A radiograph shows tooth #3 with an MO amalgam and a large radiolucency beneath the mesial portion of the restoration. Clinically there is a tiny chip at the mesial isthmus. What best explains the appearance of the restoration?

Correct answer: B. Recurrent caries beneath the restoration has undermined the marginal ridge

• A. Corrosion produces surface tarnish and marginal ditching over time; it does not create a large subsurface radiolucency.
• B. CORRECT. The radiograph is the answer: a LARGE radiolucency beneath the mesial aspect of an existing restoration is RECURRENT (secondary) caries, most commonly at the GINGIVAL margin of the proximal box where marginal breakdown and plaque stagnation coincide. As the underlying dentin is demineralised, the amalgam loses its foundation, and the marginal ridge and isthmus — the thinnest and most highly stressed part of a Class II amalgam — chip under normal function. The chip is the CONSEQUENCE of the caries, not an independent event.
• C. Heavy occlusion can fracture an amalgam at the isthmus, and the isthmus is indeed the classic fracture site of a Class II amalgam. But that explanation does not account for the large radiolucency visible beneath the restoration, which is the dominant finding.
• D. Overcarving leaves a submarginal contour and a ditched margin, which predisposes to plaque retention — a contributing factor, but not an explanation of the current radiographic picture.

Note: Diagnostic discipline: a radiolucency under a restoration must be distinguished from the normal radiolucent BASE or liner and from the 'burnout' artefact at the cervical region. Compare with previous radiographs where available, and correlate with clinical findings — marginal breakdown, softness on gentle probing, and any chip or fracture.