Endodontic irrigants and irrigant delivery systems

Endodontic treatment is a predictable procedure with high success rates. Success depends on a number of factors, including appropriate instrumentation, successful irrigation and decontamination of the root-canal space to the apices and in areas such as isthmuses. These steps must be followed by complete obturation of the root canals, and placement of a coronal seal, prior to restorative treatment. Several irrigants and irrigant delivery systems are available, all of which behave differently and have relative advantages and disadvantages. Common root-canal irrigants include sodium hypochlorite (NaOCl), chlorhexidine gluconate, alcohol, hydrogen peroxide and ethylenediaminetetraacetic acid (EDTA). In selecting an irrigant and technique, consideration must be given to their efficacy and safety. With the introduction of modern techniques, success rates of up to 98 percent are being achieved.1 The ultimate goal of endodontic treatment per se is the prevention or treatment of apical periodontitis, such that there is complete healing and an absence of infection,2 while the overall long-term goal is the placement of a definitive, clinically successful restoration and preservation of the tooth. For these to be achieved, appropriate instrumentation, irrigation, decontamination and root-canal obturation must occur, as well as attainment of a coronal seal. There is evidence that apical periodontitis is a biofilm-induced disease.3 A biofilm is an aggregate of microorganisms in which cells adhere to each other and/or to a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance. The presence of microorganisms embedded in a biofilm and growing in the root-canal system is a key factor for the development of periapical lesions.4–7 Additionally, the root-canal system has a complex anatomy that consists of arborisations, isthmuses and cul-de-sacs that harbor organic tissue and bacterial contaminants (Figs. 1a, b).8 The challenge for successful endodontic treatment has always been the removal of vital and necrotic remnants of pulp tissue, debris generated during instrumentation, the dentin smear layer, microorganisms, and micro-toxins from the rootcanal system.9 Even with the use of rotary instrumentation, the nickel-titanium instruments currently available only act on the central body of the root canal, resulting in a reliance on irrigation to clean beyond what may be achieved by these instruments.10 In addition, Enterococcus faecalis and Actinomyces prevention or treatment of apical periodontitis such as Actinomyces israelii — which are both implicated in endodontic infections and in endodontic failure — penetrate deep into dentinal tubules, making their removal through mechanical instrumentation impossible.11,12 Finally, E. faecalis commonly expresses multidrug resistance,13–15 complicating treatment. Therefore, a suitable irrigant and irrigant delivery system are essential for efficient irrigation and the success of endodontic treatment.16 Root-canal irrigants must not only be effective for dissolution of the organic of the dental pulp, but also effectively eliminate bacterial contamination and remove the smear layer — the organic and inorganic layer that is created on the wall of the root canal during instrumentation. The ability to deliver irrigants to the root-canal terminus in a safe manner without causing harm to the patient is as important as the efficacy of those irrigants. Over the years, many irrigating agents have been tried in order to achieve tissue dissolution and bacterial decontamination. The desired attributes of a rootcanal irrigant include the ability to dissolve necrotic and pulpal tissue, bacterial decontamination and a broad antimicrobial spectrum, the ability to enter deep into the dentinal tubules, biocompatibility and lack of toxicity, the ability to dissolve inorganic material and remove the smear layer, ease of use, and moderate cost. As mentioned above, root-canal irrigants currently in use include hydrogen peroxide, NaOCl, EDTA, alcohol and chlorhexidine gluconate. Chlorhexidine gluconate offers a wide antimicrobial spectrum, the main bacteria associated with endodontic infections (E. faecalis and A. israelii) are sensitive to it, and it is biocompatible, with no tissue toxicity to the periapical or surrounding tissue.17 Chlorhexidine gluconate, however, lacks the ability to dissolve necrotic tissue, which limits its usefulness. Hydrogen peroxide as a canal irrigant helps to remove debris by the physical act of irrigation, as well as through effervescing of the solution. However, while an effective anti-bacterial irrigant, hydrogen peroxide does not dissolve necrotic intra-canal tissue and exhibits toxicity to the surrounding tissue. Cases of tissue damage and facial nerve damage have been reported following use of hydrogen peroxide as a root-canal irrigant.18 Alcohol-based canal irrigants have antimicrobial activity too, but they do not dissolve necrotic tissue. The irrigant that satisfies most of the requirements for a rootcanal irrigant is NaOCl.19,20 It has the unique ability to dissolve necrotic tissue and the organic components of the smear layer.19,21,22 It also kills sessile endodontic pathogens organized in a biofilm.23,24 There is no other root-canal irrigant that can meet all these requirements, even with the use of methods such as lowering the pH,25–27 increasing the temperature28–32 or adding surfactants to increase the wetting efficacy of the irrigant.33,34 However, although NaOCl appears to be the most desirable single endodontic irrigant, it cannot dissolve inorganic dentine particles and thus cannot prevent the formation of a smear layer during instrumentation.35 Calcifications hindering mechanical preparation are frequently encountered in the root-canal system, further complicating treatment. Demineralizing agents such as EDTA have therefore been recommended as adjuvants in root-canal therapy.20,36 Thus, in contemporary endodontic practice, dual irrigants such as NaOCl with EDTA are often used as initial and final rinses to circumvent the shortcomings of a single irrigant.37–39 These irrigants must be brought into direct contact with the entire canal-wall surfaces for effective action,20,37,40 particularly in the apical portions of small root canals.9 The combination of NaOCl and EDTA has been used worldwide for antisepsis of root-canal systems. The concentration of NaOCl used for root-canal irrigation ranges from 2.5 to 6 percent, depending on the country and local regulations; it has been shown, however, that tissue hydrolyzation is greater at the higher end of this range, as demonstrated in a study by Hand et al. comparing 2.5 and 5.25 percent NaOCl. The higher concentration may also favor superior microbial outcomes.41 NaOCl has a broad antimicrobial spectrum,20 including but not limited to E. faecalis. NaOCl is superior among irrigating agents that dissolve organic matter. EDTA is a chelating agent that aids in smear layer removal and increases dentine permeability,42,43 which will allow further irrigation with NaOCl to penetrate deep into the dentinal tubules.44