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BDS3 DMS > Elastomeric Impression Materials > Flashcards

Elastomeric Impression Materials Flashcards

1
Q

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

A
  • polyethers
  • addition silicones
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2
Q

Describe the elastic behaviour of impression materials

A
  • 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
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3
Q

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

A
  • material characteristics
    • degree of accuracy required
  • patient acceptance
    • taste
    • smell
  • ease of use
    • working time
    • setting time
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4
Q

How are elastomers formed?

A

polymerisation with cross-linking of polymer chains

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5
Q

What is the result of cross-linking during polymerisation

A
  • generation of elastic properties
  • fluid to solid transition
  • production of byproducts (water, hydrogen, alcohol)
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6
Q

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

A

dimensional stability and cast compatibility are affected

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7
Q

What are the 3 different types of elastomers?

A
  1. polysulphides (no longer used)
  2. silicones (addition curing and condensation curing)
  3. polyethers
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8
Q

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

A
  • surface detail reproduction
  • flow/viscosity
    -contact angle/wettability
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9
Q

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

A
  • elastic recovery
  • stiffness/flexibility
  • tear strength
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10
Q

What are the material properties involved in practical considerations

A
  • mixing time
  • working time
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11
Q

What is the shore A hardness test?

A

a specific hardness test for impression materials

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12
Q

What is the shark fin test?

A

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

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13
Q

Why is the thermal expansion coefficient important for impression materials?

A

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

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14
Q

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

A
  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
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15
Q

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

A
  1. quality of surface interaction between material and tooth and soft tissues
  2. accuracy
  3. dealing with removal and undercuts
  4. dimensional stability
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16
Q

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

A
  • viscosity
  • surface wetting
  • contact angle
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17
Q

What are the ideal properties for accuracy?

A
  • surface reproduction
  • visco-elasticity/elastic recovery
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18
Q

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

A
  • flow under pressure
  • tear/tensile strength
  • rigidity
19
Q

What are the ideal properties for dimensional stability?

A
  • setting shrinkage
  • thermal expansion/contraction
    -storage
20
Q

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

A

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

21
Q

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

A

the material must make intimate contact with the oral tissues

22
Q

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

A

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

23
Q

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

A

small contact angle

  • results in a larger percentage of the volume making contact with the target surface
24
Q

What happens as a result of larger contact angles?

A

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.

25
Q

What happens as a result of a smaller contact angle?

A

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

26
Q

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

A

non-ionic surfactant component

  • wets tooth surface
  • more easily wetted by water containing die materials
27
Q

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

A

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

28
Q

Discuss ISO4823 in relation to surface reproduction

A

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
29
Q

What is the ideal elastic behaviour of an impression material?

A
  • load applied and material immediately stretches to the strain required
  • level of strain maintained until the load is removed
  • material returns to original dimensions
30
Q

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

A
  • 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
31
Q

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

A

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
32
Q

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

A
  • 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
33
Q

What is the shark fin test?

A

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
34
Q

Why is the shark fin test useful?

A

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
35
Q

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

A

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

36
Q

Why is rigidity important for dealing with undercuts and removal

A

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

37
Q

Why is setting shrinkage important for dimensional stability?

A

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

38
Q

Why is thermal expansion/contraction important for dimensional stability?

A

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

39
Q

Why is storage important for dimensional stability?

A

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

40
Q

Do polyethers or addition silicones set faster?

A

Polyethers set slightly faster than addition silicones

41
Q

Do polyethers or silicones have longer working lengths?

A

Addition silicones have double the working time than polyethers

42
Q

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

A
  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
43
Q

Describe the decisions that should be made from the evidence available

A

Purchase - evidence shows new material is better

No advantage - new material as good as current

Review later - insufficient evidence

Reject - unconvincing data

BDS3 DMS (10 decks)

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