility and cytotoxicity

Weaknesses

  • poor corrosion resistance
  • prone to delayed expansion
  • expensive
45
Q

What are the issues of fluoridated amalgams

A

More susceptible to corrosion

46
Q

What are the issues with consolidated silver in amalgams

A

This is where spherical alloy particles are condensed and fluoroboric acid is used to lean

Strain hardening makes condensation difficult and so higher pressures are needed to remove voids

BDS2; Dental materials (11 decks)

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