MJDF Dental Materials Science 2 — Questions and Answers
Question 1: What is the main advantage of resin-modified glass ionomer cement (RMGIC) over conventional glass ionomer?
- Improved mechanical strength and reduced early moisture sensitivity due to resin component, while retaining fluoride release and bonding (Correct answer)
- Higher fluoride release than conventional GIC
- No acid-base reaction component
- Harder than porcelain
Correct answer: Improved mechanical strength and reduced early moisture sensitivity due to resin component, while retaining fluoride release and bonding
RMGIC combines the acid-base setting of GIC (fluoride release, chemical bonding to tooth) with light-cure resin polymerisation, reducing early moisture sensitivity and improving strength and handling.
Question 2: What is the purpose of silanisation of ceramic surfaces before bonding?
- Silane coupling agent bonds to both the ceramic (via silanol groups) and the resin (via methacrylate groups) creating a chemical bridge (Correct answer)
- Etches the ceramic for micro-mechanical retention only
- Cleans the ceramic surface
- Acts as a colour modifier
Correct answer: Silane coupling agent bonds to both the ceramic (via silanol groups) and the resin (via methacrylate groups) creating a chemical bridge
Silane has dual reactivity: one end bonds to silicon oxide groups on etched ceramic; the other end co-polymerises with resin, creating a strong chemical bond between ceramic and resin cement.
Question 3: What is 'polymerisation shrinkage' and how does it affect composite restorations?
- Volume reduction during monomer-to-polymer conversion (~1.5–3%), causing contraction stress at restoration margins leading to microleakage and cuspal deflection (Correct answer)
- Expansion of the composite during curing
- Temperature-related change only
- Colour change associated with curing
Correct answer: Volume reduction during monomer-to-polymer conversion (~1.5–3%), causing contraction stress at restoration margins leading to microleakage and cuspal deflection
When monomers polymerise, Van der Waals distances are replaced by covalent bonds, reducing volume by 1.5–3%; this shrinkage stress can debond from cavity walls, open margins, and deflect cusps in large cavities.
Question 4: What is the purpose of conditioning (acid etching) enamel before composite bonding?
- Create microporosities for micromechanical resin tag formation by dissolving hydroxyapatite selectively (using 35–37% phosphoric acid for 15–30 sec) (Correct answer)
- Chemically bond to enamel directly
- Remove the smear layer from enamel only
- Increase enamel fluoride content
Correct answer: Create microporosities for micromechanical resin tag formation by dissolving hydroxyapatite selectively (using 35–37% phosphoric acid for 15–30 sec)
Phosphoric acid etching selectively dissolves enamel prism cores or peripheries, creating microporosities; resin monomers infiltrate these and polymerise in situ, forming resin tags providing micromechanical retention.
Question 5: What is the creep behaviour of dental amalgam and why is it significant?
- Slow time-dependent plastic deformation under sustained load; can cause marginal overhang and 'amalgam tattooing' of gingiva (Correct answer)
- Rapid elastic recovery after load removal
- Volume expansion under occlusal loading
- Complete recovery after stress removal
Correct answer: Slow time-dependent plastic deformation under sustained load; can cause marginal overhang and 'amalgam tattooing' of gingiva
Amalgam exhibits creep — slow plastic flow under sustained occlusal load — which can cause the restoration to extrude marginally, creating overhangs and contributing to marginal breakdown over time.
Question 6: What is the Vickers hardness test used to measure in dental materials?
- Hardness — resistance to permanent indentation — using a diamond pyramid indenter under a known load (Correct answer)
- Fracture toughness
- Modulus of elasticity
- Tensile strength
Correct answer: Hardness — resistance to permanent indentation — using a diamond pyramid indenter under a known load
The Vickers hardness test presses a diamond pyramid into the surface under a defined load and measures the diagonal of the indentation; it provides a hardness value (VHN) used to compare dental materials.
MJDF Dental Materials Science 2
What is the main advantage of resin-modified glass ionomer cement (RMGIC) over conventional glass ionomer?