A black spot on a tooth: decay or staining?

Tooth discolouration falls into two main groups. Cleveland Clinic writes that extrinsic stains affect the enamel, the outermost layer of the tooth, and mostly come from food and drink, whereas intrinsic discolouration starts inside the tooth, in the layer beneath the enamel (dentine), and is linked to causes such as trauma or certain medicines. A black spot can come from either group, so the first question is whether the spot is on the surface or inside the tooth.

The main sources of surface stains are well known. Cleveland Clinic lists dark-coloured food and drink such as coffee, tea, red wine and soy sauce, smoking and tobacco, and poor oral hygiene; it notes that stains cling to dental plaque, and that if plaque is not removed with regular brushing and flossing, the discolouration builds up. It generally links brown stains with smoking and tobacco.

Tartar can also look black. Cleveland Clinic writes that tartar takes on the colour of food and drink and can turn black in people who drink tea, coffee or red wine, that tartar below the gum line causes bleeding, and that tartar mixed with blood turns black. A hard, rough, dark deposit on the inner side of the lower front teeth or along the gum edge that does not come off with brushing fits this picture. Tartar is not scraped off at home; it is removed with a clean by a dentist.

There is also a thin black line, seen particularly in children and some adults, that forms as a row of dots on the part of the teeth close to the gum. It is called extrinsic black stain. What is interesting is that this stain has been found to have an inverse relationship with decay: in a meta-analysis pooling 14 studies, children with black stain were less likely to develop decay (odds ratio 0.67). The authors stress that it is not clear whether this is because the stain is protective, or because the mouth flora of children who already have a low risk of decay suits the bacteria that form it. Another meta-analysis reported a similar result, based on cross-sectional studies. This stain does not come off with brushing; it is removed with a clean by a dentist, but it often comes back.

On the decay side, things are different. The NHS writes that tooth decay may cause no symptoms at first, and that as it progresses it can cause toothache, sharp sensitivity to hot, cold or sweet things, and white, brown or black spots on the tooth. Cleveland Clinic notes that black spots on teeth usually indicate areas of advanced decay, and that small holes together with brown spots most likely mean untreated decay. MSD Manuals explains that the deep, narrow grooves on the back teeth harbour bacteria and cannot be cleaned, which makes them prone to decay.

There is a crucial distinction here: not all dark decay is active. The International Caries Classification and Management System (ICCMS) describes the surface of active decay as whitish or yellowish, dull and rough, and the surface of arrested (inactive) decay as whitish, brown or black, shiny and hard. In cavities that have reached the dentine, active decay has soft or leathery dentine, while arrested decay has shiny, hard dentine. In other words, a dark colour on its own does not show that decay is progressing; sometimes it is the trace of decay that stopped years ago and hardened once oral hygiene improved.

This distinction has a practical consequence. A study describing the Nyvad criteria showed that active decay that had not yet formed a cavity was at higher risk of progressing to a cavity or a filling than inactive decay. In the same research group's 1999 study, agreement between trained dentists was high, but 81.6% of the misclassifications were in lesions that had not yet formed a cavity. This means early spots are where assessment by eye is hardest; that is why X-rays and follow-up play an important part.

Metal and medicines can also darken a tooth. Cleveland Clinic writes that silver-coloured amalgam fillings can make a tooth look greyish, and that root canal treatment can sometimes cause discolouration. MSD Manuals notes that silver diamine fluoride (SDF), used to stop decay, permanently stains the decayed area black, which is why it is used mostly on baby teeth. If your child has had this applied to a tooth, the resulting blackness is an expected result.

Finally, the whole tooth darkening rather than a spot. Cleveland Clinic writes that grey discolouration can show that the nerve inside the tooth may have died, and that this can happen with trauma. A single front tooth turning grey or a dark shade over a few weeks or months, particularly if that tooth took a knock in the past, is a completely different situation from a surface stain, and the tooth needs to be assessed with nerve tests.

The situations in the first list are usually not urgent; even so, if you think it could be decay, the NHS advises seeing a dentist without waiting for your check-up date. If a point from the second list applies, bring your appointment forward.

You can do the first three steps at home and the rest at the dentist. What you do at home does not make a diagnosis; it makes it clearer what to tell the dentist at the appointment.

1. 1

   Look at where it is

   With a mirror and good light, look at where the spot is. A line running along the gum edge on many teeth fits staining or tartar better; a dark area in a groove on the biting surface of a single back tooth, or between two teeth, fits decay better.

2. 2

   Brush and look again

   Brush your teeth thoroughly and look again. A fresh surface stain clinging to plaque may lessen; a spot that does not change at all is either tartar, a stubborn stain or colour coming from inside the tooth. Do not scrape it with a hard object, a needle or a fingernail.

3. 3

   Note the symptoms

   Sensitivity to sweet, hot or cold things, pain when chewing, floss catching, whether the spot has grown in recent months and whether the tooth has taken a knock in the past are all things to tell the dentist.

4. 4

   Looking and feeling at the dentist

   Once the tooth has been cleaned and dried, the colour, shine and surface of the spot are assessed; the dentist touches it gently with a blunt probe to see whether the surface is hard or soft. In enamel, active decay is dull and rough and arrested decay is shiny and hard; if the cavity has reached the dentine, active decay has soft or leathery dentine, and arrested decay hard dentine.

5. 5

   X-ray

   MSD Manuals writes that X-rays are still the most important tool for detecting decay, working out how deep it goes and seeing decay under old fillings. Only an X-ray can show how wide the decay is under a small black dot.

6. 6

   Nerve tests if needed

   If the whole tooth has darkened, its response to cold and its sensitivity to light tapping are tested, and the tip of the root is assessed on an X-ray. Whether the nerve is alive determines the direction of treatment.

There is no single treatment for a black spot; what is done depends on where the spot comes from.

The following are wrong decisions often seen with this complaint. We set them out so that you can ask the right questions, whichever dentist you see.

The timings vary with the source of the spot. The sequence below is a general framework; the plan of the dentist who examines you comes first.

1. After a clean

   Surface stains and tartar are removed in the same session. The black line and coffee and tea stains can come back over time; regular cleaning and changing habits reduce how often they return.

2. With early decay

   After fluoride is applied, the spot is reassessed at check-ups. A spot that hardens and becomes shiny is considered arrested; a spot that widens or softens goes on to a filling.

3. After a filling

   Ask your dentist how long sensitivity after the filling is expected to last. If the sensitivity is getting worse, or turns into throbbing pain or pain that wakes you at night, go back to your dentist.

4. Regular check-ups

   MSD Manuals recommends check-ups at intervals of 3 to 12 months depending on the risk of decay. The NHS wants even people with no problems to have a check-up at least every 2 years, and under-18s once a year.

We are not giving a single figure here, because behind a black spot there may be a single-session clean, or decay that needs a filling, root canal treatment or a crown. These are the items that determine the scope:

The source of the spot

A surface stain, tartar, arrested decay, active decay and a dead nerve follow entirely different paths.

How deep the decay goes

Decay confined to the enamel is dealt with by fluoride and follow-up, decay that has reached the inner layer of the tooth by a filling, and decay that has reached the nerve by root canal treatment.

The number of teeth affected

A spot on a single tooth and decay spread across many teeth call for planning on a different scale.

The need to correct the colour

Whitening, tooth-coloured fillings or veneers on the front teeth are separate items.