dental amalgams Flashcards
What is dental amalgam?
Silver and shiny
Metal alloy powder (Ag3Sn) and liquid (Hg) capsule- safety
Conducts heat
Cheap
What are the two types of amalgams?
Conventional
High copper content (non gamma 2)
What is the composition of conventional amalgams?
Gamma phase-
~Silver, Ag 67-74%
~Tin, Sn 25-27%
~Copper, Cu ~6%
~Zinc, Zn ~2%
=alloy powder
+ triple distilled mercury, Hg
=liquid
What is the gamma phase?
Alloy
Eg. Ag3S, intermetallic compound, very narrow compositional range=3rd pure phase of Ag and Sn
How do you work out the type of alloy?
Atomic size
Covalency
Structure
What are the three types of alloy?
substitutional solid solution
interstitial solid solution
intermetallic compound
What do the size and shape of the powder particles influence?
Handling
Setting reactions
Final properties
Lathe cut (course/fine grains) OR spherical
Spherical handled easier
Fine grain sets faster
What is the setting reaction?
Ag3Sn + Hg—> Ag3Sn + Ag2Hg3 + Sn7-8Hg
Solid + liquid—> 3 solids
Alloy + triple distilled mercury—> unreacted alloy + silver mercury phase (gamma 1) + tin mercury phase (gamma 2)
What does the amalgam look like on a micrograph?
A. Outer layer of alloy particles dissolve in mercury
B. Gamma 1 phases form
C. Gamma 2 phases form (less)
D. Unreacted gamma alloy particles in a matrix of gamma 1 and 2 matrix
Why are amalgams placed?
Primary caries Secondary caries Poor margin Restoration fracture Tooth fracture Other
Why are amalgams replaced?
Tooth fracture
Recurrent caries
Gross amalgam fracture
Marginal breakdown
Why might the tooth fracture?
Undermined enamel (done via prep) Weakened tooth structure (aim for minimal removal of tooth) Residual caries (spreads and undermines cusp) + inflam of pulp
Why might recurrent caries occur?
Amalgam adults- 72%
Amalgam kids- 56%
Composites- 43%
~1985
Contamination
Poor matrix techniques (overhangs)
Poor condensation (porosity and excess Hg- reduced strength) (poor margins)
Why might there be a gross amalgam fracture?
Shallow prep Pressure points (sharp internal angles)- stress concs Not well mechanically retained proximally (reduced w grooves)
What is the tensile strength of amalgams?
69 MPa
Why might there be marginal breakdown?
Incorrect cavo-surface angles (esp acute)
Delayed expansion (if site not dry, zinc reacts w saliva to form ZnO and H2 gas- bubbles expand, amalgam expands, pressure on pulp)
Over/underfilling
Over carving
Creep and corrosion (metal dislocates and degrades)
What are problems with amalgams?
Non-adhesive Not aesthetic Not strong/tough Susceptible to creep and corrosion Biocompatibility
What is the benefit of having a stronger amalgam?
Preparing cavity simpler
Remove less tooth structure
What are the tensile strengths?
Amalgam- 60MPa
Gamma- 170MPa
Gamma-1- 30MPa
Gamma-2- 20MPa
Would be stronger if we got rid of gamma-2
What are lathe cut particles?
Machined from solid ingot
Graded chippings
Highly reactive w/o prior heating
What are spherical alloys?
Reach full strength more quickly
Easier to condense
Easier to carve and polish
Lower mercury content
What is admix?
Dispersed phase amalgams
Addition of Cu
Lathe cut + spherical= admix
AgSn alloy + AgCu alloy= AgSn +AgCu alloy
What is the dispersed phase amalgam modified reaction?
Gamma + mercury + AgCu—> gamma + gamma-1 + gamma-2
—> (gamma-2 + AgCu)—> gamma + Cu6Sn5 + gamma-1
What is the single phase high copper content formulation?
AgSnCu + Hg—> gamma-1 + (AgSnCu) + Cu6Sn5
Cu>12%
CuSn can be Cu3Sn or Cu6Sn5 depending on precise starting composition
