Dental Amalgam: Material Properties and its Use in Clinical Dentistry Flashcards
What is an amalgam?
Amalgam is any alloy of mercury with another metal or other metals.
What are the types of amalgam that are used in dentistry?
Copper amalgam no longer used
Conventional amalgam
High copper amalgam
What is the composition of the alloy powder in dispersalloy amalgam that we use?
Silver (69%), tin (18%), zopper (12%) and zinc (1%)
What is the amalgam forming reaction called?
Amalgamation
Why is the mixing time 8 seconds?
The composition with the amalgamator we use needs 8 seconds for complete mixture. If a different composition or type of amalgam was used it would require different amount of time.
What are the dentist-controlled viariables of application of amalgam?
Trituation (mixing)
Condensation (extra pressure is required to adapt to cavity wall and margin and to get rid of excess mercury)
Burnishing (Carving and burnishing)
Polishing
What happens to amalgam if it is undertriturated?
It becomes grainy and crumbly
What happens it amalgam is overtriturated?
It sets too quickly
What are the types of alloy shaped?
Lathe cut alloys and spherical alloys exist. Lathe cut require small condensers and high force
Spherical alloys
What is the purpose of burnishing?
Burnishing removes excess mercury with pressure and improves margin adaptation
Post carving improves smoothness
Combined there is less leakage?
What is the most common cause of amalgam failure?
Most important reason for failure of amalgam is premature loading. 1 hour must be given before patient eats.
What is the purpose of polishing amalgams?
Increases smoothness
Decreased plaque retention
Decreased corrosion
Clinically effective
What happens during amalgamation?
Dissolves the surface of alloy particles to form a plastic mass. Setting and hardening occur as the liquid mercury is consumed in the formation of new solid phases.
What are the uses of amalgam?
Direct, permanent posterior restorations
Large foundation restorations
Cares for croensd or fixed partial denture restorations
What are the weaknesses of amalgam?
Amalgam is brittle material (wweak under tension strong under compression)
Prone to corrosion
Creep
Why is tin used in amalgam?
Because tin gives it a soft consistency at room temperature. (high reactivity and low melting temperature)
What is the purpose of high copper in amalgams?
It removes gamma2 phase
What are the ADA specs for dimensional changes that take place in dental amalgam?
Amalgam neither contract nor expand more than 20 microns/cm between 5 minutes and 24 hours beginning of trituration
What causes contraction of amalgam?
Contraction results as the particles dissolve and the gamma1 grows.
If there is sufficient liquid mercury present to provide a plastic matrix expansion will occur when gamma1 crystals impinge against one another.
What does low Hg:alloy ratio lead to?
Lower mercury:alloy ratios and higher condensation pressures leads to less mercury in the mix and thus results in contraction.
Longer trituration times used of smaller particle size alloys leads to accelerated setting and this leads to contraction
Modern amalgams contract automatically due to low Hg:Alloy ratios
What is the most easily corroded amalgam phase?
The gamma2 phase
What is the most favourable strength characteristics of amalgam?
Resistance to compression forces is the most favourable strength characteristic of amalgam
What is creep?
Time dependent plastic deformation
