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

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

Answer 13

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

Question 14

What does mucostatic mean?

Answer 14

More fluid, so less likely to compress soft tissue

Question 15

What does mucocompressive mean?

Answer 15

More viscous, more likely to compress soft tissue

Question 16

What materials have high viscosity?

Answer 16

Putty elastomers (heavy-body)
Impression compound

Question 17

What materials have medium viscosity?

Answer 17

Regular body elastomers

Question 18

What materials have low viscosity?

Answer 18

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

Question 19

How can you measure wettability?

Answer 19

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

Question 20

Trend of wettability

Answer 20

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

Question 21

Why is good wetting needed?

Answer 21

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 22

Describe material contraction of impression materials in the mouth.

Answer 22

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 23

What is TEC in terms of tray properties?

Answer 23

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

Question 24

What can happen with partial detachment from the tray?

Answer 24

Impression distortion and decreased accuracy and improper fit

Question 25

How do we increase attachment to the tray?

Answer 25

Tray adhesives and perforated trays

Question 26

What do viscoelastic materials exhibit?

Answer 26

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.

Question 27

What is viscoelasticity?

Answer 27

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 28

What materials are viscoelastic?

Answer 28

Elastomeric and certain polymeric materials

Question 29

How are polymers viscoelastic?

Answer 29

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

Question 30

What is strain rate behaviour?

Answer 30

Rate at which you apply the load

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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)

Question 34

What is a dashpot?

Answer 34

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

Question 35

What models can model viscoelastic materials?

Answer 35

Maxwell or Voight models

Question 36

What is the Maxwell model?

Answer 36

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

Question 37

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

Answer 37

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)

Question 38

What is the extent of deformation in the Maxwell model?

Answer 38

Application load Time of application (Graph pg37)

Question 39

What is the Voight model?

Answer 39

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.

Question 40

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

Answer 40

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).

Question 41

What is recovery time dependent on in the Voight model?

Answer 41

Application load Time of application (Graph pg38)

Question 42

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

Answer 42

Yes

Question 43

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

Answer 43

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

Question 44

Are polymers strain rate sensitive?

Answer 44

Yes all polymers are strain rate sensitive

Question 45

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

Answer 45

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 46

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

Answer 46

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 47

Are silicones viscoelastic?

Answer 47

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

Question 48

Are alginates viscoelastic?

Answer 48

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.