How do recurrent or residual caries typically appear on a radiograph?

• A. A radiolucent area with diffuse margins beneath or adjacent to a restoration
• B. A radiopaque area within the restoration
• C. A radiolucent line at the cementoenamel junction
• D. A radiopaque halo around the tooth

Answer: (A)

Recurrent or residual caries show up on radiographs as a radiolucent area that exists beneath or right next to a previous restoration, often along the restoration margins. This pattern happens because caries under a restoration progresses in dentin while the restoration itself remains radiopaque, so the neighboring dentin becomes demineralized and appears darker on the film. The borders of this radiolucent zone are typically diffuse and ill-defined, reflecting irregular undermining of dentin under the sealed margin rather than a clean, clearly delineated lesion.

This explanation fits the typical radiographic sign you look for after a restoration has been placed, signaling that decay has continued or remained under the restoration. In contrast, a radiopaque spot inside the restoration would indicate the filling material itself, not caries. A radiolucent line at the cementoenamel junction is not the classic pattern for recurrent caries, and a radiopaque halo around the tooth doesn’t describe caries at all.