Amalgam Flashcards
High vs low copper amalgam
High copper has greater strength, less tarnish and corrosion, less creep ie deformation under constant stress, less sensitivity to handling variables
What are the materials present in high copper amalgam
Silver (40-60%)
Tin (27-30%)
Copper (13-30%)
Zinc 1%
Role of silver in amalgam
Cause setting expansion
Increase strength
Increase corrosion resistance
Role of tin in amalgam
Cause setting contraction
Decrease strength
Decrease corrosion resistance
Role of copper in amalgam
Increase strength
Reduce tarnish and corrosion
Reduce creep, lower case of marginal leakage
Role of zinc in amalgam
Reduce oxidation of other metals in the alloy
Setting reaction of amalgam
Excess AgSnCu + Hg —> unreacted alloy + CuSn + AgHg (gamma 1)
No SnHg ie gamma 2!!
Various strengths of amalgam
High compressive strength
Low tensile and low shear strength (must be supported by tooth structure)
Effects of creep in amalgam
Creep is slow change in shape caused by compression
Causes amalgam to flow, unsupported amalgam protrudes from margin and can fracture off and cause ditching. Overhangs can trap food. Secondary caries.
What is responsible for high levels of creep in amalga
Gamma 2 in low copper amalgam ie SnHg
What are factors that can affect dimensional change of amalgam
Mercury alloy ratio (more mercury more expansion)
Condensation technique
Mixing time
Manipulation (more manipulation, more mercury consumed, contraction)
Particle size (smaller particle, silver enters solution faster due to increased surface area, gamma 1 forms faster resulting in expansion)
Effects of galvanic corrosion in amalgam
Two dissimilar metals exist in wet environment, promoted by acid
Discolours amalgam restoration
May lead to pitting
Corrosion products may fill amalgam surface and reduce microleakage (self sealing)
Co efficient of thermal expansion of amalgam
Amalgam CLT 3x larger than that of dentine, cause microleakage as no adhesion between amalgam and tooth. Debris around margins can lead to secondary caries
Can amalgam be places in stress bearing areas
Yes, can be used in large mod cavities
Why is moisture control important in amalgam
Moisture contamination will cause corrosion of zinc. React with zinc to form H2 which collects in restoration, exerts pressure and causes delayed expansion
