amalgam Flashcards
how is it formed?
by trituration
constituents
Ag 70% Sn 25% Cu 3% Zn 1% Hg 1%
amalgam powder
50% Sn, Ag - intermetallic compound Ag3Sn - y phase Cu - strength and hardness Zn - scavenger (slag) (Hg - pre-amalgamated)
amalgam liquid
50%
Hg - triple distilled
classification
composition
- traditional
- copper enriched
particle shape and size
equation for setting reaction chemical symbols
Ag3Sn + Hg - Ag3Sn + Ag2Hg3 +Sn7Hg9
y
Ag3Sn
y1
Ag2Hg3
y2
Sn7Hg9
equation for setting reaction y
y + Hg - y + y1 + y2
why are there particles of y in the end product?
unreacted particles because insufficient Hg for complete reaction
amalgam matrix
y1 and y2
which is the strongest phase?
y
particle types
spherical/spheroidal
lathe cut
lathe cut
fill ingots
use heat tx to relieve internal stresses
spherical/spheroidal
spray molten metal into inert atmosphere
y2 properties
weak and poor corrosion resistance
effect of higher Hg content
weaker amalgam as will produce more y2
traditional (hand mixed) setting dimensional changes
initial contraction - solution of alloy particles in Hg
expansion - y1 crystallisation
modern (mechanically mixed) setting dimensional changes
small contraction
solid solution of Hg in Ag3Sn
zinc and expansion
saliva/blood: Zn + H20 - ZnO + H2
bubbles - pressure build up - expansion
downward pulpal pain, upward sits proud. Why Zn free
properties depend on:
handling factors - proportioning and trituration - condensation (remove excess Hg) - carving and polishing cavity design product corrosion
factors reducing strength
undermixing too high mercury content after condensation too low condensation pressure slow rate of packing - increments don't bond corrosion creep
amalgam disadvantages
creep - marginal breakdown
thermal properties
doesn’t bond to tooth
aesthetics
amalgam advantages
high abrasion resistance handling ok radiopaque smooth surface if polished short placement time
advantages of spherical particles
less Hg required higher tensile strength higher early compressive strength less sensitive to condensation easier to carve lower setting shrinkage
amalgam corrosion
y2 most electronegative - acts as anode of oxidation cell, will gradually dissolve
- weakens esp at margins
corrosion products may contribute to sealing margins
how to reduce corrosion
Cu enriched, polishing margins
avoiding galvanic cells
Cu enriched alloys %
> 6% Cu
non-y2
types of Cu enriched alloy
dispersion modified
single composition
dispersion modified Cu enriched alloy
Ag-Cu spheres and lathe cut alloy
y + Hg - y + y1 + y2
y2 + Ag-Cu - Cu6Sn5 + y1 (takes several days)
Cu6Sn5 halo around spheres
small insignificant amount of y2 remains
single composition Cu enriched alloy
powder Ag-Sn-Cu
Ag-Sn-Cu + Hg - Ag-Sn-Cu + y1 + Cu6Sn5
benefits of copper enriched
higher early strength less creep higher corrosion resistance increased durability of margins gets rid of y2 no zinc - no delayed expansion
disadvantages of copper enriched
surfaces more prone to tarnish
encapsulated
Hg hygiene
indications
moderate and large cavities
posterior teeth
core build ups when definitive Rxs will be indirect cast Rx
contraindications
aesthetics important
pt history of sensitivity
can’t produce retentive cavity
would need to remove excessive healthy tooth to produce retentive cavity
local reactions
lichenoid lesions
galvanic response
tooth discolouration
amalgam tattoo
lichenoid reactions
T4 hypersensitivity
replace with gold/comp
galvanic response
battery effect from 2 diff amalgams or amalgam and cast metal
tingling
tooth discolouration
corrosion products migrate into tooth surfaces - porous
amalgam tattoo
fine amalgam particles migrate into ST
confuse with melanoma
matrices
recreate cavity wall
allows adequate condensation
confines amalgam to cavity
<0.05mm thick
condensation
vertical and lateral pressure expels excess mercury adapts material to cavity walls reduces layering (homogenous) eliminates voids
inadequate condensation
lack of adaptation to cavity
poor bonding between layers
inadequate Hg expression therefore removal
inferior mechanical properties
moisture contamination
reduces strength
increases creep, corrosion, porosity
microleakage
passage of fluid and bacteria in microgaps (10 microns) between Rx and tooth
- pulpal irritation and infection
- discolouration
- secondary caries
caused by mechanical loading and thermal stresses
wedges
adaptation of matrix at cervical margin temp tooth separation aids proximal contour prevents excess amalgam gingivally prevents movement of matrix band
why should you always overfill?
increased Hg content on surface - remove by carving
retention form
features that prevent loss of Rx in any direction
resistance form
features that prevent loss of material due to distortion or fracture by masticatory forces
- sufficient depth
- gingival floor approx 90 degrees to axial wall
sealing and bonding: dual cure
no evidence of increased survival
mechanical retention
grooves/dimples within cavity design
pins - Ti/SS
pins
increase retention
into D in greatest bulk
never in E or ADJ - will crack
pack amalgam around pin
problems with pins
initial - stress, D cracking, sensitivity, risk of pin in pulp/periodontium
long-term - secondary caries: if Rx leaks it won’t fall out, caries track down pin
why should you never use pins with composites?
won’t fall out if bond fails
minimata convention?
Global treaty to protect human health and env from adverse effects
phase down
amalgam legistlative rules
encapsulated
separators - at least 95% particles
authorised waste collection
SDCEP guidance
no justification on health grounds for:
- not placing amalgam
- removal of sound Rx
don’t use in U15s, pregnant and breastfeeding unless specific medical reason e.g. lack of cooperation and inadequate moisture control
carving
recreate anatomical contour
finishing
only if need to adjust
polishing
considered unnecessary
heat
Hg risk?
removal
dam and high vol aspiration
min cutting
greatest Hg release during insertion and removal
Hg absorption
vapour into lungs
contact w skin
GIT
gingiva and mucosa
GV Black cavity classification
1 - pit and fissure 2 - approximal (posterior teeth) 3 - approximal (anterior teeth) 4 - approximal involving incisal angle 5 - cervical surfaces 6 - cusp tips
