Dental Amalgams Flashcards
What is an amalgam made out of
When mercury is mixed with another metal like gold, tin or silver
What is considered the best and most cost effective amalgam option for restorations
Mercury + silver tin alloy
What metals are found in the conventional (low copper) alloy of amalgam
67%-74% Silver (gamma phase Ag3Sn)
25-28% - Tin
0-6% copper
0-2% zinc
this alloy is added to mercury
Describe the modern high copper amalgams alloy composition
40-70% silver 21-30% tin 12-30% copper 0-1% zinc 0-0.5% palladaium 0-4% indium
alloy is added to mercury
What does adding silver in an amalgam alloy increase
- Strength
- Tarnish and corrosion resistance
- Expansion
What does adding silver in an amalgam alloy decrease
- Flow and creep
- Setting time
What does adding tin to an amalgam alloy increase
- Setting time
- Flow and creep
What does adding tin to an amalgam alloy decrease
- Strength
- Expansion
- Corrosion resistance
What does adding copper to an amalgam alloy increase
- Strength and hardness
- Tarnish and corrosion resistance
- Expansion
What does adding copper to an amalgam alloy decrease
- Flow and creep
- Setting time
What does adding zinc to an amalgam alloy do
Acts as a scavenger and decreases the oxidation of other metals
What is the role of mercury in an amalgam alloy
Activates the reaction
What does adding palladium to an amalgam alloy increase
- Tarnish and corrosion resistance
- Strength
What does adding Indium to an amalgam alloy increase
- Strength
- Expansion
- Setting time
What does adding Indium to an amalgam alloy decrease
- flow and creep
- surface tension and so the amount of mercury required it less
What shapes of particles does the dental amalgam alloy powder come in
Lathe-cut
Spherical
How are lathe cut amalgam alloys formed
Block of alloy with gamma and beta phases is homogenised at 400C for up to 8 hours to form gamma phase.
Particles of this block are annealed at 100C to relieve internal strains.
This is then surface treated with acid to increase reactivity and remove oxides from surface
What size of particle can lathe cut amalgam alloys be
Micro cut
Fine cut
Coarse cut
Between 60-100um long
10-70um wide and 10-35 thick
How are spherical amalgam alloys created
Spherical particles are produced by atomising molten alloy in an inert atmosphere.
Acid washed to increase reactivity
What is the typical particle size of spherical amalgam alloy particles
5-50um
How do spherical particles compare to lathe cut particles in amalgam alloys
- Requires less mercury - 40-45%
- Require less mixing time
- Require lower condensation pressures
- Harden more rapidly
- Smoother surface
What are the different ways that you can classify dental amalgams
- Based on Cu content
- Based on Zn content
- Based on alloy
- Based on shape
How can you classify dental amalgams based on Cu content
Low Cu alloys
High Cu alloys
How can you classify dental amalgams based on Zn content
Zn containing
Zn free
How can you classify dental amalgams based on the alloys
Binary
Ternary
Quaternary
How can you classify dental amalgams based on their shaped
- Lathe cut
- Spherical allots
- Admix (dispersed phase)
Describe the setting reaction of dental amalgams
- Mercury initially dissolves the surface of the alloy particles
- New mercury containing alloys formed
- Reaction is never complete
- Set amalgam contains unreacted Ag3Sn in a matrix of the newly formed alloys.
- A “cored structure”
What are the gamma 1 and gamma 2 compounds found in between Ag-Sn alloys (i think)
Ag2Hg3 = gamma 1 Sn7-8Hg = gamma 2
What phases are found in the low copper, conventional, set amalgam structure
Gamma phase
Gamma 1 phase
Gamma 2 phase
Describe the gamma phase of conventional low copper dental amalgams
Ag3Sn
- Unreacted alloy
- Strongest phase and corrodes the 2nd least
- Forms ~30% of volume of set amalgam
Describe the gamma 1 phase of conventional low copper dental amalgams
Ag2Hg3
- Matrix for unreacted alloy found around and in-between AgSn alloy
- 2nd strongest and corrodes the least
- Forms ~60% of volume
Describe the gamma 2 phase of conventional low copper dental amalgams
Sn7-8Hg
- Weakest and softest phase
- Corrodes fast, voids form
- ~10% of volume
- Volume decreases with time due to corrosion
- Found in between the gamma 1 phases and AgSN alloys
Describe the first setting reaction of Admixed high copper alloys
- Hg dissolves Ag and Sn from Ag3Sn particles
- In the first reaction AgCu remains unreacted
- Gamma + Hg = Gamma + Gamma 1 + Gamma 2
Describe the second setting reaction of admixed high copper alloys
- Gamma 2 (Sn7-8Hg) reacts with AgCu to form Cu6Sn5
- This forms the eta phase that surrounds the unconsumed AgCu particles as well as gamma 1 phase
Describe the structure of single phase high copper dental amalgams
Gamma (AgSn) phase coated in epsilon (Cu3Sn) coating
What happens in the first setting reaction of single phase high copper dental amalgam
Ag and Sn dissolve in Hg
What happens in the second setting reaction of dental amalgams
Gamma 1 (Ag2Hg3) crystals grow binding gamma (Ag3Sn) particles together
Eta crystals (Cu3Sn) form within gamma 1
Write down the setting reaction of single phase high copper dental amalgam
(Gamma + Eta) + Hg = Gamma + Eta + Gamma 1 + eta
What contributes to the net shrinkage of all modern amalgams
- Initial shrinkage from dissolution of the alloy
- Expansion from growth of crystalline phases
- The higher the mercury content the higher the expansion
- Final bit of shrinkage due to final contraction caused by solid state reactions
- Stable after 6-8 hours
What contributes to higher shrinkage
- Smaller particle size, more dissolution
- Higher Hg/Alloy ratio, more dissolution
- Trituration time longer and more rapid, more dissolution
- Condensation pressure higher, lower mercury and less new alloy formed
What is trituration
Mixing of dental amalgams
What is speed and time is usually used in trituration
Around 3000 rpm
For about 5-20 seconds
What is the dental amalgam powder mixed with
Powder is mixed with 40-50% mercury, higher amount for low copper
What does under triturated alloy powder look like
Dull, dry, crumbly mix
What does properly triturated alloy powder look like
Shiny mix, separates from capsule in single mass
What does over triturated alloy powder look like
Shiny, hot, wet mix, sticks to capsule
What is condensation in dental amalgams
This is when you pack the amalgam into the tooth cavity incrementally
Why is condensation used for dental amalgams
- removes excess mercury by bringing mercury to the top of the surface
- Prevents voids
- Gives optimum marginal adaptation
What is roughly the final mercury content of lathe cut and spherical dental amalgams
Lathe Cut = 45%
Spherical = 40%
Describe the relationship between condensation pressure and strength for lathe cut and spherical dental amalgams
Lathe cut - high strength under lower condensation pressures, doesn’t increase much with condensation pressure
Spherical - Needs higher condensation pressures to reach the levels of strength of lathe cut amalgams
What 3 processes of finishing can be done for amalgams
Carving
Burnishing
Polishing
What gives the better surfacee finish, spherical or lathe cut amalgams
Spherical
What is burnishing
This is the process of smoothing out surfaces with pressure, helps to remove residual mercury from the surface and improves margins
Why might a patient complain about pressure after having a zinc containing amalgams
Zinc containing amalgams undergo delayed expansion when they are contaminated with water, the zinc will react with the water and produce hydrogen
What is tarnishing
The formation of black silver sulphide that can be polished off and does not cause long term problems, has no effect on clinical lifetime
List the order of corrosion resistance by the phases found in dental amalgams
Gamma 1 > Gamma > Ag3Cu2 > epsilon > eta> gamma 2
What is the most prone phase in low copper amalgams and what are the corrosion products
Low copper gamma 2 phase is the most prone and the corrosion products are tin oxides and chlorides
What is the most prone phase in high copper amalgams and what are the corrosion products
High copper Eta phase is the most prone to corrosion, the products of corrosion = tin oxides, chlorides and copper chlorides.
High copper amalgams corrode more slowly than low copper amalgams
What is galvanic corrosion and give an example
When you have opposing dissimilar metals, where one is more electronegative than the other so one will corrode and the other won’t
e.g. amalgam with gold casting alloy - amalgam will corrode here
What is localised galvanic corrosion
Occurs between different phases as there is a difference between the electropnegativites of each phase
What is Crevice corrosion
Plaque build up increases electronegativity with respect to the clean surface
Clean surface is cathode and plaque surface is anode and the plaque surface corrodes
What is stress corrosion
Occurs under sustained tensile force in a corrosive environment
What are the mechanical properties of amalgam
- High compressive strength
- Good wear resistance
- Mercury content is critical to strength
- Must be less than 50%
How long do dental amalgams last
can last around 12 years, longer than composites
What are the disadvantages of amalgam
- Non adhesive - mechanical retention
- Conducts heat and electricity
- Corrosion and tarnish
- Poor aesthetics
- Amalgam is brittle, weak in thin section
- Amalgam is visco-elastic - creep
Describe the coefficient of thermal expansion of amalgam compared to dentin and what affect this can have
Coefficient is much higher than dentin so can result in marginal leakage
What properties of amalgams affect the marginal seal
- Coefficient of thermal expansion is much higher than dentin = marginal leakage
- Low copper amalgams corrode and result in sealing of margins, this process is slower for high copper
- For all amalgams, metal sulfides will form in tarnish and this will help marginal seal
What products are there that can contribute to the marginal seal of dental amalgam restorations
Cavity varnishes - resins in a volatile solvent, seals dentin surface and reduce bacterial inflammation
Coat surface and margins with unfilled resin, will delay microleakage until corrosion products seals
What do amalgam bonding agents do
Contain phosphonate esters or 4-META that supposed to reduce marginal leakage and postoperative sensitivity
What is a disadvantage of amalgam bonding agents
Need to condense amalgam before polymerisation of bonding agent is complete = technically challenging
How can the viscoelasticity of amalgams lead to marginal breakdown
- Creep can lead to flow over the margins and this can create a thin layer of amalgam over the margins
- These thin layers will fracture and result in marginal breakdown, “ditching”
- Over filling has a similar effect
What ways does the manufacturer control the quality of the amalgam
- Alloy composition
- Heat treatment of the alloy
- Size, shape and method of production of the alloy particles
- Surface treatment of the particles
- Supplied form (encapsulated)
- Mercury:alloy ratio
What ways does the dentist control the quality of the amalgam
- Trituration procedures
- Cavity design
- Condensation technique
- Marginal integrity
- Anatomy
What 3 forms does mercury exist in and how can each enter the body
Elemental Hg - mainly absorbed through the lungs
Inorganic Hg-S - in some industrial processes, mainly through skin and mouth
Organic Hg-(CH3)2 - Fish contaminated with methyl mercury
What are the main problems with using mercury amalgams
Toxic to patient and also very and for the environment
What is the most dangerous way to absorb mercury
mercury vapour as Hg has high vapour pressure
What is the half life of mercury
about 55 days
What levels of mercury are considered subtle and dangerous
Subtle effects = 30 ug per day
Dangerous = 82 ug per day
AY BAWS CAN I HABE DE NOTE PLZ
The dose of mercury that a patient will receive from amalgam fillings will be negligible but still needs to be noted
What can released mercury in the oral cavity do other than enter the blood
React with unreacted gamma phase and leads to expansion
Who is most at risk of dental amalgams
The dental team
What complications in the oral cavity can arise as a result of amalgams
Lichenoid lesions - usually associated with old and corroded amalgams
Amalgam tattoo - accidental implantation of silver containing compounds into oral mucosa
When might amalgam tattoo occur
- During the removal of an old amalgam
- Broken pieces-socket-tooth extraction
- Particles entering surgical would
- Amalgam dust in oral fluids
Where might you see amalgam tattoo in the oral cavity
Gingiva
Buccal Mucosa
Alveolar Mucosa
What element tried to replace mercury in amalgams
Gallium alloys, not that good tho
What were the benefits of gallium alloys
- Setting expansion - 1.6%
- Good compressive strength
- Low creep
- Rapid solidification (early polish)
- Biocompatibility and cytotoxicity similar to conventional amalgam and composites
What were the disadvantages of using gallium alloys
- Poor corrosion resistance
- Prone to delayed expansion, moisture contamination
- Expensive
- Very wet mix and difficult to handle
- Not used much but still available
What is consolidated silver made of
Spherical alloy particles
What is consolidated silver acid treated with
Fluoroboric acid
What is a problem with consolidated silver as an alternative to amalgams
Needs high pressures required to remove voids
As you condense it the particles will strain harden and made condensation difficult
What are the benefits and disadvantages of fluoridated amalgams
Released fluoride for several weeks but final product was more susceptible to corrosion
How much copper can you find in low copper amalgams
0-6%
How much copper can you find in high copper amalgams
12-30%
