Decay under a crown: how it starts and how it is found

What you might notice

Often nothing at all, for a long time. When signs do appear, they tend to be a darkening line at the edge of the crown, a bad smell on the floss or interdental brush after cleaning one particular gap, or sensitivity to sweet and cold that was not there before. Pain usually arrives late, when the decay is already close to the nerve.

A dark line at the margin is not always decay. It can be the metal of an older crown showing through, or gum that has receded to reveal a slightly different-coloured root. Only an examination separates the three, so treat the line as a reason to be seen, not a diagnosis.

Root-filled teeth decay in silence

This is the part that catches people out, and it matters most to anyone with several crowned teeth that have also had root canal treatment. A tooth without a living nerve cannot give you the early warning of sensitivity or ache that a live tooth would. Decay can therefore spread under the crown for a long time with no symptom whatever, hollowing out the tooth that the crown depends on. The first sign may be the crown coming off one day with the core of the tooth still inside it, or a tooth that snaps at the gum line while you are eating something soft. By then the choice has often narrowed to rebuilding what is left, if enough remains, or losing the tooth. On these teeth, regular examination with radiographs is worth far more than waiting for something to feel wrong, because often nothing will. Root canal on a healthy tooth explains how teeth crowned for appearance sometimes end up in this group without the patient realising.

How a dentist finds it

At a check-up the dentist runs a fine probe along the margin, feeling for a catch, a step or a soft area. Then come radiographs, usually bitewings, which show the surfaces between the back teeth and the margins of crowns there. They have a limitation worth knowing about: zirconia and metal are opaque to X-rays, so the image shows the edges of the crown and the root beneath it but not always the tooth directly underneath the ceramic. That is why the probe and the image are read together, and why a comparison with earlier radiographs is so useful. A change between two films is often clearer than either film alone.

After a full set of crowns abroad

If you came home from Turkey with ten or twenty crowns, this is the long-term risk to plan for. The work may look perfect for years while one margin quietly fails, and with many units there are many margins. A few habits make a real difference:

• Register with a dentist at home and go regularly, even when nothing hurts. Tell them exactly which teeth are crowned and which of those are root filled. If no NHS practice near you is taking adults, a private check-up once or twice a year is a small commitment next to a crown lost to decay nobody saw.
• Bring the radiographs taken after fitting. They are the baseline every later film is compared against. What your records should include lists what to ask the clinic for before you leave.
• Ask how often you should be seen. Someone with many crowns, a dry mouth or a history of decay may be advised to come more often than the usual interval, and a dentist can prescribe a higher-fluoride toothpaste where the risk justifies it.
• Clean between the teeth every day, with floss or interdental brushes sized for each gap, and angle the brush into the gum line on every crowned tooth.

Opening that relationship before anything goes wrong is what keeping your UK dentist informed is about.

If decay is found

What happens depends on how far it has gone.

1. Small and confined to the margin. It can often be cleaned out and filled from the side, with the crown left in place.
2. Running under the crown. The crown is removed, usually destroying it in the process. The decay is cleared, the tooth is reassessed and, if it can be restored, a new crown is made.
3. Reaching the nerve, or leaving too little tooth. Root canal treatment and a rebuilt core may save it. If not, the tooth is extracted and the gap becomes a separate decision.

Each remake starts from slightly less tooth than the one before, which is the argument laid out in how long crowns and veneers last. Caught at the first stage, decay under a crown is a filling. Caught at the third, it can be the end of the tooth, and the only thing separating the two is how often somebody looked.