Elastomeric Impression Materials Flashcards
What are the two main types of elastomers used in dentistry?
- polyethers
- addition silicones
Describe the elastic behaviour of impression materials
- 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
What must be considered when choosing which impression material to use?
- material characteristics
- degree of accuracy required
- patient acceptance
- taste
- smell
- ease of use
- working time
- setting time
How are elastomers formed?
polymerisation with cross-linking of polymer chains
What is the result of cross-linking during polymerisation
- generation of elastic properties
- fluid to solid transition
- production of byproducts (water, hydrogen, alcohol)
What happens to a material as a result of the production of byproducts?
dimensional stability and cast compatibility are affected
What are the 3 different types of elastomers?
- polysulphides (no longer used)
- silicones (addition curing and condensation curing)
- polyethers
What are the material properties which affect the accuracy in which surface details are captured?
- surface detail reproduction
- flow/viscosity
-contact angle/wettability
What are the material properties which affect the accuracy of the dimension and shape captured?
- elastic recovery
- stiffness/flexibility
- tear strength
What are the material properties involved in practical considerations
- mixing time
- working time
What is the shore A hardness test?
a specific hardness test for impression materials
What is the shark fin test?
a test determining the ability of an impression material to capture undercuts
Why is the thermal expansion coefficient important for impression materials?
due to the temperature gradient between the oral cavity and room temperature
What are the two ways in which twin form impression materials are delivered?
- twin cartridge
- base paste and catalyst paste
- syringe gun pushes through mixing tip
- homogenous tip delivered to impression tray
- putty
- catalyst paste and base paste
- mixed until one even colour
What are the 4 stages of impression material use that have different ideal properties?
- quality of surface interaction between material and tooth and soft tissues
- accuracy
- dealing with removal and undercuts
- dimensional stability
What are the ideal properties for the quality of surface interaction between the material and tooth and soft tissue surfaces?
- viscosity
- surface wetting
- contact angle
What are the ideal properties for accuracy?
- surface reproduction
- visco-elasticity/elastic recovery
What are the ideal properties for dealing with removal and undercuts?
- flow under pressure
- tear/tensile strength
- rigidity
What are the ideal properties for dimensional stability?
- setting shrinkage
- thermal expansion/contraction
-storage
Why is viscosity important for the quality of surface interaction between the material and oral tissues
the material must be able to flow readily without requiring too much pressure applied
Why is surface wetting important for the quality of surface interaction between the material and oral tissues
the material must make intimate contact with the oral tissues
Why is contact angle important for the quality of surface interaction between the material and oral tissues
determines hoe well the material envelops the oral tissues to record fine detail
Should the wetting angle of an impression material be large or small?
small contact angle
- results in a larger percentage of the volume making contact with the target surface
What happens as a result of larger contact angles?
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.
What happens as a result of a smaller contact angle?
a small contact angle results in no spaces between the globules of impression material so all of the surface is replicated
What is the additional component in hydrophilic silicons and what properties does it change?
non-ionic surfactant component
- wets tooth surface
- more easily wetted by water containing die materials
Why is surface reproduction important for accuracy of an impression material?
the more of a surface that can be captured, the more accurate the impression
Discuss ISO4823 in relation to surface reproduction
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
What is the ideal elastic behaviour of an impression material?
- load applied and material immediately stretches to the strain required
- level of strain maintained until the load is removed
- material returns to original dimensions
What is the realistic visco-elastic effect of impression materials?
- 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
What clinical aspect can influence the elastic recovery of a material?
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
Discuss the relationship between flow and elasticity when an impression is being taken
- 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
What is the shark fin test?
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
Why is the shark fin test useful?
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
Why is tear strength important for dealing with removal and undercuts?
the material must have large tear strength to withstand the large stresses applied during removal
Why is rigidity important for dealing with undercuts and removal
the material should be flexible (e.g. low rigidity) for ease of removal of the material, especially from undercut regions
Why is setting shrinkage important for dimensional stability?
the material should have a low setting shrinkage so it maintains the shape of the tissue being replicated
Why is thermal expansion/contraction important for dimensional stability?
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
Why is storage important for dimensional stability?
some materials experience a dimensional change during storage
- polyethers and addition silicones do not
Do polyethers or addition silicones set faster?
Polyethers set slightly faster than addition silicones
Do polyethers or silicones have longer working lengths?
Addition silicones have double the working time than polyethers
What is the 5 stage decision making approach that should be used for assessing materials?
- Know the key material properties
- be aware of sales techniques - Review product specification
- compare to rival products - Know typical values expected for specific properties
- Identify properties not mentioned
- Reject claims not supported with scientific/clinical data
Describe the decisions that should be made from the evidence available
Purchase - evidence shows new material is better
No advantage - new material as good as current
Review later - insufficient evidence
Reject - unconvincing data
