Flashcards in Dental amalgam Deck (27)
what is dental amalgam a product of?
alloy + mercury react to form silvery hard grey mass
why is it not increasingly demanded from the public?
performs better but has poor aesthetics
why is the use of dental amalgam declining?
-development of viable alternatives
-concerns on safety and environmental pollution
what are the two compositions of dental amalgam?
-conventional (products pre 1996)
-higher copper (post 1986)
what was development of amalgam stimulated by?
better understanding of structure properly relationships
what is the conventional composition of amalgam?
what is the typical high copper composition of amalgam?
what are the variations in alloy morphology?
what gamma phases is wanted and which is not wanted?
wanted - phase 1
not wanted - phase 2
Describe the dental amalgam setting reaction.
-preponderance of Ag3Sn (undergoes setting reaction)
-Cu has strengthening effect on amalgam
-Zn acts as scavenger in alloy production (may eliminate if manufacture in inert atmosphere
what is mixing termed?
Describe the setting reaction and dimensional changes.
Dimensional changes :
-30 mins small shrinkage (Hg diffuses into alloy)
-30-60 mins - expansion due to crystallisation
-overall effect is either net expansion /contraction
-ISO limit +/- 0.1% expansion/contraction
-manipulative variables help limit
why is moisture controlled during setting reaction?
when Zn combines with H20 , H2 gas is produced
how long is the setting reaction from placement to function?
how long is the setting reaction for final strength?
How does compressive strength relate to tensile and transverse strength?
compressive strength is greater than tensile and transverse
what is strength proportional to?
Mercury (Hg) content
-optimum if Hg 44-48%
Describe creep properties.
Plastic deformation under load :
clinically flows manifesting as protrusions at restoration margins
-fracture off producing ditching
-responsible: gamma 2 largely, but not exclusively
-corrosion of the alloy exacerbates (releases Hg in marginal crevice ) --> localised expansion of amalgam
Describe corrosion properties.
Electrochemical interactions of different alloy phases with salvia as electrolyte :
-gamma 2 anode
-gamma 2 breaks down to Sn and Hg ( Hg remains free and is ingested)
- ADVANTAGE- corrosion products provide marginal seal
-in high copper phase undergoing most corrosion is CU-Sn (does not corrode as much)
Describe thermal properties.
-metal has high thermal diffusivity
- three fold coefficient of thermal expansion of dentine
-in reality not a problem due to transient nature of functionally encountered thermal stimuli
Describe biological properties.
Hg compounds know to be toxic to CNS:
-risk to dental team
what risk does dental amalgam have to the patient?
-1998 placement and removal of dental amalgam restorations banned by D o H during pregnancy
-no conclusive evidence but perceived elevated risk:
Hg doses high -placement,contouring, removal
-Hg concentrates and crosses placenta (abortion/child abnormalities?)
-high concentration of Hg in urine/blood of those with amalgam restorations
what do some reports link dental amalgam to?
(removals of amalgam restorations claimed to bring about relief of symptoms)
What about environmental pollution?
-water of dental unit
-release of Hg during trituration
Describe manipulative variables
-range 5:8 to 10:8
-drier mixes achieved by mechanical mixing
-packing and condensation removes excess mercury
Describe dispensation of dental amalgam.
-weight (most accurate)
-proportioning and mixing devices
-now generally manufacturer encapsulated (accurate and most reproducible)