In a seven-year clinical trial of back-tooth fillings in children, about 94% of silver amalgam fillings and 86% of tooth-coloured composite fillings were still in service at the end. Most fillings last many years, but none lasts forever, and the size of the filling, the tooth it sits in and your risk of new decay matter at least as much as the material.
Written by Dr James Tran, dentist at Lumi Dental.
New decay at the edges ends most fillings
A filling seals a cleaned-out cavity. Over time, the margin where filling meets tooth is where trouble starts. Plaque that sits along that edge can cause fresh decay, known as secondary caries, which spreads under the filling where a toothbrush cannot reach. The other common ending is a fracture, either of the filling or of the tooth wall beside it.
A 2014 analysis by Opdam and colleagues in the Journal of Dental Research, which pooled original data on more than 2,800 composite fillings in back teeth, found that decay and fracture were the main reasons fillings failed. Fillings in people at high risk of decay, and fillings covering more surfaces of the tooth, were significantly more likely to fail.
How the materials compare
Amalgam
Amalgam has the longest track record. The trial mentioned above, published by Bernardo and colleagues in the Journal of the American Dental Association in 2007, followed 472 children aged 8 to 12 and found amalgam outlasted composite, especially in large fillings covering more than three surfaces. Secondary decay was the main cause of failure for both materials.
Amalgam is used less than it once was. The Minamata Convention on Mercury, an international treaty that came into force in 2017, commits countries to phasing down mercury use, including dental amalgam, and many patients prefer a tooth-coloured filling.
Composite
Composite resin is bonded to the tooth, matches its colour and usually needs less healthy tooth removed. A 2021 Cochrane review by Worthington and colleagues found low-certainty evidence that composite fillings in back teeth had about twice the risk of failure of amalgam fillings, mainly because of secondary decay. Most of that evidence comes from older trials in children, and composite materials and bonding techniques have continued to develop, so the gap in everyday practice may be smaller. For small and medium fillings in people who keep decay under control, composite generally performs well.
Glass ionomer
Glass ionomer releases fluoride and bonds to the tooth, but it is softer and wears faster under heavy chewing. It is often chosen for baby teeth, for fillings near the gum line, or as a temporary or interim repair.
Why two identical fillings can age differently
The same material can last very different lengths of time in different mouths. The factors that make the biggest difference are:
- Size. A filling that replaces a large part of the tooth leaves thinner walls of natural tooth around it, which are more likely to crack.
- Decay risk. Frequent sugar, a dry mouth and plaque left along the edges all raise the chance of new decay at the margin.
- Bite forces. Back teeth do most of the chewing, and grinding or clenching adds extra load.
- Habits. Chewing ice, hard lollies or pens can chip both fillings and enamel.
Signs a filling needs a closer look
Some failing fillings cause no symptoms at all, which is why they are often found on a check-up X-ray. Others give warning signs worth booking in for:
- New sensitivity to cold, hot or sweet things in a filled tooth
- A sharp pain when biting, which can point to a crack
- A rough or chipped edge your tongue keeps finding
- A dark line around the filling or a visible gap
- Food catching in the same spot every time you eat
- Part of the filling coming away
Sensitivity can have other causes too, which we explain in our article on sensitive teeth.
What replacing a filling involves
If the tooth around a worn filling is still sound, a new filling is usually all that is needed. If decay has spread or the remaining walls are thin, a larger restoration such as an onlay or a crown may better protect what is left. When decay reaches the nerve, root canal treatment may be needed before the tooth is rebuilt. Dealing with a failing filling early, rather than waiting for pain, usually keeps the repair smaller. Our guide to dental filling costs covers how the size and material affect the fee.
Helping your fillings last
The habits that prevent new decay are the same ones that protect existing fillings: brushing twice a day with fluoride toothpaste, cleaning between the teeth, keeping sugary snacks and drinks to mealtimes, and regular check-ups with your dentist or OHT so early breakdown at the edges is found while it is small. If you grind, a night guard can take some of the load off. Our article on preventing tooth decay goes into more detail.
What patients ask about fillings
Will every filling need replacing one day?
Many will over a lifetime, but a small, well-maintained filling can last for decades. Fillings are checked at each visit and replaced when they wear, crack or start to leak, not on a fixed schedule.
Should I replace my old amalgam fillings with white ones?
Not just because they are old or silver. If an amalgam filling is intact and the tooth is healthy, replacing it removes more tooth structure. It is reasonable to consider replacement when a filling has failed, the tooth has cracked, or you have a cosmetic concern, and your dentist can talk through the trade-offs.
Can a failing filling be painless?
Yes. Decay under a filling often causes no symptoms until it is quite advanced, which is why regular check-ups and X-rays matter.
If you would like your fillings checked, our team in Melrose Park can examine them and let you know if any need attention. You can read about our general dental care or see our current offers on the deals page.
This article is general information and not a substitute for personal dental advice.
