Decay Under a Crown and Bite Care: Self-Check Signs Such as Dark Lines, Trapped Food and Odor

Can decay form inside a tooth that has a crown?

Yes. The crown material itself does not decay, but if a gap forms at the edge (margin) where the crown meets the tooth, bacteria can get in and secondary decay develops in the remaining tooth [1]. Teeth that have had root canal treatment in particular give no pain signal, so it is hard to notice from symptoms, and because the inside cannot be seen from outside, it tends to be found late [1]. That is why, for crowned teeth, checking the margin and inside with X-rays at regular checkups is how problems are found [1].

When DentStat analyzed 700 patient comments from YouTube dental content on decay under crowns, margins and bite, the largest share expressed anxiety that “there’s no way to know what’s happening under the crown” [3].

Is a dark edge around my crown a cavity?

Not everything that looks dark is a cavity [1]. When gums recede with age or gum inflammation, the inner structure of a crown containing metal or the root side of the tooth can show through as a dark line, which is different from decay [1]. On the other hand, discoloration can also come from secondary decay progressing at a margin gap, so the two are distinguished by probing and X-rays, not appearance [1]. Questions asking what a dark line at the edge is appeared repeatedly in the comments [3].

If food gets stuck and there is a smell, has something gone wrong?

It does not immediately mean failure, but it can be seen as a sign to get checked [1]. Odor is often caused by food and plaque left between the margin and the tooth and may improve with floss and interdental brushes [1]. However, if food trapping that was not there when the crown was first placed starts and continues, wear at the contact with the neighboring tooth or a change in margin fit should be checked [1]. Conversely, a contact so tight that floss cannot go in at all can also make care harder, so both are checked at checkups [1].

Is it okay to trim the opposing tooth to adjust the bite?

The basic approach is to adjust the new crown, though the opposing tooth is sometimes smoothed slightly [1]. If a crown is too high, chewing force concentrates on that tooth first, which can lead to pain, sensitivity or jaw muscle discomfort, so adjusting the height is itself a necessary step [1]. Still, which tooth is adjusted and by how much is something the patient should hear explained and agree to, and the comment analysis showed much resistance to trimming a healthy opposing tooth and requests for prior explanation [3]. A checkable step is to ask why and how much will be adjusted and hear the answer before it proceeds.

How should I care for a crowned tooth?

The focus of care is the margin, not the covered part [1]. These are the basic check and care items.

• Remove plaque from the margin and between teeth with floss and interdental brushes: the basis for preventing secondary decay and gum inflammation [1]
• Check for decay underneath and margin fit with X-rays at regular checkups: the inside cannot be seen from outside [1]
• If a high or foreign feeling persists when you bite, have the bite checked [1]
• New food trapping, odor or gum bleeding are signs to move up your next checkup [1]

Since margin care and regular checkups are cited as what long-lasting crowns have in common, keeping up a checkup schedule even without symptoms is how problems are found earlier [1].

Crown materials, replacement timing and other prosthetic topics are covered in the crown guide, scaling done alongside regular checkups in the scaling guide, and regional clinic data based on HIRA public data in Find a dentist [2].