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# Why fillings fail: secondary caries, fracture and leakage

- Topic

Most fillings fail because new decay starts at their edge or because the filling or tooth breaks. Repairing is often better than replacing.

Updated Nov 12, 2026

## Key points

- Secondary caries and fracture are the main causes of failure.
- Patient risk matters as much as the material.
- Repair before replace.

## What it is

No restoration lasts for ever. The two main reasons fillings fail are secondary (recurrent) caries at the margin and fracture of the restoration or the tooth around it. Who the patient is matters as much as the material: high caries risk, heavy biting or bruxism and large fillings all shorten survival. Each replacement removes more tooth, so the first choice for a localised defect is repair.

## Key facts

- Secondary caries forms at the margin, where biofilm collects; it is a new lesion, not a leak of the old one.
- Fracture: thin cusps, large fillings and bruxism raise the risk; root-filled back teeth without cusp coverage are especially at risk.
- Staining at a margin alone is not caries and does not by itself justify replacement.
- Overhangs and open contacts trap food and plaque and harm the gum.
- Composite fillings fail somewhat more often than amalgam in high-risk patients, mostly from secondary caries.

## In clinical practice

- Replacing a whole filling for a small defect enlarges the cavity every cycle.
- Treating caries risk (diet, fluoride, hygiene) does more for the next filling than changing the material.
- Pain on biting with a large old filling suggests a cracked tooth.

## Next step / Initial management

- Check old restorations clinically and on bitewing radiographs for caries, overhangs and fractures.
- A localised defect: repair; caries under the restoration or a fractured cusp: refer to the dentist for replacement or cusp coverage.
- Recurrent failures: reassess caries risk and bruxism before the next restoration.

## Test yourself

Think of the answer, then tap the question to see it.

## What are the two leading causes of filling failure?

Secondary caries at the margin and fracture of the restoration or the tooth.

## Is a stained margin by itself a reason to replace a filling?

No: staining alone is not caries; it is monitored unless there is a cavity, softness or caries on the radiograph.

## Why is repair often preferred to replacement?

Replacing the whole restoration removes more healthy tooth each time, while repair keeps it and often lasts as long.

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## Connections

## Related 5

- [Minimal intervention: when to drill and when not](https://durgical.com/en/dentistry/minimal-intervention-when-to-drill-and-when-not/)
- [Resin composite and light curing](https://durgical.com/en/dentistry/resin-composite-and-light-curing/)
- [Dental amalgam: mercury, safety and the phase-down](https://durgical.com/en/dentistry/dental-amalgam-mercury-safety-and-the-phase-down/)
- [How caries forms: biofilm, sugar and demineralisation](https://durgical.com/en/dentistry/how-caries-forms-biofilm-sugar-and-demineralisation/)
- [Cracked tooth and vertical root fracture](https://durgical.com/en/dentistry/cracked-tooth-and-vertical-root-fracture/)

## Related conditions 1

- [Dental caries](https://durgical.com/en/conditions/dental-caries/)

[← All of dentistry](https://durgical.com/en/dentistry/)

Educational content; it does not replace assessment by a dentist or doctor and is not a clinical instruction. For facial or neck swelling with difficulty swallowing or breathing, a knocked-out tooth or bleeding that will not stop, go to an emergency department at once.

Sources: [Failure Risk of Composite Resin and Amalgam Restorations: A Systematic Review and Meta-Analysis](https://pmc.ncbi.nlm.nih.gov/articles/PMC12246595/) (International Dental Journal (PMC open access), 2025); [Reparative Dentistry: Possibilities and Limitations](https://pmc.ncbi.nlm.nih.gov/articles/PMC6244566/) (Current Oral Health Reports (PMC open access), 2018); [Dental Caries](https://www.ncbi.nlm.nih.gov/books/NBK551699/) (StatPearls Publishing (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/dentistry/why-fillings-fail-secondary-caries-fracture-and-leakage/