Chlorhexidine as a Canal Irrigant: A Review

The importance of irrigation used in root canal therapy cannot be understated. While mechanical instrumentation may remove significant num­bers of bacteria in a canal system,¹ the remaining bacteria can cause or sustain periradicular tissue inflammation.^2,3 Therefore, antibacterial irrigation is of great importance to help eliminate or reduce bacteria to a level that allows tissue healing. In various concentrations, sodium hypochlorite (NaOCl) has been used for decades as a primary endodontic irrigant. While it has excellent antimicrobial and tissue-dissolution properties, it also has some disadvantages. One of the more serious problems associated with NaOCl is that it can cause soft-tissue inflammation if expressed out of the confines of the root canal.⁴ This event is commonly re­ferred to as a sodium hypochlorite accident, which may cause extreme pain and/or widespread swelling.⁴ In addition, NaOCl has an unpleasant odor and taste if it passes the den­tal dam. Also, NaOCl will act as a bleaching agent if accidentally expressed onto clothing. Because of NaOCl’s negative properties, chlorhexidine has been studied as an alternative irrigant for root canal disinfection. This report is a review of the significant literature regarding chlorhexidine in endodontic therapy.

Chemistry

Chlorhexidine was developed in the research laboratories of Imperial Chemical Industries Ltd in the late 1940s.⁵ Chlor­hexidine is a cationic biguanide that is active at a pH of 5.5 to 7.0 and works by binding to negatively charged bacterial cell walls and extramicrobial complexes. At low concentrations, chlorhexidine has a bacteriostatic effect, causing an alteration of bacterial-cell os­motic equilibrium and leakage of potassium and phosphorous. At high concentrations, chlor­hexidine is bacteriocidal causing the cytopoasmic contents of the bacterial cell to precipitate, resulting in cell death.^6,7 While chlorhexidine is not water soluble, chlorhexidine gluconate, which is a salt formed from chlorhexidine and gluconic acid, is (Figure 1).⁸

Antimicrobial Properties of Chlorhexidine

Chlorhexidine is a broad-spectrum antimicrobial agent that has been shown to be active against vegetative bacteria and mycobacteria, has moderate activity against fungi and viruses, and inhibits spore germination. It has been shown to be most effective against gram-positive cocci, while less active against gram-positive and gram-negative rods.⁶ The antibacterial efficacy of chlorhexidine is comparable with that of NaOCl^1,9-11 and is effective against strains of bacteria re­­sistant to calcium hydroxide, such as gram-positive Entero­coccus faecalis. ¹² The concentration often used in endodontic therapy is 2% chlorhexidine. This has been found to be more effective in the least time when compared with other concentrations of chlorhexidine ranging from 0.002% to 2%.¹² In the clinical realm of practice, the goal of en­do­dontic therapy is to reduce bacterial populations in an in­fected canal to levels that are not detectable by culture procedures.² In several studies, chlorhexidine as an irrigant has been shown to lower the number of postirrigant positive bacterial cultures, as well as the number of colony-forming units remaining in positive cultures.^1,7 Because of its cationic properties, chlorhexidine can bind to surfaces covered with acidic proteins, such as the hydroxyapatite component of dentin, and be released at therapeutic levels, a phenomenon known as substantivity.^12,13 This can occur in 48 hours to 72 hours after instrumentation.¹⁴ In a study by Tanamaru Filho et al, dogs with radiographic evidence of periapical lesions exhibited better periapical tissue repair in teeth that were immediately obturated after 2% chlorhexidine irrigation than those immediately obturated after 5.25% NaOCl irrigation. It was thought that these results could be attributed to absorption of chlorhexidine into the dentin tissues, thus maintaining a pro­longed antimicrobial action and better environment for tissue healing.¹⁵

Toxicity

One of the often cited reasons for using chlorhexidine as a canal irrigant is its perceived minimal toxicity to host tissues. While chlorhexidine does not appear to cause any long-term damage to host tissues, it still may cause an inflammatory response in these tissues if expressed beyond the root canal. Yesilsoy et al injected 0.12% chlorhexidine into the subcutaneous tissues of the backs of guinea pigs to help assess short-term toxic effects. After histologic examination, they found a mild inflammatory response after 2 hours, moderated inflammatory re­sponse after 2 days, and foreign body granuloma formation at 2 weeks, which resolved over time.¹⁶ However, this study was performed using a lower concentration of chlorhexidine than is often used in endodontic therapy. To help evaluate the inflammatory response of 2% chlorhexidine, Tanamaru Filho et al separately injected 0.5% NaOCl, 2% chlorhexidine digluconate, and phosphate-buffered saline into the peritoneal cavity of mice. This study found the number of inflammatory cells resulting from 2% chlorhexidine injection was similar to the phosphate-buffered saline control at all times tested, while the 0.5% NaOCl injection resulted in a significantly larger number of inflammatory cells.¹⁷ The researchers concluded that 2% chlorhexidine was biocompatible. In contrast, a study by Faria et al found that tissue reactions to chlorhexidine injected into the subplantar space of the hind paws of mice caused inflammatory responses and tissue death positively correlated with the concentration of chlorhexidine used. Faria et al determined chlorhexidine in concentrations of 0.5% and 1% induced large foci of coagulative necrosis associated with an inflammatory infiltrate mainly composed of neutrophils and mononuclear cells, along with interstitial dermal and subcutaneous edema. The concentration of 0.25% chlorhexidine caused only small foci of tissue necrosis, while 0.125% chlorhexidine appeared to cause no necrosis.¹⁸ This study also examined the effect of chlorhexidine on tissue healing by testing different concentrations of chlorhexidine on cultured L929 fibroblasts. They found that at lower concentrations, chlorhexidine induced apoptosis of the fibroblasts and, at higher concentrations, induced necrosis and increased expression of heat-shock protein 70, an indicator of cellular stress.¹⁸ These findings seem to indicate that a 2% concentration of chlorhexidine, as commonly used in endodontic therapy, may have toxic effects on host tissues if expressed beyond the confines of the root canal and may impair healing.

Another toxicity concern with the use of chlorhexidine is the formation of para-chloranaline (PCA), which is an aromatic amine. When studied in rats, rabbits, and cats, the primary toxic effect was methemogloblin formation: Re­peated exposures to PCA led to cyanosis and methaemoglobinaemia. In humans, accidental occupational exposure to PCA produced symptoms of increased methemoglobin and sulfhaemoglobin levels, cyanosis, the development of anemia, and systemic changes from anoxia.¹⁹ While chlor­hexidine may spontaneously hydrolyze to PCA over time,²⁰ it undergoes a chemical reaction when combined with NaOCl and forms a precipitate that contains PCA.²¹ Water or alcohol can be used as an irrigant to flush NaOCl from the canal before chlorhexidine is used, thus minimizing PCA formation.²² Ethylenediaminetetraacetic acid (EDTA) may also be an appropriate substance to flush the remaining NaOCl out of a canal, as the combination of chlorhexidine and EDTA does not result in a chemical reaction. The white precipitate that is formed from the combination of EDTA and chlorhexidine has been shown to be a salt containing no PCA.²³ Further study is needed to evaluate the clinical significance of this salt.

Bonding

With the development of resin-based obturation materials, the effect of an endodontic irrigant on the resin-based ma­terials and their capacity to bond to dentin is an important consideration. While NaOCl has been shown to have a detrimental effect on resin bonding to root canal dentin, chlorhexidine has been shown to have no effect, or even a positive effect, on bonding of resin-based materials to root canal dentin.^24,25 Nascimento Santos et al theorized that the reason chlorhexidine irrigation produced mean micro­tensile bond strengths similar to that of a control trial of saline solution is because chlorhexidine is a nonoxidizing agent that does not interfere with the resin adhesion system tested.²⁴ Erdemir et al suggests that the higher bond strengths they obtained with chlorhexidine irrigation compared to the control is be­cause chlorhexidine is absorbed into the den­tin and helps facilitate the absorption of dentin bonding agents into the dentin tubules.²⁵ As discussed previously, the combination of NaOCl and chlorhexidine forms a precipitate. This should be considered if chlorhexidine and NaOCl irrigants are to be used in the same canal, as the pre­cipitate formed in the dentin tubules by the interaction of the two irrigants will significantly reduce the number of pa­tent dentin tu­bules.²² This may have an effect on resin bond­ing because a hybrid layer is required for proper sealing.²² In a study evaluating the coronal seal of teeth obturated after different irrigants, it was found that significantly more co­ronal leakage was in teeth obturated after a combination of NaOCl and chlorhexidine gel than those teeth obturated after NaOCl or chlorhexidine gel used individually.²⁶ It was thought that the precipitate that formed with the combination of NaOCl and chlorhexidine left a residual film on the canal wall, which interfered with sealing and led to increased microleakage. These findings may be useful when determining which irrigant(s) to use during endo­dontic therapy, especially if a resin-based obturation system is to be employed.

Soft-Tissue Dissolution

The soft-tissue dissolution properties should strongly be con­sidered when choosing an irrigant. Because of the complex anatomy of the root canal system, not all canal surfaces will come in contact with and be cleaned by me­chanical instrumentation.²⁷ Tissue remnants left in complex ana­tom­ical irregularities, such as lateral walls, dentinal tubules, and isthmi, can be sources of nutrients for bacteria.² This is important when considering that some bacteria, such as E faecalis, can be entombed for 12 months and still maintain viability.²⁸ Further, even nonviable bacteria left in den­tin tubules or canal irregularities shielded by a biofilm may cause an inflammatory cascade leading to tissue destruction.²⁹ While chlorhexidine has many positive qualities, it fails to have any substantial tissue-dissolution properties. Clegg et al found that 2% chlorhexidine was not capable of disrupting the biofilm generated from bacterial samples taken from teeth with chronic apical periodontitis.²⁹ Naenni et al used full-thickness palatal mucosa from pigs incubated in 10 mL of 10% chlorhexidine for 2 hours. At the end of the 2-hour period, the pig mucosa retained 90% of its initial weight while 1% NaOCl caused the pig mucosa to retain only 7% of the initial weight.³⁰ When considering that soft-tissue remnants comprise part of the smear layer left by mechanical instrumentation, it would seem that chlorhexidine would not be an effective irrigant to remove the entire smear layer. One possible way to address this problem is by using chlorhexidine in a gel base, rather than an aqueous solution. Ferraz et al³¹ showed that 2% chlorhexidine in a base of natrosol gel did a superior job of cleaning the root canal wall and leaving open dentin tubules than did 2% chlor­hexidine in aqueous solution or 5.25% NaOCl when used during biomechanical preparation of the root canal. It was thought that the better cleaning by the 2% chlorhexidine gel was a result of the gel viscosity, which caused a mechanical cleaning that compensated for the inability of chlor­hexidine to dissolve pulp tissues. Because natrosol gel is water soluble, the gel could be removed from the canal, using a final rinse of distilled water.³¹ However, this study evaluated only the cleansing ability of the different irrigants in the middle third of the root canal. Further study in the apical third of the root where more complex anatomy is often present may be needed to evaluate the cleansing ability of chlorhexidine in a gel base.

Conclusion

Many factors should be considered when choosing an irrigant(s) for endodontic therapy, including antimicrobial ac­tivity, effect on bonding properties, toxicity, and the ability of the irrigant to dissolve tissue. While chlorhexidine has been shown to be an effective antimicrobial agent capable of killing the pathogens involved in endodontic infections, it still lacks the soft-tissue dissolution properties that assist in a more complete cleansing of the canal. Until further study is done and more conclusive evidence is presented on the effectiveness of chlorhexidine as a root canal irrigant in either gel or liquid form, it is perhaps best used as an accessory irrigant to NaOCl. If employed as a final irrigant, the beneficial properties of chlorhexidine, such as antimicrobial substantivity, could be incorporated into the endodontic therapy. Because evidence indicates chlorhexidine may cause an inflammatory response and be toxic to host tissues, care should be taken not to express it beyond the confines of the root canal. If chlorhexidine is to be used as a final irrigation after NaOCl, care should be taken to flush out NaOCl from the canal prior to introduction of chlorhexidine to avoid, or at least reduce, PCA-containing precipitate formation.

References

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24. Nascimento Santos J, de Oliveira Carrilho M, Fernando De Goes M, et al. Effect of chemical irrigants on the bond strength of a self-etching adhesive to pulp chamber dentin. J Endod. 2006;32(11):1088-1090.

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31. Ferraz C, Gomes BP, Zaia A, et al. In vitro assessment of the an­ti­microbial action and the mechanical ability of chlor­hexidine gel as an endodontic irrigant. J Endod. 2001;27(7):452-455.

About the Author

Steven Ryan, DDS, Private Practice, Stockton, California