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When dental pulp undergoes pathological changes due to trauma or caries, microorganisms enter the pulp chamber and invade the anatomic irregularities of the root canal system.1 Infection of the root canal spaces occurs most frequently as a sequela to a profound caries lesion.2 The objective of endodontic treatment is to prevent or eliminate infection within the root canal. In every root canal system there are spaces that cannot be cleaned mechanically and where cleaning is dependent on thorough chemomechanical debridement of pulpal tissue, dentin debris, and infective microorganisms. Infection control is critical for the success of nonsurgical endodontic treatment.
Irrigation is complementary to instrumentation in facilitating the removal of pulp tissue and/or microorganisms. There are a number of ideal requirements of a root canal irrigant. It should provide a broad spectrum of antimicrobial activity while flushing out debris from the root canal. It should be nontoxic and biocompatible in nature, able to sterilize the canal and dissolve the smear layer. The root canal irrigant should have good lubricating action along with low surface tension to be able to flow into inaccessible areas. Finally, the irrigant should facilitate dentin removal but not weaken the tooth structure.
In light of the importance of infection control, the aim of this review is to analyze the relevant literature on root canal irrigating solutions, their actions, and interactions. For this review article the authors performed a Medline search for all English language articles published through January 2014.
Sodium Hypochlorite (NaOCl)
Sodium hypochlorite (NaOCl) was first introduced during World War I by chemist Henry Drysdale Dakin and surgeon Alexis Carrel for treating infected wounds through the use of a buffered 0.5% solution of NaOCl. It is, therefore, also known as “Dakin’s solution.” In 1936 Walker first suggested its use in root canal therapy, and in 1941 Grossman demonstrated the tissue-dissolving ability of chlorinated soda when used in double strength. Spangberg in 1973 said that 0.5% of NaOCl has good germicidal activity.3
NaOCl Mechanism of Action
At body temperature, reactive chlorine in aqueous solution exists in two forms: hypochlorite ion (OCl-) and hypochlorous acid (HOCl). At acidic or neutral pH, chlorine exists predominantly as HOCl, whereas at high pH of 9 and above OCl- predominates. Hypochlorous acid is responsible for the antibacterial activity; the OCl- ion is less effective than the undissolved HOCl. Hypochloric acid disrupts several vital functions of the microbial cell, resulting in the cell death.4 As a disinfectant, HOCl is more effective than OCl-. By controlling the pH, one can ensure that the more effective bactericide HOCl will remain the dominant species in solution. The germicidal potency of HOCl is approximately 80 times more than OCl- ion.
Concentration of Sodium Hypochlorite for Endodontic Usage
In endodontic therapy NaOCl solutions are used in concentrations that vary from 0.5% to 5.25%.5 Also available are unbuffered solutions at pH 11 to 12 in concentrations ranging between 0.5% and 5.25%, or the so-called Dakin’s solution, which is a buffered 0.5% solution at pH 9.5,6 There is no difference between these two solutions with respect to tissue dissolution or antibacterial efficiency.5,7 NaOCl dissolves pulpal remnants, organic compounds of dentin, and organic components of the smear layer.8,9 Moreover, neutralization or inactivation of lipopolysaccharides has been reported with NaOCl.10,11 However, NaOCl is not able to remove the smear layer by itself, as it dissolves only organic material.12
The dissolving capacity of NaOCl is significantly better than all other commonly used irrigants8 but is dependent on the concentration of the solution. The higher the concentration, the greater the cytotoxicity.3,13 Regular replenishing of NaOCl irrigant solution is necessary for better tissue-dissolving capability during root canal treatment. NaOCl used in concentrations of 0.5% and 1% wt/vol have enhanced tissue-dissolving capability, antimicrobial activity, and biocompatibility and are recommended for routine clinical use.5 It should be used throughout the instrumentation phase. However, use of NaOCl as the final rinse following ethylenediaminetetracetic acid (EDTA) or citric acid should be avoided because it rapidly produces erosion of the canal wall dentin.14 “Full strength” 5.25% NaOCl can have a detrimental effect on dentin elasticity and flexural strength. Hypochlorite in some situations may increase the risk of vertical root fracture.15 This is most likely due to the proteolytic action of concentrated hypochlorite on the collagen matrix of dentin.
Increasing the Efficacy of Sodium Hypochlorite
NaOCl should not be diluted with water for root canal irrigation, as this reduces its antibacterial and tissue-dissolving properties.16 The volume of irrigant is also clinically relevant, as an increase in volume correlates with reduction of intraradicular microorganisms and improved canal cleanliness.17,18 Yamada et al recommended 10 ml to 20 ml of irrigant for each canal.
With regard to timing, the greater the contact time, the more effective the NaOCl irrigant is. This is especially important in necrotic cases where 5.25% of NaOCl used for 40 minutes was found to be effective.3 The clinician should be cognizant of the working time of NaOCl, bearing in mind the fact that rotary root canal preparation techniques have expedited the shaping process.19
Another factor is temperature. Warm NaOCl dissolved organic tissues significantly better than unheated solutions.20 A rise in temperature by 25oC increased NaOCl efficacy by a factor of 100.21 The ability of 1% NaOCl at 45oC to dissolve human dental pulps was found to be equal to that of a 5.25% solution at 20oC.21 However, there are no clinical studies available at this point to support the use of heated sodium hypochlorite. Once heated but not used, NaOCl solution must be discarded immediately, because its effectiveness is exhausted.22
The use of side venting irrigation needles helps to move the irrigant sideways in a whole canal and avoid periapical extrusion. Shaping of the root canal must be to at least size #25 at the apex for irrigation needles to reach the more apical portions of the canal. The irrigant does not move apically more than 1 mm beyond the irrigation tip, therefore deep placement with small-gauge needles enhances irrigation.23 During rinsing, the needle is moved up and down constantly to produce agitation and prevent binding or wedging of the needle. Apical negative pressure irrigation will improve the irrigation dynamics and enhance interaction between NaOCl and the root canal wall in the apical portion of the canal.
Passive ultrasonic activation of NaOCl after canal preparation is an alternative approach to improve its effectiveness, as this will “accelerate chemical reactions, create cavitational effects, and achieve superior cleansing action.”24 Factors that influence ultrasonic irrigation include frequency and duration of agitation, size and position of the insert, and concentration and volume of NaOCl present inside the root canal. It appears best to insert a slim, non-cutting instrument in a controlled fashion after canal preparation.25 Laser-assisted root canal disinfection seems to have the potential to improve fluid dynamics within the root canal.
NaOCl solutions containing 0.5% and 5% chlorine stored at 4oC displayed satisfactory stability at 200 days, whereas 5% NaOCl decomposed rapidly when stored at 24oC.26 Therefore, it is recommended to store NaOCl solutions in a refrigerator and in dark bottles to avoid degradation by light.27
Sodium Hypochlorite’s Effect on Mechanical Properties
Cavalleri et al have shown that using sodium hypochlorite as an irrigating solution in root canals for short-term contact (several minutes, as is the case in clinical practice) does not alter the surface structure of the files through corrosion and does not cause any risk of fracture of nickel-titanium (Ni-Ti) instruments.28 NaOCl irrigation decreases bond strength between resin cements and dentin, because hypochlorite affects the polymerization of the resin sealer.29,30 Agents such as ascorbic acid or sodium ascorbate completely reverse the reduction of bond strength.31
NaOCl and chlorhexidine (CHX) are not soluble in each other, and a brownish orange precipitate is formed, which is a carcinogenic product, parachloroanaline (PCA). PCA has mutagenic potential. Atomic absorption spectrophotometry has indicated that the precipitate contains iron, which may be the reason for the orange development.32 This reaction coats the canal surface and significantly occludes the dentinal tubules and affects the seal of the root canal.33 It would appear prudent to minimize the formation of PCA by washing away the remaining NaOCl with alcohol or EDTA before using CHX.
Precautions to Take
Sodium hypochlorite is caustic if accidentally extruded into periapical tissue or adjacent anatomical structures such as the maxillary sinus.34 Emphysema develops within 10 to 20 minutes if accidentally injected into periapical tissue. Edema and paresthesia may result due to the tissue-dissolving capability of NaOCl. Because the potential for spread of infection is related to tissue destruction, medications such as antibiotics, analgesics, and antihistamines should be prescribed accordingly.
Chlorhexidine Digluconate (CHX)
Chlorhexidine digluconate (CHX) was developed in the late 1940s in the research laboratories of Imperial Chemical Industries Ltd. (Macclesfield, England). Amongst a series of polybisguanides synthesized to obtain antibacterial agents, chlorhexidine was the most potent of the tested bisguanides.35 It is a strong base and is most stable in the form of its salts. The original salts were chlorhexidine acetate and hydrochloride, both of which are relatively poorly stable in water.36 Hence, they have been replaced by chlorhexidine digluconate.
CHX is a potent antiseptic, which is widely used for chemical plaque control in the oral cavity in concentration of 0.1% to 0.2%,37 while the concentration of root canal irrigating solutions usually found in the endodontic literature is 2%.38 This has been found to be more effective in the least amount of time when compared with other concentrations of chlorhexidine ranging from 0.002% to 2%.39 CHX permeates the microbial cell wall or outer membrane and attacks the bacterial cytoplasmic or inner membrane or the yeast plasma membrane.5 However, similar to other endodontic disinfecting agents, the activity of CHX depends on the pH and is also greatly reduced in the presence of organic matter.
CHX cannot be advocated as the main irrigant in standard endodontic cases because it is unable to dissolve necrotic tissue remnants8 and remove biofilm and it is less effective on gram-negative than gram-positive bacteria.35,40 It is only because of these differences that CHX cannot replace NaOCl as the gold standard of root canal irrigants. Direct contact between NaOCl and CHX should be avoided, otherwise red CHX crystals will precipitate immediately (para-chloroaniline, which is known to be carcinogenic). It would appear prudent to minimize their formation by washing away the remaining NaOCl with alcohol or EDTA before using CHX.41 Several studies have compared the antibacterial effect of NaOCl and 2% CHX against intracanal infection and have shown little or no difference between their antimicrobial effectiveness.42,43 However, CHX does not cause erosion of dentin like NaOCl does as a final irrigant after EDTA, and, therefore, 2% CHX may be a good choice for maximizing antibacterial effect at the end of chemomechanical preparation.38 Some studies have indicated that CHX gel may be slightly more effective than CHX liquid, but the possible reasons for differences are unknown.44
Hydrogen Peroxide (H2O2)
Hydrogen peroxide (H2O2) is a clear, odorless liquid and is used in dentistry in varying concentrations of 1% to 30%.5 For endodontic treatment, concentration between 3% and 5% solution is used as an irrigating agent. The antimicrobial efficiency and the tissue-dissolving capacity of H2O2 are poor in comparison with NaOCl. Combining NaOCl with H2O2 produces a bubbling effect as a result of chemical reaction and reduces the effectiveness of NaOCl45 (H2O2 + NaOCl à O2 + H2O + NaCl).
H2O2 Mechanism of Action
It rapidly dissociates into H2O + [O] (water + nasant oxygen). On coming in contact with the tissue enzymes catalase and peroxidase, the liberated nasant oxygen produces a bactericidal effect, but this effect is transient and diminishes in the presence of organic debris. This causes oxidation of a bacterial sulfhydryl group of enzymes and, thus, interferences with bacterial metabolism. The rapid release of nasant oxygen on contact with organic tissue results in effervescence or bubbling action, which is thought to aid in mechanical debridement by dislodging particles of necrotic tissue and dentinal debris and floating them to the surface. H2O2 is highly unstable and easily decomposed by heat and light, and there is no scientific evidence indicating that H2O2 is superior to other irrigants.
Ethylenediaminetetracetic acid (EDTA)
EDTA is a commonly used chelating agent. It was introduced to dentistry by Nygaard-Østby in 1957 for cleaning and shaping of canals. While NaOCl is an excellent irrigating solution, it cannot dissolve inorganic dentin materials.12 EDTA complements the action of NaOCl by chelating calcium ions in dentin and making instrumentation of root canals easier, and it is effective at a neutral pH. Removal of smear layer from the root canal is an important step in endodontics as it exposes the bacteria living in the dentinal tubules to be acted upon by the disinfecting irrigants and, additionally, allows the endodontic sealer to penetrate into the dentinal tubules for a more intimate fit leading to an enhanced sealing of the canal. The formation of sealer tags into the dentin provides good adaptation between the sealer cement and the dentin interface.46 This may further reduce the microleakage that frequently results from improper obturation of the root canal.
The effect of EDTA on dentin depends on the concentration of EDTA solution and the length of time it is in contact with the dentin. EDTA as a 17% solution effectively removes the smear layer by chelating the inorganic components of the dentin.5,20 It has almost no antibacterial activity, is highly biocompatible, can demineralize intertubular dentin, and reduces the surface hardness of root canal wall dentin.47 Teixeira et al has showed equally effective smear layer removal after EDTA irrigation for 1, 3, and 5 minutes.48 Crumpton et al showed efficient smear layer removal with a final rinse of 1 ml of 17% EDTA for 1 minute.49 Ultrasonics helped in efficient smear layer removal from the apical region of the root canal.50
Prolonged exposure to EDTA may weaken root dentin51 and thereby increase the risk of creating a perforation during mechanical root canal instrumentation. Irrigation of the root canal using alternative NaOCl and EDTA appears to be very promising.52 This combination seems to enhance the tissue-dissolution capability of NaOCl14,52 and is more efficient in reducing intraradicular microbes than NaOCl alone.53 EDTA retains its calcium-complexing ability when mixed with NaOCl, but EDTA causes NaOCl to lose its tissue-dissolving capacity.54 Therefore, EDTA and NaOCl should be used separately and should never be mixed.55
MTAD (Mixture of Tetracycline, an Acid, And a Detergent)
Torabinejad et al developed an irrigant that is a mixture of 3% doxycycline, 4.25% citric acid, and detergent (Tween 80, 0.5%),56 with a pH of 2.15. The commercial product is BioPure® (DENTSPLY Tulsa Dental Specialties). It is effective in removing the smear layer due to its low pH, and it showed tissue-dissolving action as long as the canal was rinsed with NaOCl during mechanical preparation.57 Recent protocol recommends an initial irrigation for 20 minutes with 1.3% NaOCl, followed by a 5-minute final rinse with MTAD.57 MTAD works better in the apical third to remove the smear layer as compared to other root canal irrigants.58 In MTAD preparation, the citric acid may serve to remove the smear layer, allowing doxycycline to enter the dentinal tubules and exert an antibacterial effect.57
MTAD seems to adversely influence the physical properties of dentin and causes significant reduction in bond strength of both resin-based and calcium-hydroxide–based sealers due to precipitate formation.59 Concerns have been expressed regarding the use of tetracycline (doxycycline) because of possible resistance to the antibiotic and staining of tooth hard tissue, as demonstrated by exposure to light in an in-vitro model.60 However, no report of in-vivo staining has been published.
Hydroxyethylidene Bisphosphonate (HEBP)
Also known as etidronate or etidronic acid having chelating properties, hydroxyethylidene bisphosphonate (HEBP) has been proposed as a potential alternative to EDTA or citric acid because this agent shows no short-term reactivity with NaOCl.61 It is nontoxic and has been systematically applied to treat bone diseases.62 Continuous chelation irrigation protocol using 5% NaOCl with 18% HEBP optimizes bonding quality of epoxy resin sealer (eg, AH Plus) to dentin.63
Maleic Acid
Maleic acid is a mild organic acid used as an acid conditioner in adhesive dentistry.64 Effective smear layer removal takes place at 5% and 7% concentration, however at 10% or more it can result in demineralization and damage to the root canal wall.65 At 7%, maleic acid has proved to be more efficient than 17% EDTA in removal of the smear layer from the apical third of the root canal system.64 It also produces maximum surface roughness as compared to 17% EDTA, which plays an important role in micromechanical bonding of resin sealers. However, further evaluation is needed regarding the biological effects and technique of use of maleic acid on periapical tissues before routine clinical use can be employed.
Tetraclean
A mixture of doxycycline hyclate, an acid, and a detergent,56 Tetraclean (Ogna Laboratori Farmaceutici, Milano, Italy) is similar to MTAD, but the concentration of antibiotic (doxycycline 50 mg/ml) and the type of detergent (polypropylene glycol) differ from those of MTAD.66 It is able to eliminate microorganisms and the smear layer in dentinal tubules of infected root canals with a final 5-minute rinse and opens up the dentinal tubules. It has low surface tension allowing better adaptation of the mixtures to the dentinal walls66 and is effective against both strictly anaerobic and facultative anaerobic bacteria.67
Other Natural Root Canal Irrigants
Neem (Azardiracta indica) is a versatile medical plant that has a wide spectrum of biological activity. Its antioxidant and antimicrobial properties make it a potential agent for root canal irrigation as an alternative to sodium hypochlorite.68
Turmeric (Curcuma longa) has anti-inflammatory, antioxidant, antibacterial, antifungal, and antiviral activities. It is proven to be safe as a root canal irrigant, effective against Enterococcus faecalis.69
Liquorice (Glycyrrhiza glabra) is also used as a root canal irrigant. Liquorice extract, either separately or as a liquorice/calcium hydroxide (Ca[OH]2) mixture, had a potent bactericidal effect against E. faecalis.70
Noni (Morinda citrofolia) is a traditional medicinal plant that has been used in Polynesia for more than 2,000 years. It has been proven to effectively remove smear layer from root canal walls of instrumented teeth in a manner similar to that of NaOCl in conjunction with EDTA.71
Conclusion
Irrigation plays a vital role in successful endodontic treatment. Available literature and studies demonstrate advantages and limitations of each irrigant under consideration, and none of them completely satisfy the requirements of the ideal root canal irrigant. Although NaOCl is the most significant irrigating solution, no single irrigant can accomplish all the tasks required by irrigation. Understanding the mode of action of various available and commonly used solutions is important for optimal irrigation. New developments in the composition of the irrigating solutions will help to advance safe and effective irrigation.
Disclosure
The authors had no disclosures to report.
About the Authors
Amit Jena, MDS
Reader
Department of Conservative Dentistry & Endodontics
Institute of Dental Sciences
Siksha ‘O’ Anusandhan University
Odisha, India
Sanjit Kumar Sahoo, BDS
Post-Graduate Student
Department of Conservative Dentistry & Endodontics
Institute of Dental Sciences
Siksha ‘O’ Anusandhan University
Odisha, India
Shashirekha Govind, MDS
Reader, Department of Conservative Dentistry & Endodontics
Institute of Dental Science
Siksha ‘O’ Anusandhan University
Odisha, India
Queries to the author regarding this course may be submitted to authorqueries@aegiscomm.com.
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