The intention of root canal
treatment is to remove sufficient bacteria and dead bacterial toxins to prevent re-infection of the area around the tip of the root of the tooth.
Because the inside of the root canal
is not a nice uniform shape, with tapering, curves, branched canals, etc etc, not all the bacteria are removed, in fact most studies show that there has never been a 'sterilised' canal with all bacteria removed.
So, some bugs are always left behind.
All infections start with a few bugs. It depends on how many of them (and obviously the adequacy of the host's defence systems). Too few bugs and the host defence wins. More than the minimum and infection results: this has led to the concept of the 'infective dose' and comparisons between things like HIV and HepB show massive differences in the required dose (ie the minimum infective dose) to acquire an infection.
So, if too many are left behind, and remember that there are always going to be SOME left behind, some root canal
treatments will fail.
Another cause of recurrent infection is the failure of the seal of the restoration of the tooth. Research suggests that this is a greater cause of failure of root canal treatment than residual bugs.
In your case the restoration has obviously failed so bacteria are getting in: your dentist is giving very sound advice: the root filling will already be bacterially contaminated and this has to be addressed.
Your options are re-root treat. Advisable where a good (ie sealed) restoration can be placed on the tooth and the dentist is confident of good long-term survival. Your decision as to whether this gives long-term value for money.
Or extract it.
Dentist in England.