If the caries is not removed on time, it can hit the nerve (pulp), causing a severe pain that does not stop until the causative agent is removed. Sometimes the nerve loss can be slow and painless, due to slow progression. Bacteria through the root canals progress toward the bone and cause inflammation (abscess or granuloma). Inflammation of the dental pulp requires endodontic treatment ("nerve extraction"; cleaning, spreading and filling the root canal).
Later supply of tooth by fillings or crowns is impossible if root canal treatment is not performed properly, that is if the cause of infection is not removed and the root canals are not cleansed and filled. Sometimes, endodontic treatment can not be successfully performed due to thin, impassable root canals, previous poor endodontic procedures, etc. Some of these cases are successfully resolved by apicotomy (by surgically removing the root canal).