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26 Cards in this Set

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  • Back
What are the main metal components of amalgam?
silver, tin and mercury
what are the forms of dental alloy?
lathe cut and spherical
describe characteristics of lathe cut
swarf from machining
has residual stress and hence has increased internal energy and reactivity with Hg
usually annealed
What are the resulting phases after setting?
γ, γ1 and γ2
what does it mean by γ and γ1 and γ2 phases?
γ: unreacted phase
γ1 and γ2: reacted but γ2 is very reactive
What happens to the phases when large amount of Hg remains during setting?
more γ1 and γ2 are formed
Describe about initial contraction in amalgam setting.
Hg seeps into gaps between the alloy. This therefore cause contraction in terms of total volume.
What are the effects trituration?
breaks up alloy particles (increase in surface area of the reaction and removes oxide layer from the alloy)
what is the consequence of short amalgam trituration time?
only small amount of Hg diffuses alloy
less initial contraction
favours crystalisation of γ1 and γ2 => high expansion rate
what is the consequence of long amalgam trituration time?
increase rate of diffusion of Hg into dental alloy hence greater initial contraction and less expansion
What does it mean by condensation in amalgams?
packing in amalgam before it sets inside the cavity
What are the effects of amalgam condensation?
1. Increased pressure=> disturbance in crystalisation => decrease in expansion
2. inhibits initial reaction => less contraction
3. more Hg is removed from surface and less crystallization => less expansion
What happens when contaminated by moisture during trituration and condensation of amalgam?
considerable expansion takes place over 2 to 8 days
What are the effects of efficient condensation?
1. removes excess Hg
2. reduce porosity
3. creates a good marginal seal
4. Reduce post operative sensitivity
How can effective condensation be achieved?
1. use a suitably sized and shaped packer
2. add amalgam in small increments
What are the finishing touches added to amalgam restorations?
burnishing and carving
What is the purpose of burnishing?
1. increase surface hardness
2. better marginal adaption
3. decrease porosity
4. reduce corrosion (crevice)
What are the reasons for carving and polishing?
1. removes the high mercury layer on the surface
2. improves aesthetics (prevents black oxide layer)
Which of lathe cut or spherical alloys give better surface finish?
spherical
What is wrong with traditional amalgams?
suffer from lack of strength and have excessive creep and corrosion
What are the types of corrosion which occur in amalgam restorations?
1. Surface corrosion
2. Crevice corrosion
3. Electrolytic corrosion
What are the consequences of creep in amalgams?
1. flattening of contact points
2. overhanging margins
3. protrusion of restoration at the margins leading to marginal breakdown
What is the mechanism of crevice corrosion?
driven by difference in the levels of oxygen on the surface
What improvements have been made of modern amalgam?
dispersed phase amalgam
- increased compressive strength
- more rapid set at full strength
- decreased creep
- reduced corrosion
What are the common causes of amalgam restoration failure?
1. tooth fracture
2. thermal expansion and contraction
3. Recurrent caries
4. gross amalgam fracture
What are posible systemic adverse effects of a dental material?
toxic
inflammatory
immunological
allergenic