Amalgam and properties Flashcards
What are some advantages of amalgam as restorative material?
inexpensive, easily prepared direct material, margin sealing, longevity
What are some disadvantages of amalgam as restorative material?
poor aesthetics, weakening of tooth structure, no adhesive bonding, sensitive to manipulation, brittle, mercury waste
What are the two components of dental amalgam?
alloy + Hg
What is the mercury/alloy ratio?
0.5 or 1:2
What is the weight percent of mercury in an amalgam capsule?
42-45 wt%
What factors change the setting process?
composition, shape and size of alloy particles
What are the major take aways from the ADA standard No 1 requirement for amalgam?
composition, other elements cannot exceed 0.1 wt%,
small creep and dimensional change, and a high compressive strength
What are the major elements of amalgam?
Ag (primary), Sn (secondary), cpper, and zinc
What are the three phases the complex structure of the particles possess?
gamma (most important), beta, and epsilon
What is characteristic of a filling or lathe-cut?
machined from cast ingot (lump of melted metal)
What is characteristic of spherical method of particle making?
molten alloy blown through nozzle
How many sizes are typically observed from spherical method?
3
Is there a wide range of sizes from lathe-cute?
Yes
Why is there a wide range of sizes in lathe-cut?
optimum condensation
Do spherical particles that are wetted have a higher or lower mercury:alloy ratio than lathe-cut?
lower
At what percent is a product considered high-copper?
> 12
What is the advantage of high-copper products?
longevity and decreased creep
Is zinc-free alloy economically feasible?
No
Does zinc help or render machining lathe-cut particles?
it facilitates it and improves corrosion resistance
What are the benefits of heat treating alloy for dental amalgam?
eliminates non-uniformity, relieves stress, and lowers for the control of setting time
Is there free mercury found after the setting reaction?
No, only found in reaction phase
Is gamma 1 or 2 absent in set material?
2
What gamma is the major reaction phase in low and high-copper amalgams
gamma 1
What is characteristic in the first step of dental amalgam setting reaction?
form gamma 1 & 2
What is characteristic of the second step of dental amalgam setting reaction?
disappearance of gamma 2 phase and formation of nā phase
Is the setting reaction slower than for HCSS products than LCSS?
yes because of the two-step process
At what time is the total dimensional change of dental amalgams limited by ADA standard no.1 ?
24 hours
Can the dimensional changes be detected by the unaided eye?
No, the contractions cause microleaks
What are the two different setting processes of dimensional changes?
solution and crystallization (precipitation)
What is the strongest phase in dental amalgams?
incompletely consumed starting alloy particles (gamma)
What is the weakest phase in dental amalgams?
gamma 2 in low-copper amalgams (most corrosion prone)
What are the four main types of dental corrosion?
galvanic, electrochemical, crevice, and chemical
Where does galvanic corrosion take place?
at interproximal contact with gold alloys
Why does electrochemical corrosion take place?
bc multiple phases
Where does crevice corrosion take place?
at the margins due to unpolished scratches, secondary anatomy, lower oxygen concentration = plaque retention
Why does chemical corrosion occur?
reaction with sulfide ions at occlusal surface
Is limited corrosion beneficial?
yes, because reduction in microleakage - gamma 2 in low copper amalgams and nā in high-copper amalgams
How is corrosion minimized?
polishing
What element is clinically shown to have superior marginal integrity and longevity?
Zinc
What are the two mechanical properties needed to resist forces of mastication?
comprehensive strength»_space; tensile strength and proper dentinal support
Describe poor edge strength
Fracture of ledge on poorly finished restoration
AKA low tensile strength leads to fracture in bending
Would insufficient strength of amalgam leaf to and increased or decreased amount of marginal breakdown?
increased
Which product has the most rapid setting reaction?
HCSS
At what time period is the final strength of the amalgam identified?
1 week - nearly same strength as 1 days after
Define viscoelastic materials
the mechanical properties depend on the rate of loading
What is the avg compressive strength of amalgam after 24 hrs?
350Mpa or 50k psi
What is the avg tensile strenght of amalgam after 24 hrs?
70 MPa or 10k psi
Is a higher or lower ratio for compressive strength by tensile strength indicative of a brittle material?
high
Do high-copper amalgam have high or low creep?
generally low
True/False Creep is only a mechanical property.
True
it is correlated with clinical marginal fracture of low-copper amalgam
What is the creep mechanism?
grain boundary sliding of gamma 1 phase
this is blocked by the nā phase in high-copper amalgams
What is the result of mercury content?
more phases, more expansion, less strength
What is an affect of moisture and Zn-containing dental amalgam ?
delayed and excessive increase in setting expansion, decrease in strength
What is the role of trituration ?
coat each alloy article with mercury
What is the result in a small increase in trituration time?
decreases setting expansion and increase strength
What is the result of a small increase in condensation pressure?
decrease setting expansion and increase in pressure
True/False One should delay condensation after trituration.
False.
do NOT delay
True/False The government threshold limit value for sustained exposure (40hr/wk) is 0.05 mg Hg/m3
True
What is the ng of Hg per mL of blood that causes mercury poisoning?
100 ng per mL
What are the mercury hygiene recommendations by ADA?
singly use, no touch, discard old/damaged, dispose in unbreakable jar holding water w/ finely divided sulfur
