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FIG. 9-55 Diagram of handle movements during balanced force hand preparation.

Step 1: After pressureless insertion of a Flex-R or NiTiFlex K-file, the instrument is rotated clockwise 90 degrees, using only light apical pressure.

Step 2: The instrument is rotated counterclockwise 180 to 270 degrees; sufficient apical pressure is used to keep the file at the same insertion depth during this step. Dentin shavings are removed with a characteristic clicking sound.

Step 3: This step is similar to step 1 and advances the instrument more apically.

Step 4: After two or three cycles, the file is loaded with dentin shavings and is removed from the canal with a prolonged clockwise rotation.

Rotary Instrumentation

NiTi rotary instruments are an invaluable adjunct in the preparation of root canals, although hand instruments may be able to enlarge some canals just as efficiently when used in appropriate sequences (Fig. 9-56). Hand instruments should be used only after coronal preenlargement (e.g., with GG drills). After preenlargement, the access cavity and canals are flooded with irrigant, and a precurved scouting file is advanced into the canal. A lubricant can help prevent apical blockage in this early stage. Once the working length has been established (aided by an electronic apex locator and radiographically verified), apical enlargement to the desired size begins (Fig. 9-57). As stated previously, various apical preparation designs exist, and the choice is driven mostly by the desired obturation technique, whether an apical stop or an apical taper is prepared. Finally, canal taper is increased by decreasing the working length of larger instruments in 1-mm or 0.5-mm increments, producing 0.05 and 0.10 mm/mm tapers, respectively.

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FIG. 9-56 Root canal instrumentation with hand files: Part I. After the orifice has been accessed (see Figs. 9-49 and 9-54) and copious irrigation performed (1), the working length (WL) is determined. A size #10 and/or #15 K-file is advanced to the desired apical preparation endpoint, aided by an electronic apex locator (2). The apical canal areas are then enlarged with K-files (3) used in the balanced force technique (see Figure 9-55). Frequent, copious irrigation with sodium hypochlorite is mandatory to support antimicrobial therapy. Frequent recapitulation with fine K-files is recommended to prevent blockage (4). Apical enlargement is complete to the desired master apical file (MAF) size (5), which depends on pretreatment canal sizes and individual strategy. Typically, size #40 or larger may be reached in anterior teeth, as in this example. File sizes larger than #20 may be used with NiTi instruments (e.g., NiTiFlex).

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FIG. 9-57 Root canal instrumentation with hand files: Part II. Frequent irrigation with sodium hypochlorite (1) is more efficient after the working length (WL) is reached, because irrigation needles may penetrate deeper into the canal. Canal taper is increased to further improve antimicrobial efficiency and to simplify subsequent obturation. Hand instruments are set to decreasing working length in 0.5 mm increments (step-back) from the master apical file (MAF) (2 to 3). A fine K-file is used to recapitulate to WL during the procedure (4), and the MAF is used as a final recapitulation (5) to ensure that remaining dentin chips have been removed.

Copious irrigation and frequent recapitulations with a smaller file to working length may be required, and in some instances, clinicians must devise creative strategies using small crown-down and/or step-back sequences.

In many cases, hand instrumentation produces adequate shapes, but clinicians often choose NiTi rotary instruments either to enlarge curved canals or to produce wider tapers. Fig. 9-58 illustrates the development of these shapes in the mesial root canals of a mandibular molar, clearly showing that substantial areas of the root canal surface are not instrumented, even when apical size #50 or 0.09 taper are reached (see red areas in Fig. 9-58, G and I).

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FIG. 9-58 Stepwise enlargement of mesial root canal systems in an extracted mandibular molar demonstrated with micro–computed tomography (µCT) reconstructions. The buccal canal (left) was prepared with a LightSpeed (LS) instrument, and the lingual canal (right) was shaped with a ProTaper (PT) instrument. A, Pretreatment view from the mesial aspect. Note the additional middle canal branching from the lingual canal into the coronal third. B, Initial preparation and opening of the orifices, aided by ultrasonically powered instruments. C, First step of root canal preparation, up to LightSpeed size #20 and ProTaper shaping file S1. D, Further enlargement to LS size #30 and PT shaping file S2. E, Apical preparation to LS size #40 and PT finishing file F1. F, Additional enlargement to LS size #50 and PT finishing file F2. G, Superimposed µCT reconstructions comparing the initial canal geometry (in green) with the shape reached after use of the instruments shown in F. H, Final shape after step-back with LS instruments and PT finishing file F3. I, Superimposed µCT reconstructions comparing initial geometry and final shape. Note the slight ledge in the buccal canal after LS preparation and some straightening in the lingual canal after PT preparation.

LightSpeed Instruments

Since the introduction of LightSpeed instruments, the manufacturer’s guidelines have changed. This section presents a version used for the LightSpeed LS132 (Fig. 9-59).

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FIG. 9-59 Finishing of LightSpeed preparations to allow obturation. With the canal system flooded (1), apical preparation (2) is continued until an LS instrument requires 12 pecks to reach the working length (WL). The next LS instrument (3) then is used to a point 4 mm short of the WL to prepare for LightSpeed’s SimpliFill obturation system. Alternatively, canals may be flared for other root canal filling techniques by preparing with each subsequent instrument 1 mm shorter (5).

After access and coronal preenlargement with the instrument of choice, working lengths are obtained, and apical enlargement is done with at least a loose-fitting size #15 K-file. LSX instruments are then slowly advanced to working length while registering tactile feedback. The first instrument that experiences resistance 4 mm short of working length is the final apical size; it is then advanced to working length like the smaller instruments before. The next larger instrument is placed to 4 mm short of working length. This prepares the apical 5 mm for a matching SimpliFill obturator (Discus Dental). Midroot shaping is then accomplished with sequentially larger LSX instruments. Finally, the MAR is used to recapitulate to the working length.

All LightSpeed instruments are used in the following way: A slow, continuous apical movement is used until the blade binds; after a momentary pause, the blade is advanced further with intermittent (“pecking”) motions.120

ProFile

Many different techniques have been advocated for the ProFile,363 but the general pattern remains a crown-down approach with varying tapers and tip diameters. The ProFile instrument therefore can be used as an example for systems with this basic design (e.g., the HERO 642, K3, and FlexMaster). It must be noted that the manufacturers’ instructions for those systems vary somewhat, and the instructions for GT rotary, RaCe, and the Twisted File vary even more. Clinicians should always read the manufacturers’ instructions for details on working with those instruments. That said, it also must be noted that the merits of specific instructions have not been scientifically elaborated.

As with other instruments, coronal preenlargement is suggested (see Figs. 9-53 and 9-54). The working length then is determined as described previously, and an open glide path is secured with K-files up to size #15 or #20, depending on the canal anatomy. If canal size permits, canal preparation begins with #.06 taper instruments in descending tip diameters57 (Fig. 9-60). In more difficult small canals, #.06 tapers are followed by #.04-tapered instruments, also with descending tip diameters (Fig. 9-61). Apical preparation is performed either with multiple shaping waves, as suggested for GT rotary files,74 or in a step-back manner.361 Because of their superior resistance to cyclic fatigue, #.02-tapered ProFile instruments are useful for abrupt apical curves. Preparation is complete once a continuous #.06 taper with an adequate apical size is achieved. Recapitulation during the preparation with a small hand file is recommended.

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FIG. 9-60 ProFile instrumentation in a wider canal. In irrigated and flooded canals (1), a crown-down preparation is done with a sequence of #.06-tapered ProFile instruments (2). When the apical third is reached, the WL is determined and a glide path is secured (3). Apical preparation is then completed by continuing the crown-down sequence (4) up to the desired apical width at the WL. Several shaping waves may be required (5).

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FIG. 9-61 Sequence of ProFile instruments used in constricted canals. After irrigation (1) and coronal preenlargement with orifice shapers (see Figure 9-51), ProFile instruments size #25, #.06 taper; size #20, #.06 taper; and size #24, #.04 taper are used as crown-down instruments (2). After the WL has been determined and a glide path secured (3), apical preparation to the desired size begins (4). For additional taper, larger instruments may be used to a point short of the WL.

ProTaper

The approach for ProTaper instruments differs from that for most other NiTi rotary files (except the MTwo instrument marketed by VDW in Europe) in that no traditional crown-down procedure is performed (Fig. 9-62).

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FIG. 9-62 Instrumentation of root canals with ProTaper instruments. After irrigation and scouting (1 and 2), the coronal thirds are enlarged with shaping files S1 and S2. Hand files then are used to determine the WL and to secure a glide path. Apical preparation is completed with S1 and S2. Finishing files are used to the desired apical width.

Size #10 and #15 hand files are precurved to match the canal curvature and then passively inserted into the coronal two thirds of a root canal as pathfinding files, which confirm the presence of a smooth, reproducible glide path. This step is essential for ProTaper shaping instruments, because they are mostly side-cutting and have fine, fragile tips.

Shaping files S1 and S2 are then passively inserted into the scouted canal spaces, which have been filled with irrigant (preferably NaOCl). If necessary, the Sx file can be used at this stage to relocate orifices or remove obstructing dentin. After each shaping file is used, the canals are reirrigated, and a size #10 file is used to recapitulate to break up debris and move it into solution. This process is repeated until the depth of the pathfinding #10 or #15 file is reached.

After irrigation, the apical third is fully negotiated and enlarged to at least a size #15 K-file, and the working length is confirmed (see Fig. 9-62). Depending on the canal anatomy, the rest of the apical preparation can be done with engine-driven ProTaper shaping and finishing hand files. As an alternative, handles can be placed on these instruments (Fig. 9-63) so that they can be used for the balanced force technique.

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FIG. 9-63 Treatment performed with nickel-titanium rotary and hand instruments to eliminate instrument separation while maintaining biologic aims. A, Pretreatment radiograph of tooth #15. B, Posttreatment radiograph shows a significant curvature in the mesiobuccal canal and additional anatomy in the lingual root. C, Pretreatment radiograph of teeth #14 and #15. Both teeth were diagnosed with irreversible pulpitis. D, Posttreatment radiograph shows four canals in both of the treated maxillary molars. Note the wide apical preparation, particularly in the curved mesiobuccal canals.

(A-B courtesy Dr. T. Clauder; C-D courtesy Dr. H. Walsch.)

ProTapers S1 and S2 are then carried to the full working length, still in a floating, brushing motion. The working length should be confirmed after irrigation and recapitulation with a K-file, aided by an electronic apex locator and/or radiographs. Because of the progressive taper and more actively cutting flutes higher up in the ProTaper design, interferences in the middle and coronal thirds are removed at this stage.

The preparation is finished with one or more of the ProTaper finishing files, used in a nonbrushing manner; because of their decreasing taper, these files will reach the working length passively. Recapitulation and irrigation conclude the procedures (see Fig. 9-62).

Most cases requiring root canal therapy lend themselves to canal preparation with many different systems. Depending on the individual anatomy and the clinician’s strategy, various sequences may be used. Fig. 9-64 presents two cases that involved different problems and therefore were approached differently. Mesiobuccal roots of the maxillary molar can show substantial curvature; rotary instrumentation and/or hybrid techniques allow preservation of the curvature (see Fig. 9-64, A) and optimal enlargement (see Fig. 9-64, B). Often hand instruments other than ISO-normed files (see Fig. 9-63) are used in these cases to ensure a smooth, tapered shape or to eliminate ledges.

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FIG. 9-64 Instruments with increased taper that can be used by hand. A, ProTaper instruments with special handles attached to rotary instrument shanks. B, GT hand instruments.

Hybrid Techniques

For some time, combining various NiTi preparation systems have been suggested89,448 to address certain shortcomings of current instruments (Box 9-5). Although many combinations are possible, the most popular and useful ones involve coronal preenlargement followed by different additional apical preparation sequences. However, clinicians must keep in mind that anatomic variations in each canal must be addressed individually with specific instrument sequences. Most important, oval canals extend deep into the apical area,408,456,464,469 and apical foramina may in fact be oval in most cases.68 Naturally, a rotating file can produce a round canal at best, so a strategy must be devised for adequately shaping oval canals without overly weakening radicular structure (compare Figs. 9-45 and 9-47). One hybrid approach completely prepared 95% or more of all such canals and resulted in extremely wide apical sizes that may be difficult to achieve with most instrument systems.210-212

BOX 9-5 Benefits of Using a Combination of Instruments for Endodontic Therapy

♦ Instruments can be used in a manner that promotes their individual strengths and avoids their weaknesses (most important).
♦ Hand instruments secure a patent glide path.
♦ Tapered rotary instruments efficiently enlarge coronal canal areas.
♦ Less tapered instruments allow additional apical enlargement.

Histologic slides (see Fig. 9-41) and µCT reconstructions (see Figs. 9-45, 9-47, and 9-58) show critical areas that were not mechanically prepared despite the use of various individual rotary techniques. The aim of hybridizing NiTi rotary techniques, therefore, is to increase apical size using a fast and safe clinical procedure.

Various clinicians have used this type of hybrid procedure in their practices (see Figs. 9-2, 9-5, 9-15, and 9-63). The technique involves the use of a variety of instruments: GG drills and K-files for establishing straight-line access; ProTaper instruments for body shaping and apical preenlargement; NiTi K-files or LightSpeed instruments for apical widening; and various instruments for final smoothing.448

After a stainless steel file has confirmed a smooth glide path into the coronal two thirds, irrigation and mechanical preparation with a sequence of ProTaper shaping files opens and preenlarges the apical third (Fig. 9-65). Once the working length has been established, the apical third is flooded with NaOCl and further enlarged with ProTaper finishing files F1 and F2. The F3 ProTaper finishing file is relatively inflexible, and because of its side-cutting action, it should be used with caution in curved canals (Fig. 9-66). Further enlargement is possible with the F4 and F5 instruments, but these files may not be used in more acutely curved canals. The effectiveness of techniques combining different rotary instruments in enlarging canals recently was documented using superimposed root canal cross sections (Fig. 9-67). This method can help identify insufficiently prepared areas and weakening of the radicular structure.

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FIG. 9-65 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. Precurved K-files are then used to explore and determine the working length (5).

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FIG. 9-66 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).

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FIG. 9-67 Effect of a hybrid technique on root canal anatomy studied in a Bramante model. A1-A4, Both mesial canals of an extracted mandibular molar have been instrumented. Canal cross sections are shown before instrumentation (B1-D1). B2-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.)

A different approach—using, for example, NiTi K-files, .02 tapered rotaries (e.g., RaCe), or LightSpeed LSX (Fig. 9-68)—may be also advantageous if larger sizes are desired. Finally, the overall shape may be smoothed with either engine-driven or handheld instruments. Handheld ProTaper or GT instruments may aid removal of acute apical curvatures or ledges and provide access to apical canal areas for irrigants. Some hybrid systems seem to work better than others, but the deciding factors are likely the root canal anatomy and an adequate preparation goal.

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FIG. 9-68 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).

Other Systems

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 superior. Similarly, neither traditional modified handpieces192,412 nor a recently introduced system (EndoEZE AET; Ultradent, South Jordan, UT) have been shown to allow preparation of adequate canal shapes.290

Ultrasonic devices have been linked to a higher incidence of preparation errors and to reduced radicular wall thickness.234,260,488 Newer analytic systems (e.g., µCT) allow tracking of the amount of dentin removed (Fig. 9-69); however, the amount of potentially infected dentin that should be removed to maximize the chance of a successful outcome is unclear.

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FIG. 9-69 Reconstruction from micro–computed tomography 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.

Canal Cleaning Techniques

As stated earlier in this chapter, irrigants and other intracanal medicaments are necessary adjuncts that enhance the antimicrobial effect of mechanical cleansing and thus augment overall clinical efficacy.79-81 It is well established that large areas of canal walls, particularly in the apical third but also in ribbon-shaped and oval canals, cannot be cleaned mechanically,188,308,310,408,456,464 meaning that microbiota present in these untouched areas could survive (see Figs. 9-40, 9-45, and 9-47). Residual bacteria and other microorganisms exist both in these hard-to-reach spaces and in dentinal tubules.171,300,342 Chemical disinfection is an important cornerstone of a successful outcome because it is directed towards elimination of microorganisms present in dentinal tubules and in the crevices, fins, and ramifications of a root canal system.287,449 In one study, investigators prepared root canals, irrigated with saline solution, and sampled before, during, and after instrumentation.112 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 al.380 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.384

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, so care must be taken to avoid extrusion of irrigants into periapical regions.70,190

Several factors are important for efficient root canal irrigation. One 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.435,470 NaOCl and EDTA administered in larger volumes produced significantly cleaner root canal surfaces than smaller volumes.470

Irrigation Modes and Devices

There is a common consensus that root canal irrigants are indispensable aids in dissolving and inactivating organic debris and destroying microorganisms. In addition, some agents allow removal of a postpreparation smear layer in order to allow access to dentinal tubules. Several methods of employment of an irrigant inside the canal space are available.

Syringe Delivery

Application of an irrigant into a canal by means of a syringe allows exact placement, replenishing of existing fluid, rinsing out of larger debris particles, as well as allowing direct contact to microorganisms in areas that are reached by the needle tip. The actual exchange of irrigant is restricted to 1 to 1.5 mm apical to the needle tip, with fluid dynamics taking place near the needle outlet.481 This was the case even if the needle tip diameter was three ISO sizes smaller than the diameter of the apical preparation.64 Volume and speed of fluid flow are proportional to the cleansing efficiency inside a root canal. Therefore, both the diameter and position of the needle outlet determine successful chemomechanical débridement; placement close to working length is required to guarantee fluid exchange.64,185,262

The choice of an appropriate irrigating needle, therefore, is 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. 9-70). Excess pressure or wedging of needles into canals during irrigation with no possibility of backflow of the irrigant should be avoided under all circumstances190 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.107

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FIG. 9-70 Irrigation needles inserted into prepared root canals. A-B, A 27-gauge needle barely reaches the middle third. C-D, A 30-gauge, side-venting needle reaches the apical third (see Figure 9-43).

Another aspect is closeness of the needle tip or outlet to the apical endpoint of canal preparation to allow direct proximity of fresh irrigant to canal walls.64,366 In that respect, size of the irrigation needle,97 as well as apical size and taper of root canal preparation,89,264,280 play a role in allowing contact of irrigants to adjacent canal areas. 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. 9-44 and 9-70). Therefore, effective cleaning of the root canal must include intermittent agitation of the canal content with a small instrument258,437 to prevent debris from accumulating at the apical portion of the root canal (see Fig. 9-42).

Preparation size264 and taper102 ultimately determine how close a needle can be placed to the final apical millimeters of a root canal. Some needles and suction tips may be attached to the air/water syringe to increase both the speed of irrigant flow and the volume of irrigant. Examples include the Stropko Irrigator (Vista Dental Products), which is an adapter that connects to the air/water syringe and accepts standard Luer-lock needle tips for irrigant removal and application as well as air drying.

Manually Activated Irrigation

Irrigant that is placed inside the root canal more effectively reaches crevices and mechanically untouched areas if it is agitated inside the root canal. Coronoapical movements of the irrigation needle,190 stirring movements with small endodontic instruments,258,437 and manual push-pull movements using a fitted master gutta-percha cone have been recommended.186

Sonically Activated Irrigation

In one study,341 investigators suggested that both passive sonic or ultrasonic irrigation rendered root canals significantly cleaner than manual preparation. Débridement of root canals supported by sonically and ultrasonically activated irrigation was superior to passive needle application of irrigants.77,439 However, in comparison to sonic activation, ultrasonic irrigation produced significantly cleaner canals.200,341 Other investigators discovered no significant difference in débridement between sonic or ultrasonic fluid activation inside a root canal. The difference lies in the oscillating movements: sonic devices range between 1500 Hz and 6000 Hz, and ultrasonic equipment requires vibrations greater than 20,000 Hz.200,241,401 Sonic or ultrasonic irrigation may be carried out with activated smooth wires or plastic inserts, endodontic instruments, or activated irrigation needles. Examples include EndoSonor (DENTSPLY Maillefer) and EndoSoft ESI (EMS, Nyon, Switzerland) inserts, the EndoActivator System (DENTSPLY Tulsa Dental), and the Vibringe sonic syringe (Vibringe, Amsterdam, Netherlands).

Ultrasonically Activated Irrigation

Ultrasonically powered instruments have become indispensable now, with well-adapted tips from various manufacturers. During preparation, ultrasonic tips are able to remove minimal amounts of dentin, conserving as much tooth structure as possible. Visibility is better than with burs, and the tips can be diamond coated to increase their efficiency. However, all tips develop significant heat that is transferred through dentin walls and can cause necrosis of surrounding bone if used without coolant. The temperature elevation also takes place during use of ultrasonic power during root canal irrigation10,195 and improves the antibacterial effect through warming of the irrigating solution.486

Ultrasonic action is most effective if the file is able to oscillate freely inside a given root canal.240 Passive ultrasonic irrigation is defined as activation of the rinsing agent without simultaneous preparation of the root canal walls.436,486 Passive ultrasonic irrigation is believed to promote tissue removal and tissue dissolution and may be done with a smooth wire insert that will avoid damaging canal walls and altering the shape in an undesirable way.439 This strategy allows cleaning of isthmus areas, fins, or C-shaped canals by acoustic streaming and to a lesser extent cavitation, as well as (to some degree) other hard-to-reach areas such as dentinal tubules or lateral canals. Disinfection is rendered more effective, an important consideration in necrotic cases.10,90 Kuah et al.217 demonstrated that to eliminate smear layer and debris in the apical region of a prepared root canal, a 1-minute application of EDTA with ultrasonics followed by a final flush of NaOCl was the most proficient method.

Negative- and Positive-Pressure Irrigation

Another approach to afford better access of irrigation solution is so-called negative-pressure irrigation. Here, irrigant is delivered into the access chamber, and a very fine needle connected to the dental unit’s suction device is placed into the root canal. Excess irrigant from the access cavity is then transported apically and ultimately removed via suction. First, a macrocannula, equivalent to an ISO size #55, .02 taper instrument, removes coronal debris. Subsequently, a microcannula, equivalent to a size #32, .02 taper, removes particles lodged close to working length. Such a system is commercially available (EndoVac, Discus Dental) and may prove a valuable adjunct in canal disinfection.281

Another device that makes use of pressure-suction technology is the RinsEndo system (Dürr Dental, Bietigheim-Bissingen, Germany). It aspirates the delivered rinsing solution into an irrigation needle that is placed close to working length and at the same time activates the needle with oscillations of 1.6 Hz amplitude. This system has been investigated by several authors and was superior to customary needle irrigation in cleaning and disinfection.67,262 Researchers262 used 30-gauge irrigation needles in canals shaped to ISO size #40 and compared irrigation with the RinsEndo system to conventional needle irrigation and so-called manual-dynamic irrigation that involves pumping action of a fitted master gutta-percha cone inside a root canal; in this study, 200 push-pull strokes were delivered. The cleaning effect of the RinsEndo unit was found to be superior to conventional needle irrigation, but the least amount of residual debris was found after hand agitation of a master cone.

Eradication of Microorganisms

Various types of microorganisms—bacteria,205,215,270,272 yeasts,449,450 and possibly viruses339,340—can infect the pulp and may lead to apical periodontitis (see Chapters 14 and 15 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.381,387 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.112 Even when simple saline was used as an irrigant, a 10-fold to 1000-fold reduction of the bacterial load through mechanical instrumentation was demonstrated.80,112,288

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.80 In that study, 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.80 In another study, teeth that caused symptoms tended to have more bacteria than teeth with no clinical symptoms.288

Other researchers287 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 NaOCl (with and without EDTA), CHX, and hydrogen peroxide in varying concentrations when used in sequence or in combination as endodontic irrigants.181 They found that CHX and NaOCl were similarly effective in eliminating the bacteria tested. Synergistic effects were observed for some of the irrigants (e.g., CHX and IKI).

Both of the preceding studies used infected dentin specimens. When evaluating 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.169,317 Moreover, chemical interactions occur between irrigation solutions; for example, NaOCl can become ineffective if it comes in contact with EDTA161 (Fig. 9-71).

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FIG. 9-71 Interaction of sodium hypochlorite with various factors that determine its efficacy.

Some of the more difficult to remove endodontic pathogens that can cause treatment failure are enterococci, Actinomyces, and Candida organisms47,276,277,378,462 (see Chapter 15). Table 9-1 presents the results of a number of studies evaluating the effectiveness of some commonly used antimicrobial agents.

Currently, the endodontic irrigation solution with the best proteolytic effect is NaOCl,481 even though it does not meet all the requirements of an ideal irrigant (Box 9-6). It is readily available, inexpensive, and consequently a widely used irrigation solution. As mentioned previously, necrotic tissue and debris are dissolved by the breakdown of proteins into amino acids through free chlorine in NaOCl. However, because unbound chlorine is the important component, the solution must be replenished frequently during preparation to compensate for lower concentrations and to constantly renew the fluid inside the root canal. This is even more important when the root canal is narrow and small and files must carry the NaOCl to the apical third during instrumentation (see Fig. 9-44). A 1% solution is effective at dissolving tissue and providing an antimicrobial effect. The use of 6% commercial household bleach in its undiluted form causes substantial necrosis of wound surface areas and may result in serious clinical side effects (Fig. 9-72). It is diluted in 1:1 or 1:3 ratios with water to produce a 2.5% or 1% solution; both are suitable for clinical endodontic use.398,477,482

BOX 9-6 Properties of an Ideal Irrigant for Root Canal Treatment

The irrigant should:

♦ Be a highly effective disinfectant
♦ Be nontoxic locally and nonallergenic
♦ Differentiate between necrotic and vital host tissue
♦ Retain its effectiveness with dental hard tissue and when mixed with other irrigants
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FIG. 9-72 Toxic effect of sodium hypochlorite on periradicular tissues. After root canal treatment of tooth #3, the patient reported pain. A, On a return visit, an abscess was diagnosed and incised. B, Osteonecrosis was evident after 3 weeks.

Importantly, to avoid extrusion and serious damage to periapical tissues, irrigation needles should never be wedged into canals during irrigation.70 Higher concentrations of NaOCl are more aggressive toward living tissue and can cause severe injuries when forced into the periapical area (see Fig. 9-72).

Such accidents can be prevented by marking the working length on the irrigation needle with a bend or a rubber stop and by passively expressing the solution from the syringe into the canal (see Fig. 9-70). The needle should be continuously moved in an up-and-down motion. It should remain loose in the canal, allowing a backflow of fluid. The goal is to rinse the suspended, concentrated dentinal filings out of the pulp chamber and root canals as new solution is brought down into the most apical areas by the endodontic instrument and the capillary effect.

As stated before, patency files should be used carefully and should not be extended farther than the periodontal ligament, because they are possible sources of irrigant extrusion.

In one study, heating increased the antibacterial action of NaOCl.104 Heating can be done in several ways; for example, after the solution has been drawn into the irrigating syringe, a syringe warmer can be used (e.g., Syringe Warmer [Vista Dental Products, Racine, WI]) (Fig. 9-73). Heating also enhanced the antibacterial effectiveness of CHX and Ca(OH)2 solutions.131 A 0.5% NaOCl solution heated to 113° F (45° C) dissolved pulp tissue as efficiently as a 5.25% solution used as the positive control (Fig. 9-74).386 Heating to 140° F (60° C) resulted in almost complete dissolution of tissue. Studies have shown that 1 minute at 116.6° F (47° C) is the cutoff exposure at which osteoblasts can still survive; however, higher temperatures may in fact be sufficient to kill osteoblasts and other host cells.128,129 Also, warming of NaOCl to 122° F (50° C)53 or 140° F (60° C)5 increases collagen dissolution and disinfecting potential, but it may also have severely detrimental effects on NiTi instruments, causing corrosion of the metal surface after immersion for 1 hour51 (Fig. 9-75).

image

FIG. 9-73 Device for heating syringes filled with irrigation solution (e.g., sodium hypochlorite) before use.

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FIG. 9-74 Effect of heating on the ability of 0.5% sodium hypochlorite (NaOCl) to dissolve pulp tissue: NaOCl heated to 113° F (45° C) dissolved pulp tissue as well as the positive control (5.25% NaOCl) did. When the NaOCl was heated to 140° F (60° C), almost complete dissolution of tissue resulted.

(Modified from Sirtes G, Waltimo T, Schaetzle M, Zehnder M: The effects of temperature on sodium hypochlorite short-term stability, pulp dissolution capacity, and antimicrobial efficacy. J Endod 31:669–671, 2005.)

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FIG. 9-75 Corrosion of nickel-titanium files in heated sodium hypochlorite (NaOCl). A, Rotary instrument immersed for 2 hours in NaOCl heated to 140° F (60° C). B, Magnification of rectangular area in A, showing severe corrosion.

As stated before, an increase in the temperature of the irrigant may be another reason to include ultrasonic devices in canal irrigation10,486; using these devices may also increase the tissue-dissolving capabilities of NaOCl via temperature elevation,5 but this effect seems to be limited to the main canal.10 In one study, peak irrigant temperatures during ultrasonic irrigation reached 113° F (45° C) near the file tip but remained at 89.6° F (32° C) on the outer root surface of teeth prepared to a size #45.86 The effect seems to depend on the insert,486 possibly due to its oscillation patterns.

Yet another reason for using ultrasonic devices might be enhancement of canal débridement. However, some authors have reported no significant effect with ultrasonics, neither in débriding root canal walls96,108,258 nor in reducing bacterial counts.118,382 However, the majority of available studies appear to find both bacterial reduction194 and improved débridement,167 as reviewed recently.438

Smear Layer Management

EDTA is a decalcifying chelating agent used as a 15% to 17% buffered solution during instrumentation of root canals. The decalcifying efficacy of EDTA-containing pastes is variable.160,441 EDTA acts as a chelator with calcium ions and removes the dentinal debris produced on the root canal walls during preparation. It thus opens dentinal tubules, promoting better penetration of disinfectants.154,191,422,485 Whenever the wall of a root canal is instrumented, whether by hand or rotating instruments, the parts of a dentin wall touched by an instrument are covered by a surface layer called the smear layer.261,296 The smear layer, which consists of dentin shavings, cell debris, and pulp remnants,369 can be described as itself having two separate layers: a loose, superficial deposit and an attached stratum that extends into the dentinal tubules, forming occluding plugs.87

For some time, clinicians and researchers paid little attention to the smear layer, partly because it was a thin superficial layer (1 to 5 µm) that might be present or not, depending on the type of instrument and the sharpness of its cutting blades.369 Also, because acids and chelating agents dissolve the smear layer, it was removed and escaped attention in routinely processed specimens (Fig. 9-76).109 Smear layers are not seen in unprepared canal areas, which may have calcospherites, buttonlike structures that are abundant on intracanal surfaces.

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FIG. 9-76 Prepared root canal surfaces after irrigation, showing varying degrees of smear layer. A, Scanning electron micrograph (SEM, ×25) showing prepared areas with and without open dentinal tubules. The presence of calcospherites indicates that no mechanical preparation has been done laterally (arrow). B, SEM, ×400, showing thin, homogenous smear layer and scattered debris in a canal that received a final sequence of a high-volume flush of 17% ethylenediamine tetra-acetic acid (EDTA) followed by 2.5% sodium hypochlorite (NaOCl).

Some authors have reported that an overlying smear layer delays but does not eliminate the effect of medicaments.287 Others contend that a smear layer may adversely affect disinfection and may also increase microleakage after canal obturation.369 Although organic substrate in a smear layer may serve as a nutrition source for some species of bacteria,65,297 some have suggested the converse: that a smear layer can act as a beneficial barrier, preventing microorganisms from entering the dentinal tubules when a root canal is colonized by bacteria between appointments.121 The potential of intracanal disinfectants has been evaluated in vitro after removal of the smear layer with a combination of 5.25% NaOCl and 17% EDTA.171 The decalcifying effect of EDTA is self-limiting, so the solution must be replaced at intervals.191 EDTA can help open very narrow root canals and can decalcify to a depth of approximately 50 µm. Because the smear layer consists of organic and inorganic components, the combined use of NaOCl and EDTA, with time frames of 30 seconds to 60 seconds each, is considered most effective for its removal.83 In general, it seems beneficial to remove the smear layer in the later phases of endodontic therapy rather than during the early phases. Research continues on ways to improve the effectiveness of irrigation. For example, tensides were added to irrigants more than 20 years ago to reduce their surface tension, thereby improving wetability.6 The rationale for this increased wetability was to improve the penetration of irrigants into the dentinal tubules,409 and this concept is still pursued with MTAD today. One irrigation “cocktail” investigated was a mixture of 5% NaOCl and 17% EDTA with the tensioactive chemical Triton X-100; this solution was used with ProFile instruments.7,144 The study reported that apical smear layer scores were significantly lower compared with those of control groups when the tensioactive agent was used throughout the preparation process. However, a more recent study found no effect of a wetting agent on the action of EDTA, measured as calcium in eluates from root canals in vitro.484 Rather than surfactant, ultrasonic irrigation appears to promote smear layer removal by EDTA.238

Liquid disinfectants were effective against E. faecalis in dentinal tubules up to depths of 400 µm. Microbiologic analyses of split root halves showed that early removal of the smear layer resulted in significantly higher bacteria counts.121 In contrast, other researchers have acknowledged that the smear layer, while acting as a barrier, might block irrigation solutions from entering the dentinal tubules.42 Moreover, some bacteria (e.g., Bacteroides gingivalis and Treponema denticola) have the potential to dissolve smear layer proteins,432 thereby producing gaps which could promote both coronal and apical microleakage and bacterial multiplication.

Fig. 9-77 shows root canal cross sections with very little debris; irrigating solutions can penetrate the dentinal tubules in this example. Some reported that the presence of the smear layer had no significant effect on apical leakage in dye penetration testing.132,247 Others described an improved seal after removal of the smear layer.43,411 The latter study, which used a coronal leakage model, found a significantly decreased incidence of bacterial penetration (30% versus 70%) when the canals were irrigated with 17% EDTA and 5.25% NaOCl before obturation. In obturated root canals, a remaining smear layer led to bacterial leakage in 60% of the samples versus no leakage when the smear layer was removed.101 Other authors had similar results after smear layer removal with EDTA solution alone. Another investigation421 found that many lateral canals in the apical third of root canal systems cleaned with a barbed broach wrapped with MTAD-soaked cotton showed less erosion than when EDTA was used. Other studies have found that a stronger bond was present when the smear layer was removed,148 and a statistically significant reduction of microleakage was also measured.124,349,350 Still another investigation reported increased apical microleakage of the filled root canal after removal of the smear layer.419

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FIG. 9-77 Example of canals with minimal smear layer. A, Middle third after irrigation with 17% ethylenediamine tetra-acetic acid (EDTA) and 2.5% sodium hypochlorite (NaOCl). B, Apical third with some particulate debris.

As shown in Fig. 9-43, dye staining is improved when the dentinal tubules are opened and the smear layer is removed with EDTA, at least in the two more coronal levels. However, this effect is at least partly due to tubular sclerosis and the overall lower number of tubules apically, resulting in fewer diffusion pathways across dentin.292

Although the effect of the smear layer on leakage has been widely studied, its removal from root canal walls remains controversial. The apparently conflicting results of studies could stem from differences in the various microleakage test models467 and from different obturation and irrigation techniques. The problem of coronal leakage has received much attention as a major factor in determining the success or failure in root canal therapy.47,327,330,331,428

In two in vitro studies, the noninstrumentation technique, which relies on activated irrigation solutions rather than mechanical preparation, produced excellent canal cleanliness244,245 (see Fig. 9-10, A). However, preliminary clinical studies identified a need for improvement before this system can be used routinely to clean root canals.27

For the time being, root canals must be mechanically enlarged before irrigation. Larger apical preparations enhance the efficacy of irrigation, and the additional use of ultrasonic energy during cleaning and shaping may also increase the efficacy of endodontic irrigants.87,88,105,224 Ultrasonics used passively in canals with sufficiently large apical preparations may reach and better clean any uninstrumented canal areas.241,463,465 One investigation studied the débriding ability of 2.5% NaOCl in canal recesses.105,106 In 10 of 11 cases, these researchers found significantly cleaner histologic sections after ultrasonically activated irrigation. In the ultrasonically treated group, the bacteria count was reduced by 99.8%, but hand filing alone reduced the bacteria count by 99.3%, so the improvement from ultrasonic therapy was limited. With ultrasonics, root canals are débrided by shear stresses produced between the irrigant and the canal wall, with subsequent cell disruption.9

Acoustic streaming of the irrigation fluid through ultrasonic treatment has been suggested as a method of improving cleanliness. However, this effect occurs mainly in the most coronal levels; the apical areas were least affected by activated irrigation96,258 (see Fig. 9-43). Because the amplitude of the oscillation is greatest at the instrument’s tip, attenuation and constraint most significantly affect the apical part,447 where the diameter of the canal is smallest.

One investigator reported that the most effective regimen with ultrasonic energy was to activate every dose of irrigant placed in the canal.85 With this approach, roughly 18 minutes of irrigation is required per canal. Other investigators used an irrigation time of 1 minute each for EDTA and NaOCl, which seems clinically more practical.83,258 These authors stated that the use of ultrasonic energy for irrigant activation did not improve débridement compared with control groups. However, bacterial species show varying degrees of susceptibility to ultrasonication.8,38,255,390

Because of the conflicting evidence concerning the effectiveness of ultrasonics in root canal therapy, other methods of disinfecting and débriding canals properly must be studied. Such research might include better ways to deliver irrigants and disinfecting solutions.

Summary

Cleaning and shaping are important, interdependent steps in root canal treatment. Cleaning, as demonstrated by an intracanal surface free of smear layer, can be done only after root canals have been sufficiently enlarged to accommodate adequate irrigation needles. Canal preparation is optimized when mechanical aims are fulfilled and enlargement is acceptable; such aims include avoiding both significant preparation errors and weakening of the radicular structure, which can result in fractures.

Taken together and performed to a high standard, the procedures described in this chapter lay the foundation for biologic success in both straightforward (Fig. 9-78) and more complicated (Fig. 9-79) clinical cases. Recall radiographs confirm favorable outcomes or biologic success (i.e., prevention or healing of periradicular periodontitis) over the years. Similarly, adherence to the principles discussed leads to predictable outcomes for root canal treatments.

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FIG. 9-78 Clinical cases treated according to the principles detailed in this chapter. A, Pretreatment radiograph of tooth #30 with a periradicular lesion. B, Postobturation radiograph. C, Two-year follow-up radiograph shows osseous healing. D, Immediate postobturation radiograph of tooth #29 shows both a periapical and a lateral osseous lesion. E-F, One-year and three-year follow-up radiographs show progressing osseous healing. Note the imperfect obturation of tooth #30.

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FIG. 9-79 Complicated clinical cases treated with hybrid techniques. A, Pretreatment radiograph of tooth #16 indicates laceration and significant curvature of all roots. B, Posttreatment radiograph shows multiple planes of curvature. C, Pretreatment radiograph of tooth #19, which was diagnosed with irreversible pulpitis. D, Angulated posttreatment radiograph shows three canals in the mesiobuccal root canal system, all of which were prepared to apical size #50.

(A-B courtesy Dr. T. Clauder; C-D courtesy Dr. H. Walsch.)

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