AGD Dental Materials & Technology 4 — Questions and Answers
Question 1: What is the primary purpose of adding barium sulfate or strontium to composite resins?
- Improve color matching
- Provide radiopacity for radiographic detection (Correct answer)
- Increase flexural strength
- Reduce polymerization shrinkage
Correct answer: Provide radiopacity for radiographic detection
Heavy metal salts like barium sulfate absorb X-rays, making composite restorations detectable on radiographs and distinguishable from recurrent caries.
Question 2: Which factor most significantly affects the working time of polyether impression materials compared to vinyl polysiloxane?
- Polyether is more hydrophilic and unaffected by moisture
- Polyether has a significantly shorter working time due to faster polymerization (Correct answer)
- VPS sets faster in humid conditions
- Polyether requires higher mixing ratios
Correct answer: Polyether has a significantly shorter working time due to faster polymerization
Polyether materials typically offer 2–3 minutes of working time compared to the 4–5 minutes available with most VPS materials.
Question 3: Which type of stress does a cantilever pontic primarily place on the retaining abutment crown?
- Compressive stress only
- Pure tensile stress
- Torsional and bending (torque) stress (Correct answer)
- Shear stress along the occlusal table only
Correct answer: Torsional and bending (torque) stress
A cantilever pontic acts as a lever arm, generating rotational torque and bending moments that concentrate at the abutment connector.
Question 4: The Knoop hardness of enamel (~340 KHN) compared to dentin (~70 KHN) has which clinical implication for composite selection?
- Only enamel-bonded composites need filler particles
- Restorations must withstand occlusal forces against a very hard substrate requiring adequate filler content (Correct answer)
- Dentin-bonded composites require higher hardness than enamel-bonded ones
- Hardness values are irrelevant to material selection
Correct answer: Restorations must withstand occlusal forces against a very hard substrate requiring adequate filler content
Because enamel is very hard, opposing restorations and the composite itself must have sufficient hardness and wear resistance to avoid premature wear.
Question 5: What is the function of the coupling agent (silane) applied to ceramic restoration surfaces before bonding?
- Increases ceramic surface hardness
- Creates chemical bonds between the inorganic ceramic and organic resin cement (Correct answer)
- Removes residual hydrofluoric acid
- Provides antimicrobial activity at the margin
Correct answer: Creates chemical bonds between the inorganic ceramic and organic resin cement
Silane is a bifunctional molecule with methoxysilyl groups that bond to silica in glass ceramics and methacrylate groups that co-polymerize with resin cement.
Question 6: Which characteristic differentiates nanofill composites from nanohybrid composites?
- Nanofill contains only nano-sized particles with no larger filler, yielding superior polish retention (Correct answer)
- Nanohybrid has smaller particle sizes than nanofill
- Nanofill composites have higher flexural strength than nanohybrid
- Nanohybrid composites use ceramic filler exclusively
Correct answer: Nanofill contains only nano-sized particles with no larger filler, yielding superior polish retention
Nanofill composites contain only nano-sized particles (5–75 nm), allowing a smoother polished surface with better gloss retention than hybrid systems.
Question 7: In the lost-wax casting technique for metal restorations, what is the purpose of the sprue?
- Provides a channel for molten metal to flow into the mold (Correct answer)
- Creates the internal surface adaptation
- Generates the wax pattern shape
- Forms the proximal contacts
Correct answer: Provides a channel for molten metal to flow into the mold
The sprue is a wax rod attached to the pattern that, when burned out, leaves a channel through which molten alloy enters the investment mold.
AGD Dental Materials & Technology 4
What is the primary purpose of adding barium sulfate or strontium to composite resins?