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Root canal therapy, at its core, aims to eliminate apical periodontitis (AP). AP results when intrapulpal bacteria, including facultative and obligate anaerobes as well as viruses and fungi, spread into the periapical tissues.1,2 There, interactions with the immune system activate both positive and negative regulatory pathways, leading to the host-mediated destruction of apical bone.3 As endodontic treatment seeks to eliminate microbially-derived AP, root canal therapy must effectively remove said microbes to achieve its aims. Furthermore, as persistent intraradicular infection is often regarded as the main cause of post-endodontic treatment failure, the importance of effective disinfection cannot be underscored.4
Endodontic instrumentation alone is incapable of eradicating intracanal microbes. According to one study, mechanical canal preparation leaves 35% of canal walls untouched.5 Furthermore, anatomical structures, such as intercanal isthmuses and lateral canal anatomy, are often present within teeth,6 and instruments alone cannot effectively clean these areas. Consequently, additional efforts are necessary to eliminate microbes in the root canal system. Two effective means to achieve these ends are endodontic irrigants and intracanal medicaments. This article reviews the current trends in endodontic disinfection as it relates to these two entities.
Root canal irrigants are liquids that both flush the root canal system of debris and eliminate microbes. Ideal irrigants possess broad antimicrobial properties against both anaerobic and facultative organisms, inactivate lipopolysaccharide, dissolve both vital and necrotic tissue remnants, and either prevent the formation of a smear layer during instrumentation or effectively remove it once formed.7 These properties are summarized in Table 1. Unfortunately, no single liquid irrigant possesses all of the above properties. Accordingly, many practitioners employ several irrigants during root canal therapy to take advantage of their varied properties.
Both sodium hypochlorite (NaOCl) and chlorhexidine gluconate (CHX) are solutions that possess antimicrobial properties. NaOCl effectively eliminates a broad scope of intracanal species in their various living states.7 Planktonic intracanal pathogens are readily susceptible to its effects,8 as are those bacteria living in established biofilms on the canal walls.9 Furthermore, NaOCl penetrates dentinal tubules to eradicate bacteria capable of tubular spread.10 These properties make NaOCl the most commonly used endodontic disinfectant on the market today. Most members of the American Association of Endodontists (AAE) report using NaOCl at concentrations at or above 5.25%,11 as dilution decreases its antimicrobial effectiveness. This concentration is both safe for clinical use when used properly and does not increase postoperative pain when compared with more dilute solutions.12
NaOCl’s abilities, while impressive, can be reduced by interactions with other commonly used instrumentation aids and irrigants. Consequently, caution must be exercised when combining products during root canal therapy. Ethylenediaminetetraacetic acid (EDTA) is often used during root canal therapy in both lubricant preparations and as a stand-alone irrigant as it effectively removes the hard tissue component of the smear layer that NaOCl does not.13 Though NaOCl depends on its active chlorine content to eliminate microbes, interaction with EDTA reduces this active chlorine, thus inhibiting NaOCl’s desired effects.14 As a result, clinicians must design their lubrication and irrigation protocols carefully to permit maximum NaOCl effectiveness.
CHX has gained popularity as an endodontic irrigant because its antimicrobial activity is comparable to that of 5.25% NaOCl.15 It is particularly effective against Enterococcus faecalis, a microbe commonly isolated from cases of treatment failure.16 Furthermore, CHX is an attractive irrigant choice because its activity persists long after it is dried from the root canal system, a property known as substantivity. It binds to dentin, allowing its antibacterial activity to continue for as many as 48 days after exposure.17 The use of CHX is limited, however, as unlike NaOCl, it lacks the ability to dissolve both vital and necrotic tissue remnants.15 Consequently, CHX should not be used as the sole disinfectant; rather, it can serve as an adjunct in patients with NaOCl allergies or when open apices pose a significant extrusion risk.15
Like NaOCl, CHX is subject to serious chemical interactions, necessitating care with its use. CHX reacts with NaOCl to form a red-brown, carcinogenic precipitate called parachloroaniline.18 This precipitate has the ability to occlude dentinal tubules and can effectively block the root canal system.19 Consequently, if both products are used during root canal therapy, an intracanal rinse with saline is advisable before switching irrigants. Important properties of both sodium hypochlorite and chlorhexidine gluconate are reviewed in Table 2.
Several adjunctive irrigation techniques beyond simple syringe irrigation were developed in order to offer improvements in disinfection, debridement, or both with known irrigant solutions. These adjunctive techniques include vacuum-assisted irrigation, passive ultrasonic irrigation (PUI), and sonic activation (Figure 1). At this time, 45% of AAE members report using one or more of these techniques during root canal therapy.11 While research certainly justifies the use of some of these techniques to eliminate microbes, other techniques prove less effective. Vacuum-assisted irrigation, like that from the EndoVac™ system (Kerr Corporation, www.kerrdental.com), while improving canal debridement, offers no improvement in microbial reduction and should not be relied upon for these purposes.20 PUI, on the other hand, improves both debridement and disinfection. One study found PUI was as effective as a final rinse with CHX in eliminating bacteria.20 Lastly, sonic activation, like that offered by the EndoActivator® (Dentsply Sirona, www.dentsply.com), is as effective as PUI.21 Given these findings, clinicians may want to consider using one or more of these techniques during treatment.
In addition to both irrigants and their adjunctive techniques, intracanal medicaments play a role in endodontic disinfection. These commercially-available materials, meant for insertion into the canal spaces between root canal appointments, include calcium hydroxide (CaOH) and CHX gel. Multi-visit endodontic treatment necessitates the use of an intracanal medicament because without it, microbes easily recolonize the root canal system, negating the effects of earlier disinfection efforts.22 Interappointment treatment with CaOH increases the number of canal systems with undetectable levels of bacteria,23 and eliminates more lipopolysaccharide than single-visit root canal therapy.24 To achieve these ends, CaOH requires adequate exposure time in the root canal space, with one study suggesting a minimum 7-day exposure is necessary.25 Unlike irrigation with NaOCl, however, CaOH does not effectively eliminate bacteria from canal isthmuses or dentinal tubules,26 underscoring the need for irrigation throughout the procedure even if multi-visit treatment is performed. More recently, 2% CHX gels have become available to clinicians, and research indicates that these are as effective as CaOH at eliminating intracanal microbes.27
As AP results from microbial interactions with immune functions in the apical bone, root canal therapy must remove these contaminants in order to successfully eliminate AP. This article discusses several means by which clinicians can remove microbes from infected teeth. With careful selection of irrigation solutions, adjunctive irrigation techniques, and intracanal medicaments, an evidence-based treatment protocol can be developed to help improve treatment outcomes for patients.
The author has no relevant financial relationships to disclose.
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About the Author
Assistant Clinical Professor
Tufts University School of Dental Medicine
White River Junction, Vermont