L07: Impression Materials

Question 1

What is dimensional accuracy?

Answer 1

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

Question 2

What is considered under dimensional accuracy?

Answer 2

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

Question 3

What are rheological properties?

Answer 3

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

Question 4

What do you have to consider in viscosity in dentistry?

Answer 4

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.

Question 5

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

Answer 5

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.

Question 6

What is wettability?

Answer 6

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.

Question 7

How does shrinkage affect impression materials?

Answer 7

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

Question 8

How does tray adhesion affect impression materials?

Answer 8

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.

Question 9

What mechanical properties of impression materials should we consider?

Answer 9

Tear resistance, tensile strength

Question 10

What is dimensional stability?

Answer 10

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

Question 11

Does alginate have good dimensional stability?

Answer 11

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.

Question 12

What properties are part of dimensional stability?

Answer 12

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

Question 13

What does mucostatic mean?

Answer 13

More fluid, so less likely to compress soft tissue

Question 14

What does mucocompressive mean?

Answer 14

More viscous, more likely to compress soft tissue

Question 15

What materials have high viscosity?

Answer 15

Putty elastomers (heavy-body)
Impression compound

Question 16

What materials have medium viscosity?

Answer 16

Regular body elastomers

Question 17

What materials have low viscosity?

Answer 17

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

Question 18

Why is good wetting needed?

Answer 18

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

Question 19

Describe material contraction of impression materials in the mouth.

Answer 19

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.

Question 20

What can happen with partial detachment from the tray?

Answer 20

Impression distortion and decreased accuracy and improper fit

Question 21

How do we increase attachment to the tray?

Answer 21

Tray adhesives and perforated trays

Question 22

What do viscoelastic materials exhibit?

Answer 22

Permanent deformation

Question 23

What is viscoelasticity?

Answer 23

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.

Question 24

What materials are viscoelastic?

Answer 24

Elastomeric and certain polymeric materials

Question 25

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

Answer 25

Yes

Question 26

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

Answer 26

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

Question 27

Are polymers strain rate sensitive?

Answer 27

Yes all polymers are strain rate sensitive

Question 28

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

Answer 28

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.

Question 29

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

Answer 29

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.

Question 30

Are silicones viscoelastic?

Answer 30

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

Question 31

Are alginates viscoelastic?

Answer 31

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.