Amalgam Delayed Expansion — ORE Part 1 MCQ

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HardDental MaterialsAmalgam Delayed ExpansionORE Part 1

During condensation of a zinc-containing conventional dental amalgam, moisture contamination occurs. Which constituent is responsible for the subsequent classic delayed expansion?

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Correct answer: C — Zinc reacting with moisture

**A. Zinc reacting with moisture** is correct. Classic delayed expansion occurs when a zinc-containing amalgam is contaminated by moisture during manipulation. The zinc–water reaction generates hydrogen gas, and gas accumulation produces a delayed post-setting expansion. Evidence shows that water-contaminated zinc-containing alloys develop post-setting expansion, with the clinically important classic phenomenon particularly associated with conventional/low-copper amalgam. Copper may influence alloy behaviour and high-copper amalgams are less prone to classic delayed expansion, but copper is not the causative reactant. Silver and tin contribute to normal amalgam phases and dimensional change, not this moisture-induced defect. Mercury is required for amalgamation but does not independently produce classic delayed expansion on water contamination.

Reference: Osborne JW. Expansion of contaminated amalgams assessed by photoelastic resin. Quintessence International. 1999;30(10):673-681. https://pubmed.ncbi.nlm.nih.gov/10765851/