Decay under a crown: why it starts at the edge and why it is easy to miss

People are often surprised to hear that a crowned tooth can decay. The crown is ceramic or metal, after all, and bacteria cannot eat through it. But a crown is a cap on a living tooth, and that tooth is still made of enamel and dentine that plaque acids can dissolve. Decay under a crown is one of the main reasons crowns eventually need replacing, and because it starts out of sight, it can be quite advanced before anyone notices. Here is why it happens, why it is genuinely hard to see, and what actually lowers the risk.

Where it starts: the margin

The margin is the line where the edge of the crown meets the tooth. On most crowns it sits at the gum line or just under it. However well a crown fits, that junction is a seam: crown, a thin layer of cement, then tooth. Plaque collects there easily, especially between teeth where a toothbrush does not reach.

Decay that forms next to an existing filling or crown is called secondary or recurrent caries. A review prepared for European caries experts found that the crown or filling material has some influence, but that other factors seem to matter more: whether there are gaps at the edge and how big they are, the patient's own caries risk, and the experience of the dentist who placed it (Askar et al., 2020). In plain terms, a good fit and your daily habits outweigh the brand of ceramic.

Gum recession adds to the problem. When the gum shrinks back, the margin and some of the root surface become exposed. Root surfaces are more vulnerable to decay than enamel, and a review of gum recession and root caries in older adults links the two (Heasman et al., 2017), so a crown that was hidden under the gum at 30 can have an exposed, vulnerable edge at 45.

Why you may not feel it

The NHS notes that tooth decay may not cause any symptoms at first. Under a crown that is even more likely. The decay is beneath a hard shell, and many crowned teeth have had root canal treatment, so there is no live nerve to send a warning. By the time symptoms appear, the decay may have spread under a large part of the crown.

Signs that something may be happening:

• A dark or brown line at the edge of the crown that was not there before.
• A soft or catching feeling when you run floss or a fingernail along the edge.
• A bad taste or smell from one area, especially if food keeps catching there.
• Bleeding gums around one crown when the rest of your mouth is healthy.
• A crown that feels slightly loose or comes off. Our article on what to do if a crown comes off or chips covers the first steps.
• Later on, pain, sensitivity or swelling.

Not every dark line is decay. Older metal-ceramic crowns can show a grey line at the gum for other reasons. That is exactly why it needs a dentist to look rather than guesswork.

Why X-rays struggle, especially under zirconia

X-rays show decay because decayed tooth lets more X-ray through than healthy tooth, so it appears darker. Crown materials vary in how much they block X-rays, a property called radiopacity. A 2025 study measuring current milled crown materials found that radiopacity differed a lot between them, and that zirconia had the highest radiopacity of all the materials tested (Nejat et al., 2025). On an X-ray, a zirconia crown looks bright white and solid. That is useful for checking its edges and shape, but it also means the tooth inside is largely hidden behind it.

The authors of a pilot study on this exact problem put it bluntly: conventional radiographic techniques are insufficient for detecting caries under fixed restorations (Bilgin et al., 2014). In their lab test they could see decay under zirconia, lithium disilicate and metal-ceramic crowns using cone beam CT. CBCT is a 3D scan, though, not a routine check-up X-ray.

Even for decay next to fillings, detection is imperfect. A systematic review of methods for finding secondary caries found that looking, X-rays and laser fluorescence each had sensitivities of only about 50 to 59 percent, meaning they missed a fair share of lesions in the studies, many of which were done in the lab (Brouwer et al., 2016). Decay at the side of a crown, between teeth, can often be seen on a bitewing X-ray. Decay directly underneath is the hard part.

This is one reason E-max and zirconia are not simply interchangeable. Our zirconia crowns and E-max crowns pages explain the wider trade-offs.

Is it more common with some crowns?

The best evidence here is from bridges rather than single crowns. A large systematic review of tooth-supported bridges found significantly more decay in the supporting teeth under zirconia bridges than under metal-ceramic ones over five years (Pjetursson et al., 2015). The studies behind it used older materials and techniques, and the reason is not fully clear; the fit of the margin, the cement and how well patients could clean all play a part. It is still a good reason to take margin fit and cleaning seriously with any crown or bridge.

What actually prevents it

The same things that protect natural teeth protect the edge of a crown. Small, boring habits done every day do more than any special product.

Fluoride toothpaste, used properly

Brush twice a day with a fluoride toothpaste, last thing at night and one other time. Spit out afterwards but do not rinse, so the fluoride stays on the teeth. NHS advice is that adults should use a toothpaste with at least 1,350ppm fluoride. If you are at high risk of decay, UK guidance says a dentist can prescribe a higher-strength toothpaste: 2,800ppm from age 10, or 5,000ppm from age 16 for people with high risk, including those with root decay (Delivering better oral health, chapter 9).

Clean between the teeth daily

The margins most at risk are the ones between teeth. The Oral Health Foundation advises cleaning between your teeth once a day with interdental brushes or floss, and asking your dental team which sizes suit each gap. Brushes are often easier where spaces are larger; floss suits tight contacts. If food packs into one gap repeatedly, that is a separate problem worth fixing, covered in our article on food stuck between crowns.

Watch how often you have sugar

The NHS links tooth decay to having too much sugary food and drink. Frequency matters as much as quantity: sipping sweet coffee or fizzy drinks through the day gives plaque repeated acid attacks at the margin. Keep sugary things to mealtimes where you can.

Keep up check-ups

The NHS says the gap between check-ups can be anywhere from 3 months to 2 years depending on your risk. With several crowns, your dentist may want bitewing X-rays at intervals and will look closely at each margin. Comparing X-rays over time helps, because a change is easier to see than a single snapshot.

Mention dry mouth

The NHS warns that you are more likely to get tooth decay if you have a dry mouth, which some medicines and conditions cause. If your mouth often feels dry, tell your dentist, as it changes how much fluoride you may need.

If decay is found

Treatment depends on size and position. Askar and colleagues note that secondary caries can be managed either by repairing the defective part or by replacing the whole restoration. A small lesion at an accessible edge may be cleaned out and filled. If decay has spread under the crown, the crown usually has to come off, the decay is removed, and a new crown is made. If it has reached the nerve, root canal treatment may be needed first. Sometimes, when decay goes deep below the gum, the tooth cannot be saved.

What we do before and during crown treatment

At Precious Dental, existing decay is treated before crowns are made, because covering active decay is how problems start. Teeth are digitally scanned, and at the try-in we check the fit along with the bite and shade before anything is permanently fixed. After that, the margins are in your hands and your local dentist's: daily cleaning and regular check-ups. Our guarantee may be reduced or void if hygiene is neglected or check-ups are missed, and root canal treatment is not covered, which is why we explain this before treatment rather than after. For more on what crowning does to the tooth itself, see what happens to your teeth, and for everyday care, aftercare and longevity.

Worried about one of your crowns?

If you have noticed a dark edge, a bad taste or a crown that feels different, see a dentist and ask for the margins to be checked. You are also welcome to send us your X-rays and photos through our contact page or on WhatsApp, and we will tell you what we can see and what we cannot.

Common questions

Can a crowned tooth still get decay?

Yes. The crown itself cannot decay, but the natural tooth underneath can, starting where the crown edge meets the tooth. This is why crowned teeth still need brushing, cleaning between and check-ups.

Will I feel it if a tooth under a crown is decaying?

Often not at first. The NHS notes that tooth decay may not cause any symptoms early on, and a root-filled tooth has no nerve to warn you. Signs can include a dark line or soft area at the edge, bad taste, a crown that feels loose, or pain later on.

Can decay under a zirconia crown be seen on an X-ray?

Decay at the edges between teeth can often be seen. Decay hidden directly under the crown is much harder, because zirconia is very dense and blocks the X-ray. Dentists combine X-rays with looking, probing gently at the margin and checking for changes over time.

Does decay under a crown always mean a new crown?

Not always. A small lesion at an accessible edge can sometimes be repaired with a filling. Larger decay, or decay that has undermined the crown, usually means the crown is removed, the decay treated and a new crown made. Sometimes the tooth also needs root canal treatment.

Is decay under a crown covered by a guarantee?

Guarantees generally cover the work, not new disease caused by plaque. Ours may be reduced or void if hygiene is neglected or check-ups are missed, and root canal treatment is not covered. See our guarantee page for the terms.

Sources

1. Askar H et al. (2020). Secondary caries: what is it, and how it can be controlled, detected, and managed? Clinical Oral Investigations
2. Brouwer F et al. (2016). Detecting secondary caries lesions: a systematic review and meta-analysis. Journal of Dental Research
3. Nejat AH et al. (2025). Radiographic evaluation of current restorative CAD/CAM ceramics indicated for fabrication of indirect restoration. Journal of Esthetic and Restorative Dentistry
4. Bilgin MS et al. (2014). Posttreatment diagnosis of caries under fixed restorations: a pilot study. Journal of Prosthetic Dentistry
5. Pjetursson BE et al. (2015). All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? Part II: Multiple-unit FDPs. Dental Materials
6. Department of Health and Social Care and NHS England (updated 2025). Delivering better oral health: an evidence-based toolkit for prevention, chapter 9: fluoride. GOV.UK
7. NHS. Tooth decay
8. Oral Health Foundation. How to clean between your teeth
9. Heasman PA et al. (2017). Gingival recession and root caries in the ageing population: a critical evaluation of treatments. Journal of Clinical Periodontology
10. NHS. Dry mouth

Checked October 2026. Where we give our own view as a clinic, we say so.