Where the tooth carries a well-fitting crown and post that would be destroyed by removing it, where a previous retreatment has already failed, or where a fractured instrument blocks the canal — in all three, going in through the top of the tooth costs more than it recovers.
The procedure: a small incision in the gum over the root tip, the last few millimetres of root removed, the canal sealed from the end with a biocompatible cement under the microscope, and the gum closed with fine sutures. You are awake throughout and it is much less dramatic than it sounds.
Success rates with modern microsurgical technique and mineral trioxide aggregate are around 85–90% at five years, considerably better than the figures people find online, which mostly predate both. Sutures come out at a week and the bone fills in over six months.