EFDA Dental Materials & Equipment Maintenance 5 — Questions and Answers
Question 1: Which property of dental amalgam is described as the tendency to expand or contract under stress without fracturing?
- Compressive strength
- Creep (Correct answer)
- Hardness
- Tensile strength
Correct answer: Creep
Creep is the slow plastic deformation of amalgam under sustained compressive load, which can lead to marginal breakdown and ditching over time.
Question 2: Why must etchant (phosphoric acid) be completely rinsed from enamel before applying bonding agent?
- Acid residue inhibits light polymerization
- Residual acid continues to demineralize enamel and disrupts bonding chemistry (Correct answer)
- Acid darkens the composite resin color
- Rinsing is optional for self-etch systems only
Correct answer: Residual acid continues to demineralize enamel and disrupts bonding chemistry
Residual phosphoric acid must be thoroughly rinsed because it can continue etching and prevent proper resin infiltration into the enamel tubules.
Question 3: Which type of gypsum product is used to pour final working casts requiring high strength and hardness?
- Type I impression plaster
- Type II model plaster
- Type III dental stone
- Type IV die stone (Correct answer)
Correct answer: Type IV die stone
Type IV die stone (improved high-strength stone) has the highest compressive strength and lowest expansion, making it ideal for working casts and dies.
Question 4: What is the purpose of applying cavity varnish before placing zinc phosphate cement?
- Improve bonding of cement to dentin
- Seal dentinal tubules to reduce pulpal irritation from cement acidity (Correct answer)
- Increase the strength of the final restoration
- Act as a desensitizer for post-operative sensitivity
Correct answer: Seal dentinal tubules to reduce pulpal irritation from cement acidity
Cavity varnish seals dentinal tubules to protect the pulp from the low pH of freshly mixed zinc phosphate cement.
Question 5: When is biological monitoring (spore testing) of an autoclave required at minimum?
- Daily
- Weekly (Correct answer)
- Monthly
- After each load
Correct answer: Weekly
The CDC and OSAP recommend weekly spore testing of sterilizers to verify that sterilization conditions are being achieved.
Question 6: Which characteristic distinguishes polyvinyl siloxane (PVS) from polysulfide impression materials?
- PVS has a sulfur-based odor and lower dimensional stability
- PVS has superior dimensional stability and no offensive odor (Correct answer)
- PVS requires custom trays exclusively
- PVS has a longer working time than polysulfide
Correct answer: PVS has superior dimensional stability and no offensive odor
PVS (addition silicone) offers excellent dimensional stability and no byproducts, while polysulfide releases hydrogen byproduct and has a characteristic sulfur odor.
Question 7: What does the term 'film thickness' refer to when selecting a luting cement?
- The depth of the preparation the cement can fill
- The minimum thickness of the set cement layer between the restoration and tooth (Correct answer)
- The time required for the cement to reach full strength
- The amount of cement that flows out of the crown margin
Correct answer: The minimum thickness of the set cement layer between the restoration and tooth
Film thickness is the minimum thickness of cement between two parallel surfaces under load, and a low film thickness is critical to ensure complete seating of cast restorations.
EFDA Dental Materials & Equipment Maintenance 5
Which property of dental amalgam is described as the tendency to expand or contract under stress without fracturing?