Two hundred and thirty-six cases of endodontic treatment failures, none of which had advanced periodontal disease, postperforations, or root or crown fractures were analyzed clinically, radiographically, and histobacteriologically to determine the major factor(s) for treatment failures. It was found that there was a correlation between bacterial infection in the canal system and the presence of periradicular rarefaction in endodontic failures. This report provides evidence indicating that the major factors associated with endodontic failures are the persistence of bacterial infection in the canal space and/or the periradicular area and the presence of preoperative periradicular rarefaction. The apical extent of root canal fillings, i.e underfilled, flushfilled, or overfilled, seems to have no correlation to treatment failures.
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