INTRODUCTION:

Complete debridement, disinfection of the pulpal space are considered to be essential for predictable longterm success in endodontic treatment. Residual pulpal tissue, bacteria and dentin debris may persist in the irregularities of root canal systems, even after meticulous mechanical preparation (1,2). Hence , several irrigating substances have been recommended for use in combination with canal preparation, including sodium hypochlorite (NaOCl), chlorhexidine, 17% EDTA, citric acid, MTAD and 37% phosphoric acid solution. These substances might affect the penetration of the resin sealer in dentin and its polymerization too, They might also degenerate dentin if they have a negative effect on the collagen fibers (3,4). Antibacterial effects of Chemomechanical procedures can be enhanced by the subsequent placement of an antimicrobial intracanal medication, particularly in those cases of exudation, haemorrhage, perforation, root resorption, trauma or incomplete root formation (5-7).

Chlorhexidine can be applied clinically as antimicrobial agent during all phases of the root canal preparation, including the disinfection of the operatory field; enlargement of the canals orifices; removal of necrotic tissues before working length determination; in the Chemomechanical preparation prior to the foraminal patency and enlargement; as an intracanal medicament; in the disinfection of obturation cones; for modeling the main gutta-percha cone; in the removal of gutta-percha cones during retreatment; in the disinfection of prosthetic space; among others.

General Uses:

Chlorhexidine is used in disinfectants, cosmetics , and pharmaceutical product.(8)

1 It is used to kill bacteria that cause infections.

2 It is found in many medicines that are applied directly to the affected area of the body.

3 It is an antiseptic treatment.

4 It is used to treat and prevent infections.

5 Chlorhexidine has also been applied to medical devices such as dental implants, vascular catheters, needleless connectors and antimicrobial dressings. Chlorhexidine, when applied to or impregnated in medical devices kills organisms and protects against microbial colonization and subsequently biofilm development.(9)

Chemical Structure:

The structural formula of Chlorhexidine consists of two symmetric 4-chlorophenyl rings and two biguanide groups connected by a central hexamethylene chain, as illustrated in the image below:(10)

Storage:

Its shelf life is about 1 year, provided that packaging is adequate, in a dark, refrigerated bottle (10,11). Regarding the gel formulation, it may keep its pH and satisfactory antimicrobial activity for approximately 10 months after the fabrication date. Color alteration has been found in samples 1 year after the fabrication date, due to the presence of breakdown products resulting from prolonged shelf life or exposure to high temperatures.

Mechanism of Action:

Chlorhexidine is a strong base and it is more stable in the form of its salts. The salts originally employed were acetate and Hydrochlorite, both have relatively poor water solubility and largely replaced by the Digluconate in 1957 (12), which is a highly water soluble salt. Aqueous solutions of Chorohexidine are more stable within the pH range from 5 to 8. The antimicrobial activity of Chlorhexidine is pH-dependent, the optimum range from 5.5 to 7.0, within which is the pH of body surfaces and tissues (10).

The bactericidal effect of the drug is by the cationic molecule binding to extra-microbial complexes and negatively charged microbial cell walls, thereby altering the osmotic equilibrium of the cells. At low concentrations, low molecular weight substances will leak out, specifically potassium and phosphorous, resulting in a bacteriostatic effect. At higher concentrations, Chlorhexidine has a bactericidal effect due to precipitation and/or coagulation of the cytoplasm of bacterial cells, probably caused by protein cross-linking, resulting in cell death (13,14), and leaving cell debris in the root canals (15), which can be removed with a vigorous irrigation with distilled water.

Antibacterial activity:

Chlorhexidine is a broad-spectrum biocide. It is effective against Gram-positive bacteria, Gram-negative bacteria and fungi. The bactericidal effect is a result of the binding of this cationic molecule to negatively charged bacterial cell walls. At low concentrations of chlorhexidine, this results in a bacteriostatic effect, at high concentrations, membrane disruption results in cell death.(16) Chlorhexidine inactivates microorganisms with a broader spectrum than other antimicrobials and has a quicker kill rate than other antimicrobials .(17) Chlorhexidine kills by disrupting the cell membrane. (18)

Antifungal Activity:

The mechanism of action for fungi is very similar to that of bacteria. Candida species is the most common of the fungi present in both healthy and affected individuals. [19] Fungi have been found in the infected root canals that have not had any previous endodontic treatment. [20] The fungus uptakes chlorhexidine in a short amount of time and impairs the integrity of the cell wall and the plasma membrane entering the cytoplasm resulting in leakage of cell contents and cell death.[21]

Chlorhexidine and Biofilms:

Biofilm are communities of microorganisms attached to a surface, embedded in an extracellular matrix of polysaccharides. Bacterias in these microcolonies have developed into organized communities with functional heterogeneity (22). It ensures a protected mode of growth that allows survival in a hostile environment. Bacteria in such an environment differ greatly in phenotype when compared with their counterparts, and are far less susceptible to antimicrobial killing (22,23). It has been reported that microorganisms grown in biofilms could be 2-fold to 1000-fold more resistant than the corresponding planktonic form of the same organisms (24).

Several studies using a single-species biofilm model (23, 25) and apical dentin biofilm (26) have reported that, higher concentration of NaOCl and Chlorhexidine solution were effective against the tested microorganisms. The mechanical agitation improved the antimicrobial properties of the chemical substances (23). Although Chlorhexidine is effective against bacterial biofilms, NaOCl is the only irrigation solution that has the capacity of disrupting biofilms (24).

Chlorhexidine and Coronal Microleakage:

Canals medicated with Chlorhexidine retard the entrance of microorganisms through the coronal portion of the tooth into the root canal system, due to its wide antimicrobial activity and substantivity (27). Such a finding is useful in coronal restoration becomes defective or if it is lost. However, even though a temporary seal delays the entrance of saliva and microorganisms into the canal system, it does not prevent microleakage (27). Regarding coronal micro leakage during the intracoronal bleaching, it was found that Chlorhexidine is a vehicle for sodium perborate, enhanced its antimicrobial activity and did not affect dentin microhardness (28-30).

Chlorhexidine and Apical Fluid Penetration:

Canals irrigated or medicated with Chlorhexidine do not affect negatively the ability of root fillings to prevent fluid penetration into the root canal system through the apical foramen (24, 31-33).

Substantivity:

The effectiveness of Chlorhexidine stems from its capacity to absorb to negatively charged surfaces in the mouth , being slowly released from these retention sites and therefore maintaining prolonged antimicrobial activity for several hours (10). This process is known as substantivity, and only Chlorhexidine and tetracycline have this property so far (34).

Regarding its substantivity, it has been found that the use of Chlorohexdine as root canal irrigating substance prevented microbial activity from 48 h (35), 7 days (in the liquid and gel formulation) (36), 21 days (37), 4 weeks (34), up to 12 weeks (38).

Interaction with Endodontic Irrigants:

Due to its wide spectrum antimicrobial activity and its inability of dissolving organic tissues, an irrigation regimen has been proposed, in which NaOCl would be used throughout instrumentation, followed by EDTA, and Chlorhexidine would be used as a final irrigant (39).

The combination of NaOCl and Chlorhexidine has been advocated to enhance their antimicrobial properties, and the advantage of using a final rinse with Chlorhexidine would be the prolonged antimicrobial activity due to the Chlorhexidine substantivity (40).

Apart from the antimicrobial aspect, the association of NaOCl with Chlorhexidine leads to the formation of an orange-brown precipitate,a chemical smear layer that covers the dentinal tubules and may interfere with the seal of the root filling (41,42). In addition this precipitate changes the color of the tooth (43,44,44) and is cytotoxic (46).

Heling and Chandler investigated NaOCl and Chlorhexidine with and without EDTA. When used in combination asendodontic irrigants against Enterococcus faecalis, and verified that combining EDTA with NaOCl or Chlorhexidine was more effective than using with EDTA alone.(47)

Regarding the orange-brown precipitate, it occurs due the presence of NaOCl, an oxidizing agent causing chlorination of the guanidino nitrogens of the Chlorhexidine (4). Basrani et al.(50) detected the presence of para-chloroaniline (PCA) in this precipitate. On the other hand, Thomas and Sem (49), Nowicki and Sem (50) and Prado et al.(4) failed to detect it using different methodologies. Thus, after chemomechanical preparation with NaOCl, the use of Chlorhexidine as a final irrigant or as an intracanal medicament would require the removal of NaOCl from the canal (42). In summary, it is important to remove all traces of the substances used inside the root canals in order to avoid interactions between them.

Chlorhexidine diffusion into the Dentinal Tubules:

It has been shown that 2% Chlorhexidine containing medicaments is able to diffuse into the dentinal tubules and reach the outer root surface, exerting antimicrobial action. Hence root canal could be considered as a reservoir for the release of intracanal medicament to the whole dentin and to the external root surface (51).

Disinfection of Obturation Cones (Gutta-Percha and Resilon Cones):

The property of NaOCl and Chlorhexidine to be used as auxiliary chemical substances is useful as disinfectant agents of guttapercha cones do not involve additional costs to clinicians, since these substances are of general use in endodontic therapy. Chlorhexidine kills vegetative forms within short periods of time. However, this agent is not able to eliminate some spore (52,53).

As a strong oxidizing agent, NaOCl is able to cause changes in surface roughness of gutta-percha cones (54) observed by atomic force microscopy (AFM) (55,56). Rapid sterilization with 2.5% and 5.25% NaOCl removes the deposition on gutta percha cones (52,57) showing that the final rinse with distilled water is essential. Resilon cones exposed to Chlorhexidine gel presented some residual antibacterial action. The clinical importance of Chlorhexidine release in endodontic cones might be related to its immediate antimicrobial effect inside the root canal, during the obturation time (54).

Chlorhexidine and NaOCl cause an increase in the surface free energy (wettability) of the gutta-percha cones and Resilon surfaces, thereby interfering positively with the adhesion mechanism. This change can be due to chemical modifications on the surface of these materials. Comparing the two solutions, Chlor hexidine is a better disinfectant compared with NaOCl. Cones disinfected with Chlorhexidine have smaller contact angles than NaOCl (58).

Other Uses in the Endodontic Therapy:

Before 1990's, Chlorhexidine gluconate was used in Endodontics as an irrigant in a liquid form. One of the first reports of its use in Endodontics (59), demonstrates its effectiveness in enhancing radicular dentin permeability.

Kennedy et al. (60), in 1967, recommended the use of 14.6% EDTA and 0.005% Hibitane, as separate solutions or combined, for irrigation of vital and non-vital teeth. These solutions reduce the number of microorganisms in the root canals and also have the advantage of being well tolerated by soft tissue and wounds, provided the contact is not prolonged. However, according to them, chlorinated soda solution should never be used with Hibitane, as it forms a brown precipitate that stains the teeth.

Chlorhexidine can be used during all phases of the root canal treatment, due to its antimicrobial and substantivity properties. Chlorhexidine has been recommended as an alternative to NaOCl, especially in cases of open apex, root resorption, foramen enlargement and root perforation, due to its biocompatibility, or in cases of allergy related to bleaching solutions (61).

With foramen enlargement, the risk of irrigant extrusion through the apex increases, favoring the use of Chlor hexidine for being less irritating to the periapical tissues than NaOCl and not inducing pain. Chlorhexidine gel can also be used for modeling gutta-percha cones, which improves their adaptation to the apical dentin wall. The use of CHX gel during retreatment has also been investigated. In vitro, groups that used Chlorhexidine gel with manual or rotary instrumentation showed smaller debris extrusion as well as the cleanest root canal walls than the ones where solvents were used.(61)

Endotoxin Reduction:

Lipopolysaccharide (LPS, endotoxin), cell membrane component of gram negative bacteria is predominantly involved in root canal infection is an important mediator in the pathogenesis of apical periodontitis and enhancing the sensation of pain in endodontic infections (54).

Concerning Chlorhexidine detoxifying activity:

Buck et al. (62) reported little or no efficacy on inactivating the biologically active portion of the endotoxin lipid A. Furthermore, in vitro studies (56-58) inoculating Escherichia coli LPS in root canals showed the low effectiveness of Chlorhexidine in reducing LPS after chemomechanical preparation.

Gomes et al. (63) in a clinical study comparing the endotoxin levels found in primary and secondary endodontic infections reported that teeth with primary endodontic infections had higher contents of endotoxins and a more complex gram-negative bacterial community than teeth with secondary infections.

Endo et al. (64), in a clinical study with secondarily infected root canals with post-treatment apical periodontitis, used hand K-files for apical preparation, and 2% Chlorhexidine gel for root canal irrigation. They found that higher levels of endotoxin were related to the larger size of radiolucent area. Chemomechanical preparation was more effective in reducing bacteria than endotoxin .

Chlorhexidine Products:(9)

1. Mouth rises

2. Gel

3. Sprays

4. Toothpaste

5. Varnishes

CONCLUSION:

It is clear that chlorhexidine has a wide range of activity against both gram positive and gram negative bacteria. It is an effective antifungal agent . It is also used as an irrigant in root canal treatment. In rare cases chlorhexidine may cause allergic reactions.

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Received on 18.03.2016 Modified on 04.04.2016

Research J. Pharm. and Tech 2016; 9(10):1755-1760.

DOI: 10.5958/0974-360X.2016.00353.X