Dental Amalgam Flashcards
An alloy, one of the constituents of which is mercury
Silver Paste
An alloy made by mixing Hg with Ag-Sn dental amalgam alloy
Silver Paste
Increases expansion Retards setting time Enhances strength Decrease flow Resist tarnish/corrosion
Silver
Unites with Hg with difficulty Reduces setting time Increases expansion Increases strength and hardness Reduces flow
Copper
Combines readily with Hg Retards setting time Improves platsicity Reduces expansion Increases flow
Tin
Combines readily with Hg Causes expansion increases setting time Increases flow Inhibits oxidation Decreases porosity Provides better clinical performance
Zinc
Other Constituents of Dental Amalgam
Indium (In)
Palladium (Pd)
Mercury (Hg)
Decreases surface tension Reduces amount of mercury necessary Reduces emitted mercury vapor Reduces creep and marginal breadown Increases strength Must be used in admixed alloys 5%
Indium (In)
Reduces corrosion
Greater luster
e.g. Vallant PhD (Ivoclar Vivadent)
0.5%
Palladium (Pd)
Activates reaction
Only pure metal that is liquid at room temperature
Mercury (Hg)
Classification of Amalgam
Amalgam alloy particle geometry and size
Copper content
Zinc content
Irregular particle pack poorly and require a large amount of Hg (50-60%) Precapsulated amalgam (spherical) has 42-45% Hg
Alloy-Mercury ratio
Require less Hg
Smaller surface area, easier to wet
40 to 45%
Spherical Alloys
Reaction of Amalgam
Mercury + Silver-Tin -> Silver-Tin phase + Silver-Mercury phase + Tin-Mercury Phase
Called the gamma phase
Composed of unreacted alloy particle
Silver-Tin Phase
Ag3Sn
Unreacted alloy
Strongest phase and corroded the least
Forms 30% of volume set amalgam
Gamma
The amount of mercury needed to wet all particles and occupy the space between the particles range from ____________ mercury by weight
40-60%
Called the gamma one
Silver-Mercury Phase
Ag2Hg3 Matrix for unreacted alloy Second strongest phase 10 micron grains binding gamma 60% of volume
Gamma 1
Called the gamma two
Tin-Mercury Phase
Sn8Hg
Weakest and softest phase
Corrodes fast,
Voids from corrosion yeilds Hg which reacts with more gamma
10% of volume
Volume decreases with time due to corrosion
Gamma 2
2 phenomenon during hardening of amalgam
Solution
Crystallization
When Hg comes in contact with amalgam alloy, the particles are wt by mercury and begins to absorb it
The solution of mercury into silver-tin particles leads to the formation of the surface of Ag-Hg and Sn-Hg phases
The crystallisation of the gamma1 and 2 phases and their subsequent growth leads to hardening of amalgam
Crystallisation
Factors affecting Setting Time (4)
Effect of alloy/mercury ratio
Condensation pressure
Particle size
Particle shape
More mercury increases expansion - as well as setting time
Effect of alloy/mercury ratio
As the condensation pressure increases, the tendency for shrinkage increases. Increased condensation pressure results in removal of more excess mercury and a faster setting reaction
Condensation Pressure
As the particle size decreases, the total particle surface area increases. This results in a faster setting reaction, as long as no additional mercury is added. Amalgams made from smaller dental amalgam alloy particles shrink more as long as the mercury ratio is not high
Particle size
Spherical particles need less mercury because of the smaller total surface area. Thus, spherical amalgams often set faster than lathe-cut amalgams. Because of the lower mercury ratio, spherical amalgams often shrink more than lathe-cut alloys
Particle Shape
Properties of amalgam
Dimensional change
Strength
Causes amalgam to expand and contract
Diffusion of mercury into alloy particles causes an expansion
Spherical alloys have more contraction (less Hg)
Greater condensation = higher contraction
Dimensional Change
Amalgam does not develop sufficient strength to resist the forces pf mastication
Tensile strength - 7100 psi
Compressive strength - 45,500 psi
Develops slowly
Strength
Dentist Controlled Variable
Manipulation
4 properties under manipulation
Trituration
Condensation
Burnishing
Polishing
Process of manual mixing of alloy with Hg
Done by proportioning Hg and alloy into a mortar grinding the mixture with a pestle
Mixing time refer to manufacturers recommendation
Trituration
Sticks to capsule
Decrease working and setting time
Causes higher contraction
Overtrituration
Grainy, crumbly material
Undertrituration
Types of trituration
Hand
Mechanical
Done by proportioning Hg and alloy into a mortar and grinding the mixture with a pestle
Hand trituration
Use of amalgamator
Mechanical trituration
A hallow tube with rounded ends constructed as two pieces that could friction fit or screwed together
Reusable capsule
Pre-proportioned
Precapsulated
Phases of Burnishing
Pre-carve
Post-carve
Combined
Removes excess mercury
Improves margin adaptation
Pre-carve
Improves smoothness
Post-carve
Less leakage
Combined
Increased smoothness
Decreased plaque retention
Decreased corrosion
Clinically effectivev
Polishing
Uses squeeze cloth to remove excess mercury
Mercury rich mixtures
Illness associated with mercury
Allergies, chronic fatigue, depression, GI disorders, immune system suppression, endocrine disorders, periodontal disease, neurological problems, reproductive disorders, birth defects, kidney disease, heart problems, hypertension, repiratory disorder, cancer, and skin disease
Sources of mercury
Mercury vapours released from stored materials
Amalgamator aerosols
Spillage during restoration procedure
Amalgam and mercury in plumbing and sewer system
Amalgam scrap container
