ADEX Dental Biomaterials 3 — Questions and Answers
Question 1: In amalgam, the gamma-2 phase (Sn8Hg) is considered undesirable because it:
- Increases setting expansion excessively
- Is the weakest and most corrosion-prone phase (Correct answer)
- Causes mercury toxicity to pulp tissue
- Inhibits the formation of the gamma-1 phase
Correct answer: Is the weakest and most corrosion-prone phase
The gamma-2 phase is the weakest component of amalgam and is highly susceptible to corrosion, leading to marginal breakdown.
Question 2: Which initiator-accelerator system is used in chemically activated (self-cured) composite resins?
- Camphorquinone and amine
- Benzoyl peroxide and tertiary amine (Correct answer)
- Diketone and photoinitiator
- Hydrogen peroxide and UV light
Correct answer: Benzoyl peroxide and tertiary amine
Self-cured composites use a two-paste system where benzoyl peroxide in one paste reacts with a tertiary amine in the other to initiate polymerization.
Question 3: The purpose of adding silane coupling agent to composite resin filler particles is to:
- Increase the radiopacity of the restoration
- Bond the inorganic filler to the organic resin matrix (Correct answer)
- Reduce polymerization shrinkage
- Improve color stability of the material
Correct answer: Bond the inorganic filler to the organic resin matrix
Silane coupling agents form a chemical bridge between the inorganic glass filler and the organic resin matrix, enhancing composite strength and durability.
Question 4: What is galvanic corrosion in the oral cavity?
- Chemical dissolution of a restoration in acidic saliva
- Electrochemical degradation occurring when dissimilar metals contact in saliva (Correct answer)
- Oxidation of amalgam margins causing staining
- Tarnish formation on gold alloy surfaces
Correct answer: Electrochemical degradation occurring when dissimilar metals contact in saliva
Galvanic corrosion occurs when two dissimilar metals are in electrical contact via saliva, creating a battery-like cell that corrodes the more anodic metal.
Question 5: Which characteristic distinguishes Type IV gold alloys from Type I gold alloys?
- Type IV alloys contain no palladium
- Type IV alloys are harder and used for partial denture frameworks (Correct answer)
- Type I alloys are harder due to higher copper content
- Type IV alloys have lower melting temperatures
Correct answer: Type IV alloys are harder and used for partial denture frameworks
Type IV gold alloys have the highest hardness and strength among the four types, making them suitable for partial denture frameworks and thin castings requiring durability.
Question 6: What is the main advantage of using a rubber base (polysulfide) impression material over alginate?
- It is more hydrophilic and works better in wet fields
- It provides greater dimensional accuracy and longer storage stability (Correct answer)
- It sets faster with no retarder needed
- It is less expensive and easier to manipulate
Correct answer: It provides greater dimensional accuracy and longer storage stability
Polysulfide impressions are dimensionally more stable than alginate and can be poured hours after taking the impression, unlike alginate which must be poured immediately.
Question 7: The Knoop hardness test is used in dentistry to measure:
- Flexural strength of ceramic materials
- Surface hardness of mineralized tissues and restorative materials (Correct answer)
- Fracture toughness of composite resins
- Tensile strength of cements under loading
Correct answer: Surface hardness of mineralized tissues and restorative materials
The Knoop hardness test uses a diamond indenter to measure surface hardness, and is commonly applied to both tooth structure and dental materials to compare resistance to indentation.
ADEX Dental Biomaterials 3
In amalgam, the gamma-2 phase (Sn8Hg) is considered undesirable because it: