DMS - Elastomeric Impressions Flashcards
How are elastomers formed?
Polymerisation with cross linking – its the cross linking that causes the elasticity.
What are the problems of polymerisation of elastomeric impressions?
Can create water/alcohol by-products = affect dimensional stability
what are the two types of elastomeric impressions?
- Polyether
- Silicones: addition and condensation curing.
what is elastic recovery dependant on? (1)
clinical techniques – i.e. the quicker the tray is removed the less permanent deformation occurs (elastic recovery enhanced)
what properties do we want in an impression materials? (13)
- 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
what factors contribute to how the material interacts with the teeth and soft tissue? (3)
- 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.
describe how an impression material with a low contact angle acts.
= a larger volume of the material is making contact with the target tissues.
describe a impression material with a high contact angle
= more spaces between globules which leads to less surface replication.
what is added to hydrophobic silicone impression materials to allow them to make good contact with moist surfaces?
non-ionic surfactant added
how do we measure surface reproduction?
ISO 4823
what factors contribute to accuracy of an impression material? (2)
surface reproduction
elastic recovery
what factors contribute to how well an impression material can be removed? (3)
- Flow under pressure
- Tear/tensile strength = high = material can withstands large stresses during removal
- Rigidity = low value = flexible
what test do we carry out to assess the ability of an impression material to flow under pressure?
“shark fin test”
- If a material has a larger fin length it has a higher flow = will flow readily into the sulcus/undercuts
what factors contribute to the dimensional stability of an impression material? (3)
• 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
what is advantageous about Polyethers and addition silicones in terms of storage.
Polyethers and addition silicones don’t absorb/release moisture = no change in dimensions .
