Yes, you can get a cavity under a filling. Dentists call it recurrent or secondary decay, and it happens when bacteria slip into a tiny gap along the edge of an old or worn filling. It’s common, especially with fillings over 10 years old, and it’s very fixable when caught early, usually by replacing the filling with new tooth-coloured fillings and restorations.
The catch is that decay under a filling often doesn’t hurt until it’s deep. By the time you feel it, the filling may need to be upgraded to an inlay or a crown, and that’s where CEREC same-day crowns save you a second appointment and a temporary crown.
Fillings don’t last forever. They’re under constant stress from chewing, temperature swings and acids, and every material has a lifespan, which is why checking old fillings is built into the general and cosmetic dental services at every recall visit. If the decay underneath reaches the nerve, the tooth may need root canal treatment before it can be rebuilt.
This guide explains why it happens, the warning signs, how dentists spot it on X-rays, and what each treatment involves. It’s written from the chair side of a busy dental clinic in Woodbridge, where replacing worn fillings is one of the most routine jobs of the week.
What is secondary decay?
Secondary decay (also called recurrent caries) is new tooth decay that forms at the edge of, or underneath, an existing filling or crown. The original filling stays in place. The tooth around it breaks down.
It’s one of the most common reasons fillings get replaced. Some studies put it at the top of the list for failed restorations in adults.
How does a cavity form under a filling?
Think of a filling as a patch glued into a hole. The glue line is the weak point.
- A gap forms at the margin. Over years, the bond between filling and tooth can break down. Fillings also expand and shrink a tiny bit with hot and cold food.
- Bacteria get in. The gap might be thinner than a hair, but it’s big enough for plaque bacteria.
- Acid dissolves the tooth. Bacteria feed on sugar and make acid. The acid eats into the tooth beside and under the filling.
- The decay spreads quietly. Since the filling covers the area, you can’t see it, and it often doesn’t hurt early on.
Who’s more likely to get decay under a filling?
- People with fillings more than 10 to 15 years old
- Anyone with a dry mouth from medications, mouth breathing or health conditions
- Frequent snackers and sippers, especially sweet coffee or pop through the day
- People who grind their teeth, which stresses filling edges
- People with large fillings that cover most of the tooth
- Those who skip cleaning between their teeth, where many fillings sit
Signs you might have a cavity under a filling
Early secondary decay is often silent. These are the signs that tend to show up as it grows:
- Sensitivity to cold, hot or sweet in a tooth that’s been filled
- Pain when biting down on the filled tooth
- A dark grey or brown line around the edge of a filling
- Floss that catches or shreds at the filling
- A rough edge you can feel with your tongue
- A piece of the filling or tooth breaking off
- Bad taste or smell from one area
Good to know: Some staining around old silver fillings is harmless discolouration from the metal. Only an exam and X-ray can tell stain from decay, so don’t assume either way.
How dentists find decay under fillings
This is why regular checkups matter so much. Secondary decay is usually caught at a routine visit before you notice anything.
Visual and tactile exam
Your dentist checks each filling edge with good lighting and magnification, and gently feels for gaps or soft spots.
Bitewing X-rays
These show the areas between back teeth and under fillings. Decay looks like a dark shadow next to or under the bright white filling. Digital X-rays use a small fraction of the radiation of old film.
Other tools
Some offices use laser fluorescence devices or transillumination (a bright light shone through the tooth) to spot early decay.
How is a cavity under a filling treated?
The fix depends on how much healthy tooth is left once the decay is cleaned out.
Replace the filling
When decay is small, the old filling and the decay are removed and a new filling is placed. Composite (tooth-coloured) fillings bond to the tooth and seal well.
Inlay or onlay
When a bigger section is missing, a ceramic inlay or onlay fits inside or over part of the tooth. It’s stronger than a large filling.
Crown
When most of the tooth is a filling, or the tooth has cracks, a crown covers and protects the whole thing. With CEREC, the crown is designed from a digital scan and milled from a solid ceramic block during the same visit.
Root canal, then a crown
If decay reaches the pulp, the infected nerve is removed, the canals are cleaned and sealed, and the tooth is then crowned.
Extraction
If the tooth is too broken down to restore, removal followed by an implant, bridge or partial denture may be the best option.
Treatment options at a glance
| Option | When it’s used | Visits |
|---|---|---|
| New filling | Small, shallow decay | 1 |
| Ceramic inlay or onlay | Medium loss of tooth | 1 with CEREC |
| Crown | Large loss, cracks | 1 with CEREC |
| Root canal plus crown | Decay at the nerve | 1 to 3 |
| Extraction plus replacement | Tooth can’t be restored | Several |
Should you switch from silver to white when a filling fails?
When an old amalgam filling has decay under it, it has to come out anyway. That’s the natural time to talk about what goes back in.
| Factor | Composite (white) | Ceramic (CEREC inlay, onlay or crown) | Amalgam (silver) |
|---|---|---|---|
| Looks | Matches tooth | Matches tooth, holds colour well | Grey, visible |
| Bonds to tooth | Yes | Yes | No, held in mechanically |
| Tooth removed to place it | Less | Moderate | More, for retention |
| Best for | Small to medium cavities | Large repairs, cracked or weak teeth | Some back-tooth cases where moisture control is hard |
For most people replacing a failed filling in Woodbridge today, composite or ceramic is the usual choice. If a large silver filling has left thin, cracked walls, a ceramic onlay or crown protects what’s left better than another big filling would.
How long do fillings last?
Rough ranges, which vary a lot person to person:
- Amalgam (silver): often 10 to 15 years or more
- Composite (white): often 7 to 10 years or more
- Ceramic inlays and onlays: often 10 to 15 years or more
- Gold: can last 20 years or longer
Your habits move these numbers more than the material does. Someone who cleans between teeth daily and doesn’t grind can get decades out of a filling. A heavy snacker with a dry mouth might get far less.
Myths about fillings and decay
“A filled tooth can’t get another cavity.”
The filling material can’t decay, but the tooth around it absolutely can.
“If it doesn’t hurt, the filling’s fine.”
Secondary decay is often painless until it’s close to the nerve. X-rays catch it earlier than pain does.
“Old silver fillings should all be swapped out.”
Not automatically. Health Canada considers amalgam safe for most people, and drilling out an intact filling removes healthy tooth. Replace a filling when it’s failing, leaking or decayed, and talk through materials with your dentist.
“White fillings don’t last.”
Modern composites are much stronger than they were 20 years ago. For small and medium fillings, they hold up well.
A typical case
A patient came in for a routine cleaning with no complaints. Her bitewing X-ray showed a dark shadow under a 15-year-old filling on a lower molar. Clinically, the edge had a fine gap. Because it was caught early, she needed a new composite filling, done in about an hour, with no root canal and no crown.
Another patient waited after noticing cold sensitivity in a filled tooth for most of a year. When he came in, the decay had reached the nerve. He needed a root canal and a CEREC crown. Same tooth, very different outcome, mostly because of timing.
What happens if you ignore decay under a filling?
It keeps going. Decay doesn’t reverse once it’s broken through enamel and into the softer dentin underneath.
- Months 1 to 6: Often no symptoms. Maybe mild cold sensitivity.
- Next stage: The hollowed-out tooth weakens. A corner of the tooth or filling can snap off while you eat.
- Later: Bacteria reach the pulp. Pain becomes lingering and throbbing, often worse at night.
- Finally: The nerve dies and an abscess can form at the root tip, with swelling and pain on biting.
Each step adds time and treatment. Catching it at step one keeps things simple.
How to protect your fillings
- Brush twice a day with fluoride toothpaste
- Clean between your teeth every day, since many fillings sit between teeth
- Cut down on sipping sugary drinks over long periods
- Drink water through the day if your mouth feels dry
- Wear a night guard if you grind
- Keep your checkup and cleaning schedule so X-rays can catch problems early
- Ask about prescription-strength fluoride if you’ve had several cavities recently
Getting your fillings checked in Woodbridge
At Columbus Dental Centre, checking old fillings is part of every exam. We’ve served Woodbridge and the Vaughan community for over 26 years, and we’re known locally as a dentist in Woodbridge that families trust for both everyday and cosmetic care.
Our team offers the full range of treatment in one place: smile design, dentures, bridges, CEREC crowns, oral surgery, fillings, root canals, dental implants, teeth whitening, preventive care, children’s dentistry and laser dentistry. CEREC lets us restore a badly broken-down tooth with a ceramic crown or onlay in a single visit.
New patients are welcome, and we accept CDCP (Canadian Dental Care Plan) patients. We’re open Monday 10:00 AM to 8:00 PM and Saturday 9:00 AM to 4:00 PM, along with regular weekday hours.
Visit us at Suite #201, 8333 Weston Rd, Woodbridge, Ontario, call +1 647-931-1100, or email columbusdentalcentre@gmail.com. All care is provided by dentists registered with the Royal College of Dental Surgeons of Ontario (RCDSO).
Safety note: This article is general information. It can’t replace an exam. If you have pain, swelling or a broken tooth, contact a dental office promptly.
Frequently asked questions
Can a dentist see decay under a filling without an X-ray?
Sometimes, if the edge has broken down or there’s visible discolouration. Often, though, the decay hides beneath the filling, and a bitewing X-ray is the most reliable way to find it.
Does replacing a filling hurt?
The area is numbed first, so most people feel pressure and vibration rather than pain. The tooth may be a little sensitive for a few days afterward.
Why does my filled tooth hurt when I bite?
It could be a high filling, a crack, decay under the filling or inflammation in the nerve. Pain that lasts more than a few days after a new filling, or starts in an old one, should be checked.
Can decay under a filling cause an abscess?
Yes. If it’s left long enough to reach the pulp, bacteria can infect the nerve and spread to the root tip, forming an abscess.
Are tooth-coloured fillings better at preventing new decay?
Composite fillings bond to the tooth, which helps seal the edges. Both composite and amalgam can develop decay at the margins, so home care and checkups matter more than the material.
Conclusion
Cavities can form under fillings, usually where the edge has worn or leaked over time.
They’re often painless at first, so checkups and X-rays are the best way to catch them small.
Early treatment usually means a simple new filling instead of a root canal or crown.
