11. Elastomeric Impression Materials

Question 1

Types of elastomeric impression materials (2)

Answer 1

Polyethers

Addition silicones

Question 2

Why are condensation silicones not used clinically

Answer 2

They give off water during curing which affects the dimensions of the material

Question 3

Factors that affect impression material choice

Answer 3

Material characteristics
Clinical performance (patient acceptance, ease of use)

Question 4

Important elastomeric impression material properties (8)

Answer 4

Flow/viscosity
Surface detail (reproduction)
Surface wetting
Elastic recovery (%) and viscoelasticity
Stiffness (flexibility)
Tear strength
Mixing time
Working time

Question 5

Definition of viscosity

Answer 5

A measure of a material’s ability to flow (must be able to flow well)

Question 6

Function of viscosity

Answer 6

Determines a material’s potential for close intimate contact with hard/soft tissue surfaces and therefore, the accuracy of the record
Range – low, medium, high

Question 7

Definition of surface detail (reproduction)

Answer 7

Ability to flow into detail outlined by ISO standards (50-75um depending on the viscosity of the material)

Question 8

Definition of surface wetting

Answer 8

Ability to cover a surface

Question 9

Features of surface wetting (2)

Answer 9

Must be able to cover complete tooth surface so an intimate contact can be established
Must make intimate contact with teeth/mucosa so all of the surface is replicated

Question 10

Features of elastic recovery (%) and viscoelasticity (2)

Answer 10

Can be up to 99.5% elastic recovery (hard to beat)

Viscoelastic behaviour ensures some degree of permanent strain

Question 11

Ideal viscoelasticity

Answer 11

Low viscoelasticity is ideal (small deformation)

Question 12

ISO standards

Answer 12

Are not designed to establish which is the “best performing” material for a given clinical application ……(instead) they are designed to exclude unsafe and poorly performing materials from the market

Question 13

Observed viscoelastic behaviour of elastomeric impression materials

Answer 13

When an impression material, having been stretched and compressed on removal from mouth, fails to return to its original dimensions/shape, there is permanent deformation (permanent strain)

Question 14

Definition of tear strength

Answer 14

The stress a material will withstand before fracturing

Question 15

Measuring of rigidity

Answer 15

Measured by stress/strain ratio

Question 16

Ideal rigidity of elastomeric impression materials

Answer 16

Ideally, the impression material is flexible (has low rigidity) to ease its removal from undercuts/interdental regions

Question 17

Setting times of elastomeric impression materials (2)

Answer 17

Polyethers - 5mins

Addition silicones - 6mins

Question 18

Working times of elastomeric impression materials (2)

Answer 18

Polyethers - 2mins

Addition silicones - 4mins

Question 19

Elastic recovery range of elastomeric impression materials

Answer 19

98-99.5%

99.5% is ideal

Question 20

Tear strength range of elastomeric impression materials

Answer 20

1.8-9MPa