When a root canal fails: causes, signs and what can be done

A root filling is a seal against bacteria. When that seal fails — immediately or fifteen years later — the infection returns. The good news is that failures have identifiable causes, and most can be corrected.

Why root fillings fail

• A missed canal. Molars often have a fourth canal that is hard to find without magnification; left untreated it keeps the infection alive.
• Incomplete cleaning. A root filling that stops short of the root tip leaves infected tissue behind.
• Leakage from above. A cracked or worn crown, a filling that has broken down or a long delay before the tooth was restored lets bacteria back in.
• New decay under or around the restoration.
• A cracked root, which cannot be repaired.

Signs of trouble

Tenderness on biting, a dull ache, a swelling or a spot on the gum, or — most often — an area of infection at the root tip spotted on an X-ray with no symptoms at all. Pain is not a reliable guide; many failing root fillings are silent for years.

The options

1. Retreatment. The old root filling is removed, the canals — including any missed first time — are re-cleaned and the tooth is sealed again. Usually two visits.
2. Microsurgery. When the tooth cannot be reopened without harm, or a blockage near the root tip prevents retreatment, the infected root tip is removed and sealed from the outside.
3. Extraction and replacement, when the root is cracked or too little tooth remains.

Is it worth saving?

Retreatment under magnification has good success rates, and the tooth being saved is usually already crowned and functioning. The honest answer depends on the cause of failure, which is why the assessment comes first and the recommendation is put in writing. Our page on keeping your natural tooth sets out how that decision is made.

Preventing the next failure

Have the final crown or filling placed promptly after treatment, keep the gum around the tooth clean and attend your routine check-ups so that leakage is caught early.