The dentist who did mine should watch this. This was about 15 years ago I was having pain so went to dentist who referred me to an endontist my appointment was for 11:30 and I got there at 11:15 and the dentist said to his assistant "We will be out of here by 12:00!"
After numbing me up he started drilling and removing the root he was very aggressive in his work when I heard him say "Oh S**T" he had broken off the burr in the tooth.
A few months later the pain came back and I went to a different dentist when he took the x-ray he said I needed an apicoectomy because there was a broken burr in the canal.
I said that does not sound plesant.
The way its done is they cut thru the gum and do the root canal from the root end.
Ehen he got done not only did first dentist leave in the broken burr he had drilled thru the root end!
Almost all of the ones I have had done needed to be retreated except the first one I had done fifty years ago, (that one took three trips to have it done) no problems with that one.