Disinfection of the root canal system: what should the protocol be?

Sodium hypochlorite solution (NaOCl)

This chemical is considered to be the gold standard by endodontists. It is an effective antimicrobial and proteolytic agent,^15,27,28 an excellent organic tissue solvent, lubricant, is fast acting and displays a broad spectrum of antimicrobial action. It works in a number of ways by a dynamic balance as described by Estrela:²⁹

There are four factors that affect the efficacy of NaOCl, namely: concentration; volume; time; and temperature.

There are geographical variations in the concentration of NaOCl solution used for endodontic irrigation, ranging between 0.5% and 6%. In Scandinavia, lower concentrations tend to be used and in the USA, higher concentrations are favoured. Its ability to dissolve organic matter is directly related to its concentration. Irrespective of concentration, NaOCl is an effective antimicrobial, but it can kill micro-organisms more quickly at higher concentrations. It is therefore recommended that if lower concentrations are chosen, then a higher volume of solution is used to irrigate the root canal system and with a higher frequency to compensate for the decreased effectiveness.³⁰ In the past, it was commonplace for clinicians to use household bleach products. However, in contemporary practice, it is strongly recommended that only products with a CE mark for dental or medical applications are used because this is medico-legally defensible in case of any challenge. One such approved commercial product is Parcan (Septodont, Cedex, France), which is a 3% solution.

NaOCl is quickly consumed in the presence of organic matter, inflammatory exudates, tissue remnants and microbial biomass and, for this reason, it must be replenished frequently. Its antibiofilm efficiency is dependent on time and volume³¹ and its ability to penetrate dentinal tubules depends on time, concentration and temperature with it able to render dentinal tubules free of bacteria to a depth of 130 µm.³² It is therefore recommended that a dwell time of 30 minutes is allowed.³³ If the temperature of the solution is increased, more chlorine is released, and this improves its immediate tissue dissolving capacity.³⁴ However, there are no clinical studies to support use of heated NaOCl, and the temperature of the solution quickly falls to body temperature when introduced into the root canal.

Despite its many advantages, NaOCl does have some shortcomings. It is not totally effective at killing all the micro-organisms found in the root canal system.³⁵ It is highly toxic to vital tissue, and if it is inadvertently introduced into the peri-radicular tissues, it can produce some serious effects for the patient, such as severe pain (even when local anaesthesia has been used), swelling, ecchymosis and paraesthesia.³⁶ Its toxic and therapeutic doses are also undesirably close together.²⁶

Chlorhexidine digluconate

Another disinfecting irrigant is chlorhexidine digluconate (CHX), which has a broad spectrum. Chemically, this is a cationic bis-biguanide that is soluble in water and stable. At low concentrations, it is bacteriostatic by causing leakage of low molecular weight elements from the cell membrane with no permanent damage to the cell. CHX is bactericidal at higher concentrations by acting as a detergent and decomposing cell membranes leading to loss of cellular components. It displays no tissue dissolving capacity, and so has been advocated for use in teeth with open apices or in the case of perforation.

It is able to adsorb to dentine as it can bind to hydroxyapatite³⁷ and it is capable of slow release,^38,39 otherwise described as substantivity. This is the length of time that a medicament remains effective after application, and this property is considered to be one of the major advantages of chlorhexidine because its antimicrobial residual activity is extended.^40,41,42 It has been shown to display residual antibacterial action against a broad spectrum of micro-organisms for 12 weeks.^43,44 R4 (Septodont) is an example of a commercially available product, being 20% chlorhexidine digluconate in denatured alcohol. Chlorhexidine's reported disadvantages are that it may have a negative effect on peri-radicular healing⁴⁵ and concerns have been raised that it may sensitize the patient and result in anaphylaxis.⁴⁶

In many disinfection protocols CHX is used in combination with other irrigants. The use of chelating solutions (see later) prior to CHX potentiates the adhesion of CHX to dentine so it can be used as the final rinse after ethylenediamine tetra-acetic acid (EDTA) when a maximal antimicrobial effect is desirable.⁴⁷ A final flush of CHX has been shown to reduce the adhesion of micro-organisms to dentine,⁴⁸ with a study demonstrating favourable healing rates at 24 months when a one-visit procedure using NaOCl and 2% CHX as a final flush was compared with two visits with calcium hydroxide as an intra-canal dressing.⁴⁹

It is not recommended that NaOCl and CHX are used in direct contact with each other in any irrigation protocol. These two chemicals react forming the dark precipitate (flocculate), 4-parachloroaniline (PCA). This is concentration dependent, but PCA is carcinogenic, stains dentine, may penetrate into tubules, block narrow canals and hinder further penetration of NaOCl.⁵⁰ PCA may also be formed if CHX is heated.⁵¹

EDTA

EDTA is a chelating agent that scavenges bi- and tri-cationic ions such as calcium and iron, respectively. This chelator removes the instrument-created smear layer but cannot remove it alone:^15,52 NaOCl is required to remove the organic components. EDTA leaves a layer of exposed collagen on the surface of the root canal lumen, which facilitates binding of bacteria,⁵³ so the final rinse should be with NaOCl. Chelators have a temperature range within which their calcium-binding capacity is most effective. For this reason, they should not be heated.⁵⁴

Many clinicians use both EDTA and NaOCl as irrigants in their endodontic disinfection protocol. This is supported by studies that have shown that superior elimination of Enterococcus faecalis from the root canal and adjacent dentine occurs with intercalated use of 5.25% NaOCl and 17% EDTA,⁵⁵ and there is a positive synergistic effect of EDTA and NaOCl.^56,57 The use of EDTA significantly increased the odds of success of re-treatment cases by twofold.⁴⁵ When EDTA and NaOCl come into direct contact they react chemically. EDTA retains its calcium-complex forming ability, but NaOCl loses its tissue dissolving capacity as there is no free chlorine. For this reason, they should not be used sequentially without first emptying and drying the canal.⁵⁸ When EDTA is used in combination with other irrigants, it is the sequence of use that seems to be important. In one study, if EDTA was used as the first irrigant, erosion of peritubular and intertubular dentine was seen, whereas if it was used as a final rinse after 5.25% NaOCl, this did not occur.⁵⁹ Repeated cycling of NaOCl and EDTA resulted in erosion of the dentine and compromised tooth structure.⁶⁰ EDTA and CHX should not be used in direct contact as a white precipitate will form.

Citric acid

While EDTA is the most commonly used chelating agent, citric acid in a concentration of 10–50% is a suitable alternative.⁶¹ Citric acid is slightly more potent than EDTA at similar concentrations, with both acids able to detach biofilms on root canal walls.¹⁶

Iodine potassium iodide (IKI)

Iodine potassium iodide is an organic compound that releases iodine. It is a broad spectrum antimicrobial, being effective against bacteria, fungi, viruses and spores, and is also tuberculocidal. It works by attacking proteins, nucleotides and fatty acids leading to cell death.⁶² It has a low toxicity, is effective for about 2 days and is able to penetrate dentinal tubules, but may stain dentine.⁶² Some consideration needs to be given to the patient's medical history when the use of iodine-containing irrigants is being contemplated because an iodine hypersensitivity and a theoretical risk for those with thyroid and parathyroid pathoses may be present. IKI is supplied as an irrigating solution containing a 2% solution of iodine in 4% aqueous potassium iodide. Another iodine-containing chemical is povidone iodine, which is a profound antibacterial, antifungal and antiviral agent. It is widely used as a surface disinfectant but not as an endodontic irrigant per se.

Hydrogen peroxide

This chemical is used in concentrations of between 3% and 30% and is active against bacteria, viruses and yeasts.⁶³ However, it is no longer advocated over other irrigants,⁶⁴ one reason being that, due to effervescence of the chemical, gaseous oxygen may penetrate into the peri-radicular tissues causing surgical emphysema.

Chemical mixtures

Passive ultrasonic irrigation

Passive ultrasonic irrigation (PUI) has been described as an oscillating file-induced acoustic microstreaming at a frequency of more than 25 Hz⁷³ whereby a non-cutting tip is fitted to a piezoelectric handpiece and introduced into the root canal, so causing the microstreaming. A final irrigation of NaOCl with PUI has been shown to remove more debris,⁷⁴ bacteria,⁷⁵ and pulp tissue than conventional syringe irrigation.⁷⁶ Furthermore, with PUI the irrigant can penetrate any non-instrumented regions and enhances shear stress on the biofilm and tissue remnant^77,78 and causes significantly more penetration of irrigant into lateral canals than negative passive irrigation.⁷⁸ The use of PUI significantly increased NaOCl penetration into dentinal tubules⁷⁹ and, in combination with 17% EDTA, smear layer removal was enhanced.⁸⁰

Sonic activation

Activation at sonic frequencies of 1–6 kHz with a flexible tip have been reported with improved cleaning seen compared to conventional techniques.⁷⁰ Greater penetration depth occurred in the apical third with ultrasonic and sonic compared to manual dynamic activation.⁸¹ An example of a commercial product is EndoActivator (Dentsply Sirona) (Figure 7). However, other studies have shown that it provided no additional advantage over conventional irrigation.⁸²

Laser

Medium infra-red lasers with a wavelength of between 2780 and 2940 nm have also been investigated to activate irrigants in the root canal system.^83,84,85 This has been termed photon-induced photoacoustic streaming (PIPS), with an example being a PIPS pulsed Er:YAG laser.⁸⁶ A greater penetration of irrigants with PIPS has been reported.⁸¹

Ozone (O₃)

Ozone is a thermodynamically highly unstable chemical with a half-life of 40 minutes at 20°C. Its use in endodontics has been investigated for many years and there is promising in vitro evidence, but none clinically. A systematic review published in 2020 compared ozone therapy against NaOCl and concluded that ozone is not indicated either to replace or to complement the antimicrobial action of NaOCl in reducing bacterial load in patients undergoing RCT.⁸⁷

Bacterial photodynamic therapy (bacterial PDT)

This system works by the combination of a photosensitizer and light. This concept has been around since 1900,⁸⁸ but there has been a resurgence of interest in the technique of late.^89,90,91 There are a number of systems, but one example uses pharmaceutical grade tolonium chloride and laser or LED light at a wavelength of 635 ±2 nm. The photosensitizer is biocompatible and does not pose a problem if inadvertently extruded into the peri-radicular tissues. Its low surface tension enables it to wet the canal walls and penetrate into the dentinal tubules. The photosensitizer is preferentially adsorbed onto the cell wall of rapidly dividing cells. When exposed to the light of the correct wavelength, the photosensitizer becomes activated, liberating free radical oxygen species, which is a potent protoplasmic poison. This results in death of the cell with no potential for resistance or collateral damage to host cells (Figure 8). For further information on this technique and its use in clinical practice, the reader is directed to an article by Bonsor and Pearson published in Dental Update, ⁹² which, although published in 2006, is still current.

Bacterial PDT has been used in endodontics for many years with research demonstrating that it is effective in significantly reducing bacterial load as an adjunct to RCT.^{93,94,95,96,97} While it is recognized that there is insufficient evidence to definitely advocate its inclusion into an endodontic disinfection protocol, one study followed up over 700 cases over a 15-year period in a general dental practice and reported a survival rate of de novo root treatments of 97.3%, and re-root treatments of 93.7%, respectively at 10 years.⁹⁸ The cases treated in this study followed the protocol described by Bonsor et al.⁹⁴