Amalgam Flashcards by Shrey Paun

Main types of materials used for direct filling materials

Composites
Polyalkenoates (GIC)
Amalgam

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Definition of amalgam

Mixture, or blending, of mercury with another metal or alloy
Not possible to have a mercury free amalgam
Every metal can dissolve in mercury (apart from Iron) at room temperature

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Why mercury

-Every metal can dissolve in mercury (apart from Iron) at room temperature

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Classic metals in a dental amalgam

Based on the system
Silver-Mercury-Tin
Other metals are added to this system to modify its properties
Copper increases its final strength
Zinc reduces oxidation

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Conventional Amalgam constituents

Based on a powder/liquid phase

Liquid phase is simply triple distilled Hg
Powder is an alloy based on the intermetallic compound Ag3Sn
Known as the gamma phase

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Approximate % by weight and functions of metals in conventional dental amalgam

Silver: Minimum 65%. In the gamma phase

Tin: Max 29%. In the gamma phase

Copper: 6% max. Strength/hardness

Zinc: 2% max. Manufacturing

Mercury 3% max. Preamalgamation

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Manufacture of the alloy in the powder phase of conventional amalgam
Problem with it and how it overcome

-Melt components at high temperature in a reducing atmosphere to produce Ag3Sn

Silver, copper and tin oxidise easily
Use zinc as an oxygen scavenger
Then remove the zinc oxide
Final alloy always contains zinc

Melt the alloy in an inert (oxygen-free atmosphere)
Zinc free alloy (less expansion)

-Final alloy must be used in a powder form
-After the alloy is melted as a homogenous liquid, there are 2 possibilities:
Lathe cut- cooled down/mechanically grinded
Spherical- atomisation in an inert atmosphere

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What do you do after you melt the metal components together

-Either lathe cut or spherical

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Lathe cut process of the alloy manufacture

Cast into an ingot and heated at 420 degrees celcius
Cylindrical shaped alloy cut on a lathe
Power generated by further ball milling
Produces irregular size particles
Particles are stressed and elongated
Homogenised at 100 degrees celcius for 1 hour

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Spherical process of the alloy manufacture

Melt is sprayed into an inert atmosphere
Surface tension and low viscosity generate small spherical (or spheroidal) particles
Solidifies into consistent sized particles

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Different forms of particle morphology in the alloy powder

Lathe-Cut
Spherical
Mixed

Many alloy powder are formulated by mixing particles

1) Increases packing efficiency
2) Reduces Hg needed
3) Increases performance

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Differences between lathe-cut and spherical alloys

Lathe-Cut requires more mercury
Spherical requires less

Lathe cut requires more condensation force into the cavity
Spherical requires less

Lathe cut requires smaller condenser point (smaller amalgam plug required)
Spherical requires a larger point

Less easy to carve and burnish lathe cut
Easier to carve and burning as smooth surfaces in spherical

Less overhands and strong proximal contacts in lathe cut
Overhangs and weak proximal contacts in spherical

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How is the setting reaction of amalgam initiation and what is this equation
Name all the phases

Initiated by vigorous mechanical mixing
Trituration of the powder and liquid

Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg

Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2

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Phases involved in amalgam

Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2

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Setting reaction of amalgam and explanation

Initial Dissolution
-Outer surface of tin/silver particles dissolve in the triple distilled liquid mercury
Ag3Sn + Hg > Ag + Sn + Hg

Formation of Gamma 1

Silver reacts quickly to form Ag2Hg3 grains
Sticks preferentially along the alloy particles
Ag + Hg > Ag2Hg3
Gamma 1 gets dispersed into the matrix

Formation of Gamma 2

Tin reacts slowly to form Gamma 2 which is randomly distributed inside the Gamma 1 matrix
Sn and Hg> Sn7Hg

Set Amalgam
-Reaction is completely set when the Gamma 1 and Gamma 2 phases have formed a solid matrix and no mercury is left to dissolve Gamma

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Settiing reaction of amalgam and equations of each stage

Initial Dissolution:
Ag3Sn + Hg > Ag + Sn +Hg

Formation of Gamma 1
Ag + Hg > Ag2Hg3

Formation of Gamma 2
Sn + Hg > Sn7Hg

Electron Microscope Image of set amalgam

-Core of unreacted gamma particles surrounded by a matrix of Gamma 1 (high concentration) and Gamma 2 (low concentration)

Relative strength of the different phases

Assuming the amalgam has been correctly mixed, tensile strength:

Gamma > Amalgam > Gamma 1 > Gamma 2

Gamma 2 is the weakest phase and reducing it increases the strength of the restoration

Evolution of strength with time

Amalgam develops slowly (>24 hours)
Amalgam remains very weak at the time the patient leaves the surgery
Compressive strength of 50MPa after 1 hour compared to 300MPa after 24 hours

Amalgam strength compared to dental tissues

Relatively good replacement for the natural tooth substance
Hardness of amalgam is lower than that of enamel
May lead to surface facet formtion

Modulus of elasticity, compressive strength, tensile strength at 7 days and vikers hardness for enamel, dentine and amalgam

-Done in order from first to third largest

Modulus of elasticity: Enamel 1, Dentine 3, Amalgam 2
Compressive strength: Enamel 3, Dentine 2, Amalgam 1
Tensile strength after 7 days: Enamel 3, Dentine 2, Amalgam 1
Vikers Hardness Enamel 1 Dentine 3 Amalgam 2

Dimensional changes of amalgam from trituration and effects

First contraction due to dissolution of gamma phase into the mercury
Crystallisation of gamma 1 and gamma 2 leading to expansion of the amalgam
Zinc can cause dramatic expansion: zn + H20 > ZnO + H2

Contraction leads to marginal gaps
Expansion results in protrusions or even tooth cracks

Thermal Properties of Amalgam

-Metallic material
-High thermal diffusivity
-Need to protect the base of large cavity to avoid harmful affect on the pulp (pulp capping?)
Thermal expansion mismatch can cause microleakage
-Occurrence of decay in the dentine surrounding the amalgam
-Amalgams thermal diffusivity and thermal coefficient of expansion is much higher than dentines

Corrosion of Amalgam

Multiphase metallic material sitting in a wet environment
Inevitable corrosion
Gamma 2 is more elctronegative than Gamma and Gamma 1
Acts as an anode and dissolves, releasing free mercury
Corrosion can be reduced by polishing the restoration to a smooth surface
Beneficial advantage: Corrosion occurs more at the amalgam/tooth interface forming a seal which prevents microleakages

Creep of amalgam

Increase in strain after a constantly applied stress is placed on a material -Plastic deformation - Flow, or deformation, due to applied load over a long period of time and does not recover - The higher the content of gamma 2, the higher the creep - Creep causes protrusion, leading to weak, unsupported edges - Fractures at the edges - Potential ditching

How to make the amalgam stronger

- Get rid of the gamma 2 phase - Eliminate gamma 2 achieved by using a high copper concentration - Gamma 2 shows increased strength with reduced creep and corrosion -Copper is integrated in the powder blend: dispersion-modified copper enriched alloy

How to make a dispersed phase in amalgam

- Combination of 2 parts of conventional lathe cut gamma (with <6% copper) and 1 part of a Ag-Cu eutectic (72% and 28% copper) - Typical overall copper content is 12% Hg + Gamma + Cu > Gamma + Gamma 1 + Cu6Sn5 + Cu3Sn ``` n= Cu6Sn5 E= Cu3Sn ```

Dispersed phsse compared to a conventional amalgam

- Higher compressive strength - Lower creep - Lower corrosion - Faster achievement of final strength

Importance of proportioning in amalgams

- Correct Alloy:Hg ratio is necessary to obtain the optimum properties - If the mix is dry, it will not be possible to condense it - If the mix is slightly wet, the excess Hg can be removed by the correct condensing technique 5: 8 for hand mixing 10: 8 for mechanical mixing (particle dependent)

Effect of mercury concentration

- Important to reduce the Hg concentration - Final concentration should be around 40-45% - Excess Hg dramatically reduces the restorations strength

Clinical Application of amalgam restoration steps

1) Cavity Prep and pulp protection 2) Matrix and wedge application 3) Alloy selection and proportioning 4) Trituritation 5) Condensing 6) Carving and Burning 7) Wedge and Matrix Removal 8) Finishing and polishing

Trituration importance and over/under

- Essential to ensure both adequate amalgamation and the production of a plastic mix suitable for packing - Correct trituration time depends both on the alloy and the mixing system (5-20s) - Undertrituration does not allow an adequte formation of the gamma and n phases - Results in a crumbly mix - OVer tritutration results in excessive contraction

Condensation objectives and process of

- Remove excess Hg - Ensure there are no voids in the restoration - Ensure marginal integrity - Must be carried out immediately after mixing - Condense small increments - Overfill the cavity because the surface layers tend to be richer in Hg - Spherical amalgams require a lower condensation pressure than lathe cut ones

How long can you carve/finish for Carving and finishing for lathe cut or spherical Process of

- Carving of the restoration possible for 2-3 minutes, dependent on the amalgam - Spherical restorations produce a better surface than lathe cut ones - Avoid too much pressure-friction heating can cause the release of Hg - Smooth surface minimised the probability of corrosion

Limitations of amalgam

``` No adhesion to tooth substance Retentive cavity design required leading to loss of sound tooth substance Poor aesthetics Galvanic effects High thermal diffusivity ```

Mercury issues

- No adverse effects according to evidence - Pollution - Avoid spillage - High levels of mercury are associated with adverse effects in the brain and kidneys

Reactions to amalgam

- Allergic reaction to mercury rare | - Lichenoid reactions

What should u tell a patient worried about mercury from amalgam

- Very low levels - Less than total amount patient is exposed to in their daily environment or from what they eat - Removing amalgams can result in unnecessary loss of healthy parts of the tooth and can release more mercury - Only replace is worn, broken down or decay

Read about the brexit affect on the slides

Yeah ok