How does a dentist know if I have a cavity under an old crown?

One of the signs of a cavity under an old crown is a black line between the crown and the gums, like in this photograph

While sometimes dental xrays can confirm the presence of a cavity and other times a cavity can be found by using a dental tool to feel the edges of the crown, there are plenty of instances when a cavity can lurk undetected beneath what otherwise may appear to be a still serviceable crown. Dentists’ inability to diagnose “hidden” decay can confuse patients, since they may feel that dentists should have a foolproof way to detect a cavity in a tooth.

Often our decisions about evaluating old dentistry can be imprecise and dentists can differ on their diagnosis. Clearly old crowns (more than twenty years old) should be evaluated in a different light than a newly-placed crown. I find that when I do remove an older crown , I often find a partial washout of the permanent cement and/or some new cavity I was unaware of. In fact, I often say removing an old crown is a little like taking a bite out of a chocolate in a box. Just like Forrest Gump said, “You never know what you’re going to get!”

Commonly in my dental practice, I find that my older patients can have a “spot” of recurrent cavity adjacent to a crown and I have the option of just cleaning out and “spackling” the area of cavity or choosing to remove and replace the entire crown. My decision varies from patient to patient and from crown to crown. I factor in a patient’s age, finances, and the apparent fit of a crown’s edges.

If I am dealing with an elderly patient with limited finances or compromised health, I tend to be more likely to recommend a discreet repair procedure and not the replacement of their crown. If on the other hand I am dealing with a patient under seventy , who is in apparent good health, I will be more likely to recommend  replacement of their crown. If I only “patch” an existing crown, this may not last beyond a few months to a year, and I can’t always see the extent of any cavity underneath before re-sealing the edges.

Dr. Andrea Stevens is a cosmetic and family dentist in practice in Kanata, Ontario. If you have dental questions, you can call her at 613-271-7091 or visit her at Please also feel free to leave comments or questions below, and Dr. Stevens will be happy to answer!