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Dental Material Science > Amalgam > Flashcards

Amalgam Flashcards

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1
Q

Main types of materials used for direct filling materials

A
  • Composites
  • Polyalkenoates (GIC)
  • Amalgam
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2
Q

Definition of amalgam

A
  • 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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3
Q

Why mercury

A

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

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4
Q

Classic metals in a dental amalgam

A
  • 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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5
Q

Conventional Amalgam constituents

A

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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6
Q

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

A

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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7
Q

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

A

-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

or

  • 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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8
Q

What do you do after you melt the metal components together

A

-Either lathe cut or spherical

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9
Q

Lathe cut process of the alloy manufacture

A
  • 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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10
Q

Spherical process of the alloy manufacture

A
  • 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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11
Q

Different forms of particle morphology in the alloy powder

A

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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12
Q

Differences between lathe-cut and spherical alloys

A

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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13
Q

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

A
  • 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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14
Q

Phases involved in amalgam

A

Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2

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15
Q

Setting reaction of amalgam and explanation

A

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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16
Q

Settiing reaction of amalgam and equations of each stage

A

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

Formation of Gamma 1
Ag + Hg > Ag2Hg3

Formation of Gamma 2
Sn + Hg > Sn7Hg

17
Q

Electron Microscope Image of set amalgam

A

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

18
Q

Relative strength of the different phases

A

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

19
Q

Evolution of strength with time

A

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

20
Q

Amalgam strength compared to dental tissues

A
  • Relatively good replacement for the natural tooth substance
  • Hardness of amalgam is lower than that of enamel
  • May lead to surface facet formtion
21
Q

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

A

-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

22
Q

Dimensional changes of amalgam from trituration and effects

A
  • 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

23
Q

Thermal Properties of Amalgam

A

-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

24
Q

Corrosion of Amalgam

A
  • 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
25
Q

Creep of amalgam

A

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
26
Q

How to make the amalgam stronger

A
  • 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

27
Q

How to make a dispersed phase in amalgam

A
  • 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
28
Q

Dispersed phsse compared to a conventional amalgam

A
  • Higher compressive strength
  • Lower creep
  • Lower corrosion
  • Faster achievement of final strength
29
Q

Importance of proportioning in amalgams

A
  • 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)

30
Q

Effect of mercury concentration

A
  • Important to reduce the Hg concentration
  • Final concentration should be around 40-45%
  • Excess Hg dramatically reduces the restorations strength
31
Q

Clinical Application of amalgam restoration steps

A

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

32
Q

Trituration importance and over/under

A
  • 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
33
Q

Condensation objectives and process of

A
  • 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
34
Q

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

A
  • 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
35
Q

Limitations of amalgam

A 
No adhesion to tooth substance 
Retentive cavity design required leading to loss of sound tooth substance 
Poor aesthetics 
Galvanic effects 
High thermal diffusivity
36
Q

Mercury issues

A
  • No adverse effects according to evidence
  • Pollution
  • Avoid spillage
  • High levels of mercury are associated with adverse effects in the brain and kidneys
37
Q

Reactions to amalgam

A
  • Allergic reaction to mercury rare

- Lichenoid reactions

38
Q

What should u tell a patient worried about mercury from amalgam

A
  • 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
39
Q

Read about the brexit affect on the slides

A

Yeah ok

Dental Material Science (13 decks)

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