Why white fillings let back teeth down
Four ways it goes wrong
New decay grew back under the filling
What happened. A white composite filling was placed in a lower molar. Within a couple of years decay reappeared — not on a fresh part of the tooth, but right along the edge of the filling, spreading underneath where nobody could see it. The tooth had to be re-drilled, and more healthy tooth was lost to make room for the replacement.
Why it happened. Composite is held in by an adhesive layer that can shrink and break down over time, opening a microscopic gap at the margin. Composite surfaces also tend to gather more plaque bacteria than metal does, so that leaking edge becomes an easy route for a new cavity to start under the filling. Recurrent decay at the margin is one of the most common reasons back-tooth composites need redoing.
It shrank as it set — and cracked the tooth
What happened. Soon after the filling was placed, the patient had sharp sensitivity to cold on a back tooth. From above the filling looked fine, but as it hardened it had pulled slightly away from one wall, and a fine crack had run from the filling down into the tooth.
Why it happened. All composite shrinks a little as it cures (polymerisation shrinkage). In a deep back-tooth cavity that pull either opens a gap at the margin or leaves stress locked inside the tooth — which shows up as lingering sensitivity, a crack, or both. A cemented metal filling does not set by shrinking, so it never builds in that stress.
It wore down and opened a food trap
What happened. A large composite on a chewing surface slowly wore flat, dropping below the height of the tooth next to it. The tight contact between the two teeth opened, and food began packing into the gap after every meal — leading to sore, inflamed gum and a new cavity on the neighbouring tooth.
Why it happened. Composite is softer than enamel and than metal, so on load-bearing molars it wears and loses its shape faster. When a filling loses height the bite and the contact point change, and an open contact between molars is a classic cause of food impaction, gum problems and decay on the tooth beside it.
A large filling cracked and broke apart
What happened. A molar had been restored with a big composite covering most of the chewing surface. Biting on something hard, a whole section fractured away — taking a weakened wall of the tooth with it — and the restoration had to be rebuilt much larger than before.
Why it happened. The bigger a composite gets, the more it is asked to do the job of the missing tooth itself — which it isn’t strong enough to do on a molar long term. Large posterior composites fracture and de-bond more often than a cemented metal or gold inlay or onlay, which is made outside the mouth to full strength and holds the tooth walls together instead of relying on a bond.
The metal options that hold up back here
Whatever the size of the cavity, on a molar or premolar we point you at metal. Nobody sees it back there — and metal beats tooth-coloured composite on every measure that actually matters on a chewing surface: it does not shrink as it sets, it seals for longer, it resists wear and fracture, and it does not carry composite’s resin chemistry. Three options, in order of what most people choose:
- Silver amalgam filling — the best all-round choice. A century of clinical use, extremely durable, and uniquely tolerant of a slightly damp tooth — so it still seals well on molars that are hard to keep perfectly dry. It is also the lowest-cost long-lasting option, and it contains no resin and no BPA.
- Gold inlay, onlay or crown — the premium choice if the budget allows. Made outside the mouth to full strength and cemented in. It seals tightly, wears at almost the same rate as your natural enamel, and expands and contracts with heat like a real tooth — making it the gentlest option for the tooth biting against it.
- Metal inlay or onlay — for a large back-tooth cavity. A custom-milled metal restoration for when a plain filling would be pushing its luck. It holds the remaining tooth walls together and far outlasts a big composite, without the cost of gold.
A note for readers in the EU. Since 1 January 2025 the EU has banned dental amalgam (silver fillings) except in narrow medical cases, under the revised Mercury Regulation (EU) 2017/852. If you are treated in the EU, ask instead about a gold or metal inlay, onlay or crown — the same durability, and fully available. Amalgam remains a common, legal option in many countries outside the EU.