DMS - Elastomeric Impressions Flashcards

1
Q

How are elastomers formed?

A

Polymerisation with cross linking – its the cross linking that causes the elasticity.

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2
Q

What are the problems of polymerisation of elastomeric impressions?

A

Can create water/alcohol by-products = affect dimensional stability

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3
Q

what are the two types of elastomeric impressions?

A
  • Polyether

- Silicones: addition and condensation curing.

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4
Q

what is elastic recovery dependant on? (1)

A

clinical techniques – i.e. the quicker the tray is removed the less permanent deformation occurs (elastic recovery enhanced)

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5
Q

what properties do we want in an impression materials? (13)

A
  • Acceptance by the patient: taste/smell
  • Ease of use: setting time
  • Ability to capture surface detail
  • Flow/viscosity
  • Elastic recovery
  • Stiffness
  • Tear strength
  • Shore A hardness
  • Shark fin test; ability to cope with undercuts
  • Setting shrinkage
  • Dimensional stability; key to producing an accurate positive replica using gypsum
  • Thermal expansion coefficient
  • Biocompatibility
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6
Q

what factors contribute to how the material interacts with the teeth and soft tissue? (3)

A
  • Viscosity: it’s ability to flow over surfaces, this is vital for it to reach all of the dental tissues.
  • Surface wetting: the material must make intimate contact with the teeth/mucosa
  • Wettability - Contact angle: an objective measure of how well the impression material envelops the hard and soft tissues.
  • Low contact angle = a larger volume of the material is making contact with the target tissues.
  • High contact angles = more spaces between globules which leads to less surface replication.
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7
Q

describe how an impression material with a low contact angle acts.

A

= a larger volume of the material is making contact with the target tissues.

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8
Q

describe a impression material with a high contact angle

A

= more spaces between globules which leads to less surface replication.

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9
Q

what is added to hydrophobic silicone impression materials to allow them to make good contact with moist surfaces?

A

non-ionic surfactant added

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10
Q

how do we measure surface reproduction?

A

ISO 4823

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11
Q

what factors contribute to accuracy of an impression material? (2)

A

surface reproduction

elastic recovery

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12
Q

what factors contribute to how well an impression material can be removed? (3)

A
  • Flow under pressure
  • Tear/tensile strength = high = material can withstands large stresses during removal
  • Rigidity = low value = flexible
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13
Q

what test do we carry out to assess the ability of an impression material to flow under pressure?

A

“shark fin test”

- If a material has a larger fin length it has a higher flow = will flow readily into the sulcus/undercuts

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14
Q

what factors contribute to the dimensional stability of an impression material? (3)

A

• Setting shrinkage
- Should be low so that the impression material maintains the shape of what you replicated

• Thermal expansion/contraction
- Should be low to minimise the change in shape when the impression material cools down.

• Storage

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15
Q

what is advantageous about Polyethers and addition silicones in terms of storage.

A

Polyethers and addition silicones don’t absorb/release moisture = no change in dimensions .

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16
Q

List the ideal properties of an impression material. (7)

A

LOW viscosity

LOW gettability contact angle

LOW visco-elasticity

LOW stiffness

LOW thermal contraction

LOW polymerisation shrinkage

HIGH (not too high) tear resistance

17
Q

what are the advantages of using polyethers as an impression material? (3)

A
  • Set quicker
  • Shorter working time – depends on your preference
  • Can record deep undercuts
18
Q

what are the advantages of using addition silicone as an impression material? (2)

A
  • Have the most elastic recovery

- Resisting tear strength on removal