Impression materials Flashcards
Definition of Impressions [1]
Impressions are negative reproductions of dental structures
Requirements of an impression material [10]
- Capable for plastic to rigid transformation
- Acceptable for patient (Non-toxic,-irritant, setting time up to 5-7 minutes)
- Good handling properties (Easy to mix, thixotropic, adequate working and setting time)
- Accuracy and details production (Ability to replicate the intraoral surface details)
- Dimensionally stable (Ability to retain its absolute dimensional size over time)
- Resistance to mechanical stress (Elastic and rigid)
- Tear resistance ( ability to resist tearing in thin sections such as through the feather-edged material within the gingival sulcus)
- Compatible with model materials
- Resistance to disinfectants
- Cost effective
Classification of impression material [2]
- Rigid/inelastic (Old, not important)
2. Flexible/elastic (Elastomers & Hydrocolloids)
When are rigid/inelastic materials used? [1]
What is their problem? [1]
Used when there are no undercuts and for edentulous patients
Problem is any significant deformation produces a permanent deformation
What are the four rigid/inelastic materials?
- Impression compound
- Impression plaster
- Zinc oxide eugenol
- Impression waxes
What are impression compound made of? [3]
- resins (wax),
- filler (soapstone)
- lubricants (stearic acid)
Properties of impression compound [9]
- Poor surface detail
- High coefficient of thermal expansion (contracts up to 0.3% when removed from mouth to room temp.)
- Distorts when removed over undercut areas
- Mucodisplacive
- Poor dimensional stability
- Can be modified by re-heating
- Non-toxic and non-irritant
8 Good shelf life
Advantages of impression compound? [5]
Disadvantages of impression compound? [3]
Ad: o Non-toxic/ irritate o Reusable o Can be reheat and re-adapted o Support other materials for wash impressions o Mucocompressive
Disad:
o Poor surface details
o Poor dimensional stability
o Will distort if remove from undercuts
What is impression plaster’s composition? [1]
What does it mix with to set? [1]
What does it become when set? [1]
Calcium sulphate hemihydrate
Mix with water (Often adds anti-expansion solution)
Becomes dihydrate
Properties of impression plaster [8]
- Excellent at recording fine detail (because very fluid when inserted in the mouth)
- Dimensionally stable if anti-expansion solution is used
- Fractures if undercuts are present
- Mucostatic
- Needs to be treated with a separating medium (e.g. soap/varnish) before being cast in stone or plaster
- Exothermic setting reaction
- On storage dimensionally stable but a small amount of shrinkage may occur
- Non-toxic but may be unpleasant due to dryness and heat evolved during setting (exothermic)
Advantages of impression plaster? [4]
Disadvantages of impression plaster? [3]
Ad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
Disad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
What is ZOE composed of? [3+3]
Base paste:
- Zinc oxide
- Inert oils (plasticiser)
- Hydrogenated resins(increases setting time and improves cohesion)
Reactor paste
- Eugenol
- Zinc acetate (accelerator)
- Fillers (talc or kaolin)
Properties of ZOE [5]
- Recording edentulous ridges in a close fitting special tray or the patients existing dentures
- Mixed in a 1:1 paste ratio
- Used in thin sections only (2-3mm) as a wash impression
- Vaseline is used as a separating agent on those areas requiring protection (soft tissues, teeth, etc)
- Firm pressure is used during impression taking
Problem of ZOE? [1]
The set material contains both some unreacted zinc oxide and eugenol. Therefore any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression
What does ZOE setting time depend on? [3]
What is the normal setting time for ZOE? [1]
- Accelerator additives (Zinc acetate/acetic acid)
- Exposure to moisture on mixing or the addition of water will accelerate the reaction
- Increasing temperature causes a fasting setting time
Setting time is normally 4-5 minutes
Advantages of ZOE? [3]
Disadvantages of ZOE? [3]
Ad:
o Good dimensional stability
o Good surface detail
o Mucostatic/ Mucodisplacive
Disad:
o Only sets quickly in thin section
o Eugenol allergy
o Cannot be used in deep undercuts
What is the composition of impression waxes? [2]
What is the ratio? [1]
- Low melting paraffin wax and beeswax
- 3:1 ratio
When are impression waxes used? [1]
What precaution should one take after taking the impression? [1]
Used to correct small imperfections (Airblows) in other impressions such as ZOE impressions
Cast should be poured immediately (avoid distortion)
What are flexible elastic materials? [1]
When are they used? [2]
These materials can be stretched and bent to a fairly large degree without suffering any deformation
- They are used for recording the patient’s mouth where undercuts are present.
- For RPD, overdentures, implants, crowns and bridge work.
2 types of flexible elastic materials? [2]
- Elastomers
2. Hydrocolloids
What is sol state? [1]
Relatively fluidic state when the solute particles present are dispersed thoughout the medium (usually liquid)
What is gel state? [1]
Viscous and jelly-like state when the particles become attached to each other, forming a loose network with the medium which restricts movement of the solute molcules.
What are the phases between sol, gel and sol-gel states? [3]
Before gel point (sol) –>fluid
At gel point –> loss of fluidity
After gel point –> gel
When are elastomers used? [3]
What are the 2 main properties over hydrocolloid? [2]
- Used when high degree of accuracy is needed
- Bridge
- Crown
Ad:
- Good tear resistance
- Dimensional stability
Main composition of elastomers [2]
One of them contains a few other things
- Flexible matrix
- Multifunctional prepolymer, oligomer or polymer (not from monomer)
- Crosslinking agent
- Curing agent (‘catalyst or initiator)
- Modifiers (accelerators, retarders, plasticizers, flavoring agents, colorants) - Filler/extender
What are the four main groups of elastomers? [4]
- Polysulphide
- Polyether
- Polysiloxane (silicone rubber: Condensation and addition silicone)
- ‘Hybrid’