Amalgam Flashcards
what type of material is amalgam?
an alloy
how can amalgam be classified?
by its:
- composition
- particle shape & size
what are the different compositions of amalgam?
- traditional
- copper enriched
what materials is amalgam composed of?
- Ag (silver)
- Sn (tin)
- Cu (copper)
- Zn (zinc)
- Hg (mercury)
what makes up the powder aspect of amalgam?
- silver-tin (Ag3Sn)
- copper
- zinc
- mercury in powder
what is the liquid aspect of amalgam composed of?
Hg (mercury)
what is the function of the silver-tin compound found in amalgam?
it reacts with the Hg liquid to form amalgam
what is the function of copper in the makeup of amalgam?
increases strength and hardness of material
how can the mercury found in amalgam be described?
very pure (triple distilled)
what are the different particle types that amalgam can be composed of?
- Iathe cut
- Spherical, spheroidal
what is the setting reaction of amalgam?
Ag3Sn + Hg —> Ag3Sn + Ag2Hg3 + Sn7Hg9
what is the gamma phase of the amalgam setting reaction?
Ag3Sn
what is the gamma 2 phase of the amalgam setting reaction?
Ag2Hg3
what is the gamma-2 phase of the amalgam setting reaction?
Sn7Hg9 (tin-mercury)
what are the characteristics of the gamma phase of the amalgam setting reaction?
- good strength
- good corrosion resistance
what are the characteristics of the gamma-1 phase of the amalgam setting reaction?
- good corrosion resistance
what are the characteristics of the gamma-2 phase of amalgam?
- weak and poor corrosion resistance
what is the average tensile strength of amalgam?
60MPa
what amalgam phase has the highest tensile strength?
gamma phase
what amalgam phase has the lowest tensile strength?
gamma-2 phase
why is zinc not commonly used in amalgams anymore?
- zinc can interact with saliva/blood
- bubbles of H2 formed within amalgam
- pressure buildup causes expansion
(downward pressure causes pulpal pain, upwards pressure causes restoration to sit proud of surface)
what handling factors may affect the properties of amalgam?
- proportioning and titration
- condensation/how well it is packed into cavity
- carving and polishing
what are the amalgam properties dependant on?
- handling factors
- cavity design
- affected by corrosion
what type of amalgam particles are better? why is this?
spherical particles
- less Hg required
- higher tensile strength
- higher early compressive strength
- less sensitive to condensation
- easier to carve
what type of cavities are amalgam restorations particularly suited to? why is this?
- cavities in posterior teeth
- high abrasion resistance
what factors may decrease the strength of an amalgam restoration?
- undermining
- low condensation pressure (when packing into cavity)
- slow rate of packing
- corrosion
why is creep a problem in amalgam restorations?
affects marginal integrity & can contribute to microleakage
what is creep?
when a material experiences:
- low level stress levels (below elastic limit stress) which are applied repeatedly over a prolonged time period
- causes material to flow resulting in permanent deformation
how might ditching occur in amalgam restorations?
- low magnitude forces are applied to amalgam repeatedly
- amalgam flows and changes shape
- amalgam sitting proud of tooth surface
- prone to fracture, resulting in ditched margins
what is the thermal expansion of amalgam? is this good or bad?
x3 of tooth
- not ideal & may lead to margin gaps
what is the thermal conductivity of amalgam? is this good or bad?
- high which is bad
- may need to use a liner in deep cavities
what is the thermal diffusivity of amalgam? what does this mean for the material?
- high
- when a transient thermal stimuli is applied amalgams temperature rises quickly
- can damage the pulp
what are the bonding properties of amalgam?
- does not bond to tooth
- needs mechanical retention from undercuts
what type of cavity prep requires removal of MORE tooth surface, composite or amalgam?
amalgam!
what type of amalgam particles require less force when packing into the cavity?
spherical particles
what amalgam phase is most likely to experience corrosion?
gamma-2 phase
how can corrosion be reduced in amalgam materials?
- enrich with copper
- polish margins
- avoid galvanic cells
what is copper enriched amalgam?
amalgam with:
- non gamma 2 phase
- high copper (copper>6%)
what are the two types of copper enriched amalgam materials?
- dispersion modified (original)
- single composition types
what does an increased copper content provide amalgam?
- higher early strength
- less creep
- higher corrosion resistance
- increased durability of margins
what is the setting reaction of single composition copper enriched amalgam?
AgSnCu + Hg —> AgSnCu + Ag2Hg3 + Cu6Sn5
what type of amalgam (both original and copper enriched) has the highest compressive strength?
Cu single composition
what material has a higher compressive strength, amalgam or composite?
amalgam
how does the elastic modulus of amalgam compare to that of enamel?
- lower elastic modulus
- amalgam is less rigid than enamel
what are the advances of amalgam as a restorative material?
- strong
- hard
- durable
- radiopaque
- use friendly
what are some disadvantages of amalgam as a restorative material?
- corrosion
- leakage (does not bond)
- poor aesthetics
- mercury (perceived toxicity, environmental)
what can be done to an amalgam cavity prep to avoid ditching?
- ensure the CSMA is between 90-120 degrees
- if the CSMA is less than this then there is an insufficient bulk of amalgam which would make it more susceptible to fracture
- if it is more there are undermined enamel margins also making it more susceptible to fracture
