Impression Materials Flashcards
What is the function of impression materials?
To produce an accurate replica of the surface and shape of hard and soft oral tissues
What is an impression?
A negative replica of the hard and soft tissues
What is another name for dental stone?
Gypsum
Dental stone is used to fill an impression and produce a positive replica. What does this represent?
The patients mouth:
- represents the position, shape, size and orientation of each tooth
What is a positive replica used for? (2 points)
- Used for evaluating dentition when orthodontic, occlusal or other problems are involved
- In laboratory fabrication of restorations and prostheses
What can you take an impression of? (3 points)
- Single tooth
- Whole dentition
- Edentulous mouth
Treatment outcome hinges on the quality and accuracy of the initial impression. What does this require? (2 points)
- IM with the right properties
- Clinician with a good technique
What can IM be classified into in relation to clinical properties? (2 points)
- Mucostatic
- Mucocompressive
What can IM be classified into in relation to properties? (2 points)
- Elastic
- Non-elastic
What is meant by the term ‘mucostatic’?
- Fluid materials that displace the soft tissues slightly (i.e. give an impression of the undisplaced mucosa)
- Displace the soft tissues slightly
What are 2 examples of mucostatic IM’s?
- Zinc oxide eugenol
- Low viscosity alginates
What are 2 examples of mucocompressive IM’s?
- Impression compound
- High viscosity alginates/elastonomers
What is a mucocompressive IM?
- A viscous material that records an impression of the mucosa under load (i.e. give impression of displaced soft tissue)
- Displaces the soft tissue much more
What is the ideal elastic behaviour of an IM?
Once the impression material has undergone strain it should snap back to its original shape after the strain has been removed
- This is what should happen if the material is truly elastic
None of the IM’s are truly elastic so they are not a true representation of the patients dentition. What are two way we can minimise the permanent strain?
- By waiting for a time after removing the tray before the cast is poured
- If the impression is removed with a sharp pull, there is less overall permanent strain (lower deformation)
What are 2 examples of hydrocolloids?
- Agar (reversible)
- Alginate (irreversible)
What are 3 examples of elastomers?
- Polysulphides
- Polyethers
- Silicones (conventional & addition cured)
Are impression materials perfectly elastic?
- No
What are the 2 possible outcomes for impression materials that are non-elastic?
- Deformed
- Fractured
What is the correct process for ‘preparation’ of impression materials? (4 points)
- Mixing (2 pastes; paste & liquid, powder & liquid etc)
- Chemical reaction
- Heat
- Material flows & takes up shape/space available
Do some impression materials require a lining?
- Yes - clinical teacher will help with this
What are the possible setting reactions of IM’s? (3 points)
- Chemical
- Polymerisation
- Cooling
(forms solid replica)
What is included in the impression taking process?
- Preparation
- Insertion (to mouth )
- Setting
- Removal
- Storage
- Cast impression
One ideal property of an IM is accuracy. How does the flow of an IM affect this?
- Surface detail is recorded
- Needs to flow over the dentition so it can make intimate contact with the surface to record surface detail
One ideal property of an IM is accuracy. How does setting changes of an IM affect this?
- Want NO dimensional change
- Don’t want it to shrink or expand
One ideal property of an IM is accuracy. What are the ideal properties in relation to removal of the IM from the mouth? (3 points)
Want:
- NO effect on oral tissues
- NO change dimensionally (ie. no thermal contraction)
- Complete ELASTIC RECOVERY
One ideal property of an IM is accuracy. What are the ideal properties in relation to storage of the IM? (1 points)
Does not change dimensionally over time
One ideal property of an IM is accuracy. What are the ideal properties in relation to decontamination of the IM? (1 points)
Dimensions and surfaces unaltered
Does an impression material have to be compatible with the cast material?
- Yes
One ideal property of an IM is accuracy. What are the ideal properties in relation to viscosity of the IM?
Determines the ability to flow over surface & so level of detail required
One ideal property of an IM is accuracy. What are the ideal properties in relation to the setting mechanism of the IM?
Does physical or chemical processes cause dimensional contraction?
One ideal property of an IM is accuracy. What are the ideal properties in relation to the thermal expansion coefficient of the IM?
Ideally low (to cope with mouth to room temperature transition)
One ideal property of an IM is accuracy. What are the ideal properties in relation to hydrophilic/hydrophobic properties of the IM?
- Affects surface contact
- Do you need to remove saliva or will the impression material still make good contact with the tooth surface where there are remnants of saliva
One ideal property of an IM is accuracy. What are the ideal properties in relation to the elasticity of the IM?
elastic/viscoelastic behaviour
One ideal property of an IM is accuracy. What are the ideal properties in relation to the tear strength of the IM?
- Needs to withstand tearing when removed esp. from undercuts
- when pour IM into tray in the mouth if there are undercuts then the IM will flow into these regions - when remove impression tray they will be vulnerable to tearing
What are the ideal properties for an IM for patient comfort? (4 points)
- Non-toxic, non-irritant
- Acceptable taste and smell
- Short setting time
- Removable without damage to oral tissues
What are the ideal properties of an IM for operator convenience? (4 points)
- Quick, simple technique
- Convenient working/setting times
- Must be able to be decontaminated
- Cost (inexpensive) is a consideration BUT not a property
What are 2 examples of non-elastic IM’s?
- Impression comound
- Impression paste
What are 2 examples of elastic impression materials?
- Hydrocolloids
- Elastomers
What is a hydrocolloid? (2 points)
- A COLLOID is a two phase system of fine particles of one phase dispersed in another phase
- If the dispersing medium (liquid) is water then this will produce a HYDROCOLLOID
What are the 2 possible states of a hydrocolloid?
- SOL = viscous liquid state
- Gel = jelly like consistency (when sol undergoes reaction it becomes a gel)
What is an example of an irreversible hydrocolloid?
Alginate
What is the composition of alginate?
- Salt of alginic acid
- Calcium sulphate
- Trisodium phosphate
- Filler
- Modifiers (flavourings, chemical indicators)
What is the % of salt of alginic acid in alginate?
12%
What is the % of Calcium Sulphate in alginate?
12%
What is the % of salt of trisodium phosphate in alginate?
2%
What is the % of salt of filler in alginate?
70%
What is the % of salt of modifiers in alginate?
small amount
What is the function of salt of alginic acid in alginate?
Reacts with Ca ions (that have reacted with trisodium phosphate)
What is the function of calcium sulphate in alginate?
Provides calcium ions
What is the function of trisodium phosphate in alginate?
Delayes gel formation
What is the function of filler in alginate?
Cohesion, strength
What is the function of modifiers in alginate?
improves surface, taste, pH colour change
What is the setting reaction of alginate?
- 2NanAlg +nCaSO4 -> nNa2SO4 + CanAlg
- Cross-linked fibril entangling undissolved particles
Why, even when the material seems set, should you give it an extra few minutes before you remove the IM from the mouth?
- The cross-linking is still going on so don’t remove immediately
What temperature of water should be used with alginate?
Between 18-24 degrees Celsius
What does increased temperature of water do to alginate?
- Speeds up the setting
- Sets nearest tissue first
What is permanent distortion and tearing of alginate reduced by?
Reduced by using a large bulk of material - typically 5mm
What are properties of alginate that are ideal for patient comfort? (3 points)
- Non-toxic, non-irritant
- Acceptable taste, smell
- Setting time OK
What are properties of alginate that make it convenient for the operator?
- Relatively easy to use
- Setting time OK
- Storage (avoid moisture, prepare cast ASAP to avoid dimensional change)
What does syneresis mean?
Release of water
What does imbibition mean?
- Uptake of water
How is the flow of alginate?
- fine detail - OK
How are the setting changes of alginate?
OK
How is the accuracy of alginate if the patient moves while it is setting?
- This causes internal stresses and distortion
How are the elastic properties of alginate?
Nearly elastic - not 100% elasic recovery
How is the tear strength of alginate?
- Poor (avoid deep undercuts)
How is the storage property of alginate?
- Syneresis or imbibition may affect dimensions and accuracy
