dental amalgam Flashcards
Once a cavity has been prepared it needs to be restored, this can be done with either a direct or indirect restoration. What is the difference between direct and indirect?
Direct- placed directly into the prep. these materials are moldable when placed and harden with time or polymerization.
Indirect - Shaped and formed outside the oral environment then palced intraorally later
Sometimes we have to make compromises in material property so that the material can be placed more conveniently in direct placement. For example, _____ and ____ are weaker than gold but easier to place
amalgam and composite
by def. what is amalgam
an alloy of mercury with any other material. It’s made by mixing mercury with a silver-tin dental amalgam alloy, also varying amounts of copper and small amounts of zinc are added.
what are low copper (I’ll write amg as short for amalgam) amg?
What is another name for these low copper amgs?
contain only 2-5% copper, earliest successful amg. Predominanted from 1900 - 1970. Typical modern low copper amg may contain about 3.6% cu. An example is new true dentalloy.
These are often referred to as conventional amgs or traditional amgs
What is a high copper amg?
Why are they better?
Contain about 12-30% copper. the alloy particles may be filings but most often there are spheres.
They have better corrosion resistance than low cu.
What is trituration?
What is condensation?
why do we carve the amg?
Trituration- mixing the amg alloy particles with mercury and then vigorously working into a platic mass to be placed in a tooth prep
Condensation- physical compression of the unset amg to remove extra mercury rich phase.
It removes the Hg rich phase and shapes the hardening mass.
what other materials can be used other than amg?
composite, glass ionomer, cast gold, porcelain
The ADA and national institute on standards and technology have pateneted what?
A mercury free direct filling alloy that can be self welded by compaction (by hand) to create a restoration.
classification of amg and amg alloys are based on? (3)
amg alloy particle and geometry size
copper content
zinc content
How were tradititional amgs made?
mixed by proportioning components into a mortar and grinding with a pestle.
alloy was made into a brick them filed down to make filings. later, a lathe was used to produce lathe cut particles
in image, a is filings, b is spheres and c is mixed
in traditional amg alloys, why is particle size varied?
to control the reaction, produce smoother mixtures, and enhance final properties
in conventional amg, the irregular particles pack together poorly, what is an implication of this?
large amount of mercury is needed to fill in the spaces. When you pack it, you extrude some of this mercury which is good because the mercury is the weakest part so that improves the materials strength.
sphericle particles pack more efficiently. What are some implications of this?
It takes less mercury in the mixture. also the sphericle particles increase the fluidity of the mixture because they slide past eachother with less resistance
what are these?
early methods of tituration
what is this?
showing that there are lots of examples of capsules and pestles for mixing amalgam. these are put in amalgamators for automatic mixing
Modern amg is in precapsulated allow and mercury. what is in the capsule?
why is this convenient?
the components in the capsule are separated by a special diaphram that is broken when the acsule is activated or mixed. precapsulated amg is coveneint becasue you know the materials will not be contaminated or spilled before using.
Talking about modern amg, reseach in the 60’s showed that increasing the copper content above 12% was good for what reason?
It supressed the formation of teh sn-hg phase, which is prone to corrosion (or simply, it decreased corrosion). This led to a doubling or tripling of amg clinical longevity.
what did youdelis of the u of a create?
he added ag-cu spheres to conventional amg allow. this is the dispersion phase alloy, or dispersalloy
classification of amg based on _____ is the main system in use today.
copper content.
dispersed - phase amg alloys typically _____ but completely sherical amg alloys do not ______
squeak, becase they are resistant to hand condensation when placed in a prep
do not squeak, they are less resistant to condensation
what are some factors that influence choosing one material over the other?
clinical situation
personal prefrence because of handling properties.
why is zinc added? how much is is added and what is a risk associated with it?
to suppress oxidation of the metallic elements in the alloy by preferentially oxidizing first.
usually 1% or more is added, though 0.2-0.1 % is usually not oxidized and remains in the alloy. the residual amount if contaminated can react with water producing hydrogen gas and can cause delayed expansion of the amg.
how is the zinc delayed exansion problem handled?
manufacturing process that eliminated the need for zinc in the first place.
produce zinc free amalgam alloys as an alternative.
But, they have found that zinc may have some effect on amg longevity.
In conventional amg, Ag and Sn react with Hg to produce two reaction product phases, phase 1 and phase 2. These form solids and cause the mass to harden. Tell me a little bit about each phase?
Phase 1 is Ag-Hg. It is smalled, equiaxed with intermediate corrosion resistance.
Phase 2 is long, bladelike crystals that penetrate through the matrix and are prone to corrosion. Corrosion proceeds from the outside of the amg along the crystal. this creates penetrating corrosion making the amg porous and spongy. Remember phase two is bad (he said too (two) bad so sad… bit of a lame memory hanger but a memory hanger none the less).
Only a small amount of alloy is needed to react with the Hg. The remaining alloy serves to strengthen the amg. reaction products weaken the amg, so we want to try and use as little reaction products as possible.
how is the corrosion problem of phase 2 eliminated?
add a little more copper homes.
in the high copper amgs, the sn Hg reacitons (phase 2) are supressed by the pereferntial formation of cu-sn phases instead. They are not crystalline, so penetrating corrosion does not occur.
both high and low copper amgs undergo what two kinds of corrosion?
chemical
electrochemical
where is chemical corrosion usually, and what does it look like?
on the occlusal surface
produces a black tarnish film. the reaction is limited to the surface and doesn’t cimpromise any property other than esthetics.
When will electrochemical corrosion occur?
when chemically different sites act as an anode and cathode. Requires and electrolyte (saliva) and contact between two dissimilar metals. the ANODE CORRODES (this will be a q on exam), producing soluble and insoluble reaction products.
What is concentration cell corrosion?
corrosion occuring in cracks an crevies due to locally lowered oxygen and higher h ion conc. area becomes anodic and corrodes
what is stress corrosion
amg areas under stress deform making a higher energy state which is more susepticle to corrosion. in amg the combo iof mechanical and corrosion stress occurs along the margins.
why did we think that amg corrosion at margins is good?
because the crystals fill in the space and seal the Hg to the tooth
is dental amg a safe way to store Hg?
yes, one of teh safest. electrochemical corrosion will not liberate mercury from the amg.
mechanical properties of amg: high cu amg have _____ compressive strengths than those of low cu amgs
greater. compressive strength of high cu amg are similar to that of enamel or dentin.
tensile strength is important for fracture resistance. what level of tensile strength do low and high cu amg have? amg is considered to be brittle material. tht is why there has to be sufficient bulk (1.5mm)
both have low tensile strength
what is amg creep?
plastic deformation f the amg upon repeated stress. This casuses it to protrude from the prep
what is the most comon evidence of decredation on low cu amg?
marginal fracture.
what’s the halflife of a high cu amg?
about 25 yrs
