MJDF Dental Materials Science 2

Question 1

What is the main advantage of resin-modified glass ionomer cement (RMGIC) over conventional glass ionomer?

Accepted Answer

Improved mechanical strength and reduced early moisture sensitivity due to resin component, while retaining fluoride release and bonding

Answer

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?

Accepted Answer

Silane coupling agent bonds to both the ceramic (via silanol groups) and the resin (via methacrylate groups) creating a chemical bridge

Answer

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?

Accepted Answer

Volume reduction during monomer-to-polymer conversion (~1.5–3%), causing contraction stress at restoration margins leading to microleakage and cuspal deflection

Answer

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?

Accepted Answer

Create microporosities for micromechanical resin tag formation by dissolving hydroxyapatite selectively (using 35–37% phosphoric acid for 15–30 sec)

Answer

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?

Accepted Answer

Slow time-dependent plastic deformation under sustained load; can cause marginal overhang and 'amalgam tattooing' of gingiva

Answer

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.

MJDF Part 1 Practice Test

MJDF Part 1 Practice Test

MJDF Dental Materials Science 2