Influence of irrigant needle depth in removing bioluminescent bacteria inoculated into instrumented root canals using real-time imaging in vitro

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 login 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.