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elastomeric impression materials Flashcards

1
Q

2 elastomer types

A

polyether

addition silicones

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2
Q

elastic behaviour

A

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

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3
Q

how to decide IM

A

Material characteristics

Clinical performance

  • Patient acceptance
  • Ease of use
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4
Q

elastomers formed

A

formed by polymerisation with cross-linking of polymer chains

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5
Q

cross linking makes

A

ELASTIC properties
causes FLUID -> SOLID transition

POLYMERISATION
MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility

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6
Q

polymerisation issue

A

MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility

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7
Q

elastomers through the ages (3 types)

A

Polysulphides (1950s)

Silicones :

  • addition curing (1975) (polyvinylsiloxane)
  • condensation curing (1950s) (ie conventional)

Polyethers (1965)

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8
Q

how to assess materials

A

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

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9
Q

shore A hardness

A

Specific hardness test for IM

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10
Q

dimensional stability importance for IM

A

Essential for producing an accurate positive replica using GYPSUM

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11
Q

precision relates to

A

accuracy of the impression taken

not the ease of the identifying where the margins are

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12
Q

2 forms of virtual IM

Polyvinylsiloxane

A

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.

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13
Q

key features to look for IM property table (4)

A

Linear dimensional change

Recovery from deformation

Detail reproduction

Shore-A hardness

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14
Q

4 ideals for impression materials

A

quality of surface interaction between the material and tooth/soft tissue surfaces

accuracy

dealing with removal an d undercuts

dimensional stability

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15
Q

quality of surface interaction between the material and tooth/soft tissue surfaces

IM features assessed (3)

A

viscosity

surface wetting

contact angle

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16
Q

accuracy

IM features assessed (2)

A

Surface reproduction (ISO)

Visco-elasticity/elastic recovery

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17
Q

dealing with removal and undercuts

IM features assessed (3)

A

Flow under pressure (“shark fin” test)

Tear/tensile strength

Rigidity

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18
Q

dimensional stability

IM features assessed (3)

A

Setting shrinkage

Thermal expansion/contraction

Storage

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19
Q

visocsity

A

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

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20
Q

wettability

A

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.

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21
Q

ideal contact angle

A

GLOBULE of IM has a low contact angle, meaning a large percentage of its volume will make contact with the target surface.

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22
Q

large contact angle =

A

results in
spaces between globules of impression material,

so some of tooth surface not replicated

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23
Q

small contact angle =

A

NO spaces between globules of impression material,

so all of surface is replicated

24
Q

hydrophilic silicones

A

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.

25
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.
26
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
27
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
28
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
29
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
30
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).
31
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).
32
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
33
low flow =
short fin length red IM on the LHS wins. The blue IM is unsuited to recording undercuts.
34
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)
35
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.
36
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
37
tear strength ideal for IM
large IM withstands even large stresses during removal.
38
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
39
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
40
delivery system for some IMs
cartridge delivery system addition silicone and a polyether in a twin CARTRIDGE form – a BASE PASTE and CATALYST PASTE
41
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
42
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
43
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.
44
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.
45
5 points in material decision making approach
1. Know KEY material properties - be prepared for new terminology, & sales pitch. 2. Review product specification data – cf 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
46
applying DMS knowledge to decision making 1. Know KEY material properties - be prepared for new terminology, & sales pitch.
assessment - Evidence shows new material is better thus - purchase, unless cost concerns
47
applying DMS knowledge to decision making 2. Review product specification data – cf best rival products?
assessment - New materials is only as good as current one no specific advantage - REJECT
48
applying DMS knowledge to decision making 3. Know typical values expected for specific properties
assessment - insufficient evidence Review later
49
applying DMS knowledge to decision making 4. Identify properties NOT mentioned
assessment - unconvincing date REJECT
50
viscosity Ideal IM
flow across surface low
51
contact angle ideal for IM (wettability)
engagement with tooth surface/surface quality | Low wettability HIGH
52
viscoelasticity ideal for IM
deformation on removal LOW
53
stiffness ideal for IM
ease of removal from undercut LOW
54
thermal expansion ideal for IM
contraction - mouth to room temp LOW
55
polymerisation shrinkage idea for IM
contraction during setting LOW
56
tear resistance ideal for IM
ability to withstand large stresses (e.g. during removal) HIGH but not too high

BDS3 DMS (9 decks)

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