impression materials and amalgam Flashcards
broad ideal properties
pt comfort operator convenience accuracy removal and undercuts storage compatible with cast material
pt comfort
non-toxic, non-irritant acceptable taste and smell - gagging could distort setting time short removal without damage to tissues
operator convenience
easy to mix and use, quick
setting and mixing time
storage
decontaminable
accuracy
viscosity and contact angle/wettability - surface interaction
setting dimensional changes
elastic recovery
surface detail/reproduction
removal and undercuts
tear strength
rigidity
no effect on oral tissues
storage
dimensional stability
setting shrinkage
thermal expansion/contraction
alginate composition
salt of alginic acid e.g. Na/K alginate 12%
calcium sulphate 12%
trisodium phosphate 2%
filler 70%
modifiers, flavourings, chemical indicators small %
salt of alginic acid
e.g Na/K alginate
12%
reacts with Ca ions
water soluble salt
calcium sulphate
12%
provides Ca ions
reacts to form insoluble gel
trisodium phosphate
2%
delays gel formation - retarder
filler
70%
cohesion, strength
modifiers, flavourings, chemical indicators
small %
improve surface, taste, pH colour change
alginate setting reaction
Na alginate to Ca alginate
one Ca knocks out 2 Na
can’t be broken, irreversible
creation of cross-links between polymer chains
alginate pH change setting reaction
11 to 7
alginate delay
alter amount to alter setting times
trisodium phosphate preferentially reacts with Ca ions in CaSO4 until exhausted
alginate manipulation
correct powder to liquid ratio
water 18-24 degrees
perforated tray and adhesive
increased temp speeds up setting
removing alginate impression
sharp pull
elastic recovery, max tear strength
alginate and crosslinking
cross linking continues after apparent set - greater elastic properties ifs you wait for a few mins after tackiness has gone from surface
using large bulk of alginate
permanent distortion and tearing slightly reduced
alginate properties - pt comfort
non-toxic, non-irritant
acceptable taste, smell
setting time ok
alginate operator convenience
relatively easy to use setting time ok storage - avoid moisture - prepare cast asap to avoid dimensional change - syneresis - release of H2O - imbibition - uptake of H2O
alginate accuracy
not as good as elastomers for surface reproduction
setting changes and flow ok
movement - stresses, distortion. Can create stresses if pull before properly set
not 100% elastic recovery
poor tear strength - avoid deep UCs
poor long-term dimensional stability
- moist gauze over impression to reduce distortion during transit time
where can’t non-elastic impression materials be used?
UCs
impression compound
mucocompressive
thermoplastic - no chemical reaction involved
poor dimensional stability
components of impression compound
resins
waxes
plasticisers
fillers
plasticisers in impression compound
reduce brittleness
fillers in impression compound
overcome tackiness
control degree of flow
minimise shrinkage due to thermal contraction
improve rigidity
colloid
2 phase system of fine particles (1-200nm) of one phase dispersed in another phase
hydrocolloid
if dispersing medium H2O
hydrocolloid 2 states
SOL - viscous liquid state
GEL - jelly-like consistency
- agglomeration of particles
- entangled framework of solid particles enclosing liquid by capillary forces
2 major categories for assessing
material characteristics
clinical performance
- pt acceptance, ease of use
be wary of user feedback
assessing properties
flow/viscosity surface detail/reproduction contact angle/wettability elastic recovery (%) stiffness (flexibility) tear strength mixing and working time (min)
identify properties not mentioned
ISO standards
assess relevant properties of a produce to see whether it meets acceptable requirements for safe and effective use
grooves/indentations of 20/50um are replicated depending on material viscosity
don’t establish best material - designed to exclude unsafe materials from the market
but industries use them as a quality sign
viscosity
ability to flow and make close contact with tissue surfaces
surface wetting
intimate contact with tissue so all of surface is replicated
not form voids due to repulsion or moisture
contact angle
determines how well material envelops the hard/soft tissue surface
low contact angle - spread easily and adapt smoothly
tear strength
stress material will withstand before fracture
reproduction of surface detail
standard notch dimensions
A = 20um
B = 50um (ISO norm)
C = 75um
test whether material fills these notches
rigidity
stress/strain
ideally impression material low rigidity - ease its removal from UC/interdental areas
properties affecting accuracy
viscosity setting mechanism TEC - ideally low to cope with mouth to room temp transition hydrophobic/hydrophillic elasticity tear strength
thixotropy
greater accuracy of impression without excessive flow
become less viscous when subjected to an applied stress
reversible hydrocolloid
agar - no longer used
galactose sulphate, forms a colloid with H2O
better elastic recovery but needs special conditioning unit etc
irreversible hydrocolloid
alginate
alginate process
prep insertion setting removal storage cast prep
polyethers
mod dimensional change on setting
good elastic recovery
allergies
susceptible to moisture absorption
polysulfide rubber
long-term dimensional change - need to pour up quickly good elastic properties good tear strength bad taste messy
condensation-cured silicones
high setting shrinkage
quite rigid
addition-cured silicones
stability quite rigid excellent elastic recovery e.g. Virtual - PVS - tear strength - elastic recovery
viscoelastic behaviour
elastic recovery takes time
- wait before you pour cast - minimise permanent deformation
if load time is less there is less overall permanent strain so lower deformation
- remove imp tray quickly with a sharp pull
fct of impression
produce accurate (dimensional and surface) replica of surface and shape of hard and soft oral tissues
impression
a negative reproduction of tissues
tray retention
adhesive or perforated
types of impression
single tooth
whole dentition
edentulous mouth
mucostatic impression
fluid materials that prevent displacement of the STs
give impression of undisplaced mucosa
e.g. ZOE, low viscosity alginates
mucocompressive impression
stiff, viscous materials that record an imp of the mucosa under load
displaced soft tissue
may produce distorted imp in edentulous pts with a flabby ridge
e.g. compound, high viscosity alginates/elastomers
classification of impressions
clinical - mucostatic or mucocompressive
properties - elastic or non-elastic
what are elastic impressions for?
dentate
what are non-elastic impressions for?
edentulous and inter-occlusal records
if purely elastic what will happen to the shape?
will change shape instantaneously
how is the amalgam alloy formed?
by trituration
amalgam constituents
Ag 70% Sn 25% Cu 3% Zn 1% Hg 1%
amalgam powder
50% Sn, Ag Cu Zn (Hg - pre-amalgamated)
amalgam liquid
50%
Hg - triple distilled
Sn, Ag
inter metallic compound Ag3Sn. y phase
Cu
strength and hardness
Zn
scavenger (slag)
classification
composition
- traditional
- copper enriched
particle shape and size
particle types
lathe cut
spherical/spheroidal
lathe cut
fill ingots
use heat tx to relieve internal stresses
spherical/spheroidal
spray molten metal into inert atmosphere
amalgam setting reaction
Ag3Sn + Hg - Ag3Sn + Ag2Hg3 + Sn7Hg9
y phase
Ag3Sn - strongest
y1
Ag2Hg3
y2
Sn7Hg9
amalgam matrix
y1 and y2
unreacted particles
y
insufficient Hg for complete reaction
y2 properties
weak and poor corrosion resistance
higher Hg content
weaker amalgam as it will produce more y2
traditional (hand-mixed) setting dimensional changes
initial contraction - solution of alloy particles in Hg
expansion - y1 crystallisation
modern (mechanically mixed)
small contraction
solid solution of Hg in Ag3Sn
Zinc and expansion
saliva/blood: Zn +H2O - 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 Hg content after condensation too low condensation pressure slow rate of packing - increments don't bond corrosion creep
properties
early strength low then increases high abrasion resistance creep - marginal breakdown biocompatible? thermal properties - TEC x3 of tooth - conductivity high doesn't bond to tooth handling ok poor aesthetics radiopaque not anticariogenic 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
types of amalgam
Cu single - lowest creep - best compressive strength PERMITE - non-y2 encapsulated - Hg hygiene
indications
moderate and large cavities
posterior teeth
core build ups when definitive Rxs will be indirect cast Rx
corrosion
y2 most electronegative - acts as anode of oxidation cell. will gradually dissolve
- weakens esp at margins
corrosion products may contribute to sealing margins
reducing corrosion
Cu enriched, polishing margins
avoiding galvanic cells
Cu enriched alloys
non-y2
>6% Cu
dispersion modified or single composition
dispersion modified Cu enriched alloys
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 alloys
higher early strength less creep higher corrosion resistance increased durability of margins gets rid of y2 no Zn - no delayed expansion
disadvantage of copper enriched alloys
surfaces more prone to tarnish
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 lesions
T4 hypersensitivity
replace with gold/composite
galvanic response
battery effect from 2 different 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 (homogeneous) eliminates voids
inadequate condensation
lack of adaptation to cavity
poor bonding between layers
inadequate Hg expression therefore removal
inferior mechanical properties
IP caries
self-retentive box prep - remove less tooth - technically demanding proximo-occlusal prep - very retentive - destruction of tooth
moisture contamination
reduced strength
increased creep, corrosion, porosity
microleakage
passage of fluid and bacteria in micro gaps (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/fracture by masticatory forces
- sufficient depth
- gingival floor approx 90 degrees to axial wall
sealing and bonding
amalcure
no evidence of increased survival
mechanical retention
grooves/dimples within cavity design
pins - Ti/SS
placing pins
Ti/SS increase retention into dentine in greatest bulk never in enamel 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 user pins with composite?
won’t fall out if bond fails
Minimata convention
global treaty - protect human health and env from adverse effects
phase DOWN
1 - encapsulated
2 - separators - 95% or more particles
3 - authorised waste collection
SDCEP
no justification on health grounds for:
- not placing amalgam
- removal of sound Rx
don’t use in U15s, pregnant or 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 with skin GIT gingiva and mucosa D+P as metal ions?
GV black cavity classification - 1
pit and fissure
GV black cavity classification - 2
approximal (posterior teeth)
GV black cavity classification - 3
approximal (anterior teeth)
GV black cavity classification - 4
approximal involving incisal angle
GV black cavity classification - 5
cervical surfaces
GV black cavity classification - 6
cusp tips
