Why does floss catch on a crown and smell bad?

Key points in brief

• When floss catches on a crown, it is usually on something that is not smooth in one spot. The smell most often comes from dental plaque and bits of food that get trapped there.
• The cause may be an edge of the crown that sticks out, leftover cement, a contact with the neighbouring tooth that is too loose or too tight, decay under the edge, or tartar and gum inflammation. A dentist tells these apart after an examination.
• Keep cleaning the spot gently and show it to a dentist within 2–3 weeks, even if nothing hurts. A mouthwash may mask the smell, but it does not remove its cause.
• If the crown has started to move, show it to a dentist within a few days. If you have pain, swelling, pus or a fever, see a dentist the same day.
• The dentist can remove some of these causes without taking the crown off, for example by professional cleaning, removing cement or smoothing the edge.

What does it mean when floss catches on a crown?

Floss catches on a crown when, in one spot, it runs into something that is not smooth. This may be an edge that sticks out, a rough piece of leftover cement or a gap between the crown and the tooth. Cement is the material that fixes the crown to the tooth. With a well-fitting crown, dental floss passes between the teeth with slight resistance and then slides smoothly down the tooth to the gum.

As the floss slides down the tooth, most depends on the crown margin, that is the edge of the crown at the gum. A well-fitting margin blends into the tooth without a step you can feel. If the crown sticks out beyond the tooth, dentists call this an overhang. The floss then catches on the overhanging margin every time you pull it through, and it may fray or tear on it. With a large overhang, plaque builds up constantly, and long-standing gum inflammation may, over time, lead to loss of bone around the tooth.

An unpleasant smell or taste on the floss is usually a sign that plaque or bits of food are collecting in that spot. Bacteria break them down and produce sulphur compounds, which cause the smell. The gum around such a spot may be red and bleed when you floss. Bleeding gums like this usually mean inflammation, not damage to the gum from the floss.

If you notice an unpleasant smell from your whole mouth rather than from one spot, its cause may lie somewhere other than the crown. Tell your dentist, because this kind of smell needs a different examination. If floss catches or smells around a crown on an implant, mention this when you book the appointment, because the causes and the urgency can then be different.

What the symptoms say about the possible cause

When floss catches or smells unpleasant around one crown, the cause is usually an overhanging crown margin, leftover cement, a poor contact with the neighbouring tooth, a gap with decay under the margin, or tartar and gum inflammation. Sometimes two of these overlap. Your answers to a few questions after flossing will not replace an examination, but they will help you describe the symptom to your dentist more precisely. Questions 1–3 help narrow down the cause. Always ask yourself questions 4 and 5, because the answers decide how soon to book an appointment.

1. Does food get stuck in that spot after every meal?

If food gets pushed into the same place between the crown and the neighbouring tooth after every meal, the most likely cause is a contact between them that is too loose. The floss then passes between the teeth with almost no resistance, and the trapped food breaks down and smells. The contact may have been loose since the crown was fitted, or it may have loosened over time, for example when the neighbouring tooth moved. It usually does not close by itself - it needs to be corrected by the dentist.

2. Does the floss always catch at the same point near the gum?

At a single point near the gum, the floss may catch on an overhanging crown margin, leftover cement, tartar or a gap at the margin. If the symptom appeared soon after the crown was fitted, the dentist will first check whether any cement has been left behind. Leftover cement like this usually does not go away on its own, so do not wait for the catching to stop. If the crown is a few years old, the dentist more often looks for a gap and for decay under the margin.

3. Does the floss shred on the crown, between the teeth?

When floss passes between the crown and the neighbouring tooth only with difficulty and shreds there, the contact may be too tight. The contact may also be blocked by leftover cement, which stops the floss from getting through at all. The space between the teeth is then hard to clean - plaque stays there longer and the gum becomes inflamed more easily. Do not force the floss through - ease it in slowly with a back-and-forth movement.

4. Does the gum bleed, or does the tooth react to cold or sweet things?

A red, bleeding gum may come with any of these causes, because plaque trapped at the crown irritates it. Sometimes the crown is fine, and the only cause is gum inflammation with tartar at the crown, which the floss catches on. Sensitivity to cold or sweet things in a tooth with an older crown may point to decay under the margin. In that case, do not put off your appointment for longer than 2–3 weeks. A tooth that has had root canal treatment does not react to cold, so the absence of sensitivity does not rule out decay under the crown. If pain, swelling, pus or a fever appears, see a dentist the same day.

5. Signs of a leaking crown: does the crown move, or can you see a gap at the margin?

A crown is leaking when a gap forms between its margin and the tooth. A leaking crown causes symptoms late, and they are vague. So pay attention to a crown that moves and to a gap you can see at the margin.

A crown that moves when you floss or eat may have come unstuck from the tooth, and bacteria may be getting in under its margin. In that case, show it to a dentist within a few days, and until then do not bite on that side. Show a darkening margin or a visible gap around an older crown to a dentist within 2–3 weeks.

How the dentist checks the crown margin, the contact and the gum

The dentist checks the crown margin, the contact with the neighbouring tooth and the gum without taking the crown off. They may suggest removing it when the examination and an X-ray point to more extensive decay under the margin.

The dentist runs a probe, a thin instrument, along the crown margin. With it, they can feel a step, a gap or rough cement. Hard tartar, or dental calculus, at the crown margin also stops the floss and has to be removed at the dental practice.

The gum around crowns bleeds on examination slightly more often than around teeth without crowns or fillings, which is why the dentist checks it at every check-up. On an X-ray, they also assess the crown margin between the teeth and the bone around the tooth. In the mouth, however, it is not always possible to tell decay at the margin reliably from ordinary staining. When in doubt, the dentist may suggest a check-up after a shorter interval.

Professional cleaning, cement removal, smoothing or a new crown?

Treatment depends on the cause, and the dentist usually starts with the smallest procedure that removes it. Removing cement or tartar usually takes one appointment. From the simplest to the most extensive, the options are:

• professional teeth cleaning, that is removing tartar and plaque, together with learning how to clean that one spot, when the cause is gum inflammation
• removing leftover cement from under the margin or from between the teeth, with a hand instrument or an ultrasonic tip
• smoothing and polishing a small overhanging crown margin with fine rubber polishers and polishing strips
• repairing the margin without taking the crown off, when a small cavity has formed next to it that can be cleaned and filled
• improving the contact with the neighbouring tooth, for example with a filling on that tooth
• a new crown, when the cause cannot be removed without taking the old one off

Repairing the margin lets you keep the crown, but not always for long. Roughly half of such repairs need further treatment within a few years. Secondary caries, that is new decay at the edge of a crown or filling, does not always mean replacing the whole crown. Whether it needs replacing depends on how far the decay reaches, whether the margin is sealed all the way round and whether the crown is still firmly attached to the tooth.

Decay at the margin is one of the more common complications of crowns worn for years. This is why the dentist plans new dental crowns so that their margin blends into the tooth without a step. With a new crown, this spot also needs cleaning every day, because plaque builds up even at a well-fitting margin.

How to clean around the crown until your appointment

Do not stop cleaning the spot where the floss catches. When plaque stays there longer, the gum bleeds more and the smell gets stronger. If the floss snags as you take it out, let go of one end and pull it out sideways, instead of tugging it back up through the point where the teeth touch. Waxed floss or dental tape usually slides more easily and shreds less.

If the space between the teeth allows it, try an interdental brush in a size that goes in without force. Clean the spot once a day, gently, and bleeding in the first few days is not a reason to stop. Do not scrape at cement or a sharp edge with a toothpick, a file or any other sharp tool. You may scratch the crown or cut the gum.

Patients at Modern Dental & Orthodontics often ask about a mouthwash that would get rid of the smell around a crown. A mouthwash may mask or ease the smell for a while, but it does not remove plaque from under the margin, and it does not remove cement. You can use a mouthwash as well. Do not, however, put off your appointment because the smell has faded.

When should you have the crown checked by a dentist?

If you have pain, swelling, pus or a fever, see a dentist the same day, including at an out-of-hours emergency dental service. If the swelling is getting bigger or makes it hard to open your mouth, swallow or breathe, call 112 immediately or go to a hospital emergency department (A&E). Show a crown that moves to a dentist within a few days. Show the spot where the floss catches or smells to a dentist within 2–3 weeks, even if nothing hurts.

Frequently asked questions

Does a bad smell from under a crown mean decay?

A smell does not necessarily mean decay. Plaque and bits of food trapped at the crown margin or between the teeth also cause it. Decay under a crown can cause a smell too, especially when there is a gap at the margin, and at first it usually does not hurt. So a smell that keeps coming back despite careful cleaning needs to be shown to a dentist.

Can a water flosser replace floss?

A water flosser may help, but with this symptom it will not replace floss, because it will not show you where something catches. In short studies, a water flosser used in addition to brushing reduced gum bleeding to a similar extent as floss. Neither a water flosser nor floss will, however, remove cement or smooth the crown margin. Only a dentist can do that.

Can floss loosen a crown?

Floss should not loosen a crown that is well cemented. If the crown moves when you floss or eat, the cement under it may have washed out, and bacteria may be getting to the tooth. Do not try to press it back into place or glue it on. Show it to a dentist within a few days, and until then do not bite on that side.

Can a crown that has been taken off be put back?

A crown that has come loose can often be taken off by the dentist in one piece, and after the tooth has been cleaned, the dentist may cement it back on. A crown that is well cemented most often has to be cut to remove it, and then a new one is needed. So before taking a crown off, the dentist discusses with the patient whether a smaller procedure would be enough.

What if I am afraid of pain when the cement is removed?

Tell the dentist before the procedure. They will assess the spot and discuss with you whether you need an anaesthetic. If something hurts during the procedure, say so or raise your hand. Do not, however, put off the appointment out of fear of pain, because leftover cement usually does not go away on its own.

How many days after leftover cement is removed does the gum stop bleeding?

Bleeding usually lessens from week to week if the cause has been removed and the spot is cleaned every day. In studies of gum inflammation caused by plaque, bleeding fell almost to its level before the inflammation within about 3 weeks of thorough cleaning. If the gum is still bleeding after 2–3 weeks, tell your dentist. They will check whether something else has been left at the crown.

Will the floss still stick after the contact with the neighbouring tooth has been corrected?

The floss usually still passes through with slight resistance, as it does between your other teeth, but it should not shred or catch. This resistance is needed, because a contact that is too loose lets food through. If, after the correction, the floss catches or tears in the same spot again, tell your dentist at your check-up.

Read more:

• Dental crowns Warsaw
• Dental calculus — how to recognise it and remove it effectively
• Secondary caries: when an old filling really needs replacing, and when it can be left alone
• Bleeding gums when brushing — when is it normal and when is it a sign of disease?
• Professional teeth cleaning: What the procedure involves, does it hurt and what results does it deliver?

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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