Stains on teeth are generally of two varieties, extrinsic stains and intrinsic stains. They are generally considered to be of cosmetic significance only; however, a single dark tooth may indicate that the tooth is dead (necrotic), particularly if it has a history of trauma and/or infection. It should be evaluated by a dentist, who may perform vital testing to determine if it requires root canal (endodontic) treatment. Vital testing involves the application of heat, cold, and mild electrical stimulation to the teeth to see whether, and how, they respond.
Intrinsic stains occur much deeper in the tooth than extrinsic stains. Sometimes they occur when the tooth is forming, by incorporating dark pigmented molecules into the crystal structure of the tooth. Common causes include treatment with tetracycline antibiotics while the tooth is forming (Figure 1); and excess fluoride consumption during enamel formation, which can produce brownish staining or mottling.
Intrinsic stains can also occur when a tooth is injured. In such cases, excess blood flows to the tooth (hyperemia) to deliver healing and immune cells. Reddish-brown iron pigments saturate the dentin and cause the tooth to darken.