Although clinicians are aware of the microbial nature of caries and endodontic lesions, clinical correlation is often limited. Clinicians do not routinely culture necrotic root canals, and they often do not appreciate the importance of intracanal medications as an adjunct to endodontic therapy. Understanding that the proper use of intracanal medicaments will enhance the endodontic treatment outcome is paramount. Intracanal medicaments are used as antibacterial agents to eliminate residual bacteria in a root canal after instrumentation and irrigation, to render any remaining canal content inert, to dissolve tissue and act as a barrier against leakage or breakdown of the temporary filling, and to control leakage of fluids into the apical area.7
The 3 medicaments commonly used when the root canal is necrotic are NaOCL, calcium hydroxide, and chlorhexidine. NaOCL is used in conventional endodontic treatment as an irrigant, since it has antimicrobial properties and an ability to dissolve tissue. Evidence suggests that the chlorine exerts the antimicrobial effect by the irreversible oxidation of sulfhydrl groups of essential enzymes, thus disrupting bacterial metabolism.8 Harrison, et al9 found that 5.25% NaOCL was the most effective antibacterial concentration, and more dilute solutions demonstrate reduced antibacterial activity. Siqueira, et al8 demonstrated that regular exchange of the NaOCL solution during endodontic treatment helps to maintain antibacterial effectiveness. A study by Abou-Rass, et al10 demonstrated that NaOCL can dissolve both vital and necrotic tissue.
Calcium hydroxide is strongly alkaline. Its antimicrobial activity is related to the release of hydroxyl ions in an aqueous environment.11 Calcium hydroxide should be used as an intracanal medicament between appointments when necrotic tissue is present. To be effective, calcium hydroxide should be left in the canal for 7 days.12 Clinically, it is important to remove calcium hydroxide thoroughly from the canal prior to obturation. Kim, et al13 reported that calcium hydroxide left in the canal(s) may increase apical leakage of gutta-percha root fillings when a zinc oxide-eugenol sealer is used.
Chlorhexidine is a broad-spectrum antibacterial agent that has been shown to be effective against many of the strains of bacteria found in endodontic infections.14 Chlorhexidine is less toxic than sodium hypochlorite but does not dissolve tissue. It is a cationic bisbiguanide that combines with the cell wall of microorganisms and causes leakage of the intracellular component.15 One clinical advantage of using chlorhexidine over calcium hydroxide is that after removing calcium hydroxide, there is no residual antimicrobial effect, whereas chlorhexidine has been shown to have substantivity in the root canal dentin for up to 12 weeks. Also, calcium hydroxide is not clinically effective against Enterococcus faecalis and Candida albicans, whereas chlorhexidine is an effective antimicrobial agent against these organisms.14 E. faecalis is a facultative anaerobic Gram-positive cocci that is commonly identified in persistent endodontic infections, and particularly in teeth that were previously treated endodontically.16 Chlorhexidine does not have an adverse effect on the apical seal when the root canal is obturated.17,18 The use of chlorhexidine has also been shown to reduce postirrigant positive cultures when combined with NaOCL. This reduction is significant when compared to the use of NaOCL alone.19
Calcium hydroxide is used as a medicament between endodontic appointments in necrotic cases.12 Chlorhexidine is used as an intracanal medicament primarily for retreatment cases due to the increased likelihood of the presence of E. faecalis.14 With the use of new microbiological techniques such as polymerase chain reaction analysis, studies have reported higher rates of detection of E. faecalis in primary necrotic cases (untreated necrotic pulp) as well as secondary necrotic cases (failed endodontic treatment).20 It is fair to conclude that in the future, the use of chlorhexidine as an intracanal antimicrobial agent during and between endodontic treatment appointments of necrotic cases will become the standard of care.