Elastomeric Impression Materials

Question 1

What are the two main types of elastomers used in dentistry?

Answer 1

• polyethers
• addition silicones

Question 2

Describe the elastic behaviour of impression materials

Answer 2

• material places against tooth tissue and allowed to set
• on removing the impression material from contact with the tooth it has to stretch to overcome the bulbous nature of the tooth
• ideally the material is fully elastic and recovers to its original dimensions replicating the shape of the tooth

Question 3

What must be considered when choosing which impression material to use?

Answer 3

• material characteristics
  
  • degree of accuracy required
• patient acceptance
  
  • taste
  • smell
• ease of use
  
  • working time
  • setting time

Question 4

How are elastomers formed?

Answer 4

polymerisation with cross-linking of polymer chains

Question 5

What is the result of cross-linking during polymerisation

Answer 5

• generation of elastic properties
• fluid to solid transition
• production of byproducts (water, hydrogen, alcohol)

Question 6

What happens to a material as a result of the production of byproducts?

Answer 6

dimensional stability and cast compatibility are affected

Question 7

What are the 3 different types of elastomers?

Answer 7

1. polysulphides (no longer used)
2. silicones (addition curing and condensation curing)
3. polyethers

Question 8

What are the material properties which affect the accuracy in which surface details are captured?

Answer 8

• surface detail reproduction
• flow/viscosity
  -contact angle/wettability

Question 9

What are the material properties which affect the accuracy of the dimension and shape captured?

Answer 9

• elastic recovery
• stiffness/flexibility
• tear strength

Question 10

What are the material properties involved in practical considerations

Answer 10

• mixing time
• working time

Question 11

What is the shore A hardness test?

Answer 11

a specific hardness test for impression materials

Question 12

What is the shark fin test?

Answer 12

a test determining the ability of an impression material to capture undercuts

Question 13

Why is the thermal expansion coefficient important for impression materials?

Answer 13

due to the temperature gradient between the oral cavity and room temperature

Question 14

What are the two ways in which twin form impression materials are delivered?

Answer 14

1. twin cartridge
   
   • base paste and catalyst paste
   • syringe gun pushes through mixing tip
   • homogenous tip delivered to impression tray
2. putty
   
   • catalyst paste and base paste
   • mixed until one even colour

Question 15

What are the 4 stages of impression material use that have different ideal properties?

Answer 15

1. quality of surface interaction between material and tooth and soft tissues
2. accuracy
3. dealing with removal and undercuts
4. dimensional stability

Question 16

What are the ideal properties for the quality of surface interaction between the material and tooth and soft tissue surfaces?

Answer 16

• viscosity
• surface wetting
• contact angle

Question 17

What are the ideal properties for accuracy?

Answer 17

• surface reproduction
• visco-elasticity/elastic recovery

Question 18

What are the ideal properties for dealing with removal and undercuts?

Answer 18

• flow under pressure
• tear/tensile strength
• rigidity

Question 19

What are the ideal properties for dimensional stability?

Answer 19

• setting shrinkage
• thermal expansion/contraction
  -storage

Question 20

Why is viscosity important for the quality of surface interaction between the material and oral tissues

Answer 20

the material must be able to flow readily without requiring too much pressure applied

Question 21

Why is surface wetting important for the quality of surface interaction between the material and oral tissues

Answer 21

the material must make intimate contact with the oral tissues

Question 22

Why is contact angle important for the quality of surface interaction between the material and oral tissues

Answer 22

determines hoe well the material envelops the oral tissues to record fine detail

Question 23

Should the wetting angle of an impression material be large or small?

Answer 23

small contact angle

• results in a larger percentage of the volume making contact with the target surface

Question 24

What happens as a result of larger contact angles?

Answer 24

a large contact angle at the surface results in spaces between the globules of impression material meaning some of the tooth surface is not replicated.

Question 25

What happens as a result of a smaller contact angle?

Answer 25

a small contact angle results in no spaces between the globules of impression material so all of the surface is replicated

Question 26

What is the additional component in hydrophilic silicons and what properties does it change?

Answer 26

non-ionic surfactant component - wets tooth surface - more easily wetted by water containing die materials

Question 27

Why is surface reproduction important for accuracy of an impression material?

Answer 27

the more of a surface that can be captured, the more accurate the impression

Question 28

Discuss ISO4823 in relation to surface reproduction

Answer 28

universal test to measure how accurately a surface is represented by a material - material placed on surface with grooves of a specified depth (20, 50 & 75um) - even pressure applied to material - determined what level of detail can be captured by material

Question 29

What is the ideal elastic behaviour of an impression material?

Answer 29

- load applied and material immediately stretches to the strain required - level of strain maintained until the load is removed - material returns to original dimensions

Question 30

What is the realistic visco-elastic effect of impression materials?

Answer 30

- load applied and material gradually reaches the strain required - load is released and strain level gradually drops - strain does not return to original levels and permanent deformation occurs

Question 31

What clinical aspect can influence the elastic recovery of a material?

Answer 31

the speed at which the impression tray is removed from the mouth - if impression is removed with a sharp pull there is less overall permanent strain resulting in less deformation - this is due to the reduced load time

Question 32

Discuss the relationship between flow and elasticity when an impression is being taken

Answer 32

- after impression tray is placed in mouth the ability to flow diminishes - flow continually diminishes as the material sets - elasticity only starts to develop when the setting reaction has developed to a certain extent - even when the impression feels firm it will still be developing so it is beneficial to leave it for a bit longer

Question 33

What is the shark fin test?

Answer 33

a test to determine how a material flows under pressure - cylindrical container with slot of specified depth - impression material placed in the upper part of the chamber, must have a greater depth than the slot. - force applied to push the material down into the slot

Question 34

Why is the shark fin test useful?

Answer 34

to record and undercut, the material must reach the extremities of a narrow zone with a complex shape, like that between the gingiva and tooth - large fin length indicates high flow (will flow readily into undercuts) - small fin length indicates low flow

Question 35

Why is tear strength important for dealing with removal and undercuts?

Answer 35

the material must have large tear strength to withstand the large stresses applied during removal

Question 36

Why is rigidity important for dealing with undercuts and removal

Answer 36

the material should be flexible (e.g. low rigidity) for ease of removal of the material, especially from undercut regions

Question 37

Why is setting shrinkage important for dimensional stability?

Answer 37

the material should have a low setting shrinkage so it maintains the shape of the tissue being replicated

Question 38

Why is thermal expansion/contraction important for dimensional stability?

Answer 38

the material should have a low thermal expansion coefficient to minimise the change in shape as the impression material drops from 37 degrees in the mouth to 22 degrees in the dental surgery

Question 39

Why is storage important for dimensional stability?

Answer 39

some materials experience a dimensional change during storage - polyethers and addition silicones do not

Question 40

Do polyethers or addition silicones set faster?

Answer 40

Polyethers set slightly faster than addition silicones

Question 41

Do polyethers or silicones have longer working lengths?

Answer 41

Addition silicones have double the working time than polyethers

Question 42

What is the 5 stage decision making approach that should be used for assessing materials?

Answer 42

1. Know the key material properties - be aware of sales techniques 2. Review product specification - compare to rival products 3. Know typical values expected for specific properties 4. Identify properties not mentioned 5. Reject claims not supported with scientific/clinical data

Question 43

Describe the decisions that should be made from the evidence available

Answer 43

Purchase - evidence shows new material is better No advantage - new material as good as current Review later - insufficient evidence Reject - unconvincing data