elastomeric impression materials Flashcards
what are the 2 types of elastomers
- polyether
- addition silicones
why is elastic behaviour important in impression materials
When removing impression material (IM) from contact with a tooth - after its set - the IM has to stretch, flaring out at the slides to overcome the bulbous aspect of the tooth
Ideally it recovers to original dimensions, replicating the shape of the tooth ~ that is assuming it is perfectly elastic
what do you need to consider when choosing na impression material
• Material characteristics
○ Need to be aware of the degree of accuracy needed for your particular purpose
• Clinical performance ○ Patient acceptance § Working time § Setting time § Taste / smell of material ○ Ease of use for clinician
• Need to make a choice that best suits the specific circumstances
how do you assess materials
• Need to know how specific IM perform
• That means evaluating the product information available
○ For some manufacturers (eg 3M ESPE, Heraeus Kulzer), substantial documentation is provided on their websites ~ this can just be claims, better to have actual robust evidence
- Need to know what characteristics are important so you can interpret the data they give in a meaningful way
- Need to relate the data to other materials and the ideal properties
explain the chemistry in elastomers
Elastomers are formed by polymerisation with cross-linking of polymer chains
Cross-linking generates elastic properties
- Causes fluid to solid transition
• Polymerisation may produce by-products (H2O, H2, alcohol) which affect dimensional stability and cast compatibility
what are the types of elastomers
• Polysulphides (1950s)
○ We don’t use these now
• Silicones
○ Addition curing (1975)
§ This is the type we will focus on
○ Condensation curing (1950s) - ie conventional
• Polyethers (1965)
what material properties are useful for evaluation elastomers
• Surface detail (reproduction)
• Flow / viscosity
• Contact angle / wettability
These 3 affect the accuracy by which the surface features are recorded
• Elastic recovery (%)
• Stiffness (flexibility)
• Tear strength
These 3 affect the accuracy of the dimensions and shape of the final impression
• Mixing time (min)
• Working time (min)
These are practical considerations - they affect the clinician’s skill and working method, and will influence the choice, as will patient preferences
• Shore A hardness
○ This is a specific hardness test for IM
• Shark fin test (flow under pressure)
○ Relates to the ability of IM to deal with undercuts
• Setting shrinkage
○ Fundamental property
• Dimensional stability
○ Critical for producing an accurate positive replica using gypsum
• Thermal expansion coefficient
○ Plays a pivot role, as we can’t eliminate the temperature gradient between mouth and room temperature
• Biocompatibility
○ An essential requirement
○ It relates to patient’s safety
what is polyvinylsiloxane
addition silicone
when looking at a brochure for a material what do you need to ask yourself
Do you understand it
Does it make sense
What else do you want to know
what is hydrophilicity
how the material interacts in the presence of water;
what 3 properties are relevant when looking at the quality of surface interaction between material and tooth / soft tissue surfaces
- Viscosity
- Surface wetting
- Contact angle
what properties are relevant when looking at accuracy
- Surface reproduction (ISO)
- Visco-elasticity / elastic recovery
what properties are relevant when dealing with removal and undercuts
- Flow under pressure (“shark fin” test)
- Tear / tensile strength
- Rigidity
what properties are relevant when looking at dimensional stability
- Setting shrinkage
- Thermal expansion / contraction
- Storage
what does viscosity mean
A measure of material’s ability to flow
Vital for it to reach all the dental tissue’s surface area
• Determines a material’s potential for making close contact with hard / soft tissue surface –> so how well it records surface detail
- Has to flow readily without too much pressure being required
- Clearly viscosity influences the surface texture that is replicated
- Range:
- Low
- Medium
- High
what is surface wetting
Must make intimate contact with teeth / mucosa
what is contact angle
- Determines how well material envelops the hard / soft tissue surface (to record fine detail)
- The contact angle is an objective measure of how well the IM envelops the hard and soft tissue surfaces
what does the contact angle indicate
The contact angle indicates how readily the IM “wets” the tooth surface
Ie how closely the IM envelops the tooth surface
what does large contact angles result in
Large contact angle at surface, results in spaces between globules of impression material, so some of tooth surface not replicated
an IM has a high contact angle with a tooth surface is more likely to have gaps between each globule of material
so the IM won’t replicate the whole tooth surface
= not ideal
what does a low / small contact angle indicate
means a large percentage of its volume will make contact with the target surface - that is ideal
Small contact angle, no spaces between globules of impression material, so all of surface is replicated
In contrast, this IM has a low contact angle with the tooth surface, ensuring there are no gaps between each globule of the material
So the whole tooth surface is recorded = ideal
what are 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
○ To overcome this, manufacturers introduced a non-ionic surfactant component
○ These are referred to as hydrophilic silicones
what is the international standard measure for surface reproduction of an IM
• There is an international standard measure for surface reproduction of an IM - ISO 4823
This measures how accurately the surface is represented
What does this test (for surface reproduction) involve
• This test involves placing IM along a surface which has grooves of specified width: 20, 50, 75um
- Pretty narrow
To conduct the test, a uniform pressure is applied across the width of the IM
if the IM cannot fill all the grooves / notches then it cannot reach narrow niches so the IM will not give the most accurate surface detail
how does a truly elastic material behave (ideal behaviour of elasticity)
when a load is applied at T=0 (as when removing an impression tray) the material stretches instantly to the strain required
This level of strain is maintained
Until the load is removed, and the IM returns instantly to its original dimensions -
is any IM truly elastic?
No IM is truly elastic ~ they are all visco-elastic to some extent
what would a strain-time diagram look like for a viscoelastic material
a viscoelastic material when a load is applied will gradually reach the strain required
And when the load is released, its strain level gradually drops
Note the material does NOT return to its original dimensions
Instead it experiences permanent deformation (aka permanent strain)
what is the ideal elastic behaviour
• 100% elastic recovery
Or “full recovery from deformation”
- No permanent strain
- So the ideal IM undergoes 100% elastic recovery following removal of the impression tray - which is the same as no permanent strain
for a viscoelastic material, what does elastic recovery depend on
the elastic recovery depends on clinical technique
how does clinical technique affect elastic recovery
By removing the impression tray quickly, the permanent deformation experienced is reduced
In other word the material’s elastic recovery is enhanced
how do you ensure there is less overall strain for IM
if load time is less - impression removed with a sharp pull, there is less overall permanent strain (lower deformation)
what is the definition of viscoelastic behaviour
occurs when after being stretched (or compressed) a material fails to return to its original dimensions / shape
ie there is permanent deformation
what happens to the flow of the IM after placement in the tray and insertion into the patient’s mouth
after placement in the tray and insertion into the patient’s mouth the IM’s ability to flow diminishes
And this continues as its setting reaction progresses
when does the material’s elasticity begin to fully develop
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
So it pays to wait for an extra few minutes before removing it
what is the name of the test for flow under pressure / ability to record undercuts
shark fin test
what must the IM do to record an undercut
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 tooth surface}
explain the shark fin test
There is a cylindrical chamber, with a slot of specified depth
The 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 (ie pressure is applied)
an IM that flows further has a greater fin length than a material that does not flow as far
therefore the material which flows further is able to flow under more pressure and should record deeper undercuts more readily
look at diagrams cos this is kinda hard to explain w words maybe
what does a large fin length mean
high flow
will flow readily into sulcus, undercuts
can flow further in the cylindrical chamber
what is tear strength
stress material will withstand before fracturing
would you rather the tear strength be large or small
A large tear strength is clearly the ideal
As this means the IM withstands even large stresses during removal
what is the ratio for rigidity
= stress / strain ratio
what is rigidity
stress needed to cause material to change shape
what value do we want for rigidity ideally
Ideally low value (ie flexible) for each of removal of material, especially from undercut regions
Want the IM to be flexible so want low levels 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
should setting shrinkage be low or high
Should be low
If there is a little setting shrinkage then the IM will maintain the shape of the tooth or dentition you are trying to replicate
what might occur if there is a large difference between mouth and room temperature
A large difference between mouth and room temperature, may cause a change in shape
should the thermal expansion / contraction (ppm/˚C) be high or low
Ppm/˚C should be low
A low thermal expansion coefficient minimises the change in shape as the IM drops from 37˚C in the mouth to say 22˚C at the surgery worktop
what problems can arise from storage
some materials absorb / release moisture causing a change in its dimensions
For some materials storage can cause a change in dimensions - some materials absorb / release moisture (eg alginates)
This doesn’t apply to polyethers or addition silicones
compare setting and working times for polyethers and addition silicones
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 ~ your decision
what should be included in a decision making approach to choosing materials
- Know key material properties
- Be prepared for new terminology and sales pitch - Review product specification data
- Of best rival products - Know typical values expected for specific properties
- Identify properties not mentioned
- Reject claims not supported with scientific / clinical data
what decision should be made if evidence shows a new material is better
purchase new material
what decision should be made if new material is only as good as current material being used
No specific advantage; reject
Why risk changing it ?
Unless there are cost concerns
what decision should be made if there is insufficient evidence for a new material
Review later
Can’t make decision now
what decision should be made for a new material with unconvincing data
reject
what does viscosity influnece and what is it’s ideal
ability to flow across surface
ideal = low
what does contact angle (wettability) influence and what should the ideal be
Engagement with tooth surface / surface quality
ideal = low (high)
what does viscoelasticity / elasticty recovery influence and its ideal
deformation on removal
ideal = low
what does stiffness / flexibility influence and its ideal
Ease of removal from undercut
ideal = low
what does thermal expansion / contraction influence and its idedal
Contraction - mouth room temperature
ideal = low
what does polymerisation shrinkage influence and what is its ideal
contraction during setting
ideal = low
what does tear strength influence and what is its ideal
Ability to withstand large stresses (eg during removal)
ideal = High (but not too high)
what 4 categories should we use to assess IM
I) Quality of surface interaction between material and tooth / soft tissue surfaces
II) Accuracy
III) Dealing with removal and undercuts
IV) Dimensional stability
