Amalgam
Lec 12 (4-5-2011)
In previous lecture we took about general features of amalgam , its uses in dentistry and how to manipulate it .
In this lecture we will continue …..
Alloy type based on production technique :
1) lathe cut
2) spherical
3) admix
Lathe cut :
We can produce an ingot of amalgam by grinding metal ingredients ( silver , tin with other elements )
Using lathe cut machine and to make the mixture more homogenous we need to anneal it in an oven at 400 C .
Spherical:
particle size is smaller than lathe cut
Admix :
Is a mixture of both lathe cut and spherical
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Notes :
_ spherical alloy require less mercury than typical lathe cut alloys because of smaller surface area per volume
_ less mercury mean that γphase is the predominant phase since it doesn’t contain mercury and is the best phase concerning mechanical property .. so in this case the final amalgam will be a better filling material .
_ amalgam with low mercury content generally has better properties
* the first developed amalgam is called classical or conventional amalgam which contain less than 6 wt % of copper .
According to the content amalgam classified to :
1) a low copper amalgam alloy – ( <6 wt % cu )
- γ phase mixed with mercury
- γ1 & γ2 phases are also present
- nowadays it is not used that much
- it causes a lot of problem like degradation of material and creep
Creep : a deformation under constant rate of pressure .
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2) high copper amalgam alloy – (6-60) % wt Cu in normal copper alloy before mixing it with mercury.
The purpose of using high copper alloy is to remove γ2 phase by replace Hg with Cu .
High copper alloy either admix or spherical , in uni-composition almost non γ2 phase found so physical properties of this alloy is better .
* γ2 phase (Sn7-8Hg) -----> weak phase found in high amount in formation amalgam or low copper alloy so degradation occur and other phases found in high copper alloy .
* high copper alloy formed by chemical reactions which are rarely used in dentistry , for dental alloy or metals dentists only do mixing then applying the mixture in the tooth cavity .
* nowadays the metal are already prepared without any reactions they are almost be inert or we only do an oxidation .
_ plz memoriza the table about setting reaction in doctor's slide !!
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_ γ phase is the strongest one
_ γ2 phase still at boundary between γγ1
_γ2 degraded in oral cavity due to excessive heat and saliva
_the main problem in set reaction is the rest of an unconsumed mercury when mercury increase the phase become weaker .
Properties of set amalgam :
Shrinkage or expansion in micron is so minimal .
When shrinkage occur we have a space between amalgam and the tooth which is called marginal micro leakage .
_ contraction occur in all type of amalgam but in limit not exceeding 20 micro meter .
More than 20 micro contraction will push the wall of tooth and make fractures .
_ zinc ( zn ) helps in expansion beyond the limit 20 micro and could reach 50 micro .
_ 50 micro expansion could produce fracture or at least pain
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_ zinc doesn’t make any expansion only if it react with water and that cause creep so we most increase the cavity of tooth , in addition to creep the space between alloy and tooth will permit fluid and bacteria to enter the dentinal tubule so marginal (micro) leakage formed until corrosion occur by γ2 phase which is the most corrosive phase and this is the only benefit of γ2 phase .
In corrosion chemical reaction take place between amalgam and γ2 phase precipitation of γ2 phase which will fill the space between amalgam and decrease micro leakage .
_ during corrosion liberating of Hg will occur and free Hg effect gold restoration
_ gold which we are used in dentistry isn’t pure ( not inert ) so it react with Hg (mercury can weaken the gold restoration ) .
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Mechanical properties :
1. High compressive strength
Amalgam filling in the posterior teeth may stay up to tenyears (average time) , in those ten years they need to standthe cycles of occlusal forces , so if they don’t have highcompressive strength they can’t handle these huge forces ineach bite which could reach 500 N so it needs to have a
good initial compressive strength .
2. Amalgam is a viscoelastic material
Why we are concerned about initial compressive strength ??
A. To stand the condensation
B. After it sets , we do carving to get the right
anatomy of the tooth but if the anatomy was
not done accurately and if the filling is higher
than the tooth and the patient occlude on this
tooth.
If the initial strength is low , the filling will break andthis what happen in old amalgam but if the initial strength ishigh when the patient occlude he will feel that it’s incorrectand we can correct it .
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3.Tensile strength :
Tenisle strength of any brittle material it is much less than it’scompressive strength
Why ??
Any occlusal force has 2 vectors because occlusal plane is not100% parallel , these vector are :
1. Compressive stress
2. Either shear or tensile
So tensile strength in the marginal ridge if it is not high, the restorationwill be broken .
In all the applications at dentistry , we try to make the design of thecavity or the filling itself by reinforcement that most of the stresses that
exist is compressive stresses but this is not always the case sometimesthere is tensile vector or shear vector .So we need the restoration to have high tensile strength to with strandforces especially on the margin .
4.Transverse strength
the material belowthe amalgam need to have low modulus of elasticity.
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Mercury content:
The main purpose is that Hg content to be enough for the reaction withno excess .
And we obtain this in the uni-compositinal type of amalgamCondensation is easier in admix or lathe-cut alloy than spherical type.
Porosity :
Occurs as a result of :
1. γ2 phase
2. unreactive mercury in the filling
More condensation removal of excess mercury and put it atthe surface of the cavity and with curving it will be removed .
Polishing of the amalgam filling is done after 24 hours from itsset.
High copper alloy unicompoitinal can be prepared to be thefoundation restoration after 24 hours from the setting session .
Creep could happen to the amalgam restoration after awhile and thenneed replacement
We have mentioned the relation between creep and viscoelasticproperty .
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γ2 phase (viscosity phase) is the main element in causing creep andcorrosion and it shouldn’t exceed 3% .
Condensation :
We apply condensation by hands , mechanical not found in dentistryunless in some specific research institutes .
Polishing the amalgam and make it shiny
It’s not a necessary step but it reduces tarnish and amalgam could havea better stability .
But the disadvantage of this step is that no one want a shiny restorationon his mouth which make it more visible .
Is amalgam toxic ??
In the states many research and investigations took place and decidedthat there is no scientific evidence to support these clime .
In Sweden, from 1993 they prevent using amalgam in kids who areless than ten years .
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Nowadays amalgam is totally prevented in Sweden ,
Narew and (irlanda or phenlanda ) .
In Europe,most of dental schools stop teaching amalgam to the postgraduating student .
In all development countries , amalgam in declining
But in the developing countries like us . so we still use amalgam .
*Non-scientific point of view :
There is no evidence to support that all of these diseases (multiplesclerosis, renal dysfunction or any other systemic disease) are related tobetween amalgam and this rxn .
Hg is a toxic element or material , But is the Hg in theamalgam toxic ??
If it is in the form of the set amalgam , it is not .
But we have said that there is not a total consumption to all Hg in the filling .
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Forms of Hg :
1)Methyl and ethyl amalgam (organic)
2)Elemental vapor .
3)In organic compound.
The most toxic type is organic compound , the least toxic is the inorganic and the vapor is in between.
- Threshold value of mercury vapor is 50 mg/m3.
_ about hygiene recommendation & office setting the doctor only read the slide without any explanation !!
"God answers all prayers in 3 ways : "Yes"= because you deserve it! "No" = because you deserve better! "Not yet" = because the best is yet to come…" !!!
Dedicated to : lena al-ja3bary , eman z nsour & afaf haddad ..
Done by : ghidaa al-maani
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