Precipitate from a combination of sodium hypochlorite and chlorhexidine

During the cleaning and shaping of the root canal, various irrigants have been used to reduce the residual debris, necrotic tissue, and bacteria, as well as to remove smear layer. Though sodium hypochlorite (NaOCl) is the most common irrigant used in root canal treatment, chlorhexidine (CHX) has been suggested as either an alternative or an adjunct root canal irrigant because of its antimicrobial qualities and substantivity. A combination of NaOCl and CHX for root canal irrigation has been advocated to enhance their antimicrobial properties. Zehender proposed an irrigation regimen as the following: 2.5% NaOCl during instrumentation, and a final flush of the canals is performed in the sequence of 17% EDTA, 2.5% NaOCl, and 2% CHX. However, the presence of NaOCl in the canals during irrigation with CHX produces an orange-brown precipitate known as parachloroaniline (PCA) (Figure 1). The precipitate occludes the dentinal tubules and may compromise the seal of the obturated root canal. Leaching of PCA from the insoluble precipitate is of concern because it has been shown to be cytotoxic in rats and possibly carcinogenic in humans (International Agency for Research on Cancer group 2B). A