This study tested the hypothesis that the mechanical efficacy of irrigation in reducing intracanal bacteria is dependent on canal preparation size. Root canals of 30 extracted permanent cuspids were prepared consecutively to sizes 36, 60, and 77 at working length (WL) using ProFile 0.04 taper Series-29 files. Smear layer was removed and teeth autoclaved following completion of each instrumentation sequence. Root canals were inoculated with a 15-mul suspension (1x10 cells) of Pseudomonas fluorescens 5RL, a salicylate-inducible bioluminescent reporter strain. Bioluminescence was measured before inoculation (background), after inoculation and after irrigation with 6 ml of irrigant delivered 1 mm from WL using a 28G safety-ended needle. The percentage of bacteria remaining following irrigation was 26.95+/-9.71%, 10.46+/-5.87%, and 10.64+/-6.01% for sizes 36, 60, and 77, respectively (p0.05; Tukeys). Irrigation 1 mm from WL was significantly less effective in canals prepared to size 36.
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