Impression materials - desirable properties

Question 1

What viscosity should an impression material be initially?

Answer 1

Fluid

Question 2

Why should an impression material be fluid initially?

Answer 2

Flow around tissues, records details

Question 3

As a result of impression materials being fluid initially, what is used to condense it around tissues?

Answer 3

Impression trays

Question 4

What reactions significantly decreases fluidity?

Answer 4

Setting reaction

Question 5

What type of setting reactions can it be?

Answer 5

Physical or chemical process

Question 6

What type of property does the impression need to have when set?

Answer 6

Elastic for undercuts and not too rigid to remove

Question 7

How can impression materials be classified by rheological properties?
What advantages does this classification have?

Answer 7

Viscosity of initial material

Useful for judging impression accuracy

Question 8

How can impression materials be classified by mechanical properties? What is this useful for?

Answer 8

Stiffness, elasticity,

Judging whether good for undercuts, how easy it will be to remove

Question 9

How can impression materials be classified by chemical nature?

Answer 9

5 chemical classes

• pastes (zinc oxide)
• gypsum (impression plaster)
• thermoplastics (compound)
• Hydrocolloids (alginate)
• elastomers (silicone, polyether)

Question 10

What does rheological properties tell you?

Answer 10

How easily the material flows

- low viscosity flow easily and higher accuracy

Question 11

Effect of viscosity on soft tissues

Answer 11

High viscosity = displace tissues = muco-compressive

Low = mucostatic

Question 12

How does the impression tray effect viscosity?

Answer 12

If not much space between tray and tissue = less flow = increase viscosity

More space = more flow = reduce viscosity

Question 13

Properties of impression tray:

Answer 13

Correct size (effects viscosity)
Support for material (rigid)
Bond to material (limits shrinkage)

Question 14

What 2 properties are important for mechanical classification?

Answer 14

Elasticity and rigidity

Question 15

Rigidity

Answer 15

How easy it is to deform the impression material
Low stiffness = easier to deform - easier to get passes undercuts
Too stiff = difficult to remove

Question 16

3 desirable properties of impression materials

Answer 16

Accuracy
Dimensional stability
Convenience/ease of use

Question 17

What does accuracy relate to?

Answer 17

Viscosity
Wetting
Dimensional changes (setting reaction)
Thermal contraction
Adhesion to tray
Undercuts

Question 18

Wetting

Answer 18

How well the material spreads on moist surfaces

Question 19

Low viscosity =

a) fine detail
b) not detailed

Answer 19

fine detail

Question 20

When does wetting effect accuracy?

Answer 20

Hydrophobic materials - water tends to deform surfaces = bubbles = need a dry field

Question 21

How can setting effect accuracy?

Answer 21

Dimensional changes and temperature changes

Question 22

What kind of dimensional changes can occur during setting

Answer 22

Polymerisation shrinkage

Crystal growth

Question 23

What kind of thermal changes can occur during setting?

Answer 23

Thermal contraction - oral to room temp

Thermoplastic warming/cooling - impression compound heated to 55 and cooled quickly in mouth = distortion

Question 24

What effects thermal contraction?

Answer 24

Coefficient of thermal expansion - higher = more contraction

Question 25

What factors effect accuracy with undercuts?

Answer 25

Needs to be elastic

Depth of undercut - deeper = more distortion

Proper setting - allow to become full elastic

Thickness of impression material - tearing can occur