Impression Materials/Corrosion

Question 1

List 5 basic requirements of impression materials

Answer 1

• Accuracy and reproducibility
• Elasticity and flexibility
• Dimensional stability
• Easy handling
• Costs

Question 2

What is alginate?

Answer 2

A non-reversible hydrocolloid

Hydrocolloids are made up of extracts from marine algae polysaccharides.

Question 3

Are alginates visco-elastic?

Answer 3

Yes. That’s why they need to be fully set before removing.

Question 4

Tips to prevent distortion of alginate:

Answer 4

Allow 5 minutes of visco-elastic recovery during disinfection, then pour within 20 minutes to minimize distortion.

Question 5

What are the four types of elastomeric impression materials?

Answer 5

• Polysulfide
• Addition cured silicone
• Condensation silicone
• Poly-ether

Question 6

2 keys advantages of elastomeric impression materials over hydrocolloids

Answer 6

• Dimenstional stability
• Better tear strength and resistence

Question 7

Advantages and Disadvantages of Poly-sulfides

Answer 7

Good-

• Long working time
• Accurate

Bad-

• Needs to be poured ASAP for accuracy
• Messy

Question 8

Advantages and Disadvantages of ADdition cured silicone (PVS)

Answer 8

Good-

• Shorter setting time
• Good tear strength
• Extremely accurate
• Dimensionally stable

Bad-

• Hydrogen evoluation
• Latex no go
• Difficult to remove from deep undercuts

Question 9

Poly-ether materaisl advantages and disadvantages

Answer 9

Good-

• Accurate
• Tear resistance
• Hydrophilic

Bad-

• Stiff
• Bad taste
• Can’t tolerate moisture

Question 10

2 examples of retraction solutions

Answer 10

1. Aluminium hydrochloride
2. Aluminium sulfate based solution

Question 11

Solutions with epinephrine, 8-hydroxyquinoline sulphate or iron sulphate may impair setting of what material

Answer 11

Poly-ether

Question 12

Why do condensation silicone and polysulphide polymerization reactions result in the higher shrinkage

Answer 12

Reaction products

• Condentation silicone produces ethanol
• Polysulphide released water

Question 13

Do we want impression materials to be hydrophilic or hydropphobic

Answer 13

Hydrophilic = good wettability

Question 14

Two advantages of alginate over elastomeric

Answer 14

• Hydrophilic (elastomers are hydrophobic)
• Cheaper

Question 15

What filler material is usually included in elastomeric impression materials, and why

Answer 15

Colloidal silica

• Increases viscosity
• Increases dimensional stability
• Decreases setting contraction
• Decreases thermal contraction

Question 16

Why is it important to use elastomeric impression materials for crown and bridge impressions

Answer 16

High tear strength is needed for margin and undercut replication.

Question 17

What is corrosion?

Answer 17

The electro-chemical oxidation of metals in reaction with an oxidant such as oxygen

Question 18

5 reasons why corrosion is common in the oral cavity

Answer 18

• Moist
• pH fluctuation
• Temperature
• Oxygenated
• Abrasions of protective coatings

Question 19

How does corrosion occur?

Answer 19

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.

Question 20

Why is an open margin on an amalgam less of an issue that one on composite.

Answer 20

Amalgams are self sealing. Amalgams can produce corrosion products within themselves and these can seal the existing margin of a restoration.

Question 21

What is crevice corrosion

Answer 21

Occurs at material-tooth interface with O2 concentration difference. Can result in PFM smile, hence use a metal free system.

Question 22

List 4 ways corrosion can occur in the mouth

Answer 22

1. Electrochemical cell from Electrode potential difference eg gold crown next to chrome denture
2. Electrochemical cell from Oxygen concentration cell within a restoration
3. Self seal of amalgams
4. Crevice corrosion

Question 23

List 4 factors which contribute to the corrosion process

Answer 23

1. Compositional and microstructure factors
2. Stress
3. Surface roughness
4. Temperature

Question 24

3 ways to prevent corrosion in the dental setting

Answer 24

1. Choose higher electrode potential metals
2. Prevent galvanic couples
3. Prevent contact of oxidants

Question 25

2 ways of preventing corrosion of metals

Answer 25

1. Passivation (coat with metal oxide)
2. Electrocoating/polishing

Question 27

What is the active ingredient in alginate

Answer 27

Potassium salts

Question 28

What is syneresis?

Answer 28

Water expulsion

Question 30

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

Answer 30

a) Has a short setting time of 3-4.5 mins

Question 31

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

Answer 31

c)