Polymerisation stress, irrigation and occlusal guidance

Correct answer: A. The ratio of bonded to unbonded surfaces in a cavity preparation

• A. Correct. C-factor is bonded surfaces divided by unbonded (free) surfaces. Unbonded surfaces let the composite flow during polymerisation and relieve shrinkage stress; the fewer of them, the higher the stress at the adhesive interface.
• B. Depth-to-width relationships influence cavity design and stress distribution but are not what C-factor describes.
• C. The enamel-to-dentine ratio affects bond strength and marginal seal but is a different concept.
• D. Filler loading determines strength, wear resistance and shrinkage magnitude, again separate from the C-factor.

Note: A Class I preparation has five bonded walls to one free surface — C-factor 5, the worst case. This is the argument for incremental placement and for a flowable stress-breaking layer.

Correct answer: B. Chloroform

• A. EDTA is a chelator that removes the inorganic component of the smear layer and helps negotiate calcified canals.
• B. Correct — this is the exception. Chloroform is a solvent used to dissolve gutta-percha during retreatment. It is not an irrigant, and it is both cytotoxic and a suspected carcinogen.
• C. Chlorhexidine is used as an adjunct, valued for its substantivity, though it lacks tissue-dissolving capacity.
• D. Sodium hypochlorite is the primary irrigant: it dissolves organic tissue and is strongly antibacterial.

Note: Never mix hypochlorite and chlorhexidine directly — the reaction produces an orange-brown parachloroaniline precipitate. Rinse with saline between them.

Correct answer: C. Mutually protected occlusion

• A. In group function, several teeth on the working side share contact during lateral excursion — an accepted alternative, often where canine guidance has been lost to wear.
• B. Cross-arch balance is the same concept applied across the arch and is again a complete-denture goal.
• C. Correct. In mutually protected (canine-protected) occlusion, the anterior teeth disocclude the posteriors in excursions, while in centric the posterior teeth take the load and protect the anteriors.
• D. Balanced occlusion means simultaneous contact on both working and non-working sides. It is desirable in complete dentures for stability but is not the natural dentition scheme.