A root treated tooth that aches, feels tender to bite on, or shows a change on an X-ray has not necessarily reached the end of its life. Far more often, something inside the root was left untreated, and reaching that something is what re-treatment sets out to do.
Root canal treatment clears the nerve space of bacteria and seals the space that remains. The aim is to leave nowhere inside the root where bacteria can live.
Where such a space is left behind, bacteria stay in that space, and the body keeps responding at the end of the root. That response is what makes a tooth tender to bite on, sometimes many years after the original treatment.
Teeth carry more canals than a flat X-ray suggests. Upper first molars almost always hold a fourth canal, tucked behind the main canal in the mesio-buccal root, and that fourth canal is the one most often left untreated.
A canal that was never opened keeps its original contents. Nothing placed in the canals alongside reaches into that space, which is why a tooth can stay comfortable for years and then turn tender.
A periapical X-ray shows a change at the end of a root, but compresses a three-dimensional tooth into a single plane, so a canal hidden directly behind another stays hidden. A CBCT scan separates the roots and shows how far any change in the bone extends.
Inside the tooth, a dental operating microscope is what makes the difference. A canal that is invisible to the naked eye is often plainly there under magnification and light.
The old filling material comes out, the canal system is cleaned, and the whole space is sealed again. Removing the old material is what makes re-treatment slower than first-time treatment.
Different materials behave differently. Conventional root filling material softens and can be worked out; a carrier-based filling leaves a plastic carrier that has to be drawn out whole; a post, or an instrument separated in the canal, has to be dealt with before the canal below can be reached at all.
Some re-treatment is completed in one visit. Infection at the end of a root, or a large amount of old material to clear, usually points to two.
Where the rest of the root filling is sound and the problem sits at the very end of the root, surgery to treat that root end can be the better route.
Where a tooth is cracked through, or too little tooth structure is left to rebuild, the honest answer is that the tooth has come to the end. Those decisions are worth taking slowly, and are covered in the guides on root canal treatment or extraction and root canal treatment or an implant.
A re-treated back tooth needs cuspal protection, meaning a crown or an onlay, because a tooth that has been opened twice has lost structure both times.
Healing at the end of a root is slow, and is followed up over months rather than weeks. A review X-ray is taken some time after treatment rather than straight away.
A tooth root treated thirteen years earlier, tender to bite on again, where a CBCT scan showed a canal that had never been found.