Elastomers

Question 1

What are the two types of elastomers

Answer 1

• polyether

- addition silicones

Question 2

what is meant by elastic behaviour

Answer 2

to what extent does the impression material, after undergoing elastic strain, undergo elastic recovery

Question 3

How do we decide which impression material to use

Answer 3

• material characteristics

- clinical performance (patient acceptance and ease of use)

Question 4

What material properties do we look out for when assessing materials

Answer 4

• flow/viscosity
• surface detail (reproduction)
• wettability (contact angle)
• elastic recovery (%)
• stiffness (flexibility)
• tear strength
• mixing time (min)
• working time (min)

Question 5

What is the international standards organisation (ISO) standard for impression materials

Answer 5

is that grooves/indentations of either 20µm or 50µm (depending on material viscosity) are replicated

Question 6

Why would we want an impression material with a good elastic recovery

Answer 6

• easy to remove from mouth

- elastically recover from the deformation of removal

Question 7

why would we want an impression material with a high tear strength

Answer 7

reduces the risk of tearing fine margins

Question 8

why would we want an impression material with excellent wettability

Answer 8

• spread easily
• adapt smoothly to dentin and moist oral tissue
• won’t pull away leaving a void

Question 9

what is a good value for an elastomers linear dimensional change (24hrs)

Answer 9

<0.2% (will shrink by 2% after the removal)

Question 10

what is a good value for an elastomers recovery from deformation

Answer 10

> 99.5%

Question 11

what is a good value for an elastomers detail reproduction

Answer 11

20µm

Question 12

What are the ideal properties for impression materials

Answer 12

1. Quality of surface interaction between material and tooth/soft tissue surfaces
   - viscosity
   - surface wetting
2. Accuracy
   - surface reproduction
   - visco-elasticity/elastic recovery
3. Dealing with removal and undercuts
   - tear/tensile strength
   - rigidity (flexibility)
4. Dimensional properties
   - setting shrinkage
   - thermal expansion/contraction
   - storage

Question 13

Why are there international standards

Answer 13

standards 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 14

What properties determine the quality of surface interaction between material and tooth/soft tissue surfaces

Answer 14

viscosity - must be able to flow readily

surface wetting - must make intimate contact with teeth/mucosa

contact angle

Question 15

what is viscosity and what does it determine

Answer 15

• a measure of material’s ability to flow
• determines a material’s potential for making close contact with hard/soft tissue surfaces
• range: low, medium, high

Question 16

why must surface wetting be good

Answer 16

so all of surface is replicated (impression material must make intimate contact with teeth/mucosa surfaces)

Question 17

What properties determine accuracy

Answer 17

• surface reproduction

- visco-elasticity/elastic recovery

Question 18

If you have 3 notches; 1 = 20µm, 2= 50µm and 3 = 75µm, and the material flows into 2 and 3, does the material meet the ISO standard for reproduction of surface detail?

Answer 18

yes, meets the 50µm limit

Question 19

How much elastic recovery do we ideally want

Answer 19

100% elastic recovery

no permanent strain

Question 20

How does the viscoelastic behaviour of impression materials influence tray removal method

Answer 20

the load time should be as little as possible - if the impression is removed with a SHARP PULL, there is less overall permanent strain

Question 21

do we want a high or low viscoelasticity

Answer 21

low viscoelasticity (small deformation)

Question 22

What properties determine dealing with removal and undercuts?

Answer 22

• tear/ tensile strength (removal)

- rigidity (removal)

Question 23

what is tear strength

Answer 23

stress material will withstand before fracturing

Question 24

what is rigidity

Answer 24

stress/ strain

• large stress needed to cause material to change shape
• ideally impression material is flexible (we want low rigidity to ease its removal from undercut/ interdental regions)

Question 25

do we want impression materials to be low or high rigidity

Answer 25

low rigidity (we want low rigidity to ease its removal from undercut/ interdental regions)

Question 26

what have longer setting times out of polyethers and addition silicones

Answer 26

addition silicones (6 mins)
polyethers (5mins)

Question 27

what have longer working times out of polyethers and addition silicones

Answer 27

polyethers (2mins)
addition silicones (4mins)

Question 28

is ‘virtual’ a polyether or addition silicone

Answer 28

addition silicone

Question 29

how does ‘virtual’ compare to other elastomers

Answer 29

best elastic recovery (99.5%)

best tear strength (9MPa)

Question 30

what do you have to do when deciding what material you want to use

Answer 30

1. know key properties of the material
2. review product specification data
3. know typical values expected for specific properties
4. identify properties not mentioned
5. reject claims not supported with scientific and/or clinical data