Clinical Aspects Of Amalgam Flashcards
What is amalgam?
An alloy of silver, mercury and tin with other metals to modify the properties.
How is amalgam made?
Alloy powder and liquid Mercury making a paste that sets hard.
What do amalgam capsules contain?
Alloy powder, mercury and membrane separating the two.
What are the two forms of alloy powder?
Spherical - made by spraying molten metal into fine mist
Lathe cut - creating an ingot of alloy and grinding down to a powder
How do the two forms of amalgam differ in their handling?
Spherical - softer more flowable
Lathe cut - less flowable good for building up large amounts of missing tooth structure
How do the powder and mercury mix.
The mercury surrounds the particles of powder that dissolve into a solution of mercury tin and silver. Metal ions fuse with each other to make new compounds.
Ag3Sn - gamma
Ag2Hg3 - gamma 1
Sn7Hg - gamma 2
What is gamma 2?
Unwanted product Sn7Hg Leads to corrosion Cheap - changes shape under load Decrease in strength BUT can seal the cavity against microleakage
How do we reduce the amount of gamma 2?
Add copper, converts gamma 2 to a copper compound
What is admixed amalgam?
Lathe cut, spherical and copper compound mixture of amalgam
What is the composition of amalgam?
Silver - 65%
Tin - 29%
Copper - 6-13
Zinc - 2 max
What does zinc do?
Added to prevent silver from oxidising during manufacture, acts as a scavenger for oxygen but not needed if produced in a vacuum.
Can cause a large expansion or the material as it sets
How much mercury should amalgam contain?
Slightly more than 50% as less than makes the mixture too crumbly
Remove excess mercury by packing as it weakens the restoration, excess mercury is brought to the top.
Final restoration has less than 50%
Advantages
Good compressive strength Good wear resistance Kind to opposing teeth Easy to use Chemically set Cheap Radio-opaque - can see if there is secondary caries easily
Disadvantages
Not adhesive - undercuts Weak in thin sections deliberately have to remove more tooth tissue Thermal conductor Unaesthetic Occasional lichenoid reactions
Indications
Larger cavities in posterior teeth
Where heavy occlusal forces encountered - clenching
Where aesthetics aren’t important
High caries rate patients
Posterior restorations that extend subgingivally - moisture
Building up broken down teeth before crowning
What is retention?
Prevention of being pulled out of the cavity vertically
What is the cavity design for amalgam?
Infected dentine removed affected left
Clean walls
No unsupported enamel
Undercut
Cavo surface angle >90 angle between surface of the tooth and cavity
Width interproximally should be just wider than the contact area
Amalgam margin angle >70 degrees
What do pins do?
Acts as a wall to help amalgam stay in place
What are pits and grooves?
Retentive features for amalgam to lock in to
What is a bonded amalgam?
Adhesive system containing mdp or 4-META forming a hybrid layer with dentine and a chemical bond to amalgam.
Wet uncurled RMGIC can have amalgam placed directly on top
Advantages of polishing?
Less plaque retentive, reduces corrosion, looks nicer
Disadvantages of polishing
Will not increase life expectancy of restoration
Heat generated may damage the pulp
When is mercury vapour ingested?
When it is mixed, placed or removed
Released from filling when brushing and eating
Problems with mercury and the environment
Can be converted by bacteria to methyl mercury in fresh and salt water, which is highly poisonous
What was said at the minimata convention?
Phase down of amalgam by 2020 and ban mercury containing products
Call for a cease of teaching amalgam by 2015
Composite will become the most widely used type of material in posterior teeth
Recommendations for phase down of amalgam
Shall only be used in pre dosed encapsulated form
Shall not be used for deciduous teeth in children under 15, pregnant women or breastfeeding women except when strictly necessary.
All dental facilities should be equipped with an amalgam separator
Report spills
Put empty capsules into pots that are named
Clean heads in designated sinks
