What is the principal advantage of an MOD onlay over a large MOD amalgam?

Correct answer: C. It provides cuspal coverage, protecting the remaining tooth from fracture

• A. Cuspal coverage requires MORE tooth reduction — typically 1.5–2 mm occlusally — not less.
• B. Cast gold onlays are cemented rather than bonded; chemical adhesion to dentine is not a property of cast metal.
• C. CORRECT. A wide MOD preparation removes both marginal ridges and leaves the facial and lingual cusps as unsupported cantilevers that flex under load. An intracoronal amalgam does nothing to counter this — it may even act as a wedge. An ONLAY covers and SPLINTS the cusps, converting destructive tensile and shear forces into compressive forces the tooth can withstand. It also produces better proximal contours and marginal adaptation.
• D. An onlay requires indirect fabrication, at least two visits and greater cost.

Note: The usual threshold: when the isthmus width exceeds about one-third to one-half the intercuspal distance, or when a cusp is undermined, cuspal coverage is indicated.