Dental Amalgam Flashcards
What is the product of amalgamation?
Why is its use declining?
Name the two compositions:
Alloy + mercury react to form a silvery, hard, grey mass
- development of more viable alternatives
- concerns on safety and environmental pollution
- conventional: products pre 1986
- high copper: products post 1986
Today, which form of dental amalgam is used?
What is the conventional composition?
A typical high copper composition?
What are the obvious differences?
- High copper has been replaced by conventional amalgams
Conventional amalgam:
- 65% Ag, 29% SN, 6% Cu, 2% Zn, 3% Hg
Typical high copper composition:
- 40% Ag, 32% Sn, 30% Cu, 2% Zn, 3% Hg
- decline in silver (Ag) content and increase in copper (Cu)
Write down the setting reaction for both conventional and high copper dental amalgam:
What are the main differences:
Conventional:
- reacts with mercury alone
- forms a gamma 1 and a gamma 2 phase
High copper:
- requires mercury and copper to react
- does not form a gamma 2 phase, instead forms Cu6Sn5
What do each of the following contribute to the setting reaction:
a) Ag3Sn
b) Cu
c) Zn
a) Ag3Sn: undergoes setting reaction, therefore need an abundance of it
b) Cu: has a strengthening effect on amalgam
c) Zn: acts as a scavenger in alloy production complexing woth oxygen - may eliminate if manufactured in an inert atmosphere
What is trituration?
What are the dimensional changes 30 mins after mixing of amalgam?
30-60 mins after?
What is the overall effect?
How is this controlled?
Trituration is the term for mixing of amalgam.
- 30 mins after mixing: small shrinkage as Hg diffuses into alloy
- 30-60 mins: expansion due to crystallisation
- Overall effect is a net expansion/contraction
- ISO limit expansion/contraction to +/- 0.1% to prevent damage to the tooth once material is placed
- manipulation variables can help limit expansion and contraction
Explain the effects of poor mpisture control and a Zn containing alloy:
When can functional strength be reached and final strength?
Why is the compressive strength > tensile and transverse strength of amalgam?
What is the optimum Hg concentration to achieve greatest strength?
- an increased rate of expansion as Zn reacts with H2O producing ZnO + H2
- functional strength may be reached 15-20 mins after placement, however final strength is not reached until 24 hours
- amalgam is weak in thin sections, so the depth of cavity must be at least 2mm to counter for this weakness in tensile and transverse thin sections
Optimum strength if Hg 44-48%, this is achieves by correct water:powder ratio and by overpacking cavities and carving back mercury rich layer
Define creep:
Clinically, what does this mean?
What problem does this cause?
Which phase is largely responsible?
What other problem does this phase cause?
- Creep: plastic deformation under load
- Clinically this manifests as protrusions at restoration margins
- Fracture off producing ditching –> predispose to caries around the restoration
- Gamma 2 is largely but not exclusively responsible
Corrosion of the alloy, will release Hg in marginal crevice ditch around restoration, leading to lacalised ‘expansion’ of amalgam (theory of mercuroscopic expansion)
What is corrosion of the gamma 2 phase?
What does gamma 2 break down to?
Is this all negative?
- Electrochemical interactions of different alloy phases with saliva as electrolyte, the gamma 2 phase is the anode in that system
- Gamma 2 breaks down to Sn and Hg, with Hg becoming free and is ingested by the patient
- Not all bad as corrosion products provide a marginal seal
Thermal properties of dental amalgam?
Are there any problems with this?
- Amalgam has a high thermal diffusivity (heat transfers through is fast)
- threefold coefficient of thermal expansion of dentine
- In reality, not a problem because:
- place a lining material to insulate beneath amalgam to protect the pulp
- transient stimulation so not a major problem
What are the biological properties of amalgam?
What about risk for patients?
Hg compounds known to be toxic to CNS, posing a risk to the dental team and patients
- placement and removal of amalgam restorations were banned during pregnancy in 1998, no conclusive evidence but a perceived elevated risk
- high doses of mercury during placement, contouring and removal
- Hg concentrates and crosses placenta
- high concentrations of mercury in urine/blood of those with amalgam restorations in comparison to those without
What do some reports link dental amalgam to?
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- multiple sclerosis
- behavioural problems
- psychiatric disturbances
Removal of amalgam restorations are said to relieve symptoms, however, removing amalgam exposes body to mercury greatly
What environmental pollution can mercury cause and how is this managed?
- can contaminate water so all dental units have a mercury removal filter fitted
- mercury will be liberated in crematoria during cremation so many have filters in their chimneys
What are the benefits of amalgam?
- inexpensive
- easy to use
- proven track record of over 100 years use
- familiarity
- resin free, less allergies than composite
What types of amalgam are there?
- lathe cut, small condensers, high force required
- spherical alloys, large condensers, less sensitive to force
- admixture alloys, intermediate handling between lathe cut and spherical
Why do you overfill and carve back amalgam?
When can amalgam be bonded onto a tooth?
- to remove the mercury rich gamma two phase
- when there is not enough tooth substance to allow for retention of the amalgam
- to achieve a complete marginal seal, preventing ingress of bacteria
