If canal curvature is more severe, ProTapers may be used as hand instruments (see Fig. 9-63, A). Because the largest ProTaper instrument has a size #30 tip, in many cases additional enlargement is desired. This may be accomplished with LightSpeed or NiTi K-files (Fig. 9-67), which are first used to working length and then in a step-back approach. Finally, the overall shape may be smoothed with either engine-driven or hand-held instruments. Hand-held ProTaper or GT instruments may aid removal of acute apical curvatures or ledges and provide access to apical canal areas for irrigants.
Ultrasonically activated files or alternating file movements with special handpieces may be used to work canal areas that rotary instruments cannot reach. However, to date no evidence shows that canal preparation with ultrasonic instruments is clinically beneficial. Similarly, neither traditional modified handpieces279 nor a recently introduced system (EndoEZE AET; Ultradent, South Jordan, UT) have been shown to allow preparation of adequate canal shapes.279
Ultrasonic devices have been linked to a higher incidence of preparation errors and to reduced radicular wall thickness.155 Newer analytic systems (e.g., μCT) allow tracking of the amount of dentin removed (Fig. 9-68); however, the amount of potentially infected dentin that should be removed to maximize the chance of a successful outcome is unclear.
CANAL CLEANING TECHNIQUES
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Figure 9-64 Hybrid technique: Part I. After irrigation (1) and scouting (2), GG drills (3) and/or ProTaper SX files (4) are used for coronal preenlargement and to secure straight-line access to the middle third. Prebent K-files are then used to explore and determine the working length (5).
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Figure 9-65 Hybrid technique: Part II. In canal systems flooded with irrigant (1), ProTaper shaping instruments S1 and S2 (2) and then finishing instruments F1 and F2 (3) are used to preenlarge the apical third, allowing irrigants access to the canals. Finishing instrument F3 may be used if feasible (4).
Irrigants and other intracanal medicaments are necessary adjuncts that enhance the antimicrobial effect of mechanical cleansing and thus overall clinical efficacy.50-52 Several studies205,206,277,312,317 have shown that large areas of canal walls, particularly in the apical third but also in ribbon-shaped and oval canals, cannot be cleaned mechanically, meaning that microorganisms present in these untouched areas could survive (see Figs. 9-38, 9-43, and 9-45). Residual bacteria and other microorganisms exist both in these hard-to-reach spaces and in dentinal tubules.113,197,228 Chemical disinfection is an important cornerstone of a successful outcome, because it reaches bacteria or fungi present in dentinal tubules and in the crevices, fins, and ramifications of a root canal system.189,306 In one study, investigators prepared root canals, irrigated with saline solution, and sampled before, during, and after instrumentation.75 They then cultivated and counted colony-forming units. These researchers found that with instrumentation alone, progressive filing reduced the number of bacteria, regardless of whether rotary or stainless steel hand instrumentation was used. However, no technique resulted in bacteria-free canals. Siqueira et al257 confirmed this finding; they found that instrumentation combined with saline irrigation mechanically removed more than 90% of bacteria in the root canal. Many authors have stressed the importance of using antimicrobial irrigants during chemomechanical preparation to ensure complete disinfection.258
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Figure 9-66 Effect of a hybrid technique on root canal anatomy studied in a Bramante model. A1 to A4, Both mesial canals of an extracted mandibular molar have been instrumented. Canal cross sections are shown before instrumentation (B1 to D1). B2 to D2, Cross sections after preenlargement with a ProTaper F3 file (left canal) and a size #45, #.02 taper instrument (right canal). The final apical sizes were LightSpeed (LS) #50 and size #50, #.02 taper in the left and the right canal, respectively. (Courtesy Dr. S. Kuttler, Dr. M. Gerala, and Dr. R. Perez.)
Substances that have been used to rinse and chemically clean root canals have different purposes, such as dissolution of soft and hard tissues, antimicrobial effect against bacteria or other microorganisms in the root canal, and inactivation of bacterial lipopolysaccharides. These substances also should be as nontoxic as possible to protect the periradicular tissues. Unfortunately, solutions that are toxic for bacterial cells frequently are toxic for human cells as well; therefore, care must be taken to avoid extrusion of irrigants into periapical regions.43
Another critical factor is the volume of irrigant. In a study evaluating the effect of different amounts of fluids, the volume of irrigant was found to affect the cleanliness of the root canal.321 NaOCl and EDTA administered in larger volumes produced significantly cleaner root canal surfaces than smaller volumes.321 The choice of an appropriate irrigating needle, therefore, is also important. Although larger gauge needles allow the irrigant to be flushed and replenished more quickly, the wider needle diameter does not allow cleaning of the apical and narrower areas of the root canal system (Fig. 9-69). Excess pressure or wedging of needles into canals during irrigation, with no possibility of backflow of the irrigant, should be avoided under all circumstances128 to prevent extrusion of the irrigant into periapical spaces. In juvenile teeth with wide apical foramina or when the apical constriction no longer exists, special care must be taken to prevent resorption or overpreparation of the root canal.70
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Figure 9-67 Hybrid technique: Part III. Under irrigation (1), LightSpeed instruments may be used to enlarge substantially (2 and 3) and to flare the apical section (4). NiTi hand instruments (5) may be used similarly (see text for more detailed explanation).
Most root canals that have not been instrumented are too narrow to be reached effectively by disinfectants, even when very fine irrigation needles are used (see Figs. 9-42 and 9-69). Therefore effective cleaning of the root canal must include intermittent agitation of the canal content with a small instrument173; this prevents debris from accumulating at the apical end of the root canal (see Fig. 9-40). A suction system with a fine-caliber suction tip may be a valuable adjunct for removing solutions and floating debris.
Different types of microorganisms, such as bacteria,136,178,179 yeasts,306,307 and possibly viruses,227 can infect the pulp and may lead to apical periodontitis (see Chapters 14 and 24 and Fig. 9-7). These microorganisms must be reduced or eliminated to reestablish periradicular health. When bacterial samples test negative after treatment, the prognosis is improved.254,260 During mechanical root canal preparation, endodontic instruments are used to clean and enlarge root canal systems. Rotating instruments have an additional, advantageous "Archimedes screw" effect by which debris is transported in an apicocoronal direction.75 Even when simple saline was used as an irrigant, a tenfold to 1000-fold reduction of the bacterial load through mechanical instrumentation was demonstrated.52,75,190
However, as noted earlier, instrumentation alone does not produce a bacteria-free root canal. In one study, dentin samples tested positive in most of the teeth after mechanical instrumentation even though bacteria had been eliminated from the root canals in some cases.52 Bacteria persisted in seven root canals despite mechanical cleaning and saline irrigation during five consecutive appointments. Moreover, teeth with a high number of bacteria in the initial sample remained infected despite being treated five times.52In another study, teeth that caused symptoms tended to have more bacteria than teeth with no clinical symptoms.190
Ørstavik and Haapasalo189 investigated the effect of endodontic irrigants and dressings in standardized bovine dentin specimens that were infected with test bacteria. They found that bacteria were capable of colonizing the canal lumen and dentinal tubules. In the specimens used, E. faecalis rapidly infected the whole length of the tubules, whereas Escherichia coli penetrated approximately 600 μm. They also found that IKI appeared to be more effective at destroying bacteria than NaOCl, which was more effective than CHX.
Other investigators have explored the effects of sodium hypochlorite (with and without EDTA), chlorhexidine, and hydrogen peroxide in varying concentrations when used in sequence or in combination as endodontic irrigants.123 They found that chlorhexidine and sodium hypochlorite were similarly effective in eliminating the bacteria tested. Synergistic effects were observed for some of the irrigants (e.g., chlorhexidine and iodine potassium iodide).
Both of the preceding studies used infected dentin specimens; dentin is an important factor in disinfection because certain concentrations of calcium hydroxide solution, sodium hypochlorite, chlorhexidine, and iodine potassium iodide are inactivated or their activity is reduced by dentin powder112 (Fig. 9-70).
Some of the more difficult to remove endodontic pathogens, which can cause treatment failure, are enterococci and Actinomyces and Candida organisms29,182 (see Chapter 15). Table 9-1 presents the results of a number of studies evaluating the effectiveness of some commonly used antimicrobial agents.
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Figure 9-68 Reconstruction from μCT data (36 μm isotropic resolution) showing the amount of removed dentin by color coding. Maxillary molar shaped with ProTaper, apical size #25 (=F2) in mesiobuccal and distobuccal canals; palatal canal shaped to size #30 (=F3). The bar indicates the removed volume, expressed as the number of voxels. Note the red areas, which indicate dentin removal of more than 500 μm.
When reading literature about antimicrobial efficacy, clinicians must keep in mind that most disinfecting solutions are inhibited or even inactivated by contact with dentin or dentin powder during root canal preparation.112,210 Moreover, chemical interactions occur between irrigation solutions; for example, NaOCl can become ineffective if it comes in contact with EDTA107 (Fig. 9-70).
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Figure 9-69 Irrigation needles inserted into prepared root canals. A and B, A 27-gauge needle barely reaches the middle third. C and D, A 30-gauge, side-venting needle reaches the apical third (Fig. 9-40).
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Figure 9-70 Interaction of sodium hypochlorite with various factors that determine its efficacy.