Ca(OH)2 is very effective at eradicating intraradicular bacteria. Unfortunately, it is not as effective when used short term262; therefore, it requires prolonged exposure19 or higher temperatures for use as an endodontic irrigant.88
Electrochemically activated water (also known as oxidative potential water) recently was tested as a potential irrigant.110,169,265 Although this solution is active against bacteria110 and removes the smear layer,169 no evaluations of its clinical potential are available, and in vitro research indicates that NaOCl is a superior disinfectant.110
Hydrogen peroxide traditionally has been used as an irrigant in conjunction with NaOCl; however, no additional benefit to NaOCl was registered.123
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Figure 9-41 Penetration of irrigants into dentinal tubules after root canal preparation with different dentin pretreatments. Left column, Irrigation with tap water and then with blue dye. Right column, Smear layer is removed with 17% EDTA, applied in high volume and with a 30-gauge needle, followed by irrigation with blue dye. Note the comparable diffusion of dye in the apical sections, whereas dye penetrated deeper into the dentin in the two coronal sections.
Recently some have advocated the use of 0.2% or 0.5% CHX mixed in addition to sodium hypochlorite,112,123 either as an irrigant or mixed with Ca(OH)2 as an interappointment medicament. These combinations can overcome the inhibiting effect of dentin dust on conventional medicaments112,212 and can optimize their antimicrobial properties against certain resistant bacteria and yeasts.306,309 Increased effectiveness was observed when Ca(OH)2 was mixed with some common irrigating solutions. Although some authors could not confirm additive effects and even found a reduction in the antimicrobial action of CHX,119 it appears that Ca(OH)2 mixed with IKI or CHX may be able to kill calcium hydroxide-resistant bacteria260 (Box 9-4).
Box 9-4 Benefits of Using Irrigants in Root Canal Treatment
Removal of particulate debris and wetting of the canal walls
Opening of dentinal tubules by removal of the smear layer
Disinfection and cleaning of areas inaccessible to endodontic instruments
In root canal treatment, lubricants are mostly used to emulsify and keep in suspension debris produced by mechanical instrumentation. Although irrigation solutions serve as lubricants, special gel-type substances are also marketed. Two of these are the wax-based RC-Prep, which contains EDTA and urea peroxide, and the glycol-based Glyde. Another purported function of lubricants is to facilitate the mechanical action of endodontic hand or rotary files. A study evaluating the effects of lubrication on cutting efficiency found that tap water and 2.5% sodium hypochlorite solutions increased cutting efficiency compared with dry conditions.330 The authors of this study cited the ability of a lubricant to remove debris as the factor for the increased efficiency. Similarly, in recent experiments a reduction of torque scores was found when canals in normed dentin disks were prepared with ProFile and ProTaper instruments under irrigation; use of a gel-type lubricant resulted in similar torques).201
In summary, irrigation is an indispensable step in root canal treatment to ensure disinfection. The tissue-dissolving and disinfecting properties of NaOCl currently make it the irrigant of choice. EDTA should be used at the end of a procedure to remove the smear layer, followed by another flush with NaOCl or an inert solution such as physiologic saline. This minimizes inactivation of NaOCl by chemical interactions.
CLEANING AND SHAPING: CLINICAL ISSUES
Endodontists widely agree that a major biologic aim of endodontic therapy is to eliminate apical periodontitis by disinfection and sealing of root canal systems. However, considerable disagreement exists over the way this goal should be achieved (see Fig. 9-9). Although "cleaning and shaping" accurately describes the mechanical procedures,227 it should be emphasized that "shaping and cleaning" more correctly reflects the fact that enlarged canals direct and facilitate the cleaning action of irrigants and the removal of infected dentin.
Microorganisms in the pulp cavity and coronal root canal may be readily killed by irrigants early in a procedure; however, bacteria in less accessible canal areas still can elicit apical periodontitis. These bacteria can be eradicated only after root canal preparation.
Some have suggested that canals should be prepared to a uniform and continuous taper242; however, this mechanical objective facilitates obturation rather than antimicrobial efficacy. The preparation shape and antimicrobial efficacy are intimately related through the removal of infected dentin and the delivery of irrigants.
Traditionally fluids have been delivered to root canals passively by syringe and needle (Fig. 9-42); active systems such as the NIT are still in an experimental phase.166 When delivered passively, irrigants have been shown to progress only 1 mm farther than the tip of the needle.216 However, enlarged apical canals are likely to allow increasingly deeper needle placement (see Fig. 9-42), and this improves debridement and disinfection of canals.6 Nevertheless, thorough cleaning of the most apical part of any preparation remains difficult,318 especially in narrow and curved canals.122,192,219
An important mechanical objective of root canal instrumentation is full incorporation of the original canals into the prepared shape, meaning that all root canal surfaces are mechanically prepared (green areas in Fig. 9-43, A and B); however, this goal is not possible with current techniques.203
Preparation errors, such as zips and perforations, should be absent. Although these and other procedural problems (Fig. 9-44) per se may not affect the probability of a favorable outcome, they may leave parts of the root canal system inaccessible for disinfection.
Another important mechanical objective is to leave as much radicular dentin as possible so as not to weaken the root structure, thereby preventing vertical fractures. Although no definitive minimal radicular thickness has been established, 0.2 mm is considered critical.155 Straightening of canal paths can lead to minimal remaining wall thicknesses (Fig. 9-45); this underlines the need for adequate access cavity preparation and optimal enlargement of the coronal third of the root canal.
Two primary mechanical elements are the apical width and the endpoint of the prepared shape in relation to the apical anatomy. Traditional treatment has held that canal preparation and subsequent obturation should terminate at the apical constriction, the narrowest diameter of the canal. This point is believed to coincide with the cementodentinal junction (CDJ) (see Chapter 7). This definition of working length is based on histologic sections and ground specimens. However, the position and anatomy of the CDJ varies considerably from tooth to tooth, from root to root, and from wall to wall in each canal. Moreover, the CDJ cannot be located precisely on radiographs. For this reason, some have advocated terminating the preparation 0.5 to 1 mm short of the radiographic apex in necrotic cases and 1 to 2 mm short120,222,321 in cases involving irreversible pulpitis. In this way, preparation would take place inside the root canal. Follow-up studies seem to support this strategy.261,263
However, working to shorter lengths could lead to the accumulation and retention of debris, which may result in apical blockage (see Fig. 9-40). Such blockage (which consists of collagen fibers, dentin mud, and residual bacteria) inside apical canal areas is a major cause of persistent or recurrent apical periodontitis,115,256 recently labeled posttreatment disease92 (also see Chapter 24). Moreover, because of the creation of apical blockage, working to short lengths may contribute to procedural errors such as apical perforations and fractured instruments.
The electronic apex locator has helped clinicians identify the position of apical foramina more accurately; the development of this instrument made it possible to work more precisely and routinely as close as 0.5 mm to the canal terminus (see Chapter 8).
Concepts and Strategies
Two factors are closely related to the preparation length: use of a patency file and the apical width. A patency file is a small K-file (usually a size #10 or #15) that is passively extended just through the apical foramen.
Use of a patency file has been suggested for most rotary techniques. This step is believed to remove accumulated debris and help maintain working length. However, the issue is controversial, and a large number of U.S. dental schools did not teach this concept, at least not until recently.53 Moreover, Goldberg and Massone102 demonstrated that the use of patency files of varying sizes did not prevent preparation errors.
One concern with the patency file was that instead of having a cleaning effect, the file would push contaminated debris through the foramen. However, a recent in vitro study suggested that the risk of inoculation was minimal when canals were filled with sodium hypochlorite.133 No definitive evidence exists either favoring or disproving the use of a patency file. However, clinical experience suggests that this technique involves relatively little risk and provides some benefit as long as small files are used carefully.
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Figure 9-42 Irrigation and the movement of irrigants depends on the canal shape. Sequential enlargement of a canal in clear plastic block was performed with a sequence of ProFile instruments in accordance with the manufacturer's recommendations. Alternating irrigation with blue and red fluid was done after each preparation step. Note the apical presence of irrigant after sufficient shape has been provided. Note the distribution of fluid immediately after irrigation with a 30-gauge needle.
Like the position of the apical constriction, apical diameters are difficult to assess clinically.149 Some have recommended gauging canal diameters by passing a series of fine files apically until one fits snugly. However, such an approach is likely to result in underestimation of the diameter.314 This is a crucial point because the initial canal size determines the desired final apical diameter.
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Figure 9-43 Example of a desired shape with the original root canal fully incorporated into the prepared outline. A and B, μCT reconstructions in clinical and mesiodistal views of a maxillary molar prepared with a NiTi rotary system. The green area indicates the preoperative shape, and the red area indicates the postoperative shape. Areas of mixed red and green indicate no change (i.e., no removal of radicular dentin). C to E, Cross sections of the coronal, middle, and apical thirds; the preoperative cross sections (green) are encircled by the postoperative outlines (red) in most areas. (AandBfrom Hübscher W, et al:Int Endodon J 36:740-747, 2000.)
An ongoing debate exists between those who prefer smaller apical preparations combined with tapered shapes and those who favor larger apical preparations for better removal of infected dentin and to allow irrigation fluids access to the apical areas. Both sides stress the importance of maintaining the original path of the canal during preparation; otherwise, bacteria infecting the apical third of the root canal may not be reached by a sufficient bactericidal concentration of an antimicrobial agent.179 Investigators obtained a higher percentage of bacterial elimination in single-root canal systems by using a combination of significant enlargement of the apical third and sodium hypochlorite irrigation.57 Preparation errors (e.g., zips, canal transportation) can occur with wide preparations when either stainless steel or nickel-titanium instruments are used (see Fig. 9-44).
Thorough disinfection of the apical part of a root canal is essential, because this area is likely to contain intraradicular bacteria.182 Wider apical preparations remove potentially infected dentin, allowing the delivering needle and subsequently the antimicrobial irrigant to penetrate the root canal more deeply.60
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Figure 9-44 Schematic diagrams showing the most common preparation errors. A, Apical zip. B, Ledge. C, Apical zip with perforation. D, Ledge with perforation.
A study investigating rotary nickel-titanium files of three tapers (#.06, #08, and #.10) with file tips in sizes #20, #30, and #40 showed that size #20 instruments left significantly more debris in the apical third compared with size #40 instruments.299On the other hand, a study in which half the samples were prepared to a size #25 file and the other half to a size #40 file found no statistically significant difference in bacterial growth after instrumentation, with no growth observed after 1 week of treatment with a calcium hydroxide dressing.324 Another study compared step-down sequences with additional apical enlargement to ISO size #35 or a serial step-back technique with no apical enlargement. NaOCl and EDTA were used as irrigants.65 No significant difference was detected in colony-forming units with or without apical enlargement.65 These researchers concluded that dentin removal in the apical third might be unnecessary if a suitable coronal taper is achieved.
Despite the disagreement over the appropriate width of a preparation (Table 9-2), it appears that root canal preparations should be confined to the canal space, should be sufficiently wide, and should incorporate the original root canal cross sections (see Fig. 9-43). This way, routine root canal treatment results in favorable outcomes at various levels of clinicians' expertise (Fig. 9-46).