Impression materials - properties

Question 1

What is the main indication for an indirect restoration?

Answer 1

A tooth with little tooth tissue remaining.

Question 2

What are the benefits of making an indirect restoration rather than a direct one?

Answer 2

• They shrink less
• Greater tooth morphology
• More degrees of polymerisation (less likely to have shrinkage in mouth)

Question 3

Why is it important that the impressions we take are accurate?

Answer 3

To reduce the distortion that occurs when constructing the appliance.
Ensures accuracy and fit of appliance.

Question 4

What material is commonly used for indirect restorations?

Answer 4

Ceramics (rigid with high modulus of elasticity)

Question 5

What are the two types of trays that are used when imrpession taking?

Answer 5

Stock trays

Custom trays

Question 6

What two things can we do to increase the adhesion of the material to the tray?

Answer 6

• Trays with perforations

- Adhesives used

Question 7

What are the two classifications of impression materials?

Answer 7

1) Elastics

2) Non elastic materials

Question 8

Why do we want impression materials to flow?

Answer 8

So that they can capture the hard and soft tissues.

Question 9

What two factors does viscosity change with?

Answer 9

• Shear rate

- Time

Question 10

What is the relationship between shear stress and shear rate in Neutonon materia,s?

Answer 10

Shear stress is proportional to shear rate.

Question 11

Shear stress = K(shear rate)^n

If n is >1, what happens to the material with increased shear stress?

Answer 11

Shear rate will increase (material undergoes shear thinning).
This means the viscosity decreases with increased shear rate.

Question 12

Why is it not good to use either a high or a low viscosity material?

Answer 12

High = may not flow over undercuts

Low = will drip over oral cavity and can lead to gagging

Question 13

Why do we need to balance the hydrophilic and hydrophobic components of an impression material?

Answer 13

Too hydrophilic = too much water uptake which can lead to distortion from swelling

Too hydrophobic = with have limited flow in areas with lots of saliva leading to blow holes and inaccurate impression

Question 14

Will an elastic or a plastic material be better at accurately replicating the oral tissues?

Answer 14

Elastic as it has the least distortion of removal.

Question 15

What method can we use to keep shrinkage to a minimum?

Answer 15

Place a material with a high filler content into the tray and cover with a material of low filler content.

Question 16

What action do we use when removing the trays to ensure elastic recovery and least distortion?

Answer 16

Snap action!

Question 17

Ideally long or short -

Working time?

Setting time?

Answer 17

long

short

Question 18

Why do we need to make sure we are applying the tray with uniform thickness of impression material?

Answer 18

Too thick = increase in polymerisation shrinkage

Too thin = strain will lead to deformation

Question 19

What is dimensional stability and what factors affect stability?

Answer 19

Dimension stability - the ability of the impression material to not expand or contract on storage.

• Continued setting once out of mouth
• Hydrophobic/hydrophilic materials
• Distortions due to stress
• Slow elastic recovery
• Uptake of moisture

Question 20

What thermal property do we want the impression material to have?

Answer 20

High coefficient of thermal expansion.

Question 21

Why is it important that impression materials have a high tear strength?

Answer 21

This ensures no impression material is left in the PDL as it can lead to a severe reaction.

Question 22

What factors do we need to take account of when considering biocompatability?

Answer 22

• No irritation to oral mucosa
• Non toxic
• No cariogenic components present

Question 23

What are the two main ways of mixing an impression materials?

Answer 23

• By hand

- In a machine

Question 24

Why do we ideally want to use a Newtonian material?

Answer 24

No shear thinning or thickening will occur.

Question 25

When a brand comes in so sell us an impression material, what properties do we look at?

Answer 25

• Setting reaction
• Setting time
• Moisture effect
• Type of material
• Viscosity
• Type of tray needed
• Technique needed
• Cost
• How to disinfect the tray

Question 26

Why is plaster of paris a material not recommended?

Answer 26

It has low strength and can fracture easily if there are any undercuts.

Plastic material.