Elastomeric Impression Materials Flashcards
What are the material types
Polyether
Additional silicones
Why is elastic behavior required for impression materials
- When removing an impression material from contact with a tooth after it has set, the impression material has to stretch
- It flares out at the sides to overcome the bulbous aspect of the tooth
- Ideally it recovers its original dimensions, replicating the shape of the tooth
How do we select impression materials
by assessing the material characteristics and the degree of accuracy
evaluate the clinical performance
How do we evaluate the clinical performance
ease of use fo clinician
does the patient accept it
working time, setting time, taste, smell of material
How are elastomers formed
by polymerization with cross linking of polymer chains
When the elastomer undergoes cross linking generates elastic properties and it causes fluid to go to solid transition
What can polymerization of elastomers produce
Polymerisation may produce biproducts (water, hydrogen and alcohol) which can affect the dimensional stability and cast compatibility of the materia
What are the various types of elastomeric materials available
polysulphides
silicones
polyethers
What are the different types of silicones
additional curing
condensation curing
What characteristics do we look at when assessing
characteristics that effect accuracy in which surface details are recorded
characteristics effecting accuracy of dimensions and shape of the final impression
practical considerations that effect clinicians skills and working method and will influence patients preference and choice
What are the characteristics that effect accuracy in which surface details are recorded
surface detail (reproduction)
flow/viscosity
contact angle/wettability
What are the characteristics that effect the accuracy of dimensions and shape of the final impression
elastic recovery
stiffness
tear strength
What are other characteristics we look at for impression materials
shore A hardness shark fin test setting shrinkage dimensional stability thermal expansion coefficient biocompatibility
What is shore A hardness
§ Simple test for hardness of an impression material
Why is dimensional stability important
§ Crucial for producing an accurate positive replica using gypsum
Why is thermal expansion important
This plays a pivotal role as we cant eliminate the temperature difference between the mouth and room temperature
What does virtual claim
roduct is hydrophilic and so is compatible with moist dentine and gingiva
• Exhibits very low contact angles meaning it spreads and adapts easily to dentine and moist oral tissues
• Has colour contrast that makes preparation margins very easy to read
• Precise impressions
• Says it surpasses the 50 measurement for ISO norm
• Has excellent reproduction of anatomical details
• Their dimensional stability is up to two weeks so no need to pour models immediately
• Elastically recover from the deformation of removal
• Excellent tear resistance
What are the forms of IM
- It comes as two different forms - the first form is as a twin cartilage with a base and catalyst paste that requires a syringe gun to push them through the mixing tip
- You can also get two tubs where you take a spoon of each and mix them
What are the ideal properties of an impression material
quality of surface interaction between material and tooth/soft tissue surfaces
accuracy
dealing with removal and undercuts
dimensional stability
What is the quality of surface interaction between material and tooth/soft tissue surfaces dependent on
viscosity
surface wetting
contact angle
What is viscosity
§ Must be able to flow readily
□ Viscosity is a measure of a material’s ability to flow
□ It determines a material’s potential for making close contact with hard/soft tissue surfaces and so how well it records surface detail
□ It ranges from low to high
What is surface wetting
§ Must make intimate contact with the teeth and mucosa
§ The wettability depends on if the impression material is hydrophilic as the tooth is covered in saliva
What is the contact angle
§ We want the impression material to make a small contact angle with the tooth
§ A large contact angle at the surface results in spaces between globules of impression material, so some tooth surfaces are not replicated
§ A small contact angle means no spaces between globules of impression material so all surfaces are replicated
What is the accuracy dependent on
surface reproduction
visco-elasticity /elastic recover
What is disco-elasticity/elastic recovery
§ When a load is applied, it will gradually reach the strain required and when the load is released the strain level gradually drops but the material does not return to its original dimension but instead experiences permanent deformation / strain
§ However if load time is less and the impression material is removed with a sharp pull, there is less permanent strain and so lower deformation
§ Viscoelastic behaviour occurs when after being stretched or compressed, a material fails to return to tis original dimensions/shape
§ The studies also show that the elasticity increases at a certain point which is after the flow has set so its good to wait a little bit after the material is set to make use of this increased elasticity
What characteristics are for dealing with removal & undercuts
flow under pressure (shark fin test)
tear/tensile strength
rigidity
Why is the flow under pressure important
§ To record an undercut the impression material must reach the extremities of what is a narrow zone with a complex shape between the gingiva and tooth surface
What is the shark fin test
§ There is a cylindrical chamber with a slot of specified depth and the impression material is inserted in the upper part of the cylindrical chamber and so therefore must have a depth greater than that of the slot and is forced downwards so pressure has to be applied
§ If it has a large fin length then it has a high flow and will flow readily into the sulcus and undercuts
§ If it has a short fin length then it has a low flow
What is the tear/tensile strength
§ This is the stress the material can withstand before fracturing
§ Important so material can withstand large stresses during removal
What is the rigidity
§ Stress/strain ratio - stress needed to cause material to change shape
§ Ideally should have a low value for ease of removal and especially from undercut regions
What are the characteristics that determine dimensional stability
setting shrinkage
thermal expansion/contraction
storage
What is the ideal setting shrinkage
should be low
What is the ideal thermal expansion/contraction
§ If there is a large difference between mouth and room temp it may cause a change in shape
§ Therefore the ppm/C should be low
What is the ideal storage
§ Some materials absorb/release moisture causing a change in its dimensions such as alginate but this does not apply to polyethers or additional silicones
Compare polyethers and additional silicones
- Setting time - polyethers quicker
- Working time - greater for additional silicones
- Elastic recovery - better for additional silicones
- Shark fin length - greater for polyether
- Tear strength - greater for additional silicones
What does viscosity influence and what is the ideal
flow across surface
low
What does contact angle influence and what is the ideal
engagement with tooth surface and surface quality
low contact angle and high engagement with tooth surface
What does viscoelasticity influence and what is the ideal
deformation on removal
low
What does stiffness influence and what is the ideal
ease of removal from undercut
low
What does the thermal expansion influence and what is the ideal
contraction - mouth room temp
low
What does polymerization shrinkage influence and what is the ideal
contraction during setting
low
What does tear resistance influence and what is the ideal
ability to withstand large stresses e.g during removal
high but not too high
