Why the join is the weak point
A filled tooth is two materials living together in a hot, cold, wet, chemically hostile environment, under a chewing load repeated thousands of times a day. The interesting question is not why seals fail, but why they hold as long as they do.
Materials move. Tooth-coloured composite contracts very slightly as it hardens, which modern technique counters by building the filling in thin layers — but a large restoration placed in a single block starts life with more internal stress. Metal behaves differently again: it expands and contracts with temperature more markedly than tooth substance does, so decades of hot coffee followed by cold water work at the margins of an old amalgam filling like a very slow lever.
Teeth flex, and gums move. Every bite bends the tooth minutely, and in a large filling that movement concentrates at the join; grinding or clenching (Bruxismus) multiplies it, which is why a night guard (Knirscherschiene) sometimes belongs in a conversation that started with a small cavity. And if the gum has receded so that the lower margin now sits on the root surface, root dentine has no enamel armour, decays at a milder acidity and is far harder to clean.
Why the tooth stays quiet
Patients are often disbelieving when they hear a tooth needs treatment while feeling entirely fine. There are three anatomical reasons.
The first is that enamel has no nerve supply at all, so damage confined to it — or a decay front creeping under a filling but not yet deep into the dentine — has nothing to report.
The second is that the tooth defends itself. As decay advances slowly, the pulp lays down extra dentine on the inside, retreating from the advancing front. That defence works well; it also removes the early warning. A tooth quietly building barriers for two years can pass from “no symptoms” to “needs a root canal” within weeks.
The third is that the filling insulates, blocking cold and sweet stimuli from reaching the dentine — removing the very sensations that would have told you something was wrong.
How we find it
Because the tooth will not tell you, diagnosis relies on looking properly rather than waiting for symptoms.
Bitewing X-rays are the standard tool: decay between and beneath restorations shows as a darker zone where the tooth has lost mineral. One limit is worth knowing — a lesion hidden directly behind a metal filling can be masked, which is why a dentist may say “I want to see this one again in six months” rather than either drilling or dismissing it.
Clinical examination and magnification add what the image cannot show: a margin that catches a fine probe, a filling that moves under pressure, a grey shadow through the enamel, floss that shreds in the same gap. Under a microscope a hairline crack becomes visible, and decay can be removed precisely — taking out what is diseased and leaving what is sound.
A second pair of eyes. We take digital impressions with Primescan rather than trays of impression material, and we use AI support when reviewing images — alongside the dentist’s own assessment, never in place of it. On borderline findings, a system that flags a suspicious zone for a human to check is a useful safety net.
What happens when the filling comes out
Here is the part patients most want explained in advance: nobody knows the full extent until the old filling is removed. The X-ray shows a shadow; its real size becomes visible only in direct view. Three outcomes are possible, each discussed with you before anything irreversible is done.
| What we find | What it means | Practical consequence |
|---|---|---|
| Decay around the margin only, sound dentine underneath | Straightforward | A new, somewhat larger filling in the same appointment |
| Decay has undermined the walls; little sound tooth left | The tooth needs protection, not just repair | A partial crown or crown (Teilkrone / Krone), a temporary crown (Provisorium) in between, and a cost plan first |
| Decay has reached the pulp, or the nerve is inflamed | The nerve cannot be preserved | Root canal treatment (Wurzelbehandlung), then a restoration that protects the weakened tooth |
Two things follow, and neither is sales technique. An estimate for this treatment is a range rather than a fixed figure, and a plan that grows once the tooth is opened is normal; what is not normal is that growth happening without a conversation. And decay tracks sideways along the junction between enamel and dentine, so a small dark line at the edge of a filling can sit above a considerably larger cavity — not a failure of imaging, but the geometry of the disease.
Does every old filling have to be replaced?
No — and this deserves saying clearly, because worry about old fillings sends many people to the dentist unnecessarily.
A filling that is sealed, unbroken and free of decay underneath is doing its job, however old it is. Removing it means removing tooth substance with it, and every replacement cycle makes the cavity a little bigger. The judgement is about the state of the join and what lies under it, not the birthday of the restoration. Where a filling is suspicious but not clearly failed, monitoring at intervals is often the better clinical decision — the full decision path is in When does a filling need to be replaced?.
What it costs and who pays
For statutory patients, treating decay under a filling is covered treatment. The replacement filling is standard care, and since dental amalgam was phased out across the EU in 2025 the covered standard is a tooth-coloured material in front and back teeth alike. You pay something only if you choose a more elaborate technique or a laboratory-made inlay, and then only with a written additional cost agreement (Mehrkostenvereinbarung) signed beforehand.
The money question changes if the tooth has moved into crown territory. Crowns belong to the treatment and cost plan (Heil- und Kostenplan, HKP), which must reach your fund before treatment starts, and to the fixed subsidy (Festzuschuss), which rises with a complete bonus booklet (Bonusheft) — see What does caries treatment cost in Wuppertal?.
What's normal — and when to contact us
Contact us the same day if the tooth hurts spontaneously, the pain wakes you at night, biting on it is suddenly painful, or the gum or face is swollen. Those signals point at the nerve, and the options narrow the longer they are left.
Special situations
A crown rather than a filling. The same principle applies at the crown margin, usually where the gum has receded. Decay under a crown is harder to see and often shows up first as a sensitive area at the gum line or an unpleasant taste.
Several old fillings at once. Restorations placed in the same period tend to reach the end of their service life together. Sequencing matters: treat what is symptomatic or clearly decayed first, monitor the rest, and avoid a wholesale replacement programme that removes healthy tooth substance for no gain. If you also grind your teeth, a night guard protects the margins that remain.
Your German is limited. This is a topic where being able to ask questions genuinely changes the outcome, because you are consenting to a plan whose size may change mid-treatment. We explain findings in English and put the plan in writing before starting. English-Speaking Dentist in NRW covers how consultations, cost plans and insurance work for English speakers in and around Wuppertal.
No. Remineralisation only works on an intact enamel surface in the earliest stage of decay. Once bacteria are established in the dentine beneath a restoration, the tissue cannot rebuild itself and the decay must be removed. Fluoride and better cleaning slow the process; they do not reverse it there.
Two reasons. Decay may genuinely have progressed in the interval — under a filling it often advances faster than in an untouched tooth. And metal restorations cast a dense shadow that can hide a lesion directly behind them, which is why suspicious areas are re-examined at intervals rather than declared clear.
No. A dark margin is frequently staining from coffee, tea, red wine or tobacco settling in a microscopically rough join, and around amalgam it is often harmless discolouration by metal ions. It is a reason to look — with a probe, magnification and, if indicated, an X-ray — not a diagnosis in itself.
Only if the nerve is affected. Most decay found under a filling stops short of the pulp and is handled with a new filling or, where too much tooth has been lost, a partial crown. The likelihood rises the longer a tooth has been quietly decaying.
Decay under a filling is not a sign that something was done badly. It is the predictable consequence of a join between two materials living in a demanding environment for years. Because the tooth cannot warn you and the filling hides the symptoms, this is one of the few dental problems where the examination does the work that pain does elsewhere. Found early it is a slightly larger filling; found late it becomes a crown or a root canal.
Zahnarztpraxis Tsypin · Wuppertal. This article was medically reviewed and is provided for information; it does not replace an examination. Updated August 2026.
