Decay under a crown is possible even when the crown itself looks flawless, because a crown protects the outer surface of a tooth but cannot stop bacteria reaching the natural tooth structure underneath it. The most common trouble spot is the margin, the fine line where the edge of the crown meets your own tooth. Bacteria can work their way in here slowly, often without any pain at all until the decay is well established.
Key Takeaways
- A crown protects the top and sides of a tooth, but decay can still form at the margin or underneath if bacteria get in.
- Common causes include marginal leakage, gum recession exposing the edge of the crown, a cracked cement seal, or decay that was already present before the crown went on.
- Early decay under a crown often has no obvious symptoms, which is why routine check-ups and X-rays matter.
- Warning signs include sensitivity, a dull ache, bad taste or smell near the tooth, and gum swelling around the crown.
- Treatment ranges from a small filling repair to crown replacement or root canal treatment, depending on how far the decay has progressed.
- Good hygiene around the crown margin and regular dental visits are the most reliable ways to catch problems early.
Why decay can develop under a crown
A dental crown is a cap that sits over a prepared tooth, usually made from porcelain, zirconia, or a metal alloy, none of which are food for decay-causing bacteria. The problem is the join between the crown and the tooth underneath. If that seal is compromised in any way, bacteria and food particles can get underneath and start breaking down the tooth structure, out of sight.
Common reasons this happens include:
- Marginal leakage. Over time, the cement holding a crown in place can wear down or micro-gaps can form at the edge, letting bacteria seep in.
- Gum recession. As gums recede with age or gum disease, the margin of the crown that was once tucked under the gumline becomes exposed, making it easier for plaque to collect there.
- A cracked or failing cement seal. Grinding, an old crown reaching the end of its working life, or trauma to the tooth can all crack the seal.
- Decay left behind before the crown was fitted. If any decay wasn't fully removed before the crown was placed, it can continue to spread slowly underneath.
Symptoms of decay under a crown
Because the crown covers the tooth, symptoms can take longer to show up than with ordinary tooth decay. Some people notice nothing until a check-up X-ray picks it up. Others get warning signs like these. If a lingering ache from a crowned tooth is keeping you up at night, our article on why toothache feels worse at night explains why that pattern happens.
| Symptom | What it usually means |
|---|---|
| Lingering sensitivity to hot or cold | Decay may be getting close to the nerve, or the margin seal has broken down |
| A dull ache or pressure when biting | Possible decay or a loose crown affecting the bite |
| Bad taste or odour near the tooth | Bacteria breaking down tooth structure under the crown |
| Gum redness or swelling around the crown | Local irritation or infection at the margin |
| Visible dark line at the edge of the crown | Could be staining, or could be decay starting to show through, worth checking |
| A pimple-like bump on the gum near the tooth | Possible sign of infection that has reached the nerve, needs prompt review |
Signs that are usually nothing to worry about vs signs to get checked promptly
| Usually nothing to worry about | Get checked promptly |
|---|---|
| Mild, brief sensitivity to cold that settles within seconds | Pain that lingers for a minute or more after hot or cold |
| A little discomfort in the first few days after a new crown is fitted | Increasing pain weeks or months after the crown was fitted |
| Slight difference in bite feel for the first day or two | A crown that feels loose, rocks, or has shifted |
| Mild gum tenderness after a clean | Ongoing gum swelling, a bump on the gum, or a bad taste that doesn't go away |
If in doubt, it's always worth a quick check rather than waiting it out. This is general information only and isn't a substitute for an actual dental examination.
Why X-rays catch it before symptoms do
Decay under a crown often develops in a spot you simply can't see or feel until it's advanced, because the crown is in the way. X-rays let a dentist look underneath the crown at the tooth structure and the margin itself. This is one of the main reasons routine check-up X-rays matter even when a tooth already has a crown on it and feels completely fine. Catching a small area of decay early usually means a simpler, smaller fix than waiting until it causes pain.
What treatment usually involves
Treatment depends on how much of the tooth is affected and whether the decay has reached the nerve.
- Small, early decay near the margin can sometimes be treated by removing the crown, cleaning out the decay, and either placing a filling and re-cementing the same crown, or fitting a new one if the old crown no longer fits well.
- More extensive decay under the crown usually means the crown needs to be replaced once the tooth is restored, since the fit and seal need to be accurate.
- Decay that has reached the nerve may need root canal treatment before a new crown, or the same crown, can go back on. You can read more in our guide on root canal versus extraction if this is something you're facing.
- In cases where too much of the tooth structure has broken down, extraction and replacement with an implant, bridge, or denture may be discussed as an option.
A tooth that already has a crown and later needs root canal treatment is a specific situation of its own. We've covered what that looks like in more detail in root canal under a crown.
Reducing the risk of decay under a crown
- Keep up regular check-ups and X-rays, even for teeth that already have a crown.
- Brush gently but thoroughly along the gumline where the crown margin sits.
- Floss or use interdental brushes around the crown daily, since plaque builds up at the margin just like anywhere else.
- Mention any new sensitivity, ache, or odd taste near a crowned tooth at your next visit, or sooner if it's persistent.
- If you grind your teeth, ask about a night guard, since grinding can stress the cement seal over time.
- If you ever lose a filling or notice a crown feels loose, treat it seriously, our guide on what to do if you lose a filling has practical first steps that apply to a loose crown too.
Frequently asked questions
Can a tooth decay under a crown without any pain?
Yes. Because the crown covers the tooth, decay can progress for some time without pain, which is why it's often picked up on a routine check-up X-ray rather than because of symptoms.
How long can a crown last before decay becomes a risk?
Crowns commonly last many years, but the risk of decay at the margin isn't really about the age of the crown on its own, it's more about how well the seal has held up and how consistent hygiene has been around the edge.
Does a dark line around a crown always mean decay?
Not always. Sometimes it's just staining or the natural shadow of a metal margin showing through. A dentist can tell the difference on examination, so it's worth having it checked rather than guessing.
Will the whole crown need to be replaced if there's decay underneath?
Not necessarily. If the decay is small and near the edge, some dentists can access and repair it without replacing the whole crown. More extensive decay usually does mean a new crown once the tooth is restored.
Can decay under a crown turn into a dental emergency?
It can, if it's left untreated and reaches the nerve or causes an infection. Symptoms like swelling, a bump on the gum, or worsening pain are worth acting on quickly rather than waiting for your next scheduled check-up.
If a crowned tooth feels different lately, or it's simply been a while since your last check-up and X-rays, our general dentistry team can take a proper look and catch anything early. It's also worth checking our current offers to see what's available for new and existing patients.
