tives are related to their antiseptic and nontoxic effects such as efÞcacy against anaerobic facultative microorganisms (planktonic and bioÞlms), ability to inactivate endotoxin, nontoxic and noncaustic, and little potential to cause anaphylaxis.

The ideal irrigating solution to disinfect the root canal system should be a biocompatible bactericidal agent, tissue solvent, lubricant, and smear layer remover capable of physically ßushing debris, with sustained effect but without affecting the physical properties of the dentin.

The irrigating solutions in endodontics can be classiÞed as antimicrobial solutions, chelating solutions (strong or weak), combinations (antibacterial and chelating solutions combined), and solutions with detergent.

Antimicrobial effects: antiseptic solutions, topical antibiotics, bacteriostatic solutions, and bactericidal solutions.

Fig. 5.3 Percentage of responders who utilize each irrigant as their primary disinfectant agent (Reproduced with permission JOE [17])

Antiseptic Solutions

Sodium Hypochlorite

Surveys from around the world [17, 23, 94] reported that sodium hypochlorite is the most common irrigating solution used in endodontics. Figure 5.3 shows the percentage of responders who utilize each irrigant as their primary disinfectant agent in a survey by the American Association of Endodontists. It is an effective antimicrobial and proteolytic agent [48, 49], excellent organic tissue solvent [60], and lubricant with fairly quick effects. NaOCl is both an oxidizing agent and a hydrolyzing agent. Commercial sodium hypochlorite solutions are strongly alkaline and hypertonic and typically have nominal concentrations of 10Ð14 % available chlorine.

Mode of Action

Sodium hypochlorite has a pH of 11. Figure 5.4 shows the schematic interaction of the mechanism of action of NaOCl (Reproduced with permission from Estrela et al., Brazilian Endodontic Journal).

Fig. 5.4 Schematic drawing of NaOCl mechanism of action (Reproduced with permission from [20])

Estrela [20] reported that sodium hypochlorite exhibits a dynamic balance:

1: SaponiÞcation reaction:

Sodium hypochlorite acts as an organic and fat solvent that degrades fatty acids and transforms them into fatty acid salts (soap) and glycerol (alcohol), reducing the surface tension of the remaining solution.

2: Neutralization reaction:

Sodium hypochlorite neutralizes amino acids by forming water and salt. With the exit of hydroxyl ions, the pH is reduced.

3: Hypochlorous acid formation:

When chlorine dissolves in water and it is in contact with organic matter, it forms hypochlorous acid. It is a weak acid with the chemical formula HClO that acts as an oxidizer. Hypochlorous acid (HOCl−) and hypochlorite ions (OCl−) lead to amino acid degradation and hydrolysis.

4: Solvent action:

Sodium hypochlorite also acts as a solvent, releasing chlorine that combines with protein amino groups (NH) to form chloramines (chloramination reaction). Chloramines impede cell metabolism; chlorine is a strong oxidant and inhibits essential bacterial enzymes by irreversible oxidation of SH groups (sulfhydryl group) [20].

5: High pH:

Sodium hypochlorite is a strong base (pH > 11). The antimicrobial effectiveness of sodium hypochlorite, based on its high pH (hydroxyl ion action), is similar to the mechanism of action of calcium hydroxide. The high pH interferes in cytoplasmic membrane integrity due to irreversible enzymatic inhibition, biosynthetic alterations in cellular metabolism, and phospholipid degradation observed in lipidic peroxidation [20].

Concentration

In the literature, it can be found that NaOCl can be used in a concentration that ranges from 0.5 to 6 %. It was proven that the lower and higher concentrations are equally efÞcient in reducing the number of bacteria in infected root canal system but the tissue-dissolving effect is directly related to the concentration [26].

Grossman observed pulp tissue dissolution capacity and reported that 5 % sodium hypochlorite dissolved this tissue in between 20 min and 2 h. The dissolution of bovine pulp tissue by sodium hypochlorite (0.5, 1.0, 2.5, and 5.0 %) was studied in vitro under different conditions (Estrela). It was concluded that:

1.The velocity of dissolution of the bovine pulp fragments was directly proportional to the concentration of the sodium hypochlorite solution and was greater without the surfactant.

2.Variations in surface tension, from beginning to end of pulp dissolution, were directly proportional to the concentration of the sodium hypochlorite solution and greater in the solutions without surfactant. Solutions without surfactant presented a decrease in surface tension and those with surfactant an increase.

3.In heated sodium hypochlorite solutions, dissolution of the bovine pulp tissue was more rapid.

4.The greater the initial concentration of the sodium hypochlorite solutions, the smaller the reduction of its pH (Estrela).

Volume

Volume is more critical for disinfection than its concentration. Frequent exchange with fresh NaOCl is important and the use of large amount of irrigant compensates for the low concentration. It should be kept in mind that the NaOCl will inactivate its components very fast, so fresh irrigating solution should be added to the canal system constantly. (Please see chapter on irrigation dynamics to learn more about the volume.)

Time

How long does NaOCl need to kill bacteria? This question can be misinterpreted in the literature. Some articles will show bacterial killing in 30 min when 0.5 % NaOCl is used, while higher concentrations will need only 30 s to do the same job. Interpretation of results needs to be taken with caution because it will depend on the methods used to test the time. It is important to remember that the presence of organic matter, inßammatory exudates, tissue remnants, and microbial biomass consumes NaOCl and weakens its effect.

The chlorine ion, which is responsible for the dissolving and antibacterial capacity of NaOCl, is unstable and consumed rapidly during the Þrst phase of tissue dissolution, probably within 2 min [57], which provides another reason for continuous replenishment. This should especially

be considered in view of the fact that rotary root canal preparation techniques have expedited the shaping process. The optimal time that a hypochlorite irrigant at a given concentration needs to remain in the canal system is an issue yet to be answered [96].

Effect on the Dentin

As it was stated before, the dentin is composed of 22 % organic material by weight. Most of this consists of type I collagen, which contributes considerably to the mechanical properties of the dentin. NaOCl solutions may affect mechanical dentin properties via the degradation of organic dentin components.

Depth of Penetration

The depth of NaOCl penetration varied between 77 and 300 μm, and it depends on concentration, time, and temperature [99]. Figure 5.5 illustrates

Fig. 5.5 A microscope view of stained root section treated by 1 % sodium hypochlorite for 2 min (arrow) (Reproduced with permission [99])

a microscopic view of stained root section treated by 1 % sodium hypochlorite for 2 min (Published with permission).

Effect on Biofilms

Clegg et al. [12] demonstrated that 6 % NaOCl was the only agent capable of both physically removing artiÞcial bioÞlm and killing bacteria. There was a dose-dependent effect of NaOCl against bacteria, as higher concentrations were more antibacterial. Figure 5.6 illustrates the effect of different irrigants on dentin bioÞlm elimination. In summary, 3 % and 6% NaOCl showed absence of bioÞlm, 1 % NaOCl showed disruption of bioÞlm, and 2 % CHX showed intact bioÞlm (Fig. 5.6).

Limitations

Unfortunately, even though NaOCl has many ideal properties, it has some limitations such as being toxic [39, 48] (see more details in Chap. 7), nonsubstantive, ineffective in smear layer removal and corrosive. It may cause discoloration [40] and has unpleasant odor. When NaOCl is used as a Þnal rinse, bonding of the sealer to the dentin may be altered [72].

Clinical Recommendation

NaOCl in concentrations between 2.5 and 6 % should be used during the whole cleaning and shaping procedure. Pulp chamber should be used as a reservoir of fresh irrigant. Once the mechanical preparation is Þnished and a master apical Þle is determined, the protocol of irrigation should start with the activation of fresh NaOCl in each canal [27].

Fig. 5.6 (a) Scanning electron micrograph (SEM) of bacteria-free dentin on negative control specimen (original magniÞcation ×3,000). (b) SEM of positive control reveals cocci, rods, and Þlamentous organisms (original magniÞcation ×5,000). (c) SEM of dentin section treated

with 6 % NaOCl. No bacteria are visible (original magniÞcation ×5,000). (d) SEM of dentin section treated with 2 % CHX. The bioÞlm is intact with no visible disruption (original magniÞcation ×5,000) (Reproduced with permission from JOE [12])