Amalgam restorations Flashcards
Define a dental amalgam.
A dental amalgam is in any mixture, or blending, of mercury with another metal or an alloy.
Why is mercury and not other metals used in amalgam?
As every metal can dissolve in mercury at room temperature (except iron)
Mercury is the only metal that is liquid at room temperature, which allows it to form a homogeneous mixture with other solid metals. This property makes it easier to mix with other components and facilitates the handling and manipulation of the material during the filling procedure.
What is the main composition of dental amalgam?
Silver, Mercury and Tin.
What 2 metals can be added to the main composition and what do they do?
- Copper
Increases final strength.
- Zinc
Reduces oxidation
What are the 2 ways the dental amalgam alloy is manufactured?
- Lathe cut
Cooled down/mechanically grinded.
- Spherical
Atomisation in an inert atmosphere.
What are the steps to the lathe cut alloy manufacture technique?
- Cast into an ingot and heat at 420 degrees
- Cylindrical shaped alloy is cut on a lathe
- It is then power generated by further ball-milling
- Produces irregular size particles
- These particles are stressed and elongated
- Then homogenised at 100 degrees for 1 hour.
What are the steps to the spherical alloy manufacture technique?
- The melt is sprayed into an inert atmosphere.
- Surface tension and low viscosity generate small spherical (or spheroidal) particles.
- Solidifies into consistent sized particles.
In what form must the final alloy for dental amalgam be?
powdered
Why are alloy powders normally a mix of both manufacturing techniques?
- Increased packing efficiency
- Reduces the mercury needed
- Increases performance
List the differences between lathe-cut and spherical alloys.
look at notes
ow is the setting reaction initiated with dental amalgam?
Trituration
Which is vigorous mechanical mixing (amalgam mixing machine)
Where the liquid and powder is mixed.
What 2 new intermetallic compounds are formed as a result of trituration of dental amalgam?
look on notes
What are the steps in the setting reaction/trituration of dental amalgam?
The outer surface of silver/tin particles (gamma) dissolve in liquid mercury.
The silver reacts quickly to form Ag2Hg3 grains which stick preferentially along the alloy particles forming gamma 1
The tin reacts slowly to form gamma 2 which is randomly distributed inside the gamma 1 matrix
The reaction is completely set when gamma 1 and gamma 2 phases have formed a solid matrix and no mercury is left to dissolve gamma.
Why is correct trituration essential?
It is essential as it ensures both adequate amalgamation and the production of a plastic mix suitable for packing.
What does under-trituration result in?
It does not allow adequate formation of gamma and n phases and results in a crumbly mix.
What does over-trituration result in?
Excessive contraction.
Rank the tensile strength of gamma, amalgam, gamma 1 and gamma 2.
look on notes
What is the weakest phase of gamma in dental amalgam and what happemns if you reduce it?
- Gamma 2
- Reducing it will increase the strength of the restoration.
How long does it take for amalgam to develop and what does this mean in practice?
- Over 24 hours.
- Need to bring back the patient in a weeks time to polish the filling once its reached maximum strength
As enamel is harder than amalgam what can this lead to?
Surface-facet formation
How can amalgam contract and what can this lead to?
Due to dissolution of gamma phase into mercury.
Marginal gaps can be a result.
ow can amalgam expand and what can this lead to?
- Crystallisation of gamma 1 and gamma 2
- Zinc can also cause dramatic expansion
Expansion results in protrusions or even cracks.
Is amalgam’s thermal diffusivity high or low and why is this?
High
It is a metallic material.
Why is it bad for a dental material to have a high thermal diffusivity?
As this makes the pulp vulnerable to heat.
Why is it a problem that the thermal expansion coefficient is different for dentine and amalgam?
The mismatch can cause micro-leakage, so decay can form in the dentine surrounding amalgam.
How does amalgam corrode?
Gamma 2 is more electronegative than gamma and gamma 1.
It therefore acts as an anode and dissolves, releasing free Hg.
How can amalgam corrosion be reduced after insertion?
By polishing the restoration to a smooth surface.
Why can corrosion of amalgam be seen as a positive?
Corrosion occurs more at the amalgam/tooth interface.
This forms a seal which prevents microleakages.
Give 5 limitations of dental amalgams.
- No adhesion to tooth surface.
- Need retentive cavity design-loss of sound tooth substance.
- Poor aesthetics
- Galvanic effects
- High thermal diffusivity.
Define the term creep in dental restorations.
Flow, or deformation, due to applied over a long period of time and does not recover.
What factor in dental amalgam increases creep?
The higher the content of gamma 2, the higher the creep.
What can creep in dental restorations result in?
- Creep causes protrusion.
- Leading to weak unsupported edges.
- Leading to fracture of the edges
- And ditching:
How can creep be reduced in dental amalgam?
Using high copper amalgam:
- Attempt to eliminate gamma 2 by using high copper concentration.
- Gamma 2 free amalgam shows increased strength with reduced corrosion and creep.
- This is done by integrating copper in the powder blend: dispersion-modified copper enriched alloy.
Give 4 advantages of using a copper enriched alloy compared to a conventional amalgam.
- Higher compressive strength
- Faster achievement of final strength
- Lower creep
- Less corrosion
Give the 10 steps for fitting amalgam restorations.
- Cavity preparation and pulp protection.
- Rubber dam, matrix and wedges application.
- Amalgam trituration.
- Amalgam application (small quality).
- Condensation.
- Repeat 4 and 5 until cavity is filled.
- Carving and burnishing.
- Removing matrix.
- Refine carving in particular at the edges of restoration to ensure marginal seal.
- Finishing and polishing
What 3 things must you consider with cavity preparation for amalgam restorations to maximise long term success?
To not leave unsupported enamel structure.
Have a rounded cavity and not a wedged one, so the stress pattern is more uniform.
Ensure the prep is not too close to the pulp.
What are the objectives of the condensation stage of fitting amalgam?
- Remove excess mercury
- Ensure that there are no voids in the restoration
- Ensure marginal integrity
What 2 techniques are advised in the condensation stage of fitting amalgam?
- Condense small increments.
- Overfill the cavity because the surface layers tend to be richer in Hg.
How long on average do you have to carve the restoration after fitting?
2-3 minutes.
Does lathe cut or spherical amalgams produce a better surface?
Spherical
Why must you avoid using too much pressure when carving amalgam?
Friction heating can cause the release of Hg.
Give 2 environmental concerns about mercury alloys.
Require adequate measures for disposal of excess amalgam material
Hg is a polluting agent
Give 4 health concerns about mercury alloys.
1) Mercury vapors are released in small amounts from dental amalgams
2) High levels of Hg vapors are associated with adverse effects in kidneys and brain
3) FDA has reviewed scientific evidences and determined that amalgam restorations are safe for adults and children 6>
4) Consider operator exposure
What 2 things must be considered when removing dental amlagam restorations?
Patient exposure and operator exposure.
give 3 safety measures when removing dental amlagam restorations.
Safety measures:
1) Avoid removing good congruous restorations
2) Use rubber dam on patient to avoid ingestion and inhalation of Hg microparticles
3) Use appropriate PPE and control aerosol
Give 3 situations when the use of amalgam is strongly discouraged.
- Treatment of deciduous teeth.
- Children under 15.
- Pregnant of breast feeding women.
