elastomeric impression materials Flashcards
2 elastomer types
polyether
addition silicones
elastic behaviour
crucial in assessing all impression materials
Briefly, when removing an IM from contact with a tooth – after its set of course – the IM has to stretch, flaring out at the sides to overcome the bulbous aspect of the tooth. Ideally it recovers its original dimensions, replicating the shape of the tooth - assuming it’s perfectly ELASTIC
how to decide IM
Material characteristics
Clinical performance
- Patient acceptance
- Ease of use
elastomers formed
formed by polymerisation with cross-linking of polymer chains
cross linking makes
ELASTIC properties
causes FLUID -> SOLID transition
POLYMERISATION
MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility
polymerisation issue
MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility
elastomers through the ages (3 types)
Polysulphides (1950s)
Silicones :
- addition curing (1975) (polyvinylsiloxane)
- condensation curing (1950s) (ie conventional)
Polyethers (1965)
how to assess materials
surface detail (reproduction)
flow /viscosity
contact angle / wettability
elastic recovery (%)
stiffness (flexibility)
tear strength
mixing time (min)
working time (min)
and
Shore A hardness
shark fin test (flow under pressure)
setting shrinkage
dimensional stability
thermal expansion coefficient
- temp gradient between mouth and room
biocompatability
shore A hardness
Specific hardness test for IM
dimensional stability importance for IM
Essential for producing an accurate positive replica using GYPSUM
precision relates to
accuracy of the impression taken
not the ease of the identifying where the margins are
2 forms of virtual IM
Polyvinylsiloxane
Twin cartridge – with base and catalyst pastes - that require a syringe gun to push them through a mixing tip and deliver a homogeneous paste to the impression tray.
form of putty – where a spoonful of the catalyst and base pastes are extracted from the respective tubs and mixed, until the colour is uniform.
key features to look for IM property table (4)
Linear dimensional change
Recovery from deformation
Detail reproduction
Shore-A hardness
4 ideals for impression materials
quality of surface interaction between the material and tooth/soft tissue surfaces
accuracy
dealing with removal an d undercuts
dimensional stability
quality of surface interaction between the material and tooth/soft tissue surfaces
IM features assessed (3)
viscosity
surface wetting
contact angle
accuracy
IM features assessed (2)
Surface reproduction (ISO)
Visco-elasticity/elastic recovery
dealing with removal and undercuts
IM features assessed (3)
Flow under pressure (“shark fin” test)
Tear/tensile strength
Rigidity
dimensional stability
IM features assessed (3)
Setting shrinkage
Thermal expansion/contraction
Storage
visocsity
a measure of material’s ability to flow
determines a material’s potential for making close contact with hard/soft tissue surfaces
- so how well it records surface detail
range: low, medium, high
wettability
Contact Angle
is the material hydrophilic?
- ie how closely the IM envelops the tooth surface.
The GLOBULE of IM has a low contact angle, meaning a large percentage of its volume will make contact with the target surface.
- That’s ideal.
ideal contact angle
GLOBULE of IM has a low contact angle, meaning a large percentage of its volume will make contact with the target surface.
large contact angle =
results in
spaces between globules of impression material,
so some of tooth surface not replicated
small contact angle =
NO spaces between globules of impression material,
so all of surface is replicated
hydrophilic silicones
incorporate non-ionic surfactant
- wets tooth surface
- more easily wetted by water containing die materials
Initial addition silicones had some difficulties in making good contact with moist tooth surfaces.
reproduction of surface details (ISO 4823:2000)
Standard notch dimensions:-
- A=20um
- B=50um
- C = 75um
test involves placing IM along a surface which has grooves of specified width. To conduct the test, a uniform pressure is applied across the width of the IM.
75 and 50um wide grooves have been filled by the IM.
But the 20um groove is unfilled.
- We conclude that this particular IM can’t reach into such narrow niches.
Obviously an IM that records 20um grooves will give you the most accurate surface detail.
elasticity ideal behaviour
when a load is applied at T=0 – as when removing an impression tray
- the material stretches instantly to the STRAIN required. (dashed vertical line)
This level of strain is maintained (horizontal dashed line)
- Until the load is removed, and the IM returns instantly to its original dimensions – shown by the vertical dashed line at time Tf
100% elastic recovery (or “full recovery from deformation”)
- NO permanent strain
impression material - viscoelasticity recovery
a load is applied – will GRADUALLY reach the strain required. And when the load is released, its strain level GRADUALLY drops. PERMANENT DEFOMATION/STRAIN
viscoelastic behaviour of IM impact on technique
Influences tray removal method
- If LOAD time is less - impression removed with a sharp pull, there is less overall permanent strain (lower deformation)
thus Elastic recovery can be enhanced
Only when it’s setting reaction has progressed to a certain extent – does the material’s ELASTICITY begin to develop fully.
- Even when the IM appears firm to touch, it will still be developing its ELASTICITY.
it pays to wait for an extra few minutes BEFORE REMOVING IT
viscoelastic behaviour is when
Occurs when, after being stretched (or compressed) a material fails to return to its original dimensions/shape
- i.e. there is PERMANENT DEFORMATION
Only when it’s setting reaction has progressed to a certain extent – does the material’s ELASTICITY begin to develop fully.
- Even when the IM appears firm to touch, it will still be developing its ELASTICITY.
it pays to wait for an extra few minutes BEFORE REMOVING IT
flow under pressure (shark fin test)
To record an undercut, the IM must first reach the extremities of what is a narrow zone, with a complex shape between the gingiva and the tooth surface.
- objectively assessed by what’s called the SHARK FIN TEST.
A laboratory set-up is shown here. There’s a cylindrical chamber, with a slot of a specified depth. IM inserted in the upper part of the cylindrical chamber – and which has to have a depth greater than that of the slot - is then forced downwards (i.e. pressure is applied).
lab set up for shark fin test
There’s a cylindrical chamber, with a slot of a specified depth. IM inserted in the upper part of the cylindrical chamber – and which has to have a depth greater than that of the slot - is then forced downwards (i.e. pressure is applied).
high flow =
large fin length
will flow readily into sulcus, undercuts
- greater FIN LENGTH than the other material.
Therefore the RED IM is able to flow more under pressure, and should record DEEPER UNDERCUTS more read
low flow =
short fin length
red IM on the LHS wins. The blue IM is unsuited to recording undercuts.
desired fin length
LARGE not short
will flow readily into sulcus, undercuts
- greater FIN LENGTH than the other material.
Therefore the RED IM is able to flow more under pressure, and should record DEEPER UNDERCUTS more read.
The blue IM is unsuited to recording undercuts (as short fin length, low flow)
tear strength
stress material will withstand before fracturing
large TEAR STRENGTH is clearly the ideal – as this means the IM withstands even large stresses during removal.
flexibility of IM
ideally would be
low level of RIGIDITY. In other words, applying just a small stress causes the IM to undergo a lot of strain – or change in shape – allowing it to be removed easily
tear strength ideal for IM
large
IM withstands even large stresses during removal.
rigidity
= stress/strain ratio
(ie stress needed to cause material to change shape)
ideally low value (ie flexible) for ease of removal of material, especially from undercut regions
dimensional stability ideal qualities for IM
Setting shrinkage – should be low
Thermal expansion/contraction (ppm/ degrees C)
- a large difference between mouth and room temperature, may cause a change in shape (37 to 22)
- ppm/ degree C should be low
Storage – some materials absorb/release moisture causing a change in its dimensions
delivery system for some IMs
cartridge delivery system
addition silicone and a polyether in a twin CARTRIDGE form – a BASE PASTE and CATALYST PASTE
working and setting time IM comparison
POLYETHERS tend to set a little quicker. And have HALF the working time of the ADDITION SILICONES. Your clinical technique for manipulating the material may make you prefer a short or long working time
elasticity comparison for IMs
VIRTUAL is the most elastic IM with a 99.5% recovery. Flexitime is a little behind. BUT at 98% recovery, Aquasil and Impregum are much poorer performers
undercuts comparison for IMs
IMPREGUM is best at recording deep undercuts – with a shark fin length of 23mm.
Better than AQUASIL
others – there wasn’t any data available.
tear strength comparison for IMs
Resisting tearing when being removed is best achieved with VIRTUAL – with a TEAR STRENGTH of 9MPa; that is more than double the value for AQUASIL; and 5 times more than that achieved with IMPREGUM.
5 points in material decision making approach
- Know KEY material properties
- be prepared for new terminology, & sales pitch. - Review product specification data
– cf best rival products? - Know typical values expected for specific properties
- Identify properties NOT mentioned
- REJECT claims NOT supported with scientific/clinical data
applying DMS knowledge to decision making
- Know KEY material properties
- be prepared for new terminology, & sales pitch.
assessment
- Evidence shows new material is better
thus - purchase, unless cost concerns
applying DMS knowledge to decision making
- Review product specification data
– cf best rival products?
assessment
- New materials is only as good as current one
no specific advantage - REJECT
applying DMS knowledge to decision making
- Know typical values expected for specific properties
assessment
- insufficient evidence
Review later
applying DMS knowledge to decision making
- Identify properties NOT mentioned
assessment
- unconvincing date
REJECT
viscosity Ideal IM
flow across surface
low
contact angle
ideal for IM
(wettability)
engagement with tooth surface/surface quality
Low
wettability HIGH
viscoelasticity ideal for IM
deformation on removal
LOW
stiffness ideal for IM
ease of removal from undercut
LOW
thermal expansion ideal for IM
contraction - mouth to room temp
LOW
polymerisation shrinkage idea for IM
contraction during setting
LOW
tear resistance ideal for IM
ability to withstand large stresses (e.g. during removal)
HIGH but not too high
