Why root canal treatments fail — and what can be done when they do

"Root canals never work" is a myth with a grain of truth behind it: some root fillings do fail, sometimes years after they were placed. The failures, though, almost always have specific and well-understood causes. Knowing them explains both how modern treatment avoids them and what can be done when a treated tooth starts to hurt again.

How you know a root filling has failed

Sometimes there is pain on biting, a swelling or a small boil on the gum above the tooth. Often there is nothing at all, and the first sign is a shadow at the root tip on a routine X-ray at your dentist's. A tooth that has been fine for a decade can still fail, and a tooth that aches for a few days after treatment has usually not.

The main causes

A canal that was missed. This is the commonest reason. Many upper molars have a fourth canal that is almost impossible to see without magnification; lower front teeth often have two where one is expected; canals divide and rejoin. An untreated canal is a reservoir of bacteria that will eventually make itself known.

A filling that stops short. If the canal was not cleaned and sealed to within a millimetre or two of the root tip, infected tissue remains beyond the filling.

Leakage from above. Root fillings are not waterproof forever. A crown that was delayed, a filling that has broken down or new decay at its edge lets saliva and bacteria back into the canals from the top.

Persistent bacteria. Some bacteria survive in the fine side branches and tubules of the root, which is why thorough disinfection and activated irrigation matter so much.

A cracked root. A crack that runs down the root cannot be sealed. It is the one cause for which the only answer is extraction.

How modern treatment avoids them

Working under an operating microscope finds the canals that would otherwise be missed. Electronic length measurement fixes exactly where each canal ends. Flexible nickel-titanium instruments follow curves without straightening them, activated irrigation carries disinfectant into the branches, and rubber dam keeps saliva out while the work is done. Finally, a prompt crown or onlay from your dentist protects the result. None of this guarantees success — biology does not deal in guarantees — but it shifts the odds markedly.

Options when a treatment has failed

Retreatment. The tooth is reopened, the old root filling removed, the missed or re-infected canals located under the microscope and cleaned, and the system sealed again. It is usually a two-visit procedure and the first choice where the canals can be reached from above. See root canal retreatment.

Apicectomy. When a post, a sound new crown or a blocked canal makes retreatment impractical, the infection at the root tip is removed surgically and the root end sealed from below. See apicectomy.

Extraction. A split root, or too little sound tooth left to restore, means the tooth cannot be saved, and the honest advice is to remove it and plan a replacement with your dentist.

Which applies to your tooth is a matter for examination, tests and X-rays — not for guessing. A tooth-saving assessment gives you the answer in writing.