What Is the Key Role of and the Rationale for the Use of Intracanal Irrigants?

The key role of root canal irrigants is to clean the canal during the enlarging and shaping process. Specifically, the objectives of the cleaning and shaping process are to remove the vital or necrotic dental pulp tissue and neutralize or eliminate bacteria and their associated metabolic byproducts. Although the shaping of the root canal has been enhanced with advances in metal technology, the actual cleaning of the canal still relies heavily on the adjunctive use of chemical rinses and soaks to achieve these goals because of the anatomic complexity and irregularity of the tooth (Fig. 11-1). In this respect, the use of irrigants that possess multiple characteristics, such as tissue-dissolving attributes and bacteriostatic or bactericidal capabilities, is warranted at all times. Other advantageous properties include rinsing away debris created during cleaning and shaping, lubricating the instruments, and demineralizing and removing the smear layer.

FIGURE 11-1 A, Tissue debris sandwiched in the anastomoses between the mesial buccal and mesial lingual canals of the mandibular molar after cleaning and obturation. B, Tissue debris that remains in a C-Shaped canal system after cleaning and shaping (H&E stain ×10). C, Scanning electron micrograph (SEM) shows the smear layer that is present during the shaping of the root canal is packed with tissue debris, dentin chips, and bacteria (×750). D, SEM showing surface tissue debris in anatomic irregularities after shaping and cleaning the root canal with irrigants (×2000).

No one solution as yet possesses all the properties of an ideal irrigant, but it is important to emphasize that the use of neutral solutions for the irrigation process (e.g., water, saline, anesthetic solutions) serves no useful purpose in the root canal system.

The effectiveness of NaOCl in cleaning and disinfection depends on the concentration of available chlorine and the pH of the solution. Hypochlorous acid (HOCl) is a weak acid and dissociates to the hypochlorite ion (OCl⁻) and proton (H⁺) depending on the solution pH. It is generally believed that HOCl is the active species in the germicidal action, whereas the concentration of OCl⁻ is a key factor determining cleaning efficiency. This implies that the optimal pH region of the germicidal activity of NaOCl differs from that of its cleaning activity.²²

The concentrations of NaOCl that are used during root canal procedures vary among clinicians and may very well be chosen empirically. Some bacterial populations are susceptible to percentages as low as 0.5%,⁷ but these species may not reflect bacterial populations found in the tooth with a necrotic infected pulp.⁶⁴ Clinical use for both bacterial control and tissue dissolution favors percentages from 1% to 6%, with studies claiming various levels of superiority with a particular percentage. Manipulations that enhance the efficacy of NaOCl include warming the solution to at least 37°C.¹ Within the bacterial species studied, bactericidal rates for NaOCl more than doubled for each 5°C rise in temperature in the range of 5°C to 60°C.¹⁴ In particular when using steady-state planktonic E. faecalis cells, a temperature rise of 25°C increased the efficacy of NaOCl by a factor of 100.⁵¹ Within this framework of heating NaOCl, the capacity of a 1% solution at 45°C to dissolve human dental pulps was found to be equal to that of a 5.25% solution at 20°C.⁵¹ While apparently effective in a laboratory setting, there are no evidenced-based clinical studies to verify this efficacy as it relates to successful outcomes.

Using the NaOCl solutions for periods of 5 to 30 minutes in the root canal has also been advocated to enhance its effectiveness, but optimal times have not been determined.⁶⁷ The application of sonics and ultrasonics to the NaOCl solution have also been advocated,⁵⁷ but the efficacy of these combinations has shown mixed results.^27,60,63 Techniques for this approach have been detailed in literature, but here again, meaningful clinical studies are lacking.^61,62 Even with longer time periods, replenishment may be necessary to increase its effectiveness, especially in the presence of significant amounts of tissue and bacteria that may be encountered in highly irregular canal anatomies in cases of long-standing necrotic pulp. Similarly, when NaOCl has been used for long periods, the potential for altering the physical properties of dentin have been highlighted in vitro but not clarified in vivo.^13,39,49 When used in conjunction with ethylenediamine tetraacetic acid (EDTA) or 10% citric acid,³⁸ its antibacterial properties are not necessarily enhanced.⁴⁵ When these solutions are used alternatively during canal cleaning and shaping, bacterial loads may be efficiently reduced.⁷

NaOCl should always be delivered passively to the canal to prevent a forceful extrusion beyond the apical foramen. This delivery is best accomplished by avoiding wedging the delivery needle in the canal, expressing the solution slowly, and using specially tipped side-delivery needles that enhance the cleaning of the dentin walls while minimizing potential risks during its use (Fig. 11-2). The passive movement of small amounts beyond the apical foramen should not create problems for the patient. Forceful movement of greater volumes of NaOCl will, however, be deleterious, and the literature is replete with “sodium hypochlorite accidents.”³⁰ Newer technologies that prevent this extrusion but at the same time are effective in removing the smear layer include the intracanal aspiration technique, in which the irrigant is delivered from the tip of an injection needle connected to an electric apex locator (EAL) placed 2 to 3 mm short of the root end. Studies indicate that there is limited extrusion of the irrigant when applied in this manner.²¹ A further development is the EndoActivator (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA). This innovative device agitates irrigation solutions during endodontic treatment. Evidence has indicated that dynamic movement of the irrigant significantly improves débridement.⁴⁰ The system is designed to safely energize the hydrodynamic phenomenon. Claims are made that activated fluids promote deep cleaning and disinfection into lateral canals, fins, webs, and anastomoses, with minimal apical extrusion because the activator tip is placed short of the working length in the canal.¹²

FIGURE 11-2 Side-delivery needle enables delivery of irrigating solutions to the root canal walls to enhance canal cleaning.

(Courtesy Dentsply Maillefer, Ballaigues, Switzerland.)

Claims have also been made relative to the application of ultrasonic irrigation devices alone and during canal cleaning and shaping.^6,10,27 Significant débridement of canal irregularities and bacterial reductions have been noted, but the exact nature of the process has been challenged as to mechanism of action.⁶³

A development that has claimed to eliminate problems with the delivery of NaOCl is the EndoVAC (Discus Dental, Culver City, CA, USA). The device offers an apical negative pressure system that uses microcannulas to deliver irrigants to the canal and draws fluid away by evacuation. Claims are made that cleaning occurs to within 1 mm of the apex,^31,43 without apical extrusion of solution.¹² However, there are no data available at this time to indicate what happens in the last millimeter of the canal, or if this activity can penetrate into the dentinal tubules to eliminate biofilms, bacteria, or embedded endotoxins.

The most recent addition to this foray of irrigation devices is the ProUltra PiezoFlow irrigation device (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA). ProUltra PiezoFlow Ultrasonic Irrigation Needles are used to deliver irrigants by application of ultrasonic vibration. The PiezoFlow irrigation needles are used in conjunction with a piezo-electric ultrasonic energy-generating unit to provide the energy for tip oscillation. A syringe or other irrigation source is attached to the Luer-lock connection on the ultrasonic needle. Removal of irrigant is through the operatory vacuum source. Compatible irrigants include sodium hypochlorite (up to 6%), EDTA (up to 17%), Chlorhexidine (up to 2%) and BioPure MTAD, however working times with each irrigant may vary.

NaOCl contact with dentin has been questioned as to its impact on the mechanical, chemical, and structural composition of dentin.^13,39,49 Evaluations were done ex vivo using different concentrations and different time frames to determine the impact on the inorganic phase of dentin. While higher concentrations of NaOCl affected the elastic modulus and the flexural strength of dentin, no clinical relevance has been drawn or shown from these studies.

NaOCl should not be used as the final rinsing agent when resin-based bonded root canal filling materials are planned for canal obturation. The bonding of the sealer to the dentin may be altered. Alternatives are to finish with EDTA, chlorhexidine (CHX),^8,9 or BioPure MTAD (Dentsply Tulsa Dental Specialties, Tulsa OK, USA). Studies have shown that a soft chelating solution (1-hydroxyethylidene-1, 1-bisphosphonate [HEPB]) not only removes the smear layer effectively but also enhances the bond strength of intracanal resins.^11,68 However, the use of EDTA or MTAD as a final rinse may actually cause a collapse of the dentin matrix structure on the surface of the dentin, thereby impeding root canal sealer infiltration and formation of a high-quality hybrid layer bonding.^23,54,55

NaOCl has strong support as being the best intracanal irrigant⁶⁷ because it fulfills most of the ideal requirements for this purpose, but it will not dissolve and remove the smear layer created during canal enlargement and shaping (Fig. 11-3).³⁵ Other solutions (or a combination of solutio/>