Dental amalgams Flashcards
What are the two types of dental amalgams?
- Conventional
- High copper content (non-gamma 2)
Conventional dental amalgam composition
Alloy powder composition: -Silver (Ag) 67-74% -Tin (Sn) 25-27% -Copper (Cu) 6% -Zinc (Zn) 2% + Triple distilled mercury (Hg)
What is the gamma (γ) phase?
Ag3Sn
-intermetallic phase
What does shape and size of powder particles inflluence?
Handling
Setting reactions
Final properties of restoration
What are the main types of powder particles?
Lathe cute (course-grain and fine-grain) Spherical
Setting reaction for conventional amalgams
Ag3Sn + Hg (liquid) –> Ag3Sn + Ag2Hg3 + Sn7-8Hg
What happens during setting reaction for conventional dental amalgams
- Outer layer of alloy particles dissolve in Hg
- Gamma 1 phase forms within Hg liquid
- Gamma 2 phase forms where Sn reacts with Hg (less)
- Unreacted gamma alloy particles in matrix of gamma 1 (&2) matrix
How long do amalgam fillings survive?
Longest private ~ 10 years
Then students ~ 8
Then NHS ~ 4.5
Why are amalgams placed?
1 caries 2 caries Poor margin Restoration fracture Tooth fracture -Almost 50% of time replacing existing restorations
Reasons for replacement of amalgam restorations
- Tooth fracture
- Recurrent caries
- Gross amalgam fracture
- Marginal breakdown
Tooth fracture
Weakened enamel
Undermined enamel
Residual caries
How to prevent tooth fracture
Minimal removal of tooth tissue
Undermined enamel
In cavity prep, providing flat walls and floors to cavity can undermine enamel
-unsupported enamel will break off and leave gap –> may lead to recurrent caries
Tooth fracture: residual caries
- if any residual caries remains, will spread & undermine cusp –> eventually fracture
- bacterial toxins will cause inflammation of pulp
Replacement due to recurrent caries %
Amalgam adults 72%
Amalgam children 56%
Composites 43%
Recurrent caries
Contamination
Poor matrix techniques
Poor condensation
Recurrent caries: contamination
Blood & saliva in cavity prevent amalgam from adapting to surface
Recurrent caries: poor matrix techniques
-Poorly adapted matrix band –> proximal overhangs / poor contact points with adjacent teeth
–> plaque accumulation
= recurrent caries
Recurrent caries: poor condensation
–> porosity and excess Hg –> reducecd amalgam strength
If marginal adaptation is poor, can lead to marginal leakage, recurrent caries and corrosion
Gross amalgam fracture
Shallow preparations
Non-retentive proximal boxes
Sharp internal angles
Gross amalgam fracture: shallow preps
- Amalgams have low tensile strength (60MPa)
- If placed as thin sections, subjected to bending forces –> break
Gross amalgam fracture: non-retentive proximal boxes
- frequent
- partially due to low tensile strength
- reduce risk by cutting retention grooves
Gross amalgam fracture: sharp internal line angles
- -> stress concentrations
- increase risk of fracture of tooth & restoration
Marginal breakdown
- incorrect cavo-surface angles
- delayed expansion
- overfilling, underfilling, overcarving
- creep and corrosion of amalgam
Marginal breakdown: wrong cavo-surface angle
- Primary cause of marginal breakdown
- Occurs more readily with acute angles
Marginal breakdown: delayed expansion
-In Zn containing alloys
-if prep site is not kept dry, Zn can react with saliva
Zn + H2O –> ZnO + H2 -bubbles of H2 form within amalgam, which expand so amalgam expands
-pressure on pulp (pain)
-restoration sits proud
Marginal breakdown: overfilling, underfilling, overcarving
Overfilling (without carving back) –> ledge –> fracture
Underfilling/ overcarving –> acute amalgam angles –> marginal breakdown
Marginal breakdown: creep and corrosion
Long term failure
- unavoidable (intrinsic to material properties)
- only way to avoid is to change material
Define creep
Slow movement with repeated force
Define corrosion
Chemical reaction with environment
Problems with amalgam restorations
- lack of aesthetics
- non-adhesive
- lack of strength and toughness
- susceptible to corrosion
- biocompatibility
How to get round problem of non-adhesive amalgams
- cut retention grooves
- use adhesive e.g. Panavia (but then maybe you should use composite)
What is amalgam strength linked to?
Its constituents (gamma 1 and 2 are the weak phases)
- amalgam 60MPa
- gamma 170 MPa
- gamma 1 30MPa
- gamma 2 20MPa
How do we reduce the amount of gamma 1 and 2 to make amalgam stronger?
Use better condensation (packing) technique
-spherical particles require less mercury because easier to mix
Lathe cut particles
- Machined from solid ingot
- Graded chippings
- Highly reactive without prior heating
Spherical alloy benefits
- Reach full strength more quickly
- Easier to condense
- Easier to carve and polish
- Lower mercury content
Spherical alloy history
Introduced In ~1962
Prepared by atomization process
Same composition as lathe cut alloy
How are spherical alloys prepared?
-By spraying molten material into inert atmosphere
What is Admix?
High Cu Dispersed Phase Amalgam Production
-Combination of lathe cut and spherical
AgSn + AgCu alloy
Admix setting reactions (modified)
γ + Hg + AgCu –> γ + γ1 + AgCu
γ2 + AgCu –> Cu6Sn5 + γ1
γ + Hg + AgCu –> γ + γ1 + AgSnCu
Same plus silver copper phase. Tin mercury reacts with silver copper to make copper tin plus silver mercury. Note no overall tin mercury γ2
High Cu Dispersed phase amalgam microstructure
Constituents more separated
Traditional amalgam: corrosion
Galvanic cell: electrochemical difference in electrolyte --> metal anode corrodes -γ1 slightly electro+ CATHODE -γ2 slightly electro- ANODE -electrolyte = saliva Therefore no γ2 phase - no corrosion
High Cu Dispersed phase amalgam benefits
- Reach full strength more quickly
- Easier to condense
- Easier to carve and polish
- Lower mercury content
- Higher strength
- More resistant to corrosion
High Cu Single Phase Amalgams setting reaction
AgSnCu (Cu>12%) + Hg –> AgSnCu + γ1 + Cu6Sn5
High Cu Single Phase Amalgams benefits
Same as Dispersed Phase
-no lathe cut particles
Why do we use any lathe cut particles if advantages are the same with single phase amalgams?
All to do with handling
Takes time to change practices
Dental amalgams: variations
Conventional: lathe cut (Ag3Sn), spherical (Ag3Sn)
High Cu dispersed: lathe cut Ag3Sn + spherical AgCu // lathe cut AgSnCu + spherical AgSnCu
High Cu single phase: all spherical AgSnCu // lathe cut AgSnCu
Which amalgams have highest longevity?
Gamma2 free amalgams
Biocompatibility
Maybe some truth but a lot of bad press has lack of evidence behind it
-mercury itself is very dangerous
Amalgam safety
- personal protection
- patient protection
Potential Symptoms and hazards of Hg exposure
- respiratory failure
- kidney impairment
- cognitive disturbances
- reduced visuoperceptual and constructional skills
- memory loss
- hypertension
- headaches
Potential sources of Hg contamination
- Spills
- Leaky dispensers or capsules
- Removing or polishing amalgams
- Sterilising Hg-contaminated instruments
Avoiding Hg contamination
- Use pre-capsulated alloys
- Avoid direct skin contact with dental amalgam
- Use high volume evacuation when finishing or removing amalgams
- Wear mask when removing amalgams
Hg storage and disposal
- Store bulk Hg in unbreakable containers
- Store amalgam scrap under radiographic fixer soln
- Dispose of contaminated items in sealed bag
- Report & clean spilled Hg immediately using clean-up kit
- Wear professional clothing only in dental surgery
Additional precautions for Hg
- provide proper ventilation
- monitor Hg vapour levels in dental surgery
- use proper work area design
- use amalgamator with cover
