40 Questions
What is the primary function of the special water cooled perforated tray?
To circulate cold water at 20°C
What is the advantage of using this material in terms of dimensional stability?
It is dimensionally unstable due to syneresis and imbibition
What is the purpose of heating the gel to 100°C?
To create a reversible hydrocolloid
What is the composition of the gel?
85% water and 15% agar
What is the elasticity of the material?
It is viscoelastic with 98.8% elastic recovery
What is the purpose of the syringe and tray?
To manipulate and set the reaction
What type of impression is Plaster of Paris classified as?
Non-elastic impression
What is the disadvantage of the material in terms of storage?
It is dimensionally unstable due to syneresis and imbibition
What is the application of alginate impression material?
Primary impression
Which impression material is hydrophobic?
Polysulfide and addition silicone
What type of impression is classified according to the mode of setting?
Thermoset and Thermoplastic
What is the main characteristic of Non-elastic impressions?
They fracture or are permanently deformed on removal
Which impression material is used for edentulous patients without bony undercut?
Plaster of Paris
What is the primary application of impression plaster?
To make secondary impressions for edentulous patients without bony undercut
What type of impression is classified according to the interaction of the material with water and moisture?
Hydrophobic and Hydrophilic
Which impression material is an example of Thermoplastic impression?
Impression compound
What is the composition of impression plaster?
β Ca SO₄.½ H₂O and water
What is the main characteristic of Elastic impressions?
They recover their original shape after removal
What is the setting time for impression plaster?
5 minutes
What is a disadvantage of using impression plaster?
It absorbs water, causing dry mouth sensation
Why is a separating medium required when using impression plaster?
To separate the plaster from the stone cast
What is the dimensional stability of impression plaster?
0.06% change on setting
What type of impression material is impression compound?
Rigid, reversible impression material
What is a characteristic of impression compound?
It has thermoplastic properties
What is the process that occurs when a low Mw polymer is mixed with a suitable reactor at room temperature?
Polymerization
What is the main difference between hydrophilic and hydrophobic impressions?
Their wetting properties to soft tissues and gypsum material
Which type of elastomer can be electroplated?
All of the above except polyether
What is an advantage of elastomers over hydrocolloids?
Higher dimensional stability in storage
What is the application of polysulfide impression materials?
Secondary full mouth, quadrant, or single impressions
What is the result of polymerization of a low Mw polymer?
Increase in molecular weight
What is a characteristic of hydrophobic impressions?
Decreased wetting to soft tissues and gypsum material
What is a type of elastomer?
Polysulfide
What is the primary function of an impression material?
To create a replica of the oral tissues
What is the purpose of using an impression tray?
To carry the impression in its plastic form
What is the ideal property of an impression material in terms of its setting time?
It should have a suitable setting time
What is the purpose of step 7 in the impression process?
To disinfect the impression
What is the term for the areas under the inclined planes?
Under cut
What is the purpose of using a vibrator during impression mixing?
To remove air bubbles from the mixture
What is the ideal property of an impression material in terms of its mechanical properties?
It should have acceptable strength to withstand forces
What is the step that follows impression disinfection in the impression process?
Mixing gypsum materials
Study Notes
Impression Materials
- Impression material is used to record the shape and dimensions of oral tissues.
Impression Trays
- Impression trays are used to carry the impression material in its plastic form to be inserted into the patient's mouth and removed after setting.
Steps of Impression Taking
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- Tray selection
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- Impression mixing
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- Hand mixing and vibrating
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- Loading the impression into the tray
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- Insertion of the impression into the patient's mouth
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- Removal of the impression from the patient's mouth after setting
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- Impression disinfection
Ideal Properties of Impression Materials
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- Non-toxic, non-irritant, and insoluble in oral fluids
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- Pleasant odor, taste, and color
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- Suitable setting time
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- Good reproduction of fine details (good wetting of oral tissues)
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- Minimal dimensional changes on setting
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- Dimensional stability after setting
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- Compatibility with model and die materials
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- Adequate mechanical properties
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- Adequate shelf life
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- Reasonable cost
Classifications of Dental Impressions
- I. According to the mode of setting:
- Thermoset impressions (hardening by irreversible chemical reaction)
- Thermoplastic impressions (softening and hardening by reversible physical reaction)
- II. According to behavior after setting:
- Non-elastic (rigid) impressions
- Elastic impressions
- III. According to the interaction of the material with water and moisture:
- Hydrophobic impressions
- Hydrophilic impressions
Impression Materials
- Rigid impressions:
- Impression Plaster
- Impression Compound
- Zinc Oxide and Eugenol (ZnO/E)
- Elastic impressions:
- Hydrocolloids
- Elastomers + Polysulfide + Condensation silicone + Addition silicone + Polyether
Rigid Impression Materials
- Definition: Materials that cannot record undercut areas
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- Impression Plaster
- Used for edentulous patients without bony undercuts
- Used for occlusal bite registration
- Has good dimensional stability (0.06% change on setting)
- Requires a separating medium
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- Impression Compound
- Definition: Rigid impression material that sets by a physical reaction
- Composition: Natural resin and waxes
- Presentation: Gel in sealed tubes or jar
- Manipulation and setting reaction: Reversible hydrocolloid, physical reaction
- Has thermoplastic properties
Hydrocolloids
- Definition: Impression material that sets by a physical reaction
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- Agar
- Composition: Natural polymer extract from seaweeds
- Presentation: Powder to be mixed with water
- Manipulation and setting reaction: Reversible hydrocolloid, physical reaction
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- Alginate
- Application: Primary impression
- Presentation: Powder to be mixed with water
- Composition: Natural marine plant
- Polymerization: Low Mw polymer + reactor → high Mw
Elastomers
- Definition: Rigid impression material that sets by a chemical reaction
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- Polysulfide
- Consistency: Light, Medium, Heavy, Putty
- Chemistry: Low Mw polymer + reactor → high Mw
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- Condensation silicone
- Consistency: Light, Medium, Heavy, Putty
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- Addition silicone
- Consistency: Light, Medium, Heavy, Putty
- Hydrophilic properties
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- Polyether
- Consistency: Light, Medium, Heavy
Advantages of Elastomers over Hydrocolloids
- Higher tear strength
- More dimensionally stable in storage
- Ability to be electroplated (except polyether)
- Compatible with model and die materials
Hydrophilic and Hydrophobic Impression Materials
- Hydrophilic: Proper wetting to the soft tissues and gypsum material
- Hydrophobic: Decreased wetting to the soft tissues and gypsum material, can be electroplated
Learn about impression materials used in dentistry to record the shape and dimensions of oral tissues. Understand the different types of impression trays and their uses.
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