Dental Amalgam COPY

Question 1

What is the product of amalgamation?

Why is its use declining?

Name the two compositions:

Answer 1

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

Question 2

Today, which form of dental amalgam is used?

What is the conventional composition?

A typical high copper composition?

What are the obvious differences?

Answer 2

• High copper has been replaced by conventional amalgams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)

Question 3

Write down the setting reaction for both conventional and high copper dental amalgam:

What are the main differences:

Answer 3

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

Question 4

What do each of the following contribute to the setting reaction:

a) Ag₃Sn
b) Cu
c) Zn

Answer 4

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

Question 5

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?

Answer 5

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

Question 6

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?

Answer 6

• 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

Question 7

Define creep:

Clinically, what does this mean?

What problem does this cause?

Which phase is largely responsible?

What other problem does this phase cause?

Answer 7

• 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)

Question 8

What is corrosion of the gamma 2 phase?

What does gamma 2 break down to?

Is this all negative?

Answer 8

• 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

Question 9

Thermal properties of dental amalgam?

Are there any problems with this?

Answer 9

• 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

Question 10

What are the biological properties of amalgam?

What about risk for patients?

Answer 10

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

Question 11

What do some reports link dental amalgam to?

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Answer 11

• multiple sclerosis
• behavioural problems
• psychiatric disturbances

Removal of amalgam restorations are said to relieve symptoms, however, removing amalgam exposes body to mercury greatly

Question 12

What environmental pollution can mercury cause and how is this managed?

Answer 12

• 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