A crown is sometimes thought of as a permanent cap that makes a tooth immune to decay. The porcelain, zirconia or metal cannot rot, but the tooth it is cemented onto is still natural tooth, and it can. Dentists call this recurrent or secondary decay. It usually starts at the edge of the crown, and it is one common reason a crown eventually has to be redone.
The weak point is where crown meets tooth
A crown is made to fit closely over a prepared tooth, and the line where its edge finishes against the tooth is called the margin. Depending on the tooth, that line usually sits near the gum, just above it, level with it or slightly under it. It is the one place where tooth, cement and crown all meet, and it is also a natural ledge for plaque to settle against.
Over years, a few things can open that area up to bacteria. The cement at the edge can wash out slightly, the gum can shrink back and expose root surface below the crown, or the margin can simply be hard to reach with a brush and floss. Root surface is softer than enamel and tends to decay more readily once it is exposed. Decay that starts at the margin can then spread underneath, where it is shielded by the crown.
Some crowned teeth are at higher risk
Anyone with a crown can develop decay under it, but the odds go up with:
- a dry mouth, whether from medicines, medical conditions or breathing through the mouth, since saliva helps neutralise acid and repair early damage
- frequent sugary snacks or drinks, including sweetened coffee or soft drinks sipped through the day
- gum recession around the crowned tooth
- a history of new cavities elsewhere in the mouth in recent years
- crowns whose edges are hard to clean, such as those next to a gap or at the very back of the mouth
Our article on why cavities appear even when you brush explains how saliva, diet and bacteria interact, and the same principles apply at a crown edge.
It is often silent, especially in root-filled teeth
Many crowned teeth have had root canal treatment and no longer have a live nerve. Those teeth cannot send the usual warning of sensitivity to cold or sweet, so decay underneath may go unnoticed until it is extensive or the crown comes loose. Crowned teeth that still have a nerve may become sensitive, but often only once decay is well advanced.
When there are signs, people tend to notice a new ache or twinge, gum at the crown edge that bleeds or feels tender, a bad taste from one spot, a dark line or shadow at the gumline, or a crown that suddenly feels loose or comes off. None of these proves decay, and some have other explanations, but each is worth a visit. If a crowned tooth has started to feel sensitive, our guide to sensitivity after a crown covers the other possibilities.
How a dentist checks under something you cannot see through
At a check-up the dentist or OHT looks and feels along the crown margins with a fine probe, checks the gum around them, and looks for any movement in the crown. Bitewing X-rays, the small films taken from the side, show the edges of crowns on back teeth and can pick up decay starting there. They have a limit: metal and dense ceramic crowns block X-rays, so decay sitting directly under the middle of the crown may be hidden until it reaches the edge. That is one reason to keep up regular examinations even when nothing hurts.
Small repair or a new crown
What happens next depends on how far the decay has gone. A small, shallow lesion at an accessible edge can sometimes be cleaned out and sealed with a filling while the crown stays in place. When decay has spread under the crown, or the crown is loose, it usually needs to come off so the tooth can be cleaned out properly. The dentist then checks how much sound tooth is left. Often a new crown can be made, sometimes after a core build-up or root canal treatment. If too little tooth remains, removing it and replacing it with an implant, bridge or denture may need to be discussed.
The choice between crowns, onlays and fillings depends on how much healthy tooth is left, as described in our guide to inlays, onlays, crowns and fillings. For root-filled teeth, our article on crowns after root canal treatment explains why the crown is there in the first place.
Looking after the margin day to day
A crowned tooth needs the same care as any other tooth, with a little extra attention at the gumline. Angle the brush so the bristles reach just under the gum edge around the crown, and clean between the teeth every day with floss or interdental brushes sized to the gap. Fluoride toothpaste helps protect exposed root surface; spitting out rather than rinsing after brushing leaves more of it on the teeth. If you have a dry mouth or keep getting new decay, ask about higher-strength fluoride products and how often you should be seen.
Common questions
Can I get a cavity in the crown itself?
No. Crown materials do not decay. They can chip, wear or crack, but a cavity always forms in the natural tooth underneath or at the edge.
Will my crown need to come off if decay is found?
Not always. Small decay at an edge can sometimes be repaired with the crown in place, but deeper or wider decay usually means removing the crown.
Why didn't I feel anything?
The crown shields the tooth from many of the triggers that cause sensitivity, and root-filled teeth have no live nerve to respond, so decay under a crown is often painless until late.
If it has been a while since your crowns were checked, a routine examination with X-rays is the simplest way to find out how they are going. You can book through our general dentistry page, and new patient offers are listed on our current offers page.
This article is general information and not a substitute for an individual dental assessment.
