What is the reason of post endodontic disease ?
Either there is persistent or reintroduce intracanal microorganisms.. that means if I do improper instrumentation, improper cleaning, improper disinfect to canal previously treated, there is missed canal not cleaned properly or cleaned to full length but not completely disinfect or improper enlargement. So, I leave the infections inside the canal. In case it reintroduce coronally, maybe there is lateral or accessory canals, which make reinfection inside the canal or presence of extraradicular infection.