L12 Dental Silver Amalgams 2 Flashcards
Describe the manipulation of amalgam (characteristics)?
Ease of manipulation
What is the marginal adaptation of amalgam like (material characteristics)?
Acceptable marginal adaptation (how well it locks to the surfaces of tooth):
Will shrink slightly (not as much as composite). If shrink exceeds bond strength then you can form gaps causing invasion of bacteria.
Lock and key mechanical retention of cavity, so less conservative
Describe the technique sensitivity of amalgam (characteristics)?
Technique insensitivity
Far less technique sensitive than composites
Describe the sealing nature of amalgam (characteristics)?
Self-sealing? Textbooks say corrosion products fill gaps at the interface (older amalgams). But obvs don’t want corrosion products in the mouth.
Describe the biocompatibility of amalgam (characteristics)?
Not biocompatible - ambiguous. Toxic but if placed correctly there should be no health concerns unless they are allergic.
Describe the wear resistance of amalgam (characteristics)?
Good fracture toughness and is wear resistant
Metallic, so strong
Describe the cost of amalgam (characteristics)?
Cheaper to place. Not the cost of materials, the time to place - might take 4x longer to place a resin composite.
Describe the caries risk with amalgams (characteristics)
Silver amalgams are cariostatic. Bacteria don’t like the amalgam surface unlike in resin composite.
Ideal range of mercury content by weight
44-48%
What happens if >55% weight of mercury?
Decreased strength, increased corrosion, increased creep, therefore:
Poor condensation techniques and delay between trituration and condensation
Is it elemental or residual Hg in mercury?
Residual, NOT free elemental Hg so will not be released to cause negative effect.
Where is the highest residual Hg and why?
Highest residual Hg at surface and margins due to the process of condensation
Condensation squashes the particles together to squeeze and make it a smaller volume. Should never have excess that rises to the top.
What are the particle types of amalgam?
Spherical, lathe, or admix
What does surface area of particles affect?
Surface area affects wettability. Spherical only - don’t need as much mercury
Are amalgams fast setting materials?
No, takes more than 24hrs to reach 100% of mechanical properties, due to the precipitation of the different phases it takes a while.
Do spherical or lathe cut amalgams set faster?
Spherical alloys set faster since require less Hg
Is set amalgam brittle?
Set amalgam exhibits brittle failure:
- intermetallic alloy
- can snap if in thin section, especially without support
- tensile strength less than compressive strength
- weak in think sections
- unsupported edges fracture easily
What is creep?
Time dependent response of a solid to stress
How does creep change with melting point in amalgam?
Creep increases as melting point is reached
Which phases result in creep?
Phases containing Hg in rich concentrations result in creep
What is the consequence of creep in amalgam?
Amalgam flows over marginal ridge and thins
Thin sections fracture easily
Marginal deterioration = ‘ditching’
Which phase decreases corrosion resistance?
y2 phase decreases corrosion resistance
which phase is the most electronegative and what does this mean?
y2 phase is more electronegative than y and y1
It acts as an anode in solution and slowly dissolves
In what way is the corrosion reaction insidious?
Aggressive unstopping reaction
Free Hg allows further reaction with y to produce more y1 and y2
What is the corrosion reaction?
Sn7Hg (y2) + oral fluids —> tin salts + free Hg
What is the dimensional change of amalgams?
Small contraction in the first 30mins as mercury diffuses into alloy particles
Expansion as crystallisation starts
What is net contraction in modern amalgams?
Slight contraction, might be enough to form gaps
How are gaps in amalgam usually formed?
Gaps are mainly formed due to placement processing and condensation pressure
What factors affect micro leakage?
2-20micrometre gap
Affected by poor condensation technique
Corrosion by-products - mercury will be at the sealing edge
Differences in TEC - amalgam is more similar to teeth than resin composites
Type of particle
How does type of particle affect micro leakage?
Spherical exhibit increases in micro leakage - more technique sensitive and harder to condense?
Lathe cut exhibit better marginal adaptation
What are operator variable?
Trituration
Condensing
Carving/polishing
What is trituration?
To coat each particle with Hg. Thorough amalgamation ensured. Use recommended times.
How does condensation affect amalgams?
Better condensation achieves better marginal adaptation. Eliminates voids. Removes excess Hg. Spherical alloys more difficult to condense.
What does carving/polishing do?
A function of particle morphology
Polishing increases hardness and decreases porosity (might form airborne particles)
What are the problems with amalgam?
Lack of aesthetics
Non-adhesive - so less conservative
Lack of strength and toughness (last long in mouth)
Susceptible to corrosion
Lack of biocompatibility (most risk during placement or removal)
What are the environmental hazards with amalgam?
Natural emission
Human activity - methyl Hg accumulation in food chain, chlor-alkali production
How much amalgam is used in dent?
0.05-0.27%
How do we minimise/eliminate environmental Hg release?
Excess amalgam must be stored appropriately
Units with filters and/or separators
Environmental concerns around the disposal of amalgam. Where is excess amalgam found?
Major amalgam particles - surplus in trituration capsules and carved surplus
Minor amalgam particles - produced during carving, polishing (sucked up and transported via vacuum)
Lost or extracted teeth (solid waste)