Amalgam Flashcards
What is dental amalgam?
Alloy formed by the reaction of
Mercury (liquid)
Sliver, tin, copper and other metals (powder)
What are the biggest metal components in amalgam?
Sliver
Tin
What are the possible particle types of amalgam? And how are they made?
Lathe cut
-coarse, medium, fine
-formed by filing ingots
Spherical, spheroidal
-range of particle sizes
-formed by spraying molten metal into inert atmosphere
What is the gamma phase of amalgam and what is it composed of?
Good strength and corrosion resistance
Made out of sliver and tin
What is the gamme 1 phase?
Good corrosive resistance
silver and mercury
stronger than gamma 2 but weaker than gamma
What is the gamma 2 phase?
Weak and poor corrosive resistance,
tin and mercury
(copper is added to counteract it)
Why was zinc removed?
Interaction of zinc with water in saliva/blood forms hydrogen bubbles and creates a pressure build up causing expansion
Downward pressure causes pulpal pain
Why is copper present?
increases strength & hardness
eliminates gamma 2 phase
What method is used to make amalgam?
trituration
What are the advantages of spherical particles?
- less Hg required
- higher tensile strength
- higher early compressive strength
- better packing and density
What are the mechanical properties of amalgam?
- Strength
– generally consider compressive but others important
– early (1hr) - Traditional materials, poor(ish)
– late (> 24hrs) - OK - Abrasion Resistance
– high, suitable for posterior teeth – too high for deciduous - Creep
– amalgam visco-elastic
– high in traditional materials
– affects marginal integrity
NB also marginal integrity depends on: * cavity design
What are the factors that could decrease strength?
- undermixing
- too high Hg content after condensation
- too low condensation pressure
- slow rate of packing
– increments do not bond - corrosion
What is creep?
When a material experiences low-level stress levels (ie below elastic limit stress) which are applied repeatedly over a prolonged time period, it may change shape, resulting in PERMANENT DEFORMATION
What can creep cause?
as the amalgam changes shape and the edges of the cavity protrude, the margins become vulnerable to fracture
What is the biocompatibility concerns about amalgam?
concern about mercury toxicity
“Although mercury is released from dental amalgams the quantities released are very small and do not cause verifiable effects on human beings.
While the current evidence supports the concept that existing dental restorative materials are safe, it must be recognised that the supporting data are incomplete.”
What are the thermal properties of amalgam?
- Thermal expansion - x 3 of tooth
- Thermal Conductivity – high, may need to use liner / varnish in deep cavities
- Thermal diffusivity – high, so when a transient thermal stimuli is applied (eg drinking tea) amalgam’s temperature rises quickly
What are the bonding properties of amalgam?
does not bond to tooth
- needs mechanical retention
What is the viscosity of amalgam?
condensed and packed into cavity
What is the corrosive properties of amalgam and how are they reduced?
gamma 2 component most electronegative- weakens material particularly at margins
* reduce by – copper enriched, polishing margins – avoiding galvanic cells
What are the other properties of amalgam?
(aesthetics, radiopaquy, anticarigenic, smooth surface, setting shrinkage?)
- Aesthetics – poor
- Radiopaque - yes
- Anticariogenic - no
- Smooth surface - yes, if polished well, may deteriorate over time
- Setting shrinkage - modern materials tend to have net overall shrinkage - see earlier
What are the types of copper enriched amalgam?
- Dispersion modified – original type
- Single composition types
– introduced to increase uptake by profession
What is the dispersion modified method?
- originally Ag-Cu spheres + conventional lathe cut alloy to eliminate gamma 2 in two step setting reaction
- takes several days as two step setting reaction
What is the single composition formulation?
- powder Ag-Sn-Cu, copper is already incorporated in the alloy
- can be spherical or lathe cut
- one step
What are the benefits of copper enriched amalgam?
- Higher early strength
- Less creep
- Higher corrosion resistance
- Increased durability of margins
What amalgam type has the least compressive strength and the most creep?
traditional lathe cut
What amalgam brand is best?
permite
What qualities is amalgam worse in comparison to natural tissues?
elastic modulus
hardness
thermal expansion
What are advantages of amalgam?
Advantages
* strong
* hard
* durable
* radiopaque
* user friendly
What are disadvantages of amalgam?
Disadvantages
* corrosion
* leakage - does not bond
* poor aesthetics
* mercury:
- perceived toxicity
- environmental impact
