elastomeric impression materials

Question 1

what are the 2 types of elastomers

Answer 1

• polyether

- addition silicones

Question 2

why is elastic behaviour important in impression materials

Answer 2

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

Question 3

what do you need to consider when choosing na impression material

Answer 3

• 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

Question 4

how do you assess materials

Answer 4

• 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

Question 5

explain the chemistry in elastomers

Answer 5

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

Question 6

what are the types of elastomers

Answer 6

• 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)

Question 7

what material properties are useful for evaluation elastomers

Answer 7

• 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

Question 8

what is polyvinylsiloxane

Answer 8

addition silicone

Question 9

when looking at a brochure for a material what do you need to ask yourself

Answer 9

Do you understand it
Does it make sense
What else do you want to know

Question 10

what is hydrophilicity

Answer 10

how the material interacts in the presence of water;

Question 11

what 3 properties are relevant when looking at the quality of surface interaction between material and tooth / soft tissue surfaces

Answer 11

• Viscosity
• Surface wetting
• Contact angle

Question 12

what properties are relevant when looking at accuracy

Answer 12

• Surface reproduction (ISO)

- Visco-elasticity / elastic recovery

Question 13

what properties are relevant when dealing with removal and undercuts

Answer 13

• Flow under pressure (“shark fin” test)
• Tear / tensile strength
• Rigidity

Question 14

what properties are relevant when looking at dimensional stability

Answer 14

• Setting shrinkage
• Thermal expansion / contraction
• Storage

Question 15

what does viscosity mean

Answer 15

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

Question 16

what is surface wetting

Answer 16

Must make intimate contact with teeth / mucosa

Question 17

what is contact angle

Answer 17

• 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

Question 18

what does the contact angle indicate

Answer 18

The contact angle indicates how readily the IM “wets” the tooth surface
Ie how closely the IM envelops the tooth surface

Question 19

what does large contact angles result in

Answer 19

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

Question 20

what does a low / small contact angle indicate

Answer 20

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

Question 21

what are hydrophilic silicones

Answer 21

• 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

Question 22

what is the international standard measure for surface reproduction of an IM

Answer 22

• There is an international standard measure for surface reproduction of an IM - ISO 4823
This measures how accurately the surface is represented

Question 23

What does this test (for surface reproduction) involve

Answer 23

• 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

Question 24

how does a truly elastic material behave (ideal behaviour of elasticity)

Answer 24

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 -

Question 25

is any IM truly elastic?

Answer 25

No IM is truly elastic ~ they are all visco-elastic to some extent

Question 26

what would a strain-time diagram look like for a viscoelastic material

Answer 26

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)

Question 27

what is the ideal elastic behaviour

Answer 27

• 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

Question 28

for a viscoelastic material, what does elastic recovery depend on

Answer 28

the elastic recovery depends on clinical technique

Question 29

how does clinical technique affect elastic recovery

Answer 29

By removing the impression tray quickly, the permanent deformation experienced is reduced In other word the material's elastic recovery is enhanced

Question 30

how do you ensure there is less overall strain for IM

Answer 30

if load time is less - impression removed with a sharp pull, there is less overall permanent strain (lower deformation)

Question 31

what is the definition of viscoelastic behaviour

Answer 31

occurs when after being stretched (or compressed) a material fails to return to its original dimensions / shape ie there is permanent deformation

Question 32

what happens to the flow of the IM after placement in the tray and insertion into the patient's mouth

Answer 32

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

Question 33

when does the material's elasticity begin to fully develop

Answer 33

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

Question 34

what is the name of the test for flow under pressure / ability to record undercuts

Answer 34

shark fin test

Question 35

what must the IM do to record an undercut

Answer 35

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}

Question 36

explain the shark fin test

Answer 36

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

Question 37

what does a large fin length mean

Answer 37

high flow will flow readily into sulcus, undercuts can flow further in the cylindrical chamber

Question 38

what is tear strength

Answer 38

stress material will withstand before fracturing

Question 39

would you rather the tear strength be large or small

Answer 39

A large tear strength is clearly the ideal | As this means the IM withstands even large stresses during removal

Question 40

what is the ratio for rigidity

Answer 40

= stress / strain ratio

Question 41

what is rigidity

Answer 41

stress needed to cause material to change shape

Question 42

what value do we want for rigidity ideally

Answer 42

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

Question 43

should setting shrinkage be low or high

Answer 43

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

Question 44

what might occur if there is a large difference between mouth and room temperature

Answer 44

A large difference between mouth and room temperature, may cause a change in shape

Question 45

should the thermal expansion / contraction (ppm/˚C) be high or low

Answer 45

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

Question 46

what problems can arise from storage

Answer 46

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

Question 47

compare setting and working times for polyethers and addition silicones

Answer 47

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

Question 48

what should be included in a decision making approach to choosing materials

Answer 48

1. Know key material properties - Be prepared for new terminology and sales pitch 2. Review product specification data - Of best rival products 3. Know typical values expected for specific properties 4. Identify properties not mentioned 5. Reject claims not supported with scientific / clinical data

Question 49

what decision should be made if evidence shows a new material is better

Answer 49

purchase new material

Question 50

what decision should be made if new material is only as good as current material being used

Answer 50

No specific advantage; reject Why risk changing it ? Unless there are cost concerns

Question 51

what decision should be made if there is insufficient evidence for a new material

Answer 51

Review later | Can't make decision now

Question 52

what decision should be made for a new material with unconvincing data

Answer 52

reject

Question 53

what does viscosity influnece and what is it's ideal

Answer 53

ability to flow across surface ideal = low

Question 54

what does contact angle (wettability) influence and what should the ideal be

Answer 54

Engagement with tooth surface / surface quality ideal = low (high)

Question 55

what does viscoelasticity / elasticty recovery influence and its ideal

Answer 55

deformation on removal ideal = low

Question 56

what does stiffness / flexibility influence and its ideal

Answer 56

Ease of removal from undercut ideal = low

Question 57

what does thermal expansion / contraction influence and its idedal

Answer 57

Contraction - mouth room temperature ideal = low

Question 58

what does polymerisation shrinkage influence and what is its ideal

Answer 58

contraction during setting ideal = low

Question 59

what does tear strength influence and what is its ideal

Answer 59

Ability to withstand large stresses (eg during removal) ideal = High (but not too high)

Question 60

what 4 categories should we use to assess IM

Answer 60

I) Quality of surface interaction between material and tooth / soft tissue surfaces II) Accuracy III) Dealing with removal and undercuts IV) Dimensional stability