LAUSR

Introduction: This clinical study evaluated the influence of the apical preparation size using nickel‐titanium rotary instrumentation and the effect of a disinfectant on bacterial reduction in root canal–treated teeth with apical periodontitis. Methods: Forty‐three teeth with posttreatment apical periodontitis were selected for retreatment. Teeth were randomly divided into 2 groups according to the irrigant used (2.5% sodium hypochlorite [NaOCl], n = 22; saline, n = 21). Canals were prepared with the Twisted File Adaptive (TFA) system (SybronEndo, Orange, CA). Bacteriological samples were taken before preparation (S1), after using the first instrument (S2), and then after the third instrument of the TFA system (S3). In the saline group, an additional sample was taken after final irrigation with 1% NaOCl (S4). DNA was extracted from the clinical samples and subjected to quantitative real‐time polymerase chain reaction to evaluate the levels of total bacteria and streptococci. Results: S1 from all teeth were positive for bacteria. Preparation to the first and third instruments from the TFA system showed a highly significant intracanal bacterial reduction regardless of the irrigant (P < .01). Apical enlargement to the third instrument caused a significantly higher decrease in bacterial counts than the first instrument (P < .01). Intergroup comparison revealed no significant difference between NaOCl and saline after the first instrument (P > .05). NaOCl was significantly better than saline after using the largest instrument in the series (P < .01). Conclusions: Irrespective of the type of irrigant, an increase in the apical preparation size significantly enhanced root canal disinfection. The disinfecting benefit of NaOCl over saline was significant at large apical preparation sizes. HighlightsThe influence of apical enlargement on bacterial reduction was evaluated.Irrigation was done with either sodium hypochlorite (NaOCl) or saline.The larger the apical preparation size, the greater the bacterial reduction.NaOCl was superior to saline only at large apical preparation sizes.