What is the most common reason for failure of an amalgam restoration?

Correct answer: D. Inadequate preparation design, particularly insufficient depth at the isthmus

• A. True mercury allergy is extremely rare and manifests as a contact reaction, not as restoration failure.
• B. Polishing improves corrosion resistance and marginal integrity — it is protective, not a cause of failure.
• C. Galvanic currents cause a transient sharp sensation on contact between dissimilar metals; they do not cause the restoration to fail.
• D. CORRECT. The most frequent cause of bulk fracture is a preparation fault — most characteristically inadequate DEPTH at the ISTHMUS. Amalgam needs at least 1.5–2 mm of bulk to withstand occlusal load; a shallow isthmus leaves a thin section that fractures under normal function. A sharp, unbevelled axiopulpal line angle compounds the problem by concentrating stress at the same site.

Note: The hierarchy of amalgam failure: preparation design (depth and outline) first, then condensation faults, then moisture contamination, then trituration errors. The isthmus is the weak point of every Class II amalgam.