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