Your questions, answered · Bolton Park Dentistry · 9 min read
Yes, you can get a cavity under a crown. The crown itself can’t decay, but the natural tooth underneath still can, usually right at the edge where the crown meets your tooth near the gum line. It’s a fairly common problem, it often has no symptoms early on, and it’s very treatable when a dentist at Bolton Park Dentistry spots it during a routine check.
Think of a crown as a cap that covers most of the tooth. “Most” is the key word. There’s a thin seam where porcelain or metal ends and real tooth begins, and plaque loves that seam. Regular cleanings with the hygiene team keep it clear and give your dentist a chance to look for trouble before you feel any.
If decay sneaks deep under the crown, it can reach the nerve. That might mean root canal therapy and a new crown. In the rare case the tooth can’t be saved, implant restoration can replace it with something that looks and works like a natural tooth.
Most people never get that far. A crown that’s checked twice a year can last 10 to 15 years or longer, and you can book a checkup online whenever you’re due.
Quick answer
Decay under a crown starts at the margin, the line where the crown meets your natural tooth. It’s common in older crowns, in mouths with dry mouth or gum recession, and when flossing gets skipped. Caught early, it can sometimes be repaired. Caught late, the crown usually needs to be replaced.
01How does a cavity form under a crown?
A crown is cemented onto a tooth that has been shaped to receive it. The cement fills the tiny gap between the two.
Over years, a few things can open that seal:
- The cement slowly washes out at the edge
- Gums recede and expose natural tooth below the crown edge
- Chewing forces flex the tooth and the crown slightly differently
- A small chip at the crown edge creates a ledge where plaque collects
Once bacteria get into that gap, they do what they always do. They eat sugars, make acid, and dissolve tooth structure. The difference is that the crown hides the damage, so it can grow quietly.
02Who is most at risk?
Some mouths are more prone to this than others. You’re at higher risk if:
- Your crown is more than 10 years old
- You have dry mouth from medications, mouth breathing, or certain health conditions
- Your gums have receded around the crowned tooth
- You snack or sip sugary drinks often during the day
- You have a history of lots of cavities
- Flossing around the crown is hit or miss
Dry mouth is a big one. Saliva washes away food and neutralizes acid. Many common medications, including some for blood pressure, allergies, and mood, reduce saliva flow. If you take daily medications, tell your dentist.
03Signs of decay under a crown
Early on, there may be no signs at all. That’s why X-rays and exams matter so much for crowned teeth.
When symptoms do show up, they often look like this:
- Sensitivity to hot, cold, or sweets on a crowned tooth that used to feel fine
- A dark line or grey shadow at the gum line around the crown
- A bad taste or smell coming from one area
- Floss that shreds or catches at the crown edge
- Gum that bleeds or looks puffy around just that one tooth
- A crown that feels loose or shifts when you bite
- Pain when you bite down, if decay has reached the nerve
Safety note
If a crown comes off completely, keep it in a clean container and call your dentist. Don’t glue it back with household adhesive. Superglue and similar products are toxic to the tissues and can trap bacteria under the crown.
Why doesn’t it always hurt?
If the tooth had a root canal before the crown was placed, there’s no living nerve inside. Decay can spread under that crown without any pain at all. These teeth need regular X-rays more than any others.
04How dentists find hidden decay
Your dentist uses a few tools together, since none of them is perfect alone.
- Visual exam of the crown margin, looking for staining, gaps, or rough spots.
- Probing the edge gently with an instrument. A sound margin feels smooth. A leaking one catches.
- Bitewing X-rays, which show decay between teeth and under crown edges.
- Periapical X-ray if there’s any chance the nerve or bone is involved.
- Bite test to see whether the crown is loose or the tooth is cracked.
Metal and some ceramic crowns block X-rays, so decay right under the middle of the crown can be hard to see. The edges show up much better, and that’s where most decay starts anyway.
05Treatment options: what happens next?
The plan depends on how much decay there is and where it sits.
A repair filling is the smallest and quickest fix, but it only works when the decay is shallow and easy to reach. Your dentist will tell you clearly which option makes sense for your tooth.
Will I need a new crown?
Often, yes. If decay has crept under the crown more than a millimetre or two, there’s no way to clean it out properly without removing the crown. Leaving it would only let the decay keep spreading.
The good news: replacing a crown is a routine procedure. The tooth gets numbed, the old crown is removed, the decay is cleaned out, and an impression is taken for the new one. A temporary crown protects the tooth for about 2 weeks until the final crown is ready.
06Questions to ask about an older crown
If you have a crown that’s been in your mouth for a decade or more, these questions are worth bringing to your next visit:
- Does the edge of the crown still fit tightly against the tooth?
- Is there any recession exposing natural tooth below the crown?
- When was this tooth last X-rayed?
- Did this tooth have a root canal? If so, how often should it be checked?
- Would a fluoride varnish or prescription toothpaste help in my case?
- Are there signs of grinding that could stress the crown?
Your answers shape how closely the tooth should be watched. A crown with a tight edge and healthy gums might only need routine checks. A crown with recession and a history of decay nearby may need X-rays more often.
What crown materials are common today?
Most crowns placed today are one of three types:
- All-ceramic or porcelain: The most natural look, often used on front teeth.
- Zirconia: Very strong and tooth-coloured, popular for back teeth.
- Porcelain fused to metal: A metal base with porcelain on top. Strong, but a dark line can show at the gum over time.
Any of them can develop decay underneath if plaque sits at the margin. The material choice is about strength and appearance, and daily cleaning is what protects the tooth below.
07Myths about crowned teeth
Myth: Once a tooth has a crown, it’s protected from cavities forever.
Fact: The crown can’t decay, but the tooth underneath can, especially at the gum line.
Myth: If a crowned tooth doesn’t hurt, it’s fine.
Fact: Teeth with root canals have no nerve to warn you. Decay can grow silently.
Myth: Crowns need to be replaced every 5 years.
Fact: Many crowns last 15 years or more with good care. There’s no automatic expiry date.
08How to protect your crown for the long haul
A few habits make a real difference.
- Floss every day around the crown. Slide the floss down, wrap it around the tooth, and pull it out sideways if it catches. A floss threader or water flosser helps with bridges.
- Use a fluoride toothpaste and brush along the gum line at a 45 degree angle.
- Limit sugary sips and snacks between meals. Frequency matters more than amount.
- Ask about prescription fluoride if you have dry mouth or a history of decay.
- Wear a night guard if you grind. Grinding stresses the crown edge and can open the seal.
- Keep up with cleanings and X-rays on the schedule your dentist suggests.
10 to 15 years is a common lifespan for a well-cared-for crown.
The margin is where almost all decay under crowns begins.
Every 6 months is the usual exam schedule for most adults.
09A quick real-world example
A patient comes in for a regular cleaning. Their 12-year-old crown on a lower molar feels perfectly fine. The hygienist notices the floss shreds on one side, and the X-ray shows a small shadow at the crown edge.
Because it was caught at the edge, the dentist replaces the crown before decay reaches the nerve. One appointment to prepare, one to place. No root canal, no extraction. That’s the best-case outcome, and it happens because someone looked.
10Getting care from a dentist in Bolton
If you have crowns, you want a dentist in Bolton who keeps a close eye on them. Bolton Park Dentistry is a trusted family dental clinic with a calm, welcoming office designed to ease anxiety. If dental visits make you tense, sedation options are available.
The clinic accepts new patients from Bolton and surrounding communities. When something feels wrong, like a crown that suddenly feels loose or a tooth that starts throbbing, same-day appointments are offered when available for urgent needs.
Care at the clinic covers hygiene and preventive visits, amalgam-free and bonded fillings, crowns, bridges, extractions, root canal therapy, implant restoration, porcelain veneers, teeth whitening, athletic sports guards, TMJ/TMD therapy, and non-surgical gum therapy. Consultations for implants and braces are free.
All dentists are licensed by the Royal College of Dental Surgeons of Ontario (RCDSO), so treatment follows provincial standards for safety and informed consent.
Call 647-496-2336, email reception@boltonparkdentistry.com, or visit the office at 14 Parr Blvd Unit 5, Bolton, Ontario L7E 4H1. You can also learn more at boltonparkdentistry.com.
11Frequently asked questions
Can decay under a crown be fixed without removing it?
Sometimes. If the decay is small and sits at an exposed edge near the gum, a dentist can clean it out and seal it with a bonded filling. If it has spread under the crown, the crown usually has to come off.
How long can decay under a crown go unnoticed?
It can grow for months or even years without pain, especially on a tooth with a past root canal. Regular X-rays are the most reliable way to catch it early.
Why does my crowned tooth smell bad when I floss?
A bad smell from one spot often means bacteria are trapped at the crown edge or there’s decay or gum inflammation nearby. It’s worth an exam soon, even if nothing hurts.
Can a cavity under a crown cause an infection?
Yes. If the decay reaches the nerve, the nerve can become infected and form an abscess at the root tip. Signs include swelling, throbbing pain, and a bad taste. This needs prompt treatment.
Are some crown materials better at preventing decay?
Fit and home care count most. A well-fitted crown with a smooth margin, cleaned daily, has the best chance no matter whether it’s porcelain, zirconia, or metal.
12Conclusion
Crowns protect teeth well, but the natural tooth underneath still needs daily care.
Most decay under crowns starts at the edge and can be caught early with regular exams and X-rays.
Keep up with flossing and checkups, and your crown can serve you comfortably for many years.