The short answer
A crown (Krone) cannot decay — ceramic, zirconia and metal are not food for bacteria. The natural tooth underneath it can. Decay under a crown almost always begins at one specific place: the crown margin (Kronenrand), the fine line where the edge of the crown meets your own tooth at the gum. Because the crown covers everything above that line, this kind of decay is easy to miss and often causes no pain at all until it is well advanced — particularly on a tooth that has already had root canal treatment (Wurzelbehandlung) and no longer has a nerve to complain. That is why the check-up and the bitewing X-ray matter more once you have crowns, not less.
The crown is not the tooth
Patients often assume that once a tooth is crowned, it has been taken out of circulation — armoured, closed, finished. It is a comforting idea and it is half true.
What a crown does is cover the visible part of the tooth in a material bacteria cannot dissolve. What it does not do is remove the tooth. Underneath the ceramic there is still living dentine, a root, a nerve in many cases, and a border where the artificial part stops and your own tooth continues.
Dentists call decay at that border secondary caries — decay next to or beneath an existing restoration. It is one of the most common reasons crowns and bridges have to be replaced, and it has nothing to do with the quality of the crown itself. A perfectly made crown sits on a tooth that still ages, in a mouth that still has bacteria in it.
Why the margin is the weak point
Several things converge at that one thin line.
Plaque collects there. The transition between crown and tooth is a slight step, however finely it is made, and a step at the gum line is a place a toothbrush glides over rather than into.
The gum recedes over the years. When it does, it exposes root surface below the crown edge. Root surface is not enamel — it is cementum and dentine, softer and considerably quicker to decay. This is the single biggest reason older crowns develop decay: the tooth changes shape around a crown that stays the same.
Cement can wash out. The layer holding a crown in place is thin. Over many years it can dissolve at the edge, leaving a microscopic gap where bacteria settle in a space no brush can reach.
Grinding loosens things. Teeth grinding (Bruxismus) applies forces a crown was never designed for, and repeated flexing works away at the cement seal.
Old crowns were made differently. A crown fitted decades ago, from an impression taken with a tray, has a margin made to the standards and tools of its time. Digital impressions and work under a microscope have narrowed those tolerances considerably.
None of this means a crown is a bad idea. It means a crowned tooth needs its margin looked after, the way a well-made window still needs its seal checked.
Why you often feel nothing
This is the part that catches people out.
On a tooth with a live nerve, early decay under a crown may show up as sensitivity to cold or sweetness. That is a useful warning and worth acting on.
On a root-treated tooth, there is no warning at all. The nerve is gone; the tooth cannot report pain from decay. Decay can quietly progress through the dentine, and the first sign the patient notices is the crown coming loose — because there is no longer enough sound tooth holding it. Since a large share of crowned teeth are root-treated, this silence is common rather than exceptional.
The signs that do reach you are indirect, and worth knowing:
- Floss shreds or catches at the same spot every time
- A persistent bad taste or odour from one particular tooth
- Food packing into the same gap after every meal
- The gum bleeding at one isolated point, while the rest of your mouth is calm
- A dark line appearing at the gum edge of the crown
- The crown feeling slightly loose, or a click when biting
- A dull ache on biting in a tooth you thought was “done”
One of these on its own is not a diagnosis. Any of them is a reason to book a look.
How we find it before you feel it
Bitewing X-rays are the workhorse. They show the areas between teeth and around crown margins that no examination by eye can reach. Metal crowns cast a shadow that hides part of the picture, which is one reason X-rays are read alongside a careful clinical examination rather than instead of it.
The microscope changes what the examination itself can see. Under magnification, a marginal gap, a hairline crack in the tooth or a softened edge is visible in a way it simply is not with the naked eye.
Probing and flossing the margin tells us whether the edge is smooth, whether it catches, and whether the gum around it is healthy or chronically irritated.
A 3D X-ray (DVT) is used where a specific question needs it — a suspected root fracture, an unclear finding at the root tip — not as routine. In our practice a DVT costs 178 €.
AI-assisted review of scans flags areas worth a second look. It supports the dentist’s own assessment; it never replaces it, and every finding is confirmed by a person before anything is proposed.
If decay is found: what actually happens
Be prepared for a slightly uncomfortable truth. In most cases, the crown has to come off. There is no reliable way to treat decay hidden under a crown while leaving the crown in place, and patching over the top would seal bacteria in.
What happens next depends on what is underneath:
Enough healthy tooth remains. The decay is removed, the tooth is built up again, and a new crown is made. This is the ordinary outcome and the reason early detection is worth so much.
A lot of tooth is gone. The nerve may now be involved, in which case root canal treatment comes first, often followed by a post to give the new crown something to hold on to.
Decay has run below the bone level, or the root is fractured. Then the tooth may not be restorable, and the conversation turns to extraction and replacement — a bridge (Brücke) or an implant (Implantat). Which of those makes sense is a separate decision: Crown or implant: which one does your tooth need? (EN).
On the money side, keep one point in mind: a replacement crown needs its own treatment and cost plan (Heil- und Kostenplan, HKP), submitted to your statutory fund before the work starts, and it attracts the fixed subsidy (Festzuschuss) in the same way the first crown did — between 60 % and 75 % of the cost of standard care depending on your bonus booklet (Bonusheft), and up to 100 % under the hardship provision. Our own dentures carry a two-year warranty; new decay years later is a new finding rather than a defect in the earlier work, and it is treated as such.
Prevention that actually works at the margin
General advice — “brush twice a day” — is not enough here, because the risk is concentrated in a place a brush barely touches.
Clean between the teeth every day, with the right size. An interdental brush that matches your gap does more for a crown margin than any toothbrush. Sized too small it passes through without touching the surfaces; too large it traumatises the gum. Have the size checked rather than guessed.
Use floss threaders or superfloss under bridges. A bridge has an underside that ordinary flossing never reaches. That underside is where problems start under bridgework.
Keep to your professional cleaning intervals. Professional cleaning (professionelle Zahnreinigung, PZR) reaches biofilm at the gum line and just below it, and the appointment doubles as a close inspection of every margin you have. How often you need it depends on your gums and your risk profile, not on a fixed rule.
Fluoride, deliberately. Fluoride toothpaste is the baseline; where root surfaces are exposed, a high-fluoride gel or varnish is often recommended. German dental associations consistently place fluoride at the centre of caries prevention.
Deal with grinding. If you grind or clench, a night guard protects the crown, the cement seal and the tooth underneath.
Watch the frequency of sugar, not just the amount. Six sweetened coffees spread across a working day are harder on a crown margin than one dessert.
Take dry mouth seriously. Saliva is your natural rinse. Many common medications — for blood pressure, allergies, depression — reduce it. If your mouth is dry, say so; it changes the prevention plan.
Smoking worsens gum health and speeds up recession, which exposes exactly the surfaces we are trying to protect.
Who is at higher risk
- Anyone whose gums have receded, exposing root surface at the crown edge
- Patients with a history of gum disease / periodontitis (Parodontitis)
- Root-treated teeth, because they give no pain warning
- People with several crowns and bridges — more margins, more surface to look after
- Grinders and clenchers
- Patients with dry mouth, whatever the cause
- Poorly controlled diabetes, which affects gum health and healing
- Anyone whose crowns are twenty years old or more
If two or three of these apply to you, your check-up interval should reflect that.
What's normal — and when to book an appointment
Only in unusual situations, for example where the decayed spot lies on the exposed root surface just below the crown edge and the crown itself still fits perfectly. In most cases the extent of the decay only becomes visible once the crown is off, and leaving hidden decay in place is not an option.
There is no single interval that fits everyone; it depends on your risk profile, the age of the crowns and the state of your gums. Your dentist should be able to say why your interval is what it is. X-rays are taken when there is a clinical reason, not routinely on a calendar.
The material of the crown is not what decides whether the tooth underneath decays. Precision of fit at the margin, your gum health and daily cleaning between the teeth decide it. Materials differ in appearance, strength and how they behave against the opposing tooth — that is a different comparison.
Yes. “Feels fine” is exactly what a root-treated crowned tooth with decay underneath feels like. An examination plus an X-ray is a short appointment and tells you whether the margin is still sealed.
Sometimes, if the tooth underneath is sound and the crown is undamaged. But a crown that comes off after years usually comes off for a reason — decay, a failed cement seal, or a fracture. That reason has to be found before anything is stuck back on.
Usually not simply that. Receding gums, dry mouth from medication, grinding and the sheer age of a restoration all contribute, and none of them is a matter of effort. What you can influence is daily cleaning between the teeth and turning up for check-ups.
The crown is safe. The tooth underneath is not automatically safe with it. Everything worth doing is concentrated on one thin line at the gum: clean it daily with an interdental brush that actually fits, keep your professional cleaning appointments, and take seriously any spot where floss shreds, food packs or the gum bleeds on its own. Caught early, decay under a crown means a new crown. Caught late, it can mean losing the tooth — and that difference is usually decided at a check-up, not in the treatment chair.
Zahnarztpraxis Tsypin · Wuppertal. This article was medically reviewed and is provided for information; it does not replace an examination. Updated August 2026.
