Key points in brief
- A black line around a crown is usually the metal edge of an older crown, a root exposed by a receding gum, a discoloured gum, or a stained or leaking margin under which decay may be developing.
- What needs treatment above all is decay under the crown and an inflamed, bleeding gum. Visible metal next to a healthy gum is mainly a matter of appearance, so the decision to replace the crown is yours.
- The cause is identified by a dentist, because in the mirror all four look similar. If the line has not changed for a long time, show it at your next check-up.
- Show a new or widening line, or a sensitive tooth, to a dentist within a few weeks. If there is pain, swelling or pus around the tooth, see a dentist the same day.
- A new crown helps mainly when the cause is metal or decay. If the gum is receding, the edge of a new crown may also become exposed over time, which is why the gum is assessed first.
Where does a black line around a crown come from?
A dark line around a crown usually has one of four causes: the metal edge of the crown, an exposed root, a discoloured gum, or a stained or leaking margin. The crown margin - that is, its edge at the gum - is where the crown, the tooth and the gum meet. A change in any of these three parts casts a similar shadow.
If your gums are receding around many teeth, including teeth without crowns, start with what gum recession involves and when it is treated. With a crown on an implant the causes are partly different, because there is no natural tooth under the crown.
Metal crown margin
A porcelain-fused-to-metal crown has a thin metal framework under the porcelain. At the gum it can show as a narrow grey or black strip, especially if the gum has dropped even slightly. Metal showing at a crown is not a sign of disease, as long as the gum is healthy.
An exposed root
When the gum recedes, part of the root emerges from under the crown. The root is not covered with enamel, so it has a darker, more yellow shade than the crown, and after root canal treatment it can be grey. The strip of root between the crown and the gum then looks like a dark line, even with a crown that contains no metal.
A discoloured gum
A bluish or grey gum above a crown is sometimes only a shadow. The metal framework of the crown, or the dark root of a tooth after root canal treatment, then shows through a thin gum, even though neither the metal nor the root itself can be seen. When it is the root that shows through, a new crown will not change this, because the crown ends at the gum and the grey root lies beneath it.
A black gum above a crown is sometimes also caused by tiny particles of metal that got into the gum while old metal crowns or silver amalgam fillings were being drilled away. A metal tattoo of this kind looks like a grey or dark blue spot, does not hurt and is not an infection. If the gum is also swollen, see a dentist the same day. If it bleeds when you brush, show it to a dentist within a few weeks, because this may be inflammation.
If a dark patch on the gum is new or getting bigger, show it to a dentist within a few weeks. Only the dentist can tell metal particles apart from other changes.
A stained or leaking margin
Sometimes only the margin of the crown becomes stained, with no decay underneath. The tooth under a crown can still decay, however, and the decay then starts at the margin. When a gap forms between the crown and the tooth, the cement that holds the crown in place may gradually wash out. Food stains and bacteria settle in the gap. A dark, sometimes rough line can then be the first visible sign of decay under the crown. It is the same secondary caries that develops under old fillings.
Studies of zirconia crowns and bridges suggest that, years later, decay at the margin is among the most common problems of the tooth under the crown, alongside problems with the pulp, that is the nerve and blood vessels inside the tooth. Decay under a crown needs treatment, even if nothing hurts.
What you can check in the mirror before your appointment
Compare the crown with the neighbouring tooth in daylight - ideally by a window, with a small hand mirror. Look out for a few things:
- whether the dark strip is on the crown itself or on the gum above it
- whether the line is new
- whether the gum around the crown is red or bleeds when you brush
- whether dental floss catches or shreds at the edge of the crown
- whether the tooth reacts to cold or to sweet things
These observations are not enough to establish the cause. They do, however, help the dentist decide where to start the examination, so mention them at the beginning of the appointment.
Polishing, gum treatment or a new crown?
What will help depends on the cause of the line. A few popular beliefs can be misleading here.
| Common belief | What is usually the case | What helps |
|---|---|---|
| It is definitely decay | It may also be metal, an exposed root or a discoloured gum. | an examination at the practice and an X-ray |
| A thorough clean is all it needs | Deposits and tartar around the crown come off during a professional clean, that is cleaning and polishing at the practice. Metal, the root and a discoloured gum remain. | a professional clean, then an assessment of what is left |
| A new crown will get rid of the line straight away | It helps when the cause is metal or decay. | an assessment of the gum first, then a decision about the crown |
| If it doesn't hurt, nothing is happening | Decay under a crown causes no symptoms for a long time. | check-ups and an X-ray |
| A dark spot on the gum is an infection | It may be metal particles in the gum, which do no harm. Even so, show a new or growing patch to a dentist. | an assessment by a dentist, then removal with a laser if it bothers you |
Whatever the cause, the order of treatment is usually the same. The dentist treats decay and gum inflammation first and deals with appearance last. A new crown for the sake of appearance is made only once the gum is healthy and no longer changing position.
Every replacement means reshaping the tooth again - only slightly, but irreversibly - so when the crown still seals well, a decision to replace it for appearance alone is worth thinking through calmly with your dentist. If you decide to replace it, there are now dental crowns without a metal framework, made of zirconia or all-ceramic, and the material is easier to choose once you compare zirconia, all-ceramic and metal-based crowns.
When the cause is a receding gum
If a black line around a crown has appeared because the gum has receded, it is worth showing it to a dentist, even if nothing hurts. An exposed root is more prone to decay than enamel, and in some people it reacts to cold with pain.
If the exposed strip is narrow and the gum is healthy, switching to a gentler brushing technique and keeping an eye on it at later visits is often enough. If the gum keeps receding or the exposed root is sensitive, a periodontist, that is a dentist who treats gum disease, may cover the root with a connective tissue graft. This is a thin piece of tissue, usually taken from the palate. When the gum has receded at a single tooth, this is the method dentists choose first. For a tooth with a crown, the procedure is planned together with a prosthodontist, that is the dentist who makes crowns, because the crown sometimes has to be replaced after the graft.
The gum around a crown can also recede when the edge of the crown sits deep under the gum and constantly irritates it. Around ceramic crowns, thin gums recede more often than thick ones, so before a new crown is planned, ask your dentist how thick your gum is.
When the dentist suspects a leaking margin or decay
A dentist suspects decay under a crown when the dark line feels rough and a fine instrument catches in the gap between the crown and the tooth. Other signs that point to a leaking margin are:
- sensitivity of that tooth to cold or to sweet things
- an unpleasant smell or taste around the crown
- food that keeps getting trapped at the edge
- a crown that has started to feel loose
- a red gum despite careful brushing
A stained margin on its own is not yet decay, so when there is only staining, with no gap that can be felt and no symptoms, the crown is usually not replaced. During the examination, however, it can be difficult to tell staining from decay. People are more at risk of decay at the crown margin if they often eat or drink something sweet, have a dry mouth, or have a crown in a place that is hard to clean thoroughly.
The dentist can sometimes fill a small, easily reached cavity at the edge without removing the crown. With more extensive decay, the crown is taken off, the decay is removed and a new crown is made. If the decay has reached the pulp, the tooth is first given root canal treatment. When decay has destroyed a large part of the tooth under the crown, it sometimes cannot be saved.
How the dentist examines the crown and the tooth
The dentist looks at the dried crown margin in good light, often under magnification. When planning treatment at our practice in Warsaw's Wola district, we pay particular attention to comparing the tooth with older photographs of the patient's smile, if the patient brings them.
Next, the dentist uses a fine instrument to check whether the edge is smooth and well sealed. They also measure the pockets around the tooth, that is they use a thin probe to check how deep the gum comes away from the tooth and whether it bleeds when they do so. Finally, they usually take an X-ray. On the X-ray, metal and zirconia hide the inside of the tooth under the crown, so what it mainly shows are the margins on the sides facing the neighbouring teeth.
No single test reliably shows decay at the margin. Each of them can also sometimes point to decay where there is none. Sometimes certainty comes only from what the dentist sees after taking the crown off the tooth. This is usually done when several signs point to decay, because a crown that has been taken off often has to be replaced with a new one.
When the examination shows neither a gap nor decay and the gum is healthy, monitoring is usually enough. The dentist then compares the crown margin at later check-ups and, if something raises doubt, repeats the X-ray after a few months. If the black line around the crown does not change during that time, it is usually left untreated.
What should you do if you notice a dark crown margin?
If there is pain, swelling or pus, see a dentist the same day. If the crown feels loose, show it to a dentist within a few days. If the line or patch is new or widening, if the tooth reacts to cold or sweet things, or if the gum next to it bleeds, show the tooth to a dentist within a few weeks. If the line has looked the same for a long time, show it at your next check-up. If you do not know how long it has been there, treat it as new. Note down when you first noticed it, and bring older photographs of your smile to the appointment if you have any.
Consult your case with an expert and get your treatment plan
Frequently asked questions
Can a dark crown margin be removed without replacing the crown?
A dark margin can sometimes be removed without a new crown. Deposits and tartar at the edge come off during a professional clean with polishing, and the dentist may cover an exposed root with a gum graft or, if the strip is small, with a tooth-coloured filling. Metal showing from under the porcelain, however, usually cannot be masked permanently without a new crown. A metal tattoo in the gum is removed separately, for example with a laser.
Will whitening lighten a dark crown margin?
No. Whitening agents lighten only enamel and dentine, that is the tissues of your natural teeth. Porcelain, zirconia and metal keep their colour, and so does a discoloured gum. After whitening, the neighbouring teeth may even become lighter than the crown. So if you are planning to replace a crown, the teeth are usually whitened first, and the shade of the new crown is matched to the whitened teeth.
Why did the black line around my crown appear only years later?
A line often appears years later because the gum slowly drops and exposes the edge of the crown or the root. Over time the cement at the margin may also wash out, and stains build up in the gap this leaves. So a crown that looked good for a long time was not necessarily badly made. It is still worth showing it to a dentist, who will check whether the margin is still sealed.
Does an all-ceramic crown prevent a dark line?
An all-ceramic crown prevents a dark line only partly. It contains no metal, so it will not produce a grey strip from a framework and will not darken a thin gum. The gum can still recede around this kind of crown too, exposing its edge and the root. This happens with some all-ceramic crowns on front teeth, and more often in people who smoke.
Can I remove the discolouration around a crown myself?
No. Scraping at the line with a fingernail, a toothpick or a sharp tool easily damages the gum and the edge of the crown. Whitening and abrasive toothpastes will not remove metal or staining under the margin, and they may dull the surface of the crown. What helps at home is careful brushing along the gum with a soft toothbrush and cleaning around the crown every day with floss or an interdental brush.
Does the gum look different for the first few weeks after a crown is replaced?
Yes, the gum usually looks different for a few weeks. After the old crown has been removed, the tooth reshaped and an impression or scan taken, the gum can be red and slightly swollen, and it may bleed when you brush. It gradually settles around the new crown. If after 2 weeks the gum still hurts or bleeds heavily, come in for a check. A dark spot caused by metal particles in the gum will not disappear on its own after the crown is replaced.
How long after a gum procedure can you see the final result?
The final appearance is usually assessed after a few months. After the procedure, the gum may go on changing position for a few months, so the dentist checks it at later check-ups. A new crown, especially on front teeth, is planned only once the gum has stabilised. Until then, a temporary crown can protect the tooth.
Read more:
- Dental crowns Warsaw
- Gum recession — why the gum recedes and when surgical treatment is needed
- Secondary caries: when an old filling really needs replacing, and when it can be left alone
- Zirconia, all-ceramic or metal-based crown? A comparison of modern prosthetic solutions
- Gingival phenotype — thin or thick? Why it matters for treatment
Content and liability disclaimer
This article is informational and educational in nature and does not constitute medical advice, a diagnosis or a therapeutic recommendation — it does not replace a consultation with a specialist. If you are experiencing symptoms, have doubts or are facing a decision about treatment, consult a dentist. The methods described and the data cited are general in nature. The results of scientific studies relate to populations, not to an individual patient. The course and outcome of therapy depend on the individual clinical situation and may vary between patients. No information contained in this article constitutes a guarantee of result. The content has been prepared with due care, based on publicly available medical knowledge and the scientific publications indicated in the Sources section. We do not advise taking or refraining from any health-related action solely on the basis of the content of this article, without prior consultation with a doctor.
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