Impression materials - properties Flashcards
What is the main indication for an indirect restoration?
A tooth with little tooth tissue remaining.
What are the benefits of making an indirect restoration rather than a direct one?
- They shrink less
- Greater tooth morphology
- More degrees of polymerisation (less likely to have shrinkage in mouth)
Why is it important that the impressions we take are accurate?
To reduce the distortion that occurs when constructing the appliance.
Ensures accuracy and fit of appliance.
What material is commonly used for indirect restorations?
Ceramics (rigid with high modulus of elasticity)
What are the two types of trays that are used when imrpession taking?
Stock trays
Custom trays
What two things can we do to increase the adhesion of the material to the tray?
- Trays with perforations
- Adhesives used
What are the two classifications of impression materials?
1) Elastics
2) Non elastic materials
Why do we want impression materials to flow?
So that they can capture the hard and soft tissues.
What two factors does viscosity change with?
- Shear rate
- Time
What is the relationship between shear stress and shear rate in Neutonon materia,s?
Shear stress is proportional to shear rate.
Shear stress = K(shear rate)^n
If n is >1, what happens to the material with increased shear stress?
Shear rate will increase (material undergoes shear thinning).
This means the viscosity decreases with increased shear rate.
Why is it not good to use either a high or a low viscosity material?
High = may not flow over undercuts
Low = will drip over oral cavity and can lead to gagging
Why do we need to balance the hydrophilic and hydrophobic components of an impression material?
Too hydrophilic = too much water uptake which can lead to distortion from swelling
Too hydrophobic = with have limited flow in areas with lots of saliva leading to blow holes and inaccurate impression
Will an elastic or a plastic material be better at accurately replicating the oral tissues?
Elastic as it has the least distortion of removal.
What method can we use to keep shrinkage to a minimum?
Place a material with a high filler content into the tray and cover with a material of low filler content.
What action do we use when removing the trays to ensure elastic recovery and least distortion?
Snap action!
Ideally long or short -
Working time?
Setting time?
long
short
Why do we need to make sure we are applying the tray with uniform thickness of impression material?
Too thick = increase in polymerisation shrinkage
Too thin = strain will lead to deformation
What is dimensional stability and what factors affect stability?
Dimension stability - the ability of the impression material to not expand or contract on storage.
- Continued setting once out of mouth
- Hydrophobic/hydrophilic materials
- Distortions due to stress
- Slow elastic recovery
- Uptake of moisture
What thermal property do we want the impression material to have?
High coefficient of thermal expansion.
Why is it important that impression materials have a high tear strength?
This ensures no impression material is left in the PDL as it can lead to a severe reaction.
What factors do we need to take account of when considering biocompatability?
- No irritation to oral mucosa
- Non toxic
- No cariogenic components present
What are the two main ways of mixing an impression materials?
- By hand
- In a machine
Why do we ideally want to use a Newtonian material?
No shear thinning or thickening will occur.
When a brand comes in so sell us an impression material, what properties do we look at?
- Setting reaction
- Setting time
- Moisture effect
- Type of material
- Viscosity
- Type of tray needed
- Technique needed
- Cost
- How to disinfect the tray
Why is plaster of paris a material not recommended?
It has low strength and can fracture easily if there are any undercuts.
Plastic material.
