L07: Impression Materials Flashcards

1
Q

What is dimensional accuracy?

A

Key properties of material upon mixing/activation (prior to setting), and after setting

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2
Q

What is considered under dimensional accuracy?

A

Rheological properties, viscosity, hydrophobicity, wettability, shrinkage, tray adhesion, mechanical properties

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3
Q

What are rheological properties?

A

How it flows (before and after it comes into contact with the dental arch)

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4
Q

What do you have to consider in viscosity in dentistry?

A

Too viscous might not conform to small fine details, too low viscosity means it might make patient gag and flow down throat.
Edentulous patients - are soft tissues going to be influenced by the material? Less viscous are less likely to compress soft tissue whereas more viscous means there might be displacement of soft tissues. Problematic in denture design.

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5
Q

What is the relevance of hydrophobicity of impression materials in dentistry?

A

The mouth is a moist environment. Alginate is a hydrolytic reaction so is hydrophilic whereas silicones and elastomeric materials are hydrophobic (but have optimal mechanical properties.

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6
Q

What is wettability?

A

How a liquid wets a solid surface - how well a liquid spreads out over the surface of the material. Surface energy of the solid and surface tension of the liquid.

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7
Q

How does shrinkage affect impression materials?

A

Alginate has a cross-linking reaction. May be associated with change in volume, affecting accuracy.

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8
Q

How does tray adhesion affect impression materials?

A

Tray adhesion is important so there is no distortion of impression as it sets or is removed from the mouth. E.g., adhesive and perforated trays creating a mechanical interlock.

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9
Q

What mechanical properties of impression materials should we consider?

A

Tear resistance, tensile strength

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10
Q

What is dimensional stability?

A

Maintenance of the original accuracy.
It needs to last within the time of the impression being taken away

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11
Q

Does alginate have good dimensional stability?

A

No, alginates are bad in terms of stability due to hydraulic based, and water evaporates. If leaving it, it’ll shrink. Needs to be placed in damp cloth to stop evaporation/absorption of water.

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12
Q

What properties are part of dimensional stability?

A

Elastic recovery (time implication of returning back to its original dimension)
Residual stresses on cooling
Volatile components (water evaporation)
Tray adhesion

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13
Q

How are hard and soft tissue impressions affected by the viscosity of the impression material?

A

Hard tissue impressions - fine detail may be influenced
Soft tissue impressions - degree of tissue compression

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14
Q

What does mucostatic mean?

A

More fluid, so less likely to compress soft tissue

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15
Q

What does mucocompressive mean?

A

More viscous, more likely to compress soft tissue

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16
Q

What materials have high viscosity?

A

Putty elastomers (heavy-body)
Impression compound

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17
Q

What materials have medium viscosity?

A

Regular body elastomers

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18
Q

What materials have low viscosity?

A

Light body elastomers (wash)
Impression plaster
ZnO-eug paste
Hydrocolloid - agar (reversible) and alginate (irreversible)

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19
Q

How can you measure wettability?

A

Can measure by measuring the angle with which the liquid forms against the solid interface.

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20
Q

Trend of wettability

A

The smaller the contact angle, the higher the wetting properties of that liquid on that particular surface.

21
Q

Why is good wetting needed?

A

Hydrophilic characteristics, meaning:
- more hydrophilic attracts saliva, so has a smaller contact angle
- fewer voids
- less entrapment of oral fluids
- less bubbles in dies and models
- fewer retakes
- vice versa for hydrophobic materials

22
Q

Describe material contraction of impression materials in the mouth.

A

When tray is sat in mouth and material is well adhered, shrinkage vectors will move towards the fixed surface. Excessive contraction means impression material will shrink towards the tray and impression will end up being slightly larger than the tooth is. We want to reduce shrinkage.

23
Q

What is TEC in terms of tray properties?

A

Thermal expansion coefficient
Can cause stress and deboning if temperature is quite off

24
Q

What can happen with partial detachment from the tray?

A

Impression distortion and decreased accuracy and improper fit

25
Q

How do we increase attachment to the tray?

A

Tray adhesives and perforated trays

26
Q

What do viscoelastic materials exhibit?

A

They can exhibit permanent deformation of the polymer coil. If too much force is applied dependent on the time it is applied for, you can end up with significant deformation.

27
Q

What is viscoelasticity?

A

The property of a substance exhibiting both elastic and viscous behaviour. The application of stress causing temporary deformation if the stress is quickly removed but permanent deformation if it is maintained.

28
Q

What materials are viscoelastic?

A

Elastomeric and certain polymeric materials

29
Q

How are polymers viscoelastic?

A

In polymers, there are lots of different chains. Cross-links will stretch out and contract when a force is applied.

30
Q

What is strain rate behaviour?

A

Rate at which you apply the load

31
Q

What happens to a fixed spring that exhibits an elastic response on application and removal of load?

A

Load applied: instant extension, no time dependence
Load removed: immediate recovery (unless go past yield point), no time dependence

32
Q

What is the extent of deformation to a fixed spring exhibiting an elastic response when a load is applied?

A

The extent of deformation is instant, constant while the load is in place and then an instant decrease (graph on pg 36 to show)

33
Q

What happens to a fixed dashpot that exhibits a viscous response on application and removal of load?

A

Load applied: opens slowly, movement of the piston is dampened by the viscosity of the liquid. Strain becomes a function of time.
Load removed: no energy to return the piston to where it was back originally. Pot remains open. No recovery, permanent deformation/set.
(Graph pg36)

34
Q

What is a dashpot?

A

A cylinder with a piston inside it, within the cylinder is a viscous liquid.

35
Q

What models can model viscoelastic materials?

A

Maxwell or Voight models

36
Q

What is the Maxwell model?

A

Spring and dashpot placed in series, follows a similar principle but in combination

37
Q

What happens upon the application and removal of a load in the Maxwell model?

A

Load applied: instant extension of spring to the load, slow opening of pot (time depending opening of pot due to viscous liquid inside)
Load removed: immediate recovery of spring (elastic response), permanent deformation of pot (because there is not energy to push piston back though the liquid)

38
Q

What is the extent of deformation in the Maxwell model?

A

Application load
Time of application
(Graph pg37)

39
Q

What is the Voight model?

A

Arranging the spring and dashpot in parallel to each other, so they are sharing the load. Load has a consequence for both of them at the same time.

40
Q

What happens on the application and removal of load in the Voight model?

A

Load application: slow opening of spring (load is shared, some energy of the spring is provided to the piston moving through the viscous liquid). Dampening by pot.
Load removal: slow elastic recovery (energy stored in the spring is enough to bring the piston back to how it was initially, but slower).

41
Q

What is recovery time dependent on in the Voight model?

A

Application load
Time of application
(Graph pg38)

42
Q

Is there a time-dependent response to the load you apply to a set impression placed in the patients mouth?

A

Yes

43
Q

What leads to the least chance of plastic deformation when taking impressions?

A

A rapid load rate removal of an elastomeric impression material leads to the least chance for plastic deformation (aka remove impression material fast!)

44
Q

Are polymers strain rate sensitive?

A

Yes all polymers are strain rate sensitive

45
Q

What happens if the stress if applied quickly to the impression?

A

If the stress is applied quickly, energy is stored elastically and the material will not undergo plastic deformation. This is particularly important in the region of greatest deformations, e.g., the margins.

46
Q

Why do slow removal rates cause plastic deformation and distort the impression in critical areas?

A

It allows viscoelastic materials to react as a strain response. Polymer chains will not be able to go back to how they were, and cannot go back to original impression.
Decreased dimensional accuracy.

47
Q

Are silicones viscoelastic?

A

Silicone materials exhibit 97% elastic response and are good at responding to loads and returning to how they were originally.

48
Q

Are alginates viscoelastic?

A

Alginates have higher levels of viscoelasticity than silicones, and may only return back to 91% of their normal form, especially exacerbated if stretched over a longer period.