Yes, you can absolutely get a cavity under a dental crown. The crown itself can’t decay, but the natural tooth underneath it remains vulnerable to bacteria, especially along the margin where the crown meets the tooth. This is called recurrent or secondary decay, and it’s one of the most common reasons crowns eventually need to be replaced.
How Decay Gets Under a Crown
A crown sits over your natural tooth like a cap, and the seal between the two depends on a thin layer of dental cement. Over time, that cement can fracture or dissolve, creating a microscopic gap at the crown’s edge. Bacteria colonize that gap, feed on sugars and starches that get trapped there, and produce acid that eats into the tooth underneath. An overhang or poor fit at the margin makes this worse by giving plaque a sheltered place to accumulate where your toothbrush can’t reach.
Gum recession plays a major role. As gums pull back, they can expose the root surface below the crown’s edge, leaving unprotected tooth structure in direct contact with bacteria. This is especially common as you age, and root surfaces are softer and more decay-prone than enamel. The older a crown gets, the more likely the seal has weakened. The average crown lasts about 10 to 15 years depending on the material, and margin integrity tends to decline well before the crown itself looks damaged.
Why It’s Hard to Detect Early
One of the most frustrating things about decay under a crown is that it often goes unnoticed until it’s advanced. Standard dental X-rays have real limitations here. Metal and porcelain-fused-to-metal crowns block the X-ray beam, making the tooth structure inside essentially invisible on the image. Even when decay is present at the margin, the X-ray has to be angled precisely to catch it. A slight change in sensor position between visits can hide a small cavity that was visible on a previous film.
Detecting early decay at a crown margin requires a high degree of diagnostic skill. Your dentist may use a combination of X-rays taken at multiple angles, visual inspection with magnification, and a dental explorer (a fine-tipped instrument) to probe the crown’s edges. But small areas of decay can still be missed, which is why regular checkups matter even when a crowned tooth feels fine.
Symptoms to Watch For
Decay under a crown often produces no symptoms in its early stages. As it progresses, you may notice:
- Sensitivity to hot, cold, or air around the crowned tooth, particularly if gum recession has exposed part of the root
- A dull ache or constant toothache in the area, which can signal that decay is approaching the nerve
- Pain when biting down, sometimes accompanied by gum swelling, which may indicate infection
- A bad taste or smell near the crown, suggesting bacteria are active underneath
If the decay reaches the inner pulp of the tooth, you may develop a full infection with more intense pain, swelling, and even fever. At that point, a root canal is typically necessary to save the tooth.
How It’s Treated
Treatment depends entirely on how much damage the decay has done by the time it’s found.
When decay is caught early and limited to a small area at the margin, your dentist may be able to remove the crown, clean out the decay, place a filling, and reattach the same crown if it still fits properly. This is the best-case scenario and the least invasive option.
If the existing crown no longer seats well or shows signs of wear, it needs to be replaced. The dentist removes the old crown, cleans out any underlying decay, reshapes the tooth, and fits a new crown. This is common with older crowns that have loosened over time.
When decay has penetrated deep enough to reach the tooth’s pulp (the living tissue containing nerves and blood vessels), root canal therapy becomes necessary. The infected tissue is removed, the inner chamber is cleaned and sealed, and a new crown is placed on top. This sounds more dramatic than it typically feels. Modern root canals are comparable to getting a filling, and the tooth can often last many more years afterward.
In severe cases where the decay has destroyed too much tooth structure to support any restoration, extraction is the only remaining option. This is more likely when decay has gone undetected for a long time.
What Affects Your Risk
Some people are more prone to decay under crowns than others. The biggest risk factors are straightforward: how old the crown is, how well it fits, and how effectively you clean around it. Crowns older than 10 to 15 years deserve extra attention during dental exams, even if they look and feel normal.
Dry mouth significantly raises your risk. Saliva neutralizes acid and washes bacteria away from tooth surfaces, so anything that reduces saliva flow (certain medications, medical conditions, or simply aging) leaves crown margins more exposed. A high-sugar diet feeds the specific bacteria that produce decay-causing acid, particularly when sugary or starchy foods sit against the crown margin between brushings.
Keeping a Crowned Tooth Healthy
The margin where your crown meets the gumline needs the same attention you’d give any other tooth, arguably more. Brush twice daily with fluoride toothpaste, and pay particular attention to the gumline around crowned teeth. Floss or use an interdental cleaner daily to clear plaque from the spaces your brush misses. A water flosser can be helpful for getting underneath crown edges that are hard to reach with traditional floss.
If your dentist considers you high-risk for decay (due to dry mouth, a history of cavities, or significant gum recession), prescription-strength fluoride toothpaste can offer extra protection. These contain roughly three times the fluoride concentration of over-the-counter toothpaste and are specifically designed to protect vulnerable root surfaces and crown margins. Fluoride varnish applied at your dental office every few months is another option for high-risk patients. Limiting sugary snacks and acidic drinks between meals reduces the acid attacks your crowned teeth face throughout the day.
Regular dental visits are critical not because your dentist will always catch early decay under a crown, but because consistent monitoring over time gives the best chance of spotting subtle changes in the margin, the surrounding gum tissue, or the X-ray appearance before damage becomes extensive. If you notice any new sensitivity, pain, or a change in how a crown feels when you bite, bring it up at your next appointment rather than waiting.