Aim To test the hypothesis that the mechanical efficacy of irrigation in reducing bacteria in the root canal is dependent on depth of placement of the irrigation needle.
Methodology The root canals of 30 permanent cuspids were instrumented to apical size 60 using a crown-down technique. A suspension of the bioluminescent reporter strain Pseudomonas fluorescens 5RL was inoculated into each canal of sterilized teeth. Emission of bioluminescence (photons s−1) from each tooth was quantified on four sequential occasions using luminometry and bioluminescence imaging: (i) background, (ii) after inoculation, (iii) after irrigating the inoculated teeth with 3 mL of a nonantimicrobial irrigant delivered either 1 mm (group 1, n = 15) or 5 mm (group 2, n = 15) from working length (WL) using a 28G safety-ended irrigating needle, (iv) after an additional 3 mL irrigation (total 6 mL). Intragroup and intergroup comparisons were made using Wilcoxon matched pairs and Mann–Whitney tests, respectively.
Results In group 1, there was a mean log10 decrease in bacteria of 0.68 ± 0.26 after 3 mL of irrigant compared with 1.19 ± 0.48 after 6 mL (P < 0.001); in group 2 the mean log10 decrease was 0.58 ± 0.28 after 3 mL of irrigant compared with 0.69 ± 0.35 after 6 mL (P < 0.02) (Wilcoxon matched pairs). Using 3 mL of irrigant, needle depth did not have a significant effect on reduction of intracanal bacteria (P = 0.407), but the effect became significant when 6 mL of irrigant was used (P < 0.002) (Mann–Whitney tests).
Conclusions The mechanical efficacy of 6 mL of irrigant in reducing intracanal bacteria was significantly greater when delivered 1 mm compared with 5 mm from WL.