Developing an Extracted Tooth Model for Evaluating Endodontic Irrigants

Objectives: To develop a reproducible model for the evaluation of endodontic irrigants using single-rooted extracted human teeth and compare this to results from a biofilm model.

Methods: Extracted human teeth were root-prepared using a step-back, crown-down method and sealed at the apex before being mounted in cold-cure silicone and autoclaved. A standardised liquid culture of Enterococcus faecalis (NCTC 775) was inoculated into the root canal, which was then incubated at 37ºC for 48 hours. The root canals were irrigated with 1ml of 1% sodium hypochlorite (NaOCl) or phosphate buffered saline (PBS) using an electronic dispensing pipette to deliver a quantifiable and reproducible flow of irrigant into the root. Following irrigation, the root was similarly flushed with a neutralising broth. Sterile paper points were then used to sample the root canal and the number of viable bacteria remaining therein was determined by serial dilution and plating onto nutrient agar.

Results: The root canals of the eight single-rooted teeth used in this model were each capable of holding 20µl of the E. faecalis culture. The mean microbial load in the root cavity at inoculation was 5.15x10⁷ cfu ml^-1, which proved sufficient to detect a 4-log reduction in cell counts upon application of 1% NaOCl as an irrigant. The median number of viable bacteria recovered from the tooth after irrigation with PBS was 1.56x10⁵ cfu ml^-1, whilst irrigation with 1% NaOCl yielded 8.66x10² cfu ml^-1.

Conclusions: The extracted tooth model described here is a useful tool for evaluating the efficacy of endodontic irrigants since it standardises the microbial load and irrigant flow parameters. When compared to a biofilm model, the recalcitrance of E. faecalis within the root is assumed to result from mass transfer / mixing limitations between the root canal and lateral canals.