A number of prognostic studies from many areas of the world have shown the overall success rate of endodontic treatment to be around 90%. Studies which in particular dealt with the results obtained with the standardized technique, found a success rate of 91%. The conceivable influence of a wide variety of biological and technical factors on the outcome of endodontic treatment has been studied, and a fairly clear picture exists at this time as to the relative importance of the various factors.
Age. No difference has been found in the results of endodontic treatment in younger (< 35 years) or older (> 35 years) patients.
Health. With the exception of patients with AIDS, it has not been found that impairment of general health implies a prognostic risk for endodontic treatment.
Preoperative diagnosis. The overall success rate of endodontic treatment of vital teeth and nonvital teeth without radiographically visible apical periodontitis is 90–95%. The success rate drops by 15–20% in nonvital teeth with apical radiolucencies. The size of the radiolucency tends to be of importance for the prognosis. No difference is seen in the results of endodontic treatment of teeth with asymptomatic apical periodontitis or of teeth with symptomatic apical periodontitis; nor does the presence of a fistula influence the prognosis.
Root canal morphology. Endodontic treatment may be performed with a high degree of success in all groups of teeth. It might seem logical that the results would be better in anterior teeth than in molars where the root canal morphology is more complicated. However, several studies have shown the opposite results, namely that endodontic treatment is more successful in teeth with 3 roots (90%) than in teeth with 2 roots (80%), and better in teeth with 2 roots than in single-rooted teeth (70%). These rather surprising results are probably due to the fact that the relatively narrow root canals in multirooted teeth are more thoroughly instrumented than the wider canals in single-rooted teeth. The results of a study with the standardized technique, where the apical third of the root canals was enlarged considerably more than in the previous studies, seem to support this hypothesis. In this study the results were the same in molars, premolars, and anterior teeth (91%). However, morphological characteristics of the teeth are mirrored in the results of this study as well: the teeth which in morphometric investigations appeared to be the least suited for the standardized technique had the poorest results (mandibular incisors; maxillary first premolars). The results obtained in maxillary central and lateral incisors were also poorer than one might have expected, probably because, at the time of the treatment, one was unaware of how wide the apical part of the root canal of these teeth really is, and to which size it ought to be enlarged. Thus, a thorough knowledge of root canal size and morphology clearly is necessary to obtain the best possible results in endodontic treatment.
Root canal instrumentation. The technical aspects of endodontic treatment have a great influence on the prognosis. A prerequisite for effective disinfection and successful obturation of the canal is adequate instrumentation. It is difficult or nearly impossible by clinical means to determine if the apical part of the root canal is actually adequately instrumented. With the standardized technique, therefore, an attempt is made to combine radio-graphic findings and the clinical feel of the operator with a thorough knowledge of the actual width of the apical part of the root canal in the various types of teeth. As judged by the long-term results with this technique, this appears to be a worthwhile approach.
Another factor in canal instrumentation which appears to be of prognostic importance is enlargement of the apical foramen or over-instrumentation of the canal. This affects the prognosis negatively and stresses the fact that the instrumentation should not include the foraminal area, but should terminate inside the root canal. Teeth with root canals which cannot be instrumented to the desired apical level due to canal obliteration have a goo/>