Can a tooth under a crown still get a cavity?

Why a crown does not make a tooth decay-proof

A crown is a tooth-shaped cap that replaces the part of your tooth above the gum line. It is usually placed because too little tooth remained for a filling, because the tooth was cracked or weakened, or after root canal treatment. The cap itself is ceramic, metal or a combination, and none of those materials can get a cavity, because decay is a disease of tooth tissue: acids made by bacteria in plaque dissolve minerals out of enamel, dentin and root surface. Crown material simply has no minerals to lose.

What the crown cannot change is what sits underneath and around it. The tooth was trimmed to a smaller core and the crown was cemented or bonded over it. That core is natural tooth. So is the root, and in many crowned teeth the nerve inside is still alive. Every crown also ends somewhere, usually at or just below the gum, at a line called the margin. From that line down, the tooth is as exposed to plaque as any other tooth in your mouth.

Where decay starts on a crowned tooth

At the margin

The margin is the junction between two different materials, and plaque collects along it just as it collects along the gum line of any tooth. Decay that develops at the edge of an existing restoration is called secondary or recurrent decay. It is the most common reason restorations are replaced, and it usually forms on the gum side of the margin, where cleaning is hardest. A crown whose edge has chipped, whose cement has washed out, or that was never fully sealed can leave a gap wide enough to trap plaque. Narrow, microscopic gaps on their own do not appear to cause decay; wider openings may.

On the root

Gums can recede over time; the guide on receding gums explains the common causes. When they do, root surface appears below the crown's edge. Root surface has no enamel covering, so once it is exposed it decays more easily than the enamel-covered part of the tooth. A tooth that was crowned years ago with its margin neatly hidden under the gum can later show a band of root that nobody planned for.

Under the crown

Decay that begins at the margin need not stay there. It can run inward along the junction and spread beneath the cap, hidden from view, toward the center of the tooth. If the nerve is still alive, the process can eventually inflame or infect it, and an abscess can form at the root tip. The crown above may look perfect the entire time.

Why it is usually painless

Early decay usually causes no symptoms at all. On an uncrowned tooth, you or your dentist might at least see a dark spot or feel a rough area. On a crowned tooth the cap hides the surface, so the usual visual clues are gone and the first sign is often a twinge, a loose crown or an X-ray finding. By the time a crowned tooth hurts, the decay is often deep. The same point applies to any tooth, and the guide on cavities without tooth pain explains why pain is a late signal rather than an early one.

Warning signs worth a visit

• Nothing at all. This is the most common finding, which is exactly why crowned teeth are examined at check-ups rather than only when something feels wrong.
• Sensitivity to sweet, cold or hot at a crowned tooth that used to feel fine. Dentin exposed at the margin or root can react this way; so can decay.
• A dark line, gray shadow or brown spot at the gum edge. This may be surface stain, the crown's edge becoming visible as the gum recedes, or decay. Only an examination can tell which.
• Floss that catches, shreds or snags on one crown, or an edge your tongue finds rough. A ledge or an open gap can collect plaque.
• A bad taste or odor from one area, or food that packs around a crown after meals. Trapped plaque and food at a defective margin can cause this, and a persistent bitter taste or breath odor is also among the signs of a tooth abscess.
• A crown that feels loose, rocks or lifts. Cement that has failed or decay that has softened the core underneath can both loosen a crown.
• Pain on biting, a throbbing ache, or gum swelling beside the tooth. These suggest the nerve is involved or an infection has formed. Contact a dentist the same day.

If swelling is spreading quickly into the face or neck, if you have any difficulty breathing or swallowing, or if facial swelling comes with fever, go to the nearest emergency department or call 911 before contacting a dental office. The guide on facial swelling with tooth pain explains these situations.

How decay under a crown is found

At a check-up the dentist looks at each margin, feels along it with a fine instrument for softness, roughness or a gap, and checks whether the crown is firm. X-rays find what the eye cannot: decay between teeth, below the gum line and under existing restorations. How often you need X-rays depends on your decay history, how many restorations you have and other risk factors, not on a fixed calendar. Someone with several older restorations is usually checked more often than someone with none.

What is done when decay is found

What happens next depends on how far the decay has traveled, which the examination and X-ray determine.

• Small, fully reachable decay at the edge. In selected cases the decayed area can be cleaned out and filled while the crown stays in place. This works only when the dentist can see and reach the full extent of the decay.
• Decay running under the crown. The crown is usually removed so the decay can be cleaned out completely, the core rebuilt and a new crown made. The practice's approach to restoration, described on the biomimetic dentistry page, emphasizes preserving sound tooth structure, which is one more reason finding decay small matters.
• Decay that has reached the nerve. Root canal treatment removes the inflamed or infected pulp (the nerve and blood vessels inside the tooth), and the tooth can often be saved. Depending on the condition of the existing crown, the dentist will advise whether treatment can be done through it or whether a new crown is needed afterward.
• Too little tooth left to rebuild. Occasionally a crowned tooth cannot be restored and is removed. The guide on replacing one missing tooth covers the options that follow.

How to protect a crowned tooth

• Brush twice a day with fluoride toothpaste, angling the bristles toward the gum line so they reach the margin rather than skating over the crown.
• Clean between teeth once a day with floss or another interdental cleaner, and take the cleaner down to the gum on both sides of the crowned tooth. If floss shreds on a crown, mention it at your next visit rather than skipping that tooth.
• Use fluoride. Fluoride helps enamel and root surface take minerals back up and resist acid. If you have exposed roots, a dry mouth or a history of new cavities, ask whether a prescription-strength toothpaste or fluoride varnish applied at the office makes sense for you.
• Watch how often you have sugar, not just how much. Research links tooth decay to the total amount of free sugars eaten, and frequent sugary snacks or sweetened drinks add to that total while giving teeth repeated acid attacks with little time to recover between them.
• Tell your dentist about a dry mouth. Saliva neutralizes acid and washes away food; a dry mouth, often caused by medications, raises the risk of decay, including on roots.
• Protect the crown from damage. Avoid chewing ice or very hard objects, and if you grind or clench, ask whether a night guard is worth considering. Chipping, fracture and crowns coming loose (what studies call loss of retention) are among the technical complications reported in reviews of single crowns.
• Keep your check-ups and X-rays on the schedule recommended for you, and ask which of your crowns, if any, is being watched.

The next step

If a crowned tooth has not been examined on the schedule recommended for you, or you are not sure when it was last checked, or if you have noticed any of the signs above, request an appointment and mention which tooth concerns you. Bring previous X-rays if you have them from another office so the new images can be compared with them. If a crowned tooth hurts now or feels loose, say so when you contact the office so the visit can be arranged promptly.