Impression Materials/Corrosion Flashcards
List 5 basic requirements of impression materials
- Accuracy and reproducibility
- Elasticity and flexibility
- Dimensional stability
- Easy handling
- Costs
What is alginate?
A non-reversible hydrocolloid
Hydrocolloids are made up of extracts from marine algae polysaccharides.
Are alginates visco-elastic?
Yes. That’s why they need to be fully set before removing.
Tips to prevent distortion of alginate:
Allow 5 minutes of visco-elastic recovery during disinfection, then pour within 20 minutes to minimize distortion.
What are the four types of elastomeric impression materials?
- Polysulfide
- Addition cured silicone
- Condensation silicone
- Poly-ether
2 keys advantages of elastomeric impression materials over hydrocolloids
- Dimenstional stability
- Better tear strength and resistence
Advantages and Disadvantages of Poly-sulfides
Good-
- Long working time
- Accurate
Bad-
- Needs to be poured ASAP for accuracy
- Messy
Advantages and Disadvantages of ADdition cured silicone (PVS)
Good-
- Shorter setting time
- Good tear strength
- Extremely accurate
- Dimensionally stable
Bad-
- Hydrogen evoluation
- Latex no go
- Difficult to remove from deep undercuts
Poly-ether materaisl advantages and disadvantages
Good-
- Accurate
- Tear resistance
- Hydrophilic
Bad-
- Stiff
- Bad taste
- Can’t tolerate moisture
2 examples of retraction solutions
- Aluminium hydrochloride
- Aluminium sulfate based solution
Solutions with epinephrine, 8-hydroxyquinoline sulphate or iron sulphate may impair setting of what material
Poly-ether
Why do condensation silicone and polysulphide polymerization reactions result in the higher shrinkage
Reaction products
- Condentation silicone produces ethanol
- Polysulphide released water
Do we want impression materials to be hydrophilic or hydropphobic
Hydrophilic = good wettability
Two advantages of alginate over elastomeric
- Hydrophilic (elastomers are hydrophobic)
- Cheaper
What filler material is usually included in elastomeric impression materials, and why
Colloidal silica
- Increases viscosity
- Increases dimensional stability
- Decreases setting contraction
- Decreases thermal contraction
Why is it important to use elastomeric impression materials for crown and bridge impressions
High tear strength is needed for margin and undercut replication.
What is corrosion?
The electro-chemical oxidation of metals in reaction with an oxidant such as oxygen
5 reasons why corrosion is common in the oral cavity
- Moist
- pH fluctuation
- Temperature
- Oxygenated
- Abrasions of protective coatings
How does corrosion occur?
If you put two metals together with electrode potential differences, you get a classic battery effect which increases corrosion risk. Further apart on electrochemical chart, the more likely.
Sets up an electro-chemical cell with the two materials being the anode and cathode, saliva being the electrolyte and forming a circuit.
Why is an open margin on an amalgam less of an issue that one on composite.
Amalgams are self sealing. Amalgams can produce corrosion products within themselves and these can seal the existing margin of a restoration.
What is crevice corrosion
Occurs at material-tooth interface with O2 concentration difference. Can result in PFM smile, hence use a metal free system.
List 4 ways corrosion can occur in the mouth
- Electrochemical cell from Electrode potential difference eg gold crown next to chrome denture
- Electrochemical cell from Oxygen concentration cell within a restoration
- Self seal of amalgams
- Crevice corrosion
List 4 factors which contribute to the corrosion process
- Compositional and microstructure factors
- Stress
- Surface roughness
- Temperature
3 ways to prevent corrosion in the dental setting
- Choose higher electrode potential metals
- Prevent galvanic couples
- Prevent contact of oxidants
2 ways of preventing corrosion of metals
- Passivation (coat with metal oxide)
- Electrocoating/polishing
What is the active ingredient in alginate
Potassium salts
What is syneresis?
Water expulsion
Which is false regarding polyether
a) Has low, medium and high viscosity types
b) High tear strength
c) Good dimensional stability
d) Has a short working time of 1 minute
e) has a setting time of 3-4.5 minutes
a) Has a short setting time of 3-4.5 mins
Which is false regarding PVS:
a) Has low medium and high and putty viscosity range
b) Has a lower tear strength that polyether
c) Has a permanent deformation higher than polyether
d) Has good dimensional stability
e) May be disinfected in sodium hypochlorite
c)
