A patient presents with a dark line visible at the cervical margin of an anterior crown. The crown is well adapted and the periodontium is healthy. What is the MOST likely cause?

Correct answer: D. The metal substructure or collar is too thick at the margin

• A. A shade error affects the overall colour of the restoration, not a discrete dark band confined to the cervical margin.
• B. No patient requests a grey line at the gumline; this is a technical/material limitation.
• C. Over-reduction produces a weakened tooth and possible pulpal insult — it does not itself create a dark marginal line.
• D. Correct. A dark cervical line on a porcelain-fused-to-metal crown is the classic METAL COLLAR / opaque show-through problem. Where the metal substructure is too thick, or insufficient porcelain thickness covers it at the margin, the grey metal telegraphs through the thin marginal gingiva. It becomes progressively more visible as the gingiva recedes with age. Solutions include an all-ceramic restoration, a porcelain butt-joint margin, or moving the margin subgingivally.