Bacteria in the root canal system provoke the formation of periapical inflammatory lesions.1 The aim of root canal treatment is to eliminate bacteria from the infected root canal and to prevent re-infection. Biomechanical cleaning and shaping of the root canal greatly reduces the number of bacteria.2 Nevertheless, studies have shown that bacteria often persist.3 Therefore, irrigation with strong antibacterial agents is imperative to complete the cleaning and shaping process.
Irrigants have traditionally been delivered into the root-canal space using syringes and metal needles of different size and tip design. Clinical experience and research have shown, however, that this classic approach typically results in ineffective irrigation. It is the purpose of this article to present an overview on irrigating solutions in endodontics and some new devices to improve the delivery to the apical portion of the root canal system.
To comprehend how irrigation works, it is important to understand the two main objectives of irrigation: mechanical and biological. The mechanical objective involves the following: (1) flushing out debris, (2) lubricating the canal, (3) dissolving organic and inorganic tissue and (4) bleaching. The biological function of the irrigants is related to their antimicrobial effect.4
Efficacy of root canal irrigation in terms of debris removal and eradication of bacteria depends on several factors such us:5
Penetration depth of the needle: It has to be considered that the size and length of the irrigation needle (in relation to root canal dimensions) is of utmost importance for the effectiveness of irrigation (Fig. 1).
Diameter of the root canal: Several studies showed that apical preparation to file #40 and adequate taper (.04) is needed for the irrigant to be exchange at the apical portion.6,7
External diameter of the needle is of relevance for the depth of introduction into the root canal and for rigidity of the tip, which is important for irrigation of curved canals. Common injection needles have an external diameter of 0.40 mm (27 gauge), but special irrigation tips with external diameters of 0.30 mm (30 gauge) are available as well5.
Internal diameter determines the necessary pressure for moving the syringe plunger and the velocity with which the irrigant is extruded. Narrow needles need more pressure onto the plunger and extrude the irrigant with higher velocity than large needle sizes, which otherwise extrude larger amounts of irrigants but cannot be introduced as deep. There are nickel titanium needles that improve penetration into curved root canals.4
Type and orientation of the bevel of the needle: To improve safety of irrigation and prevent apical extrusion of the irrigant, some needle release the solution via lateral openings and have a closed, safe-ended tip. The orientation of the bevel is crucial to produce a turbulence effect on the dential wall of the canal.
The biological function of the irrigants is related to their antimicrobial effect. The ideal endodontic irrigant should possess the following characteristics:8 be an effective germicide and fungicide, be non-irritating to the periapical tissues, remain stable in solution, have a prolonged antimicrobial effect, be active in the presence of blood, serum, and protein derivates of tissue, have low surface tension, should not interfere with repair of periapical tissues, not stain tooth structure, be capable of inactivation in a culture medium, should not induce a cell mediated immune response.
None of the available irrigating solutions can be regarded as optimal. Using a combination of products in the correct irrigation sequence and technique contributes to a successful treatment outcome.
Sodium hypochlorite (NaOCl) is the most commonly used irrigating solution. It is an excellent antibacterial agent, able to dissolve necrotic and vital pulp tissue, the organic components of dentin as well as biofilms.9
MODE OF ACTION
When hypochlorite contacts tissue proteins, nitrogen, formaldehyde, and acetaldehyde are formed within a short time, hydrogen in the imino groups (-NH-) is replaced by chlorine (-N.Cl-) forming chloramines, which plays an important role for the antimicrobial effectiveness. Thus, the necrotic tissue and pus are dissolved. Sodium hypochlorite is a strong base (pH>11). Temperature increase will significantly improve the antimicrobial effect of sodium hypochlorite.10 Estrela10 reported that NaOCl exhibits a dynamic balance that acts as an organic and fat solvent degrading fatty acids, transforming them into salts (soap) and glycerol (alcohol), that reduces the surface tension of the remaining solution (saponification reaction).
There is considerable variation in the literature regarding the effective concentration of NaOCl as an endodontic irrigant. NaOCl is used in concentrations between 0.5 and 5.25% solutions. Clinical studies have shown both low and high concentrations to be equally effective in reducing bacteria from the root canal system.11,12 NaOCl in higher concentrations has a better tissue-dissolving ability,13 however, even in lower concentrations when used in high volumes it can equally be effective.14,15 Higher concentrations of NaOCl are more toxic than lower concentrations.16
TISSUE DISSOLUTION CAPACITY
The dissolution of bovine pulp tissue by NaOCl (0.5, 1.0, 2.5, and 5.0%) was studied in vitro under different conditions.10 It was concluded that: 1) the velocity of dissolution of the bovine pulp fragments was directly proportional to the concentration of the NaOCl solution and was greater without the surfactant; 2) with elevation of temperature, dissolution of the bovine pulp tissue was more rapid; 3) the greater the initial concentration, the smaller was the reduction of its pH10.
Several mishaps during root canal irrigation have been described in the dental literature. These range from damage to the patient's clothing, splashing the irrigant into the patient's or operator's eye, injection through the apical foramen, and allergic reactions to the irrigant, to inadvertent use of an irrigant as an anesthetic solution5 (Fig. 2).
Chlorhexidine gluconate (CHX) is a broad-spectrum antimicrobial agent that has been advocated for root canal disinfection.17,18 When used as an irrigant or intracanal medication, its antibacterial efficacy is comparable to that of NaOCl,19-21 and it is effective against certain NaOCl-resistant bacterial strains.19,22 Prolonged exposure of the root dentin to CHX may impart a residual antimicrobial property to the dentin surface.19,21,23,24 CHX has a low grade of toxicity;25 however, its inability to dissolve organic matter maybe a drawback in its clinical use26 (Fig. 3).
MODE OF ACTION
CHX is a wide-spectrum antimicrobial agent, active against Gram-positive and Gram-negative bacteria, and yeasts.27 Due to its cationic nature, CHX is capable of electrostatically binding to the negatively charged surfaces of bacteria,28 damaging the outer layers of the cell wall and rendering it permeable.29-31 Depending on its concentration, CHX can have both bacteriostatic and bactericidal effects. At high concentration CHX acts as a detergent, and by damaging the cell membrane it causes precipitation of the cytoplasm and thereby exerts a bactericidal effect.
CHLORHEXDINE APPLICATION IN ENDODONTICS
In endodontics, CHX has been studied as an irrigant and intracanal medication, both in vivo32-35 and in vitro.23,24,36-39 In vitro, CHX has at least as good, or even better antimicrobial efficacy than Ca(OH)2.40 Notably, 2% CHX was very effective in eliminating a biofilm of E. faecalis.41 In vivo, it inhibits experimentally-induced inflammatory external root resorption when applied for four weeks.42 In infected root canals, it reduces bacteria as effectively as Ca(OH)2 when applied for one week.32 Unlike Ca(OH)2, CHX has substantive antimicrobial activity that, if imparted onto the root dentin, has the potential to prevent bacterial colonization of root canal walls for prolonged periods of time.23,39 This effect depends on the concentration of CHX, but not on its mode of application, which maybe either as liquid, gel or a controlled release device.36 CHX and dentine bonding was also studied in details, on the whole, because of its broad-spectrum MMP-inhibitory effect, CHX can significantly improve the resin-dentine bond stability.
A smear layer is formed during preparation of the root canal. The smear layer consists of both an organic and an inorganic component. No clear scientifically based understanding exists on whether this layer must be removed or can be left. However, a multitude of opinions have been offered on both sides of this question. In addition to weak acids, solutions for the removal of the smear layer include carbamide peroxide, aminoquinaldinium diacetate (i.e., Salvizol), and EDTA.4
EDTA, CITRIC ACID
EDTA is normally used in a concentration of 17%. It removes smear layers in less than 1 minute if the fluid is able to reach the surface of the root canal wall (Fig. 4). The decalcifying process is self-limiting, because the chelator is used up. Although citric acid appears to be slightly more potent at similar concentration than EDTA, both agents show high efficiency in removing the smear layer. In addition to their cleaning ability, chelators may detach biofilms adhering to root canal walls (Kishor Gulabivala, personal communication). This may explain why an EDTA irrigant proved to be highly superior to saline in reducing intracanal microbiota, despite the fact that its antiseptic capacity is relatively limited.
Antiseptics such as quaternary ammonium compounds (EDTAC) or tetracycline antibiotics (MTAD) have been added to EDTA and citric acid irrigants, respectively, to increase their antimicrobial capacity. The clinical value of this, however, is questionable. EDTAC shows similar smear-removing efficacy as EDTA, but it is more caustic. Chelating agents can be applied in liquid or paste-type form. Urea peroxide is usually used as a vehicle. Research is showing that instead of lowering physical stress on rotary instruments as advocated, carbowax-based lubricants, depending on instrument geometry, have either no effect or are even counterproductive.43
Surface-active agents have been added to several irrigant to improve the surface tension and improve wettablity. Smear clear, Chlor-Xtra and CH plus are some examples on the addition of detergents to EDTA, NaOCl and CHX respectively. No sufficient data in the better antibacterial action is found in the literature.44
Q mix is an irrigation solution used as a final rinse. It is a combination of CHX with EDTA and a surfactant solution to improve penetration in dentinal tubules. It is in the market for very short time, so, there is no research available yet.
A compound has been developed with combined chelating and antibacterial properties.45,46 MTAD (BioPure MTAD; Dentsply Tulsa Dental Specialties, Tulsa, OK) is a mixture of doxycycline, citric acid, and Tween 80.46 It is applied as a 5-minute final rinse after canal instrumentation and irrigation with 1.3% NaOCl47. Preliminary in vitro studies have suggested effective elimination of root canal bacteria by MTAD,48-51 subsequent in vivo studies did not support those results.45,52-55
INTERACTIONS OF IRRIGATION SOLUTIONS
Unfortunately some negative interactions occur when NaOCl and CHX are mixed. The byproduct formed is PCA (parachloroanoline), and because of its potential toxicity, its formation should be avoided56-58 (Fig. 5). Grawehr59 studied the interactions of EDTA with NaOCl. They concluded that EDTA retained its calcium-complexing ability when mixed with NaOCl, but EDTA caused NaOCl to lose its tissue-dissolving capacity and virtually no free chlorine was detected in the combinations. Clinically, this suggests that EDTA and NaOCl should be used separately. In an alternating irrigating regimen, copious amounts of NaOCl should be administered to wash out remnants of the EDTA. The combination of CHX and EDTA produces a white precipitate. Rasimick et al60 determined if the precipitate involves the chemical degradation of CHX. The precipitate was produced and redissolved in a known amount of dilute trifluoroacetic acid. Based on the results, CHX forms a salt with EDTA rather than undergoing a chemical reaction.
NEW IRRIGATION AND DISINFECTION TECHNIQUES AND DEVICES
Many techniques and devices have been proposed to increase the flow and distribution of irrigating solutions within the root canal system. Because of the intricate nature of root canal anatomy chemomechanical cleaning and shaping are not enough to predictable eliminate bacterial loads from fins, lateral canals and even deltas at the apical portion. Throughout the history of endodontics, actions have continuously been made to develop more effective irrigant delivery and agitation systems for root canal irrigation. These systems might be divided into two broad categories, manual agitation techniques and machine-assisted agitation devices.61
Conventional syringes with various needles diameter, manual brushes and fitted gutta percha cones are suggested as a manual method to get the irrigant deep in the canal. Other than these conventional irrigation techniques, additional techniques for disinfection of the endodontic cavity have been proposed and tested, including laser systems and gaseous ozone. Recently several new devices for endodontic irrigation and/or disinfection have been introduced, among which are The Self Adjusting File (SAF, ReDent, Raanana, Israel), the Endoactivator System (DENTSPLY Tulsa Dental Specialties), Passive ultrasonic irrigation and EndoVac (Discus, Culver City, CA, USA).
MAUNAL DYNAMIC IRRIGATION
Manual dynamic irrigation has been described as a cost-effective technique for cleaning the walls of the entire root canal. It involves repeated insertion of a well-fitting gutta-percha cone to working length of a previously shaped canal. The gutta-percha cone is applied in short, gentle strokes to hydrodynamically displace and activate an irrigant (Fig 6).62
PASSIVE ULTRASONIC IRRIGATION
Ultrasonic irrigation of the root canal can be performed with or without simultaneous ultrasonic instrumentation. When canal shaping is not undertaken, the term passive ultrasonic irrigation (PUI) can be used to describe the technique. Passive ultrasonic irrigation can be performed with a small file or smooth wire (size 10-20) oscillating freely in the root canal to induce powerful acoustic microstreaming. PUI can be an important supplement for cleaning the root canal system and, compared with traditional syringe irrigation; it removes more organic tissue, planktonic bacteria and dentine debris from the root canal. PUI is more efficient in cleaning canals than ultrasonic irrigation with simultaneous ultrasonic instrumentation. PUI can be effective in curved canals and a smooth wire can be as effective as a cutting K-file. The taper and the diameter of the root canal were found to be important parameters in determining the efficacies of dentine debris removal. Irrigation with sodium hypochlorite is more effective than with water and ultrasonic irrigation is more effective than sonic irrigation in the removal of dentine debris from the root canal. The role of cavitation during PUI remains inconclusive. No detailed information is available on the influence of the irrigation time, the volume of the irrigant, the penetration depth of the instrument and the shape and material properties of the instrument (Fig. 7).63
An irrigation system called EndoVac (Discus Dental, Culver City, CA) might better deliver the irrigant to apical areas of canals and into root canal irregularities. The EndoVac system uses a suction needle placed at working length (WL). With negative pressure, the irrigant flows down from the pulp chamber into the canal to the apical areas. A study by Nielsen and Baumgartner showed significantly better debridement at 1 mm from WL on extracted teeth by using the EndoVac compared with conventional needle irrigation. Shin et al. also showed that the EndoVac left significantly less debris behind than conventional needle irrigation (Figs. 8 & 9).64
Elimination of bacterial load from the root canal system is the main goal of the endodontic treatment. Unfortunately, manual and rotary instruments and conventional irrigation is not enough to achieve this goal. Recently, new combination of irrigants and new delivery devices were introduced using pressure, vacuum, oscillation, and/or a combination with suction. For most of these devices and techniques, no data on the in vivo efficacy, risk, frequency, and intensity of apical extrusion of the irrigant or the problems encountered with the use of pressure and negative pressure or oscillation are available.OH
Dr. Basrani is Assistant Professor and Program Co-Director, MSc Endodontic Program, Department of Endodontics, Faculty of Dentistry, University of Toronto.
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