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
Dimensional Stability during storage
Factors that affect it
- Should not expand or contract, absorb moisture or dehydrate
- Minimum or no change in dimension should occur during storage of the impression
Affected by:
- Continued setting reaction
- Slow elastic recovery may continue after withdrawal of the impression
- Distortions due to recovery of internal stresses
- Loss of volatile components
- Uptake of moisture
Thermal Properties important
- Coefficient of thermal expansion
- Impression materials and stock tray should have low or similiar values
- Thermal contraction leads to inaccurate impressions
- Going from mouth temperature to room temperature
Tear Strength importance
- Resistance to teat when removing or handling it
- Thin sections should withstand the stresses generated during removal of impressions
- Tear strength should be high since external force is applied to remove the impressions
- Compresive strength must be adequate
- Important for sulvus reproduction and sulcus response
- Impression material can get stuck in the sulcus if it tears leading to periodontal reaction and ruined impression
-How to measure tear is on slides
Biocompatibility and other factors
- Acceptable taste and odour
- Non toxic
- In vitro cytoxicity testing
- Ames test for mutagenicity (in contact with oral tissues)
- Mucous membrane irritation (Hamsters pouch test)
- Sensitization test (animal models)
- Caricinogenicity
- Non-toxic
- Non irritant
- Long shelf life
- Cheap
Delivery systems for impression materials
Pad-Mix
Auto-mix- less bubbles produced in the impression
Syringes- Keep the diameter narrow to maximise pseudoplasticity effect
Bulk Fillers
Automixed reduce number of voids compared to pad mixing so auto mixed silicone impression materials are claimed to have excellent properties
Different types of proeprties on the shear stress shear strain rate curves
- Look at graph
- Pseudoplastic and Dilatant materials
- Dilatant- viscosity increases as you mix it . shear thickening
- Pseudoplastic- viscosity decreases as you mix it
Which kinds of materials should be mixed pads v automixing
Pad-Mix
-Dilatant and Newtonian fluids eg. silicone
Automix: Pseudoplastics eg. polyether and polyvinyl siloxane
When buying impression material it is important to consider:
- Type of material (chemical type and setting rxn)
- Viscosity of impression material for the tray
- Viscosity of the light-body wash material
- Technique used
- Cost
- Disinfection regime
Examples of non-elastic impression materials
- Plaster of Paris/Impression Plaster
- Impression compound
- Zinc oxide-eugenol
- Impression waxes
Impression Plaster and important to note
- B calcium sulphate hemihydrate
- Mixing with water yield rigid calcium sulphate dihydrate
- Expansion may occur on setting which can be reduced by the addition of potassium sulphate
- Potassium sulphate accelerates the rxn thus borax is added to retard the rate of reaction
- Can record fine details, initially has a low viscosity, small dimensional changes may occur however the material is very brittle
- **Treatment of impression plaster before casting a plaster model is cruicial otherwise impossible to separate the two **
Impression compound classification and definition
Thermoplastic in nature: Consists of resins such as colophony, shellac, talc as fillers and lubricants (stearic acid)
-Type I: Lower fusing materials
Available as thick sheet, can be used for an edentulous material
- Type II: Higher fusing materials
- Used as tray materials
Zinc oxide components and functions
-Consists of 2 pastes
Paste 1:
ZnO - Reactive ingredient
Olive/Linseed oil- Paste formation
Zinc acetate or water: Accelerators for setting
Paste 2:
Eugenol - Reactive
Talc or kaolin - unreactive filter
2 molecules of eugenol react with zinc oxide to yield zinc eugenolate
Pastes of 1mm thickness are used to minimize dimensional changes
Major problem is lack of elasticity
Set material has a tendency to distort or fracture when removed from undercuts
