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Dental Materials > Amalgam 1 > Flashcards

Amalgam 1 Flashcards

1
Q

Amalgams are set by … and is a reaction of …

A
  • amalgamation
  • metals with mercury
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2
Q

Amalgam has been used in dentistry for …, used since …

A
  • 1000
  • 1830s
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3
Q

Amalgam requires the mixing of …

A
  • 2 or more metals including mercury
  • powder (alloy) and liquid (mercury)
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4
Q

For dental amalgam, what is mixed?

A
  • alloys have Ag, Sn, Cu (Zn, Hg)
  • mercury is pure/triple distilled
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5
Q

… and … are the major components of alloy
How are these controlled?

A
  • Ag and Sn
  • concentration controlled to form Ag2Sn (gamma) phase
  • intermetallic compound
  • limit corrosion
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6
Q

Role of Cu in alloy

A

increase strength of set material

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7
Q

Too much copper can lead to …

A

excessive expansion

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8
Q

Zinc has what role in an alloy?

A
  • scavenger
  • reduces oxidation during alloy casting
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9
Q

Some products have mercury in the alloy itself - what is this called?

A

pre-amalgamated

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10
Q

Why is standardising alloy composition important?

A
  • different products have different alloys
  • leads to inconsistent performance
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11
Q

How to control alloy composition

A
  • ISO standards defined concentration ranges - more consistency
  • on weight percentage at least 65 silver, max 29 tin, max 6 copper, max 2 zinc
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12
Q

ISO specification pre 1986

A
  • termed traditional amalgams
  • high incidence of premature clinical features
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13
Q

Give the 3 steps of the setting reaction of amalgam

A
  • powder and mercury is mixed
  • step 1 is dissolution of small particles of alloy, diffusion of mercury into surface of larger particles
  • stage 2 is a reaction to form gamma 1 and 2
  • stage 3 is crystallisation of gamma 1 and 2
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14
Q

Set amalgam has a … structure
This consists of what?

A
  • composite
  • particles of unreacted alloy (Ag3Sn gamma phase)
  • matrix of reaction products and mercury (Ag2Hg3 and Sn7Hg gamma 2 phase)
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15
Q

Alloy particles are responsible for … in set amalgam
explain

A
  • strength
  • need to control mercury concentration for mixing
  • add enough mercury to make plastic mass but not too much which causes too much gamma 1 and 2 production
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16
Q

In MPa, tensile stregnth of phase
- gamma
- gamma 1
- gamma 2
- amalgam ?

A
  • 170
  • 30
  • 20
  • 60
17
Q

Mechanical properties of set amalgam

A
  • similar to enamel and dentine
  • good compromise to withstand oral forces
18
Q

Why can there be premature failures of set amalgam?

A
  • low tensile strength
  • margins susceptible to breakdown (called ditching)
19
Q

How to increase tensile strength in set amalgam?

A
  • beware of thin sections in bending
  • need proper cavity design
20
Q

In the past, was ditching common? What was the solution?

A
  • common
  • dentists would revise restoration
  • but would facilitate secondary caries
  • early signs of mechanical failure
  • revised restorations are larger and fail faster
21
Q

2 theories for causing ditching

A
  • creep model
  • corrosion model
22
Q

Explain the Creep model of marginal breakdown

A
  • creep is slow deformation under stress
  • stress is often below yield stress, deformation is plastic/permanent (dynamic or static)
  • model proposed amalgam is slowly deformed in the cavity, amalgam would extend below cavity margins and amalgam above margins will break under tension
23
Q

Testing of components showed … is the main culprit for creep marginal breakdown

A

gamma 2 phase

24
Q

Explain the corrosion model of marginal breakdown

A
  • corrosion occurs when theres 2 or more dissimilar metals in contact, electrolytes which saliva is, amalgam contains multiple metals
  • corrosion results in discolouration, mercury release, weaken thr material, metallic taste, delayed expansion but can seal margins
25
Q

Gamma 2 phase forms what?

A

anode

26
Q

The Gamma 2 phase is the … and … phase
This is when … and … happens

A
  • softest. weakest
  • creep and corrosion
27
Q

How have modern amalgams reduced the gamma 2 phase to reduce creep and corrosion?

A
  • increased copper
  • increases final strength
  • two phases form e and n phase which is slower than gamma 1 and 2
  • too much can lead to expansion but good otherwise
28
Q

How to achieve increased copper in amalgam?

A
  • admixed amalgams
  • two alloy powders - traditional and a 72% silver/28% copper - eutectic
29
Q

How is the setting reaction different in increased copper amalgam?

A
  • gamma 1 and 2 form as normal
  • more copper means gamma 2 becomes unstable
  • e and n form over time
30
Q

How have we developed to have no gamma 2 phase at all?

A
  • contain precious metals like palladium
  • even lower creep and corrosion
31
Q

Alloy powder is made by 2 methods. Name them

A
  • lathe cut
  • atomisation (termed spherical alloys)
32
Q

Explain lathe cut made powder

A
  • alloy cast into cylinders (ingots)
  • lathe used to cut ingots into flakes - called grains
  • different grain sizes possible
33
Q

Explain atomisation of alloy powder

A
  • termed spherical alloys
  • molten alloys sprayed into inert atmosphere
  • solidifies into spheres
34
Q

What are admixtures?

A

combination of lathe and spherical particles

35
Q

Explain process of dimensional changes in setting amalgam

A
  • initial contraction (diffusion/dissolution stage)
  • expansion (crystalisation stage)
  • small overall change due to expansion and contraction balance (0.1-0.2%)
36
Q

Excess dimensional change on setting causes what?

A
  • protrusion or cracking of teeth
37
Q

Effect of water contamination on zinc

A
  • delayed expansion
  • combines to make zinc oxide and hydrogen
  • corrosion too
38
Q

Role of zinc in amalgam

A
  • oxides easily form on molten alloys - these are weak and reduce quality
  • zinc oxidises faster than silver, tin and copper so acts as a scavenger for oxygen
  • zinc oxide is scraped of the molten alloy
  • atomised powders formed without oxygen - don’t need zinc

Dental Materials (17 decks)

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