Secondary Cavity Guide: Why Decay Forms Again at the Margins of Resin, Inlays and Crowns, and How It Is Treated

A secondary cavity is decay that forms again in an already treated tooth along the margin where the restoration meets the natural tooth, and is also called recurrent decay. The restoration itself does not decay, but decay begins from microscopic gaps at the margin or plaque that was not cleaned away and spreads under the restoration. Even when the tooth looks fine on the outside, decay often grows inside, so if you have treated teeth it is important to have them checked on X-rays at regular checkups.

Published 2026-11-08 · Updated 2026-11-08 · DentalStatics editorial team

Key facts

Long-term use of resin	Problems such as fracture or secondary decay can occur with long use (source: Korea Disease Control and Prevention Agency (KDCA, 질병관리청) National Health Information Portal)
Uses of resin treatment	Includes treating secondary decay around existing restorations and replacing non-aesthetic restorations (source: Korea Disease Control and Prevention Agency (KDCA, 질병관리청) National Health Information Portal)
Amalgam margin problems	If the margin corrodes or secondary decay enlarges the defect, it can be replaced with a gold inlay (source: Korea Disease Control and Prevention Agency (KDCA, 질병관리청) National Health Information Portal)
Characteristic of full porcelain crowns	Bacteria adhere less easily, so the risk of secondary decay is low (source: Korea Disease Control and Prevention Agency (KDCA, 질병관리청) National Health Information Portal)
Checkups and diet after treatment	Checkups every 6 months after recent treatment; avoid sugary snacks if possible if you had cavity treatment in the last 3 years (source: Korea Disease Control and Prevention Agency (KDCA, 질병관리청) National Health Information Portal)

Why do secondary cavities form?

A very small margin line forms between a restoration and the tooth, and over time, if the material wears or the bond weakens, a gap can open. When plaque builds up in the gap, acid dissolves the tooth at the margin by the same principle as the first cavity.

The KDCA National Health Information Portal explains that problems such as fracture or secondary decay can occur when resin is used for a long time, and in its explanation of gold inlays it introduces that amalgam restorations can have enlarging defects from margin corrosion or secondary decay. If the mouth environment that made cavities form easily in the first place has not changed, the same risk continues in treated teeth.

Microscopic gaps and weakened bonding at the restoration margin
Plaque building up at the margin and insufficient floss use
Habit of frequently eating sweet snacks and drinks
Conditions that reduce saliva, such as dry mouth

What symptoms should make you suspect one?

In the early stage there are almost no symptoms. As it progresses, changes appear such as the treated tooth becoming sensitive to cold again, aching when chewing, or food often getting trapped at the margin with an odor. The resin margin may look brown or black, but telling it from simple staining requires an examination.

Decay that has spread under a crown or inlay cannot be seen from the outside and is often found on X-rays. By the time symptoms appear it may already have progressed close to the nerve, so finding it at a checkup when there is no pain is advantageous for keeping the tooth.

How is a secondary cavity treated?

The existing restoration is removed, the decay beneath is cleared, and the tooth is restored again. The National Health Information Portal lists treating secondary decay around existing restorations among the uses of composite resin, and names the ease of maintenance by refilling only part of it as an advantage of resin.

If the decay is wide and the remaining tooth walls have weakened, the extent is enlarged to an inlay or crown, and if it has reached the nerve, root canal treatment is needed. With each retreatment the tooth gets a little smaller, so finding it early while small is the key to keeping the tooth a long time.

How do you keep it from forming again?

Clean the margin of treated teeth carefully with the tip of the toothbrush held upright, and manage the margins between teeth with floss or interdental brushes. Use fluoride toothpaste, and if cavity risk is high, getting fluoride applied at the dentist is also an option.

The National Health Information Portal recommends a checkup every 6 months if you have recently had oral disease treatment, and avoiding sugary snacks and drinks if possible if you had cavity treatment in the last 3 years. Material choice also has an effect, and the same material explains that full porcelain crowns have a low risk of secondary decay because bacteria adhere less easily.

What should you look at for each type of restoration?

Resin margins tend to wear and change color, so at checkups gaps and staining are looked at together. With inlays and crowns, the margin can open when the bonding cement ages, so it is good to mention any place where floss catches or shreds.

With bridges, plaque easily builds up under the connected artificial teeth and at the abutment tooth margins, so they must be cleaned separately with interdental brushes. When the margin of an old amalgam corrodes, a gap easily forms, so have it regularly checked for cracks or lifted edges.

Common myths and facts

Myth: A tooth that has been treated once will not decay again.

The restoration itself does not decay, but decay can form again in the natural tooth at the margin. The more treated teeth you have, the more important margin care is.

Myth: If you are treated with an expensive material, you need not worry about secondary cavities.

How much bacteria adhere differs by material, but with any material the tooth at the margin can decay. Brushing, floss, diet and regular checkups matter more than the material.

Frequently asked questions

The area around my resin has turned black. Is it a secondary cavity?

It may be simple staining from food pigments or it may be decay at the margin. The National Health Information Portal explains that discoloration around resin can be removed by polishing, so first have an examination to check for decay.

How are secondary cavities under a crown found?

They cannot be seen from the outside, so they are found with X-rays and an examination of the margin. If the gums around the crown swell, there is an odor, or it aches when chewing, be sure to mention it at a checkup.

If a secondary cavity forms, does the crown have to be redone?

If it is small and at part of the margin, sometimes the crown is left in place and only that part is filled. If it has spread widely under the crown, it is often removed, treated and a new one made.

If I change amalgam to resin, will secondary cavities decrease?

Changing the material alone does not make secondary cavities disappear. If there is decay or a defect at the existing amalgam margin, replacement is considered, and if there is no problem, it is often left in place and monitored.

Is retreatment of a secondary cavity covered by health insurance?

It depends on the material and method of treatment. Resin fillings for permanent-tooth cavities up to age 12 are covered, while adult resin, inlays and crowns are uninsured and costs differ by dental clinic.

References

Korean version →