Amalgam Flashcards
Main types of materials used for direct filling materials
- Composites
- Polyalkenoates (GIC)
- Amalgam
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
Why mercury
-Every metal can dissolve in mercury (apart from Iron) at room temperature
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
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
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
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
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
What do you do after you melt the metal components together
-Either lathe cut or spherical
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
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
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
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
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
Phases involved in amalgam
Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2
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
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
