Amalgam 10

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    Dental alloy(aka. Gamma phase Ag3Sn)

    - Alloy being a combination of 2 metals

    o

    Metals are in its solid rather than ionic form

    o 65% Ag; 35% Sn

    - Not pliable (means easily bent/flexible)

    - Needs Hg to help pack it into the tooth

    o Hg converts y to y1and y2which increases the pliability of amalgam

    o y it self is too porous/ brittle

    -

    Not perfectly soluble

    -

    Not homogenous (sections of just Ag and sections of just Sn within Ag3Sn)

    o If an oxide layer on the surface of Ag3Sn becomes too thick prevents binding of

    Hg (passivated layer)

    Passivated layer: Prevents the metal to corrode any further

    2 types of cut of Ag3Sn:

    -

    Lathe cutproperties are varied, not as effective for amalgamation

    o Shuave Spirals that are produced by puttingAg3Sn on a lathe machine These

    spirals are later crunched up to smaller pieces

    -

    Spherical cutparticles are regular spherically sized so reaction with Hg is more reliable

    and it is more readily condensed into a cavity

    Amalgam Steps1.

    Trituration (mixing)

    2. Condensation (packing)

    3. Carving

    4.

    Burnishing5.

    Polishing

    TriturationOverall Reaction:

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    *y1 and y2 are inferior in strength, corrosion resistance and creep to y phase limit y1 an y2 by

    limiting the amount of Hg

    Trituration Process:

    - Add Hg to gamma phase

    o Initial contraction occurs (volume of Hg and gamma mixture is less than the total of

    the initial volumes of the two mixtures)o This is as y acts as a sponge absorbing the Hg

    - Shake mixture to break up the alloy removes oxide layer and increases surface area for

    Hg to react with gamma

    o Increase SA as you break up the alloy Break off gamma particles off the big

    unreacted gamma particle (breaking off anita)

    o Hg reacts with Ag rich area to form gamma 1

    o Hg reacts with Sn rich area to form gamma 2

    o Continued shaking ensures that you remove the newly form y1 and y2 particles

    Short Trituration Time: Stop shaking after a short period of time

    - y1 and y2 dont fall off the y particle (no new surface area at y for Hg to react diffusion of

    Hg into y particles are suppressed)- Gamma 1 and 2 crystallise upon formation (nucleation) and results in expansion

    o Nucleation = Forming the new phase of y1 and y2 from y results in expansion

    *Crystallisation: Forming solid crystalsy1 and y2 gets BIGGER

    *Too much expansion = Bad = Cracking of tooth

    *Also not enough y1 and y2 forms Amalgam not very packable as y itself is brittle/hard/not pliable

    Long Trituration Time: Stop shaking only after a long time

    - For a long time; gamma 1 and 2 are continually removed from the surface of gamma and a

    new surface is available for reaction with Hg

    o More gamma 1 and 2 are formed resulting in a smaller gamma core

    o This results in a greater contraction

    *Too much contraction = Bad = Leakage around margins of tooth and restoration

    *Too much y1 and y2 = Inferior to y

    AIM of trituration

    - Need to achieve a balance between the contraction and expansion:

    o Dont want : Formation of gamma 1 and 2 with no gamma remaining (completed

    reaction)

    o We want:

    Some gamma 1 and 2 formation to keep the superior properties of gamma

    in the product

    Limit formation of gamma 2 (mainly) as gamma 2 has weaker properties

    Easy to handle in oral cavity

    Cannot become too porous (dont want toomuch product) so that it

    withstands forces

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    But it needs to still be pliable enough to allow packing into oral

    cavity

    Must be able to set in a wet environment (within saliva)

    *Time of trituration hence is the most important factor

    Limiting the formation of gamma 2- Reduce trituration time and increase condensation force

    o Reduce trituration less surface area available (and less contact time) for Hg to

    react with gammacan only be reduced to a certain extent as amalgamation

    MUST occur

    o Increase condensationpushes excess Hg to the surface and removes it less Hg

    is present to react with gamma to form gamma 2 after amalgam is set in mouth

    (reduces post-setting expansion)

    - Add a dispersed alloy (gamma alloy also containing other metals eg. Ag and Cu)

    o Ag reacts with Hg to form more gamma 1

    o

    Cu competes with Hg in gamma 2 to form a beta phase (Cu and Sn)- Mechanical properties are improved BUT more galvanic corrosion occurs as more metals are

    presentneed a balance between the two

    *Y2 properties are inferior to gamma and gamma 1

    *rate of formation is directly dependent on the amount of Hg present

    Condensation-

    Condensation: Pressure applied with a packing instrument when placing the amalgam into a

    cavity

    - Increase condensation pressure will

    o

    Counteracts setting expansion

    This is as the initial diffusion layer of Hg is disrupted and more Hg diffusion

    takes place

    o Also packs the amalgam alloy particles closer together leaving less room for Hg

    Inhibits contraction from initial reaction? Sponge absorbing Hg

    Excess Hg moves to the surface and is removed (less likely to react after it

    sets) Prevent further formation of y1 and y2 (which tends to counteract

    expansion)

    - Good condensation will remove as much of the mercury rich y1 and y2 phases

    -

    Inefficient condensation:

    o Dont pack amalgam strong enoughincreased porosity which

    can lead to 30% increase in creep

    Restoration failureConsequences of Creep

    -

    Creep: Is the flow of setamalgam which causes

    o Flattening of contact points between the amalgam restoration and neighbouring

    tooth

    o Overhang Margins

    o Biting on amalgam can cause amalgam to flow up/over to the margins and can chip

    off and cause leakageo Amalgam can protrude into tooth margins which leads to marginal breakdown

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    *Overhang Margins: An excess of dental filling material beyond the cavity margin

    Consequences of Corrosion?

    - Corrosion: Amalgam undergoes chemical reaction with its environment forming breakdown

    products

    - Corrosion reaction: y2+ O2Tin Salts and free Hg

    o

    From this reaction; a passivating film of oxides is formed Insulate amalgam fromfurther reaction/corrosion?

    o However, biting in amalgam removes the oxide layer and exposes amalgam Draw

    picture

    Gamma 2 is most reactive and a small electrolytic cell is created (galvanic

    corrosion)

    Gamma 2 is lost into the saliva as it corrodes (higher reduction potential

    gain electrons and will be released as tin salts and free Hg)

    o Zinc can be added to react with O2preferentially to reduce corrosion (prevents

    corrosion/oxidation of y2)

    If it is contaminated it results in expansion

    *Continued corrosion causes staining of dentine and stains amalgam as a dark grey/blackTooth fracture

    -

    Cavity preparations with sharp internal line angles are usually needed to increase

    mechanical retention for amalgam

    o There is very little bonding strength between amalgam and the tooth

    o However sharp line angles = act as stress raises are contribute to tooth fracture

    o COMPROMISE: use a slightly rounded cavity that also provides sufficient mechanical

    retention

    -

    Round off the acute cavosurface angles (unsupported enamel rods) to decrease risk of

    chipping/marginal breakdowns Cavosurface angles should be 90o

    - Amalgam does not reinforce remaining cusps on teeth

    o

    Cusps are more likely to fracture/chip offCoefficient of thermal expansion/contraction

    - Large discrepancy between tooth (11.4) and amalgam (28)

    o Expands and contracts very differently to tooth upon exposure to heat

    o Contraction = leakage

    o Expansion = cracking of tooth

    - Amalgam also has a high coefficient of thermal conductivity

    o Composite has similar thermal expansion/contraction as amalgam but much lower

    thermal conductivity heat transfer rate is low; expands/contracts slower

    -

    Opening the interface between tooth and restoration Allows bacterial ingress and

    recurrent caries

    Contamination of cavity

    - Need to work in a dry environment Want amalgam to bond directly to the tooth (nothing

    between tooth and amalgam which can cause failure at margins of restoration)

    - Contamination with moisture (saliva) during packing can cause expansion (along with short

    trituration and poor condensation)

    o This can cause cracking of mesial/distal margins of teeth with Class 1 restorations

    Amalgam failure

    o Restoration is too thin relative to the surface

    Cracks easily when biting on it

    o Line angles in the cavity are too sharp

    o Cavity preparation has poor mechanical retention

    Amalgam restoration can fall out of cavity

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    Gross Amalgam Fracture

    - Caused by shallow preparations (shallow = not enough strength), non-retentive cavity preps

    or sharp line angles

    Extra Points

    *Can control size of Ag3Sn to minimize the amount of Hg required e.g. smaller y particles will have alarger SA for reaction with Hg (so can decrease amount of Hg required)

    *Less seconday caries occur in amalgam compared to composite

    *Last 20-30 years; Composite a few years