Chelating Agents
The last category of root canal irrigants is chelating agents. Chelants are chemicals that form soluble complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale. In dentistry, chelating agents bind with calcium and carry it out of the canal. The chelating agent most used in endodontics is EDTA. The smear layer, which covers the dentinal tubules, is created during the instrumentation procedure and is composed of dentin, liquid from the irrigants, and tissue debris (Figure 8). Removal of the smear layer from the root canal wall is a key step in endodontics. We would like to remove the smear layer and expose the dentinal tubules for 2 reasons (Figure 9). One reason is to allow the endodontic sealer to penetrate into the tubules for a more intimate fit, hopefully better sealing of the canal. Secondly, by removing the smear layer, and consequently opening up the tubules, we can expose the bacteria that are living there and causing apical periodontitis. Once exposed, the bacteria are susceptible to the disinfecting irrigants we use.
All these irrigants must be able to come into direct contact with the bacteria in order to kill them. If the smear layer is intact, it is impossible for either NaOCl or CHX or any other material to come in direct contact with the bacteria or biofilm. The next question then becomes: How long do we leave the EDTA in contact with the canal to remove the smear layer? Teixeira et al32 showed that EDTA irrigation for one, 3, and 5 minutes were equally as effective in removing the smear layer. Crumpton et al33 showed efficient removal of the smear layer with a final rinse of one mL of 17% EDTA for one minute. Therefore, I use 17% aqueous EDTA for at least one minute in each canal, and I also activate the EDTA by using PUI
