Apical periodontitis, an acute or chronic inflamination around the apex of the tooth, is caused by bacteria in the root canal. In Sweden the dentists devote around 10X of their total time to treating this disease. The treatment usually requires 3 to 5 sessions. The treatment may fail in up to 25X of the cases. In the present study various treatment regimens were evaluated. One hundred and forty singlerooted teeth with apical periodontitis were treated. The importance of mechanical instrumentation, irrigating solutions and antibacterial dressings in eliminating bacteria from the infected root canals was studied using bacteriological techniques. The healing of the apical periodontitis after treatment was followed for 2 to 5 years on recall radiographs. Bacteria were found in all 140 root canals at the beginning of the treatment. Most of these bacteria were anaerobes and they represented a restricted group of bacteria compared to the bacteria present at other sites in the oral cavity. Mechanical instrumentation with files and reamers in combination with saline irrigation reduced the number of bacterial cells in the root canal 100- to 1000-fold during one treatment session. Bacteria could be eliminated from about half the number of root canals if this treatment was performed at 4 sessions. Mechanical instrumentation and irrigation with 0.5X or 5X sodium hypochlorite solutions or with the 5X solution in combination with 15X EDTA solution wa3 more efficient and the bacteria were eliminated from about half the treated canals after one treatment session. The bacteria which persisted in the root canal after this treatment usually increased in number during the interval up to the next session and reached levels which were often as high as in the initial sample at the previous session. All bacteria persistent in the root canals after the previous treatment regimens were with 2 exceptions eliminated by dressing the root canals for 1 to 2 months with calcium hydroxide paste. Thirty-four out of 35 root canals treated at the first session with mechanical instrumentation, irrigation with sodium hypochlorite solution and dressed with calcium hydroxide paste were free of bacteria at the second session. Calcium hydroxide paste was superior to camphorated phenol and camphorated paramonochlorophenol as dressing. Healing of 79 out of the 140 treated teeth was followed for 2 to 5 years. The majority of the lesions healed completely or decreased in size in such a way that they could be expected to heal. There was no or only an insignificant decrease in the size of the lesions in 5 cases. In 2 of these cases bacteria were demonstrated in the periapical tissues and in a third case dentin chips. Periapical lesions may thus fail to heal in a few cases due to an establishment of bacteria outside the root canal, and in that site the bacteria are inaccessible to conventional endodontic treatment. The present study showed that treatment of the majority of infected non-vital teeth can be completed in only 2 sessions, if mechanical instrumentation, sodium hypochlorite irrigation and calcium hydroxide dressing are combined. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1986, härtill 5 uppsatser.</p> / digitalisering@umu