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Dental Biomaterials > Amalgams > Flashcards

Amalgams Flashcards

1
Q

What is an amalgam?

A

A trituration - vigorously mixing the components of amalgam to bring them into contact with mercury

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2
Q

Why is amalgam a quaternary alloy?

A

It consists of mercury, silver, tin and copper

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3
Q

What are the properties of these alloys?

A

Layers slide over each other making material ductile and malleable

  • have high compressive and tensile strength
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4
Q

Why is amalgam an intermetallic compound?

A
  • Does not have a lattice, ions are joined in a specific crystal structure with a set ratio of different ions
  • Does not have sheets of ions that can slide past each other
  • Typically more than one phase (or ratio of ions)
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5
Q

What are the properties of intermetallic compounds?

A
  1. Less ductile and malleable - more brittle
  2. Set structures and fixture ratios
  3. Conductive - sea of free electrons
  4. Huge compressive strength, low tensile strength
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6
Q

What are the pros of amalgam?

A
  • Used for 200yrs so have a lot of data on it
  • Comparatively easy to handle
  • Good working time
  • Adhesive not needed
  • Good longevity and safety data
  • Favourable handling and clinical properties
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7
Q

What is amalgam made of?

A

Silver, Tin, Mercury, Copper, (Zn in processing)

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8
Q

What is the set up of the amalgam capsule?

A

Supplied in a capsule with a membrane in the middle
Elemental mercury and alloy powder separated
Sometime a little mercury is mixed in with the alloy powder
Put in amalgamator

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9
Q

What are the proportions of different metals in amalgam?

A

50% mercury
50% everything else

67-74 silver
25-28 - tin
0-6 - copper

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10
Q

What are amalgams used for?

A
  1. Direct restorations
  2. Posterior regions (aesthetics, strength and other mechanical properties)
  3. Larger restorations
  4. Where composite or other materials have failed
  5. Cost - cheap and takes less time to complete filling.
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11
Q

What are the lathe cut alloys of amalgam like?

A
  1. Solid ingot bar of alloy cut using a lathe or a similar instrument to create irregular chippings
  2. Graded to select a range of sizes
  3. Production creates highly stressed, highly reactive particles
  4. Stressed particles must be heat treated to relieve some of the stress so that the particles do not react too quickly.
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12
Q

What are the spherical alloys like (dominant form)?

A
  1. Metals molten and then sprayed into an inert atmosphere where they condense as spherical droplets
  2. Easier to incorporate copper with spherical alloys
  3. Easier to triturate as particles wetted with liquid mercury more easily
  4. Flows more readily so easier to adapt than lathe cut
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13
Q

What are the dispersed (admix) phase alloys like?

A
  • Both spherical and lathe cut particles mixed together
  • Lathe cut particles are silver-tin
  • Spherical particles are silver-copper
  • Combine the properties of the spherical and lathe-cut
  • Most amalgams are dispersed phase nowadays
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14
Q

How is amalgam handled?

A
  • No touch technique
  • Worked into cavity and condensed with an instrument
    -Bring XS mercury to the surface where it can be removed
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15
Q

What are the 3 phases of amalgam?

A
  • Mixing mercury and alloy initiates reaction
  1. Silver-tin = gamma
  2. Silver-mercury = gamma 1
  3. Tin-mercury = gamma 2
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16
Q

Why does gamma 2 phase cause issues?

A
  • Low in strength
  • Prone to galvanic corrosion
17
Q

How is the gamma 2 phase eliminated?

A

Copper is added to eliminate this phase

This converts gamma 2 phase to gamma 1

Modern amalgams are all high copper

18
Q

Describe the setting of amalgam

A

Initial small contraction followed by expansion and gamma 1 forms -> net small contraction

19
Q

How does amalgam bond to tooth?

A
  • Amalgam is no adhesive to tooth tissue
  • Undercut cavity design required which leads to the destruction of healthy tissue
  • Can be used in conjunction with adhesives suitable for alloys, e.g. Panavia
20
Q

What are the advantages of amalgam?

A
  • Cheap
  • Quick to use
  • Lasts a long time
  • Not as moisture sensitive as other direct materials
  • Strong in compression e.g. in larger posterior restorations
21
Q

What are the disadvantages of amalgam?

A
  • Aesthetics
  • Not adhesive - more destructive cavity prep
  • Patient safety issues
  • High thermal conductivity
  • Not v strong in tension and brittle
  • High coefficient of thermal expansion (expands on heating)
  • Passes heat from the top of the amalgam to the base so can be problematic if near pulp
  • Linings in larger restorations for insulation to protect from temperature changes
  • Expansion can cause remaining tooth structure to be pushed outwards causing fatigue and stress in enamel and dentine
  • Different brands for different parts of enamel
22
Q

Describe the issues with the low tensile strength of enamel

A
  • Can lead to fracture at isthmus
  • Once fractured, retention is compromised and there are more fractures
  • Enamel has low tensile strength
  • Enamel/dentine have lower coefficient of therm expansion than amalgam
  • Amalgam expands when heated, stressing adjacent enamel
  • Enamel becomes fatigued and fractured over time
23
Q

Describe ditching and creeping

A
  • Biting forces can cause enamel to flow or creep to the edges
  • Parts sticking out can snap due to low tensile strength and becoming brittle
  • Causes ditches in sides of restorations and acts as a food trap allowing for secondary caries to occur
24
Q

What is ditching a consequence of?

A
  • Slow flow of amalgam (some ductability)
  • Eventually pieces protrude
  • Easily snaps off
25
Q

What issue is caused by amalgam not adhering to tooth tissue?

A

Microscopic gap means that secondary caries can occur

26
Q

Describe aesthetic failure of amalgam

A
  • Sometimes possible to cover with a layer of composite
  • Tarnish over time allows things to become stuck
27
Q

Describe the anxiety related failure

A
  • Health concerns
  • Removal leads to short-term exposure to much more mercury
28
Q

What are the health and safety considerations when it comes to enamel?

A
  • Elemental mercury is toxic and vaporises rapidly
    -BUT amalgam is an alloy, not elemental mercury
  • Some Hg released into the body over lifetime
  • Pt most at risk during placement and removal
  • Greater release in bruxist/chewing
  • Rare - pts can have allergic reactions to amalgam
  • Galvanic corrosion can release Hg
  • Dental team more at risk than pt
29
Q

Comparative strengths of amalgam

A
  1. Amalgam = higher compressive strength of enamel
  2. Amalgam has higher tensile strength than composite
    - Alginate has lower modulus of elasticity than enamel (more flexible)
    - co-efficient of thermal expansion of enamel is lower than GIC (quite similar)

Dental Biomaterials (5 decks)

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