With the advent of rotary instrumentation, the amount of intracanal contact time with an endodontic irrigant may be reduced.  Objective: The purpose of this study was to determine the contact time of 6% sodium hypochlorite (NaOCl) required to achieve a 99.9% reduction of Enterococcus faecalis in the root canal system of mandibular bovine incisors.  Methods: After removing the crowns, root canals were prepared with rotary instrumentation using 6% NaOCl and 17% REDTA, and dried with paper points. After sealing the apex with composite resin, roots were mounted in a glass vial and sterilized using ethylene oxide. The root canals were filled with sterile trypticase soy broth (TSB).   Fifteen teeth in three experimental groups (n=15) plus the positive control (n= 3 or 4) were inoculated with E. faecalis suspension.  Teeth were incubated at 37°C for one week and re-inoculated daily.  Teeth in each of the three experimental groups were irrigated with 6% NaOCl at working length for 10, 12.5 or 15 minutes.  For the positive and negative controls, the irrigant was sterile water. D/E Neutralizing Broth was placed into the canal at working length to stop further reaction of NaOCl.  Canals were dried with sterile paper points that were then transferred to 5-mL of TSB.  TSB was vortexed, plated on Trypticase Soy Agar with 5% sheep's blood (TSAII), and incubated at 37°C for 24 hours.  Mean colony forming units (CFUs) and standard deviations were determined per group and analyzed with a Kruskal-Wallis Test (alpha=0.05). See table.  Results: No significant difference was found between the contact times (p=0.278). Conclusion: A 12.5 minute exposure of 6% NaOCl achieved a 99.91% reduction of E. faecalis in the bovine root canals.

Contact Times	15 Minutes	12.5 Minutes	10 Minutes
Positive Control Mean CFU (st dev)	1.20x105 (1.00x105)	1.87x105 (2.79x105)	1.09x105 (1.01x105)
Experimental Mean CFU(st dev)	5.57x101 (1.05x102)	1.74x102 (2.60x102)	2.14x102 (3.89x102)
Percent reduction	99.98%	99.91%	99.80%

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