When the bacteria penetrate the inside of the tooth, inflammation of the dental pulp is caused, which often results in severe pain. Sometimes nerve atrophy can be slow and painless, due to slow progression of the infection. Bacteria pass through the root canals towards the bone and cause inflammation there (abscess or granuloma). Inflammation of the dental pulp requires endodontic treatment. Subsequent treatment of tooth with fillings or crowns is impossible if root canal treatment was not performed properly, i.e. if the cause of infection was not removed and the root canals are not cleared and filled. Mild pain and dental sensitivity is a normal reaction after the endodontic procedure and is due to the reaction of the immune system to the residual microorganisms. Sometimes endodontic treatment cannot be successfully performed due to thin, unpassable root canals, previous poor endodontic interventions, and the like. Some of these cases are successfully resolved with apicoectomy (surgical removal of the root tip).