DMS - Elastomeric Impressions

Question 1

How are elastomers formed?

Answer 1

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

Question 2

What are the problems of polymerisation of elastomeric impressions?

Answer 2

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

Question 3

what are the two types of elastomeric impressions?

Answer 3

• Polyether

- Silicones: addition and condensation curing.

Question 4

what is elastic recovery dependant on? (1)

Answer 4

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

Question 5

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

Answer 5

• 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

Question 6

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

Answer 6

• 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.

Question 7

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

Answer 7

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

Question 8

describe a impression material with a high contact angle

Answer 8

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

Question 9

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

Answer 9

non-ionic surfactant added

Question 10

how do we measure surface reproduction?

Answer 10

ISO 4823

Question 11

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

Answer 11

surface reproduction

elastic recovery

Question 12

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

Answer 12

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

Question 13

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

Answer 13

“shark fin test”

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

Question 14

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

Answer 14

• 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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

LOW viscosity

LOW gettability contact angle

LOW visco-elasticity

LOW stiffness

LOW thermal contraction

LOW polymerisation shrinkage

HIGH (not too high) tear resistance

Question 17

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

Answer 17

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

Question 18

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

Answer 18

• Have the most elastic recovery

- Resisting tear strength on removal