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BDS3 DMS > elastomeric impression materials > Flashcards

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
Q

reproduction of surface details (ISO 4823:2000)

A

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
Q

elasticity ideal behaviour

A

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
Q

impression material - viscoelasticity recovery

A

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
Q

viscoelastic behaviour of IM impact on technique

A

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
Q

viscoelastic behaviour is when

A

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
Q

flow under pressure (shark fin test)

A

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
Q

lab set up for shark fin test

A

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
Q

high flow =

A

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
Q

low flow =

A

short fin length

red IM on the LHS wins. The blue IM is unsuited to recording undercuts.

34
Q

desired fin length

A

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
Q

tear strength

A

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
Q

flexibility of IM

A

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
Q

tear strength ideal for IM

A

large

IM withstands even large stresses during removal.

38
Q

rigidity

A

= 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
Q

dimensional stability ideal qualities for IM

A

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
Q

delivery system for some IMs

A

cartridge delivery system

addition silicone and a polyether in a twin CARTRIDGE form – a BASE PASTE and CATALYST PASTE

41
Q

working and setting time IM comparison

A

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
Q

elasticity comparison for IMs

A

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
Q

undercuts comparison for IMs

A

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
Q

tear strength comparison for IMs

A

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
Q

5 points in material decision making approach

A
  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
Q

applying DMS knowledge to decision making

  1. Know KEY material properties
    - be prepared for new terminology, & sales pitch.
A

assessment
- Evidence shows new material is better

thus - purchase, unless cost concerns

47
Q

applying DMS knowledge to decision making

  1. Review product specification data
    – cf best rival products?
A

assessment
- New materials is only as good as current one

no specific advantage - REJECT

48
Q

applying DMS knowledge to decision making

  1. Know typical values expected for specific properties
A

assessment
- insufficient evidence

Review later

49
Q

applying DMS knowledge to decision making

  1. Identify properties NOT mentioned
A

assessment
- unconvincing date

REJECT

50
Q

viscosity Ideal IM

A

flow across surface

low

51
Q

contact angle
ideal for IM

(wettability)

A

engagement with tooth surface/surface quality

Low
wettability HIGH

52
Q

viscoelasticity ideal for IM

A

deformation on removal

LOW

53
Q

stiffness ideal for IM

A

ease of removal from undercut

LOW

54
Q

thermal expansion ideal for IM

A

contraction - mouth to room temp

LOW

55
Q

polymerisation shrinkage idea for IM

A

contraction during setting

LOW

56
Q

tear resistance ideal for IM

A

ability to withstand large stresses (e.g. during removal)

HIGH but not too high

BDS3 DMS (9 decks)

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