To have successful endodontic treatment, roots have to be processed and filled to the end. For that purpose, is required proper equipment (microscope, rubber dam, endomotor, apex locator, device for warm gutta-percha, digital x-ray device) without which is not possible to imagine modern endodontic treatment, but is possible to be done classical treatment.
With classical treatment which is widely spread in our country, (because of financial reasons) canals can not be properly processed and filled. For example bellow we have some x-ray records on which we can see fillings in the canals with white color, and the rest of the canals which is not processed and filled with black color. That part of the canal is like a paradise for the bacteria, there they are having all conditions for reproduction, and from there they can very easily find the way to the circulation and cause lot of chronic diseases.
That is the reason why classical root canal treatment is not successful!