Amalgams Flashcards
What is an amalgam?
A trituration - vigorously mixing the components of amalgam to bring them into contact with mercury
Why is amalgam a quaternary alloy?
It consists of mercury, silver, tin and copper
What are the properties of these alloys?
Layers slide over each other making material ductile and malleable
- have high compressive and tensile strength
Why is amalgam an intermetallic compound?
- Does not have a lattice, ions are joined in a specific crystal structure with a set ratio of different ions
- Does not have sheets of ions that can slide past each other
- Typically more than one phase (or ratio of ions)
What are the properties of intermetallic compounds?
- Less ductile and malleable - more brittle
- Set structures and fixture ratios
- Conductive - sea of free electrons
- Huge compressive strength, low tensile strength
What are the pros of amalgam?
- Used for 200yrs so have a lot of data on it
- Comparatively easy to handle
- Good working time
- Adhesive not needed
- Good longevity and safety data
- Favourable handling and clinical properties
What is amalgam made of?
Silver, Tin, Mercury, Copper, (Zn in processing)
What is the set up of the amalgam capsule?
Supplied in a capsule with a membrane in the middle
Elemental mercury and alloy powder separated
Sometime a little mercury is mixed in with the alloy powder
Put in amalgamator
What are the proportions of different metals in amalgam?
50% mercury
50% everything else
67-74 silver
25-28 - tin
0-6 - copper
What are amalgams used for?
- Direct restorations
- Posterior regions (aesthetics, strength and other mechanical properties)
- Larger restorations
- Where composite or other materials have failed
- Cost - cheap and takes less time to complete filling.
What are the lathe cut alloys of amalgam like?
- Solid ingot bar of alloy cut using a lathe or a similar instrument to create irregular chippings
- Graded to select a range of sizes
- Production creates highly stressed, highly reactive particles
- Stressed particles must be heat treated to relieve some of the stress so that the particles do not react too quickly.
What are the spherical alloys like (dominant form)?
- Metals molten and then sprayed into an inert atmosphere where they condense as spherical droplets
- Easier to incorporate copper with spherical alloys
- Easier to triturate as particles wetted with liquid mercury more easily
- Flows more readily so easier to adapt than lathe cut
What are the dispersed (admix) phase alloys like?
- Both spherical and lathe cut particles mixed together
- Lathe cut particles are silver-tin
- Spherical particles are silver-copper
- Combine the properties of the spherical and lathe-cut
- Most amalgams are dispersed phase nowadays
How is amalgam handled?
- No touch technique
- Worked into cavity and condensed with an instrument
-Bring XS mercury to the surface where it can be removed
What are the 3 phases of amalgam?
- Mixing mercury and alloy initiates reaction
- Silver-tin = gamma
- Silver-mercury = gamma 1
- Tin-mercury = gamma 2
Why does gamma 2 phase cause issues?
- Low in strength
- Prone to galvanic corrosion
How is the gamma 2 phase eliminated?
Copper is added to eliminate this phase
This converts gamma 2 phase to gamma 1
Modern amalgams are all high copper
Describe the setting of amalgam
Initial small contraction followed by expansion and gamma 1 forms -> net small contraction
How does amalgam bond to tooth?
- Amalgam is no adhesive to tooth tissue
- Undercut cavity design required which leads to the destruction of healthy tissue
- Can be used in conjunction with adhesives suitable for alloys, e.g. Panavia
What are the advantages of amalgam?
- Cheap
- Quick to use
- Lasts a long time
- Not as moisture sensitive as other direct materials
- Strong in compression e.g. in larger posterior restorations
What are the disadvantages of amalgam?
- Aesthetics
- Not adhesive - more destructive cavity prep
- Patient safety issues
- High thermal conductivity
- Not v strong in tension and brittle
- High coefficient of thermal expansion (expands on heating)
- Passes heat from the top of the amalgam to the base so can be problematic if near pulp
- Linings in larger restorations for insulation to protect from temperature changes
- Expansion can cause remaining tooth structure to be pushed outwards causing fatigue and stress in enamel and dentine
- Different brands for different parts of enamel
Describe the issues with the low tensile strength of enamel
- Can lead to fracture at isthmus
- Once fractured, retention is compromised and there are more fractures
- Enamel has low tensile strength
- Enamel/dentine have lower coefficient of therm expansion than amalgam
- Amalgam expands when heated, stressing adjacent enamel
- Enamel becomes fatigued and fractured over time
Describe ditching and creeping
- Biting forces can cause enamel to flow or creep to the edges
- Parts sticking out can snap due to low tensile strength and becoming brittle
- Causes ditches in sides of restorations and acts as a food trap allowing for secondary caries to occur
What is ditching a consequence of?
- Slow flow of amalgam (some ductability)
- Eventually pieces protrude
- Easily snaps off
What issue is caused by amalgam not adhering to tooth tissue?
Microscopic gap means that secondary caries can occur
Describe aesthetic failure of amalgam
- Sometimes possible to cover with a layer of composite
- Tarnish over time allows things to become stuck
Describe the anxiety related failure
- Health concerns
- Removal leads to short-term exposure to much more mercury
What are the health and safety considerations when it comes to enamel?
- Elemental mercury is toxic and vaporises rapidly
-BUT amalgam is an alloy, not elemental mercury - Some Hg released into the body over lifetime
- Pt most at risk during placement and removal
- Greater release in bruxist/chewing
- Rare - pts can have allergic reactions to amalgam
- Galvanic corrosion can release Hg
- Dental team more at risk than pt
Comparative strengths of amalgam
- Amalgam = higher compressive strength of enamel
- Amalgam has higher tensile strength than composite
- Alginate has lower modulus of elasticity than enamel (more flexible)
- co-efficient of thermal expansion of enamel is lower than GIC (quite similar)
