Impression Materials Flashcards
Indirect restorations definition
- Restoration of teeth with rigid materials
- Materials being rigid are fabricated outside the mouth
- Often in labs
- Model replica of the tooth/teeth needs to be first constructed
How are indirect restorations made
- Record an accurate impression
- Casting up
- Investment of the pattern to a mould (lost wax technique)
- Construction of an appliance
How has CAD-CAM changed the game in terms of production of indirect restorations
-No need for that impression stage
When is an indirect restoration indicated
- Significantly less natural tooth remaining
- Need for a more rigid material/restoration
- Greater morphological control
- Allows use of machined or pressed ceramics
- Higher degrees of polymerisation
How does the DC change when curing a composite directly/indirectly and why
- Greater DC when indirect curing
- Can achieve high temperatures and higher pressures when out of the mouth
- Much higher degree of monomer conversions
- These temps and pressures would not be tolerated intra-orally
- Only light curing
Importance of accurate impressions
- Vital that no distortion occurs in the successive stages
- Small errors in impressions get worse over the steps to come
- Amplifies mistakes
- Accurate model of the prepared tooth required
- Any defects or errors will get carried in the subsequent steps translating into an unsuccessful indirect restoration
Properties of materials for indirect restorations
Examples of materials
Rigid Materials
-High modulus of elasticity that can resist high stress without permanent distortion. Many behave as a brittle material
Ceramics:
-Rigid and brittle
Pure gold:
-Ductile
Prefabricated composites:
- Improved properties
- Control on shrinkage
The role of impression materials and it what time of dental appliances would you take an impression
- Record and reproduce the form and relationship of teeth and oral tissues (hard and soft tissues)
- Record accurate impressions for appliances constructed outside the mouth
- Dentures
- Crowns
- Inlays
- Onlays
- Bridges
- Orthodontic appliances
-All require models of the hard and soft tissues
Role and types of impression tray
Stock Trays:
- Metals (reusable)
- Polystyrene
- Nylon (disposable)
Custom Trays:
-Disposable
-Impression materials are transferred to the mouth in impression trays
Why are trays perforated
- One part of the impression material will touch the tray whereas another part of the tray should touch patients hard/soft tissues
- No movement should be permitted between the tray and impression material
- Placement of adhesive/perforations in the stock trays
- Once the impression is set in the tray, it should not move
- Or it would be ruined
Classification of Impression Materials and examples of each
Elastic Materials:
- Synthetic elastomeres
eg. Silicones, polyethers - Hydrocolloids (natural)
eg. Agar, Alginates
Non-Elastic Materials:
- Impression plaster
- eg. Impression compound, zinc oxide pastes and impression waxes
Ideal Requirements of Impression materials
- Accurate
- Rheological properties (flowability)
- Dimensional stability (short and long term)
- Adequate thermal properties
- Adherence to the tray
- Disinfection
- compatibility with the model and die materials
Physical and Mechanical Properties to be aware of in Impression Materials
Flow Materials (Rheology)
Wettability
Working and setting time
Mechanical Properties (compressive and tear strength)
Working Humidity
Dimensional Accuracy
Stability
Consistency
Biocompatibility
Compatibility with die and cast materials
Shrinkage
Definition of accuracy and how achieved
- Ability to replicate the intraoral surface details
- Accurate recording of the impression of the oral tissues is very important
- 100% elastic materials do not exist, so 99% viscoelastic materials will have to do
- Material should have good flow characteristics and be elastic in nature
- Viscoelastic materials undergo some flow but most will recover
How does an impression material set
- Usually by chemical change
- Some undergo a physical change of state
Rheology and how measured
- Study of flow or the deformation of materials
- Involves the measurement of viscosity and its variation with factors such as shear rate and time
- Viscosity= Shear stress/Shear rate
-Stress Applied to the liquid/How fast you apply it
Time Dependent Viscosity importance
Ideal Graph Line time v viscosity
-You don’t want a change of viscosity right away because during the mixing and placing on the tray, you stil want it to be runny
- Look at graph on page
- Ideally you want one that gives you a good working time (no change in viscosity for a bit) then a sudden, sharp set
- Low viscosity important so it can flow over the oral tissues
Different types of viscositys of pastes and issues/pros of each
High Viscosity
May not flow over undercuts
Provides a body for impression tho
Med Viscosity
- Flow improves
- May not be adequate for very accurate impressions
Low Viscosity
- Good flow
- Should be moderate tho but it may drip
Combo can be used
- High viscosity placed on the tray, with lower viscosity placed on the tissues
- Lower viscosity gives you good impression
- Higher viscosity supports lower viscosity and prevents drip
Process of taking an impression of a tooth if using a combo of high and low viscosity
How is shrinkage managed
- Tray loaded with heavy body impression material
- Crown prep covered with light bodied material
- Although light body will shrink more, the heavy body will be on top and control the shrinkage
Affects of shrinkage during setting
What is the clinical result
- During setting shrinkage, the distortion of the impression is towards the tray
- Dies tend to be oversized and thus casting may be oversized
Strain rate sensitive elastomeres significance
- Distortion is minimal if you do a snap removal
- Fast strain rate
- If you do a slow removal, there is a lot more distortion occuring
Look at stress strain curves
Importance of wettability in an impression material
- Need to be able to flow over the saliva
- If hydrophobic, they will be repelled by moisture blow holes
- If hydrophilic, they have a good flow but may swell on storage (take up moisture from the environment)
- Hydrophilic more suited to oral environment
- But absorb moisture and undergo dimensional changes on storage
- Need a balance
Working/setting time for impressions
- Working time should be adequate to allow proper mixing and dispensation
- Setting time should be short to prevent patient fatigue
-Temperature of mouth assists a more rapid set
Dimensional stability in the mouth
- Uniform thickness of impression material required
- Allows equal dimensional change during setting
- Thickness of 2-4mm
- Too thick, increased polymerisation shrinkage
- Too thin, increased % strain during removal and more viscoelastic flow possible
- Possible tears
