Broaches

Barbed broaches are produced in a variety of sizes and color codes. They are manufactured by cutting sharp, coronally angulated barbs into metal wire blanks. Broaches are intended to remove vital pulp from root canals, and in cases of mild inflammation, they work well for severing pulp at the constriction level in toto. The use of broaches has declined since the advent of nickel-titanium rotary shaping instruments, but broaching occasionally may be useful for expediting procedures and removing materials (e.g., cotton pellets or absorbent points) from canals.

K-Files

K-files were manufactured by twisting square or triangular metal blanks along their long axis, producing partly horizontal cutting blades (Fig. 9-19). Noncutting tips, also called Batt tips, are created by grinding and smoothing the apical end of the instrument (see Fig. 9-19). Investigators introduced a modified shape, the Flex-R file, which was manufactured fully by grinding so that the transitional angles were smoothed laterally between the tip and the instrument’s working parts.³³⁵ Similar techniques are required to manufacture NiTi K-files⁴¹³ such as the NiTi-Flex (DENTSPLY Maillefer, Ballaigues, Switzerland). NiTi K-files are extremely flexible and are especially useful for apical enlargement in severe apical curves. They can be precurved, but only with strong overbending; this subjects the file to excess strain and should be done carefully. Because of their flexibility, the smaller NiTi files (sizes up to #25) are of limited use.

FIG. 9-19 Flute geometry and tip configuration of a hand file (insert) and a NiTi rotary instrument. A, K-file with sharp cutting edges (arrow) and Batt tip (arrowhead). B, GT rotary file with rounded, noncutting tip (arrowhead), smooth transition, and guiding radial lands (arrow).

Cross-sectional analysis of a K-file reveals why this design allows careful application of clockwise and counterclockwise rotational and translational working strokes. ISO-normed K- and Hedström files are available in different lengths (21, 25, and 31 mm), but all have a 16-mm-long section of cutting flutes (see Fig. 9-17). The cross-sectional diameter at the first rake angle of any file is labeled D₀. The point 1 mm coronal to D₀ is D₁, the point 2 mm coronal to D₀ is D₂, and so on up to D₁₆. The D₁₆ point is the largest diameter of an ISO-normed instrument. Each file derives its numeric name from the diameter at D₀ and is assigned a specific color code (see Fig. 9-17).

Another aspect of ISO files is the standard taper of 0.32 mm over 16 mm of cutting blades, or 0.02 mm increase in diameter per millimeter of length (#.02 taper) (see Fig. 9-17). Thus a size #10 instrument has a diameter of 0.1 mm at D₀ and a corresponding diameter of 0.42 mm at D₁₆ [0.1 mm + (16 × 0.02 mm)]. For a size #50 instrument, the diameters are 0.5 mm at D₀ and 0.82 mm at D₁₆.

The tip size increases by 0.05 mm for file sizes #10 to #60; for sizes #60 to #140, the absolute increase is 0.1 mm (see Fig. 9-18). Recalculation of these diameter increments into relative steps (in percentages) reveals dramatic differences: the step from size #10 to #15 is 50%, whereas the increase from size #55 to #60 is less than one fifth of that change (see Fig. 9-18).

In very small files (sizes #6 to #10), the problem is partly resolved by several key points: (1) apical dimensions are such that a size #6 file does not significantly remove dentin other than in severely calcified cases; (2) a size #8 file taken 0.5 to 1 mm long to establish patency (discussed later in the chapter) contacts the desired endpoint of the preparation with a diameter approaching the tip size of a #10 file; (3) similarly, placing a size #10 file just minutely through the foramen eases the way for passive insertion of the subsequent #15 file to full length.³³⁷

The ISO specifications inadvertently complicated the cleaning and shaping of root canal systems. The ISO-normed design is a simplification that has specific disadvantages, and it may explain the clinical observation that enlarging from size #10 to #15 is more difficult than the step from size #55 to #60. The introduction of Golden Medium files (DENTSPLY Maillefer), which have tip sizes between the ISO-stipulated diameters, seemed to solve the problem. However, their use is not that important clinically, because the approved machining tolerance of ± 0.02 mm negates the intended advantage. Moreover, although ± 0.02 mm tolerance is stipulated by the ISO norm (see Fig. 9-17), most manufacturers do not adhere to it.^{208,365,404,487}

A subsequent modification involved tips with a constant percentage of diameter increments, the Series 29. The first ProFile instruments (DENTSPLY Tulsa Dental, Tulsa, OK, USA) followed this design, with a nominal diameter increase of 29%. This sizing pattern creates smaller instruments that carry less of a workload. However, the intended advantage is offset by larger diameters, because the 29% increase between successive files is actually greater than the percentage change found in the ISO file series.

Hedström Files

Hedström files are milled from round, stainless steel blanks. They are very efficient for translational strokes,³⁵⁵ but rotational working movements are strongly discouraged because of the possibility of fracture. Hedström files up to size #25 can be efficiently used to relocate canal orifices and, with adequate filing strokes, to remove overhangs. Similarly, wide oval canals can be instrumented with Hedström files as well as with rotary instruments. On the other hand, overzealous filing can lead to considerable thinning of the radicular wall and strip perforations (Fig. 9-20). As with stainless steel K-files, Hedström files should be single-use instruments.³⁹⁷

FIG. 9-20 Result of an overenthusiastic attempt at root canal treatment of a maxillary second molar with large stainless steel files. Multiple strip perforations occurred; consequently, the tooth had to be extracted.

Gates-Glidden Drills

Gates-Glidden (GG) drills are important instruments that have been used for more than 100 years without noteworthy design changes. These instruments, especially the nickel-titanium FlexoGates model (DENTSPLY Maillefer),¹⁵¹ usually work well for preenlargement of coronal canal areas.^114,261 However, when misused, GG drills can dramatically reduce radicular wall thickness.^149,197,239

GG instruments are manufactured in a set and numbered 1 to 6 (with corresponding diameters of 0.5 to 1.5 mm); the number of rings on the shank identifies the specific drill size. GG instruments are available in various lengths and made by several manufacturers. Each instrument has a long, thin shaft with parallel walls and a short cutting head. Because of their design and physical properties,⁶⁶ GG drills are side-cutting instruments with safety tips; they can be used to cut dentin as they are withdrawn from the canal (i.e., on the outstroke).³³⁷ Used this way, their cutting action can deliberately be directed away from external root concavities in single-rooted and furcated teeth.⁴ GG instruments should be used only in the straight portions of the canal, and they should be used serially and passively.⁴²⁴

Two procedural sequences have been proposed: with the step-down technique, the clinician starts with a large drill and progresses to smaller ones; conversely, with the step-back technique, the clinician starts with a small drill and progresses to larger ones. With the step-down approach, the clinician must select a GG instrument with a diameter that allows introduction into the respective orifice and progression for about 1 mm. The subsequent smaller instruments progress deeper into the canal until the coronal third has been preenlarged. This technique efficiently opens root canal orifices and works best when canals exit the access cavity without severe angulations. Opened orifices simplify subsequent cleaning and shaping procedures and help establish a smooth glide path from the access cavity into the root canal system.

With the step-back approach, a small GG instrument is introduced into the canal, and dentin is removed on the outstroke. This process is repeated with the next larger GG instrument, which is again worked shorter than the preceding smaller one. In this way, the coronal third of the root canal is enlarged and dentin overhangs are removed.

As stated earlier, when used adequately, GG instruments are inexpensive, safe, and clinically beneficial tools. High revolutions per minute (rpm), excessive pressure, an incorrect angle of insertion, and the use of GG instruments to aggressively drill into canals have resulted in mishaps such as strip perforation. Also, cyclic fatigue may cause GG instruments to fracture when used in curved canal areas, and the short cutting heads may fracture with high torsional loads. Gates-Glidden drills may be used safely and to their fullest potential at 750 to 1500 rpm. As with nickel-titanium rotary instruments, GG drills work best when used in electric gear reduction handpieces rather than with air motors.

Nickel-Titanium Rotary Instruments

Since the early 1990s, several instrument systems manufactured from nickel-titanium (NiTi) have been introduced into endodontic practice. The specific design characteristics vary, such as tip sizing, taper, cross section, helix angle, and pitch (Fig. 9-21). Some of the early systems have been removed from the market or play only minor roles; others, such as ProFile (DENTSPLY Tulsa Dental, DENTSPLY Maillefer), are still widely used. New designs continually are produced, but the extent to which clinical outcomes (if any) will depend on design characteristics is difficult to forecast.³⁰³

FIG. 9-21 Design characteristics of nickel-titanium rotary instruments. A, Lateral view showing the details of the helix angle, pitch (p), and the presence of guiding areas, or radial lands (rl) (scanning electron micrograph [SEM], ×25). B, Ground working part of the instrument in A, showing U-shaped excavations and the dimension of the instrument core (c).

Most of the instruments described in this section are manufactured by a grinding process, although some are produced by laser etching and others by plastic deformation under heating. Precision at the surface quality is not really at a high level, whereas the tolerances are. Surface quality also is an important detail (see Fig. 9-21), because cracks that arise from superficial defects play a role in instrument fracture.¹⁶ Superficial defects such as metal flash and rollover are common in unused NiTi instruments.^125,251,489

Attempts have been made to improve surface quality by electropolishing the surface and coating it with titanium nitride.^325,353 The latter process also seems to have a beneficial effect on cutting efficiency.³⁵³

In essence, two properties of the NiTi alloy are of particular interest in endodontics: superelasticity (Fig. 9-22) and high resistance to cyclic fatigue (discussed later). These two properties allow continuously rotating instruments to be used successfully in curved root canals. Many variables and physical properties influence the clinical performance of NiTi rotaries.^{218,302,370,413}

FIG. 9-22 Deformation of endodontic instruments manufactured from nickel-titanium alloy. A and B, Intact and plastically deformed ProFile instruments (arrow indicates areas of permanent deformation). C, ProFile instrument placed on a mirror to illustrate elastic behavior.

Much of what is known about NiTi instruments, including reasons for instrument fracture³³ and instrument sequences, has been gleaned from clinical practice. In vitro research continues to clarify the relationship between NiTi metallurgy and instrument performance, but already NiTi rotary instruments have become an important adjunct in endodontics.³⁰¹

NiTi rotary instruments have substantially reduced the incidence of several clinical problems (e.g., blocks, ledges, transportation, perforation), but they are also believed to fracture somewhat more easily than hand instruments. This does not by itself predispose a case to posttreatment disease; rather, a retained instrument fragment limits access of disinfecting irrigants to the root canal system, possibly impeding sufficient elimination of microorganisms.¹⁷²

The following sections describe the instruments most widely used in the United States and Europe for root canal preparation. Most basic strategies apply to all NiTi rotary instruments, regardless of the specific design or brand. However, three design groups need to be analyzed separately: group I, the LightSpeed; group II, rotary instruments with #.04 and #.06 tapers, which includes the ProFile and many other models; and group III, rotary instruments with specific design changes, such as the ProTaper (DENTSPLY Maillefer) and RaCe (FKG, La Chaux-de-Fonds, Switzerland).

LightSpeed and LightSpeed LSX Instruments

The LightSpeed file, developed by Dr. Steve Senia and Dr. William Wildey in the early 1990s and now also known as LS1, was introduced as an instrument different from all others because of its long, thin noncutting shaft and short anterior cutting part. The same design principles apply to the recently developed LSX instrument (Discus Dental, Culver City, CA, USA) (Fig. 9-23) that is manufactured not by milling but by stamping. A full set consists of 25 LightSpeed LS1 instruments in sizes #20 to #100, including half sizes (e.g., 22.5, 27.5); LSX does not have half sizes, and a set includes sizes #20 to #80.

FIG. 9-23 Design features of a LightSpeed instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

The recommended working speed for LS1 instruments is 1500 to 2000 rpm and for LSX, 2500 rpm. Both variants should be used with minimal torque,³² owing to the thin shaft.¹²⁰

Cross sections of LightSpeed LS1 cutting parts show three round excavations, the U-shape design common to many earlier NiTi instruments, whereas the LSX is shaped like a flat chisel in cross section (see Fig. 9-23). Because of the thin noncutting shaft, both types of LightSpeed instruments are considerably more flexible than any other instrument on the market. In addition, cyclic fatigue is lower than with all other instruments, allowing the use of higher rpm speeds. All LightSpeed instruments feature a noncutting tip.

Because of their design, LightSpeed LS1 and LSX require specific instrumentation sequences to produce canal shapes amenable to root canal filling. The current recommendation calls for an apical 4-mm zone to be prepared to a cylindrical, nontapered shape. This section may then be filled with the proprietary SimpliFill system (Discus Dental). Different sequences are required for lateral compaction or other filling techniques.

The original LightSpeed is a widely researched NiTi rotary instrument,^{49,319,321,375,415,416} and most reports have found that the system has a low incidence of canal transportation and preparation errors. Loss of working length was also minimal in most of these studies. Data regarding the LSX are sparse. One report found similar shaping abilities for LSX and LightSpeed LS1 assessed with a double-exposure technique.¹⁹⁶

ProFile

The ProFile system (DENTSPLY Tulsa Dental) was introduced by Dr. Ben Johnson in 1994. In contrast to the LightSpeed instrument with its thin, flexible shaft, ProFile instruments have increased tapers compared with conventional hand instruments. The ProFile system was first sold as the “Series 29” hand instruments in .02 taper, but it soon became available in .04 and .06 taper (Fig. 9-24). The tips of the ProFile Series 29 rotary instruments had a constant proportion of diameter increments (29%). Because of the nonstandardized diameters, obturation was performed with nonstandardized gutta-percha cones, using either lateral compaction or thermoplastic obturation of gutta-percha (see Chapter 10). Later, another ProFile series with ISO-sized tips (DENTSPLY Maillefer) was developed and marketed in Europe. This set was believed to better accommodate standardized gutta-percha cones, which are predominantly used in Europe. Subsequently, instruments with even greater tapers and 19-mm lengths were introduced, and a .02 variant was added.

FIG. 9-24 Design features of a ProFile instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

Cross sections of a ProFile instrument show a U-shape design with radial lands and a parallel central core. Lateral views show a 20-degree helix angle, a constant pitch, and bullet-shaped noncutting tips. Together with a neutral or slightly negative rake angle, this configuration facilitates a reaming action on dentin rather than cutting. Also, debris is transported coronally and is effectively removed from the root canals.

The recommended rotational speed for ProFile instruments is 150 to 300 rpm, and to ensure a constant rpm level, the preferred means is electrical motors with gear reduction rather than air-driven motors.

ProFile instruments shaped canals without major preparation errors in a number of in vitro investigations.^{72,73,417,418} A slight improvement in canal shape was noted when size .04 and .06 tapered instruments were used in an alternating fashion.⁷¹ Loss of working length did not exceed 0.5 mm^{71-73,417,418} and was not affected by the use of .06 tapered instruments.⁷¹ Comparative assessments in vitro suggested that ProFile prepared mesial canals in mandibular molars with less transportation than K3 and RaCe.¹⁴

A very recent addition to the ProFile family of instruments is the Vortex (DENTSPLY Tulsa Dental). The major change lies in the non-landed cross section, whereas tip sizes and tapers are similar to existing ProFiles. Manufactured using M-Wire, Profile Vortex also have varying helical angle to counteract the tendency of non-landed files to thread into the root canal. At this point, comparatively little clinical or experimental data are available for ProFile Vortex.

GT and GTX Files

The Greater Taper, or GT file, was introduced by Dr. Steve Buchanan in 1994. This instrument also incorporates the U-file design and was marketed as ProFile GT. The system was first produced as a set of four hand-operated files and later as engine-driven files. The instruments came in four tapers (.06, .08, .10, and .12), and the maximum diameter of the working part was 1 mm. This decreased the length of the cutting flutes and increased the taper. The instruments had a variable pitch and an increasing number of flutes in progression to the tip; the apical instrument diameter was 0.2 mm. Instrument tips were noncutting and rounded (Fig. 9-25); these design principles are mostly still present in the current incarnation, the GTX instrument. The main differences are the NiTi alloy type used (M-Wire, manufactured by SportsWire, Langley, OK) and a different approach to instrument usage, emphasizing the use of the #20 .06 rotary.

FIG. 9-25 Design features of a GT-file. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

The GTX set currently includes tip sizes 20, 30, and 40, in tapers ranging from .04 to .010 (see Fig. 9-25). The recommended rotational speed for GT and GTX files is 300 rpm, and the instrument should be used with minimal apical force to avoid fracture of the tip.

Studies on GT files found that the prepared shape stayed centered and was achieved with few procedural errors.^{149,178,310,316,479} A shaping assessment using µCT showed that GT files machined statistically similar canal wall areas compared with ProFile and LightSpeed preparations.³¹⁰ These walls were homogeneously machined and smooth.^294,479

HERO 642, Hero Shaper

First-generation rotary systems had neutral or slightly negative rake angles. Several second-generation systems were designed with positive rake angles, which gave them greater cutting efficiency. HERO instruments (MicroMega, Besançon, France) are an example of a second-generation system; the original system known as HERO 642 has now been replaced by HERO Shaper, with very little difference in the instrument design.

Cross sections of HERO instruments show geometries similar to those of an H-file without radial lands (Fig. 9-26). Tapers of .02, .04, and .06 are available in sizes ranging from #20 to #45. The instruments are relatively flexible (the acronym HERO stands for high elasticity in rotation) but maintain an even distribution of force into the cutting areas.^430,431 HERO instruments have a progressive flute pitch and a noncutting passive tip, similar to other NiTi rotary systems. The instruments are coded by handle color.

FIG. 9-26 Design features of a HERO instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

Research with HERO files indicates a shaping potential similar to that of the FlexMaster¹⁸⁹ (DENTSPLY VDW, Munich, Germany) and the ProFile,¹⁴⁷ although in one study the HERO induced more changes in cross-sectional anatomy.¹⁵⁷ HERO instruments also were found to cause some aberrations when used in simulated canals with acute curves⁴¹⁴ but were safer than Quantec SC instruments (SybronEndo, Orange, CA).¹⁹³ More recently, HERO Shapers were found to have a better centering ability compared to RaCe instruments in resin blocks.²⁸ Comparing earlier HERO 642 and current HERO Shaper rotaries, no differences were found assessing cross sections before and after shaping in a modified Bramante technique.⁸⁴

ProTaper Universal

The ProTaper system is based on a unique concept and originally comprised just six instruments: three shaping files and three finishing files. This set is now complemented by two larger finishing files and a set designed for retreatment procedures. The instruments were designed by Dr. Cliff Ruddle, Dr. John West, and Dr. Pierre Machtou. In cross section, ProTaper shows a modified K-type file with sharp cutting edges and no radial lands (Fig. 9-27); this creates a stable core and sufficient flexibility for the smaller files. The cross section of finishing files F3, F4, and F5 is slightly relieved for increased flexibility. A unique design element is varying tapers along the instruments’ long axes. The three shaping files have tapers that increase coronally, and the reverse pattern is seen in the five finishing files.

FIG. 9-27 Design features of a ProTaper instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

Shaping files #1 and #2 have tip diameters of 0.185 mm and 0.2 mm, respectively, 14-mm-long cutting blades, and partially active tips. The diameters of these files at D₁₄ are 1.2 and 1.1 mm, respectively. The finishing files (F1-F5) have tip diameters of 0.2, 0.25, 0.3, 0.4, and 0.5 mm, respectively, between D₀ and D₃, and the apical tapers are .07, .08, .09, .05, and .04, respectively. The finishing files have rounded noncutting tips.

The convex triangular cross section of ProTaper instruments reduces the contact areas between the file and the dentin. The greater cutting efficiency inherent in this design has been safely improved by balancing the pitch and helix angle, preventing the instruments from inadvertently threading into the canal. The instruments are coded by colored rings on the handles. ProTaper instruments can be used in gear reduction electrical handpieces at 250 to 300 rpm, in accordance with universally recognized guidelines. Two usage characteristics have been recommended for ProTaper. The first is the preparation of a glide path, either manually²⁹⁸ or with special rotary instruments.⁵² An enlargement to a size approaching the subsequent rotaries’ tips prevents breakage and allows assessment of the canal size.²⁹⁸ The second specific recommendation is the use of a more lateral “brushing” working stroke. Such a stroke allows the clinician to direct larger files coronally away from danger zones and counteract any “threading-in” effect.⁵⁸ Both usage elements should be considered good practice for other instruments, particularly more actively cutting ones.³¹⁴

In a study using plastic blocks, the ProTaper created acceptable shapes more quickly than GT rotary, ProFile, and Quantec instruments⁴⁷⁹ but also created somewhat more aberrations. This was recently corroborated comparing preparations of mesial root canals in mandibular molars ex vivo with ProTaper Universal to Alpha (Brasseler Komet, Lemgo, Germany).⁴⁴⁰ In a comparison of ProTaper and K3 instruments (SybronEndo, Orange, CA), Bergmans et al.⁴⁸ found few differences, with the exception of some transportation by the ProTaper into the furcation region. A study using µCT showed that the ProTaper created consistent shapes in constricted canals, without obvious preparation errors, although wide canals may be insufficiently prepared with this system.³⁰⁸ It has been recommended that ProTaper be combined with less tapered, more flexible rotaries to reduce apical transportation.¹⁹⁹

K3

In a sequence of constant development by their inventor, Dr. McSpadden, the Quantec 2000 files were followed by the Quantec SC, the Quantec LX, and the current K3 system (all by SybronEndo). The overall design of the K3 is similar to that of the ProFile and the HERO in that it includes instruments with .02, .04, and .06 tapers. The most obvious difference between the Quantec and K3 models is the K3’s unique cross-sectional design (Fig. 9-28): a slightly positive rake angle for greater cutting efficiency, wide radial lands, and a peripheral blade relief for reduced friction. Unlike the Quantec, a two-flute file, the K3 features a third radial land to help prevent threading-in.

FIG. 9-28 Design features of a K3 instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

In the lateral aspect, the K3 has a variable pitch and variable core diameter, which provide apical strength. This complicated design is relatively difficult to manufacture, resulting in some metal flash (see Fig. 9-28).

Like most other instruments, the K3 features a round safety tip, but the file is about 4 mm shorter than other files (although it has the same length of cutting flutes) because of the Axxess handle. The instruments are coded by ring color and number.

Tested in vitro, K3’s shaping ability seems to be similar to that of the ProTaper⁴⁸ and superior to that achieved with hand instruments.³⁵⁷ More recently, when curved canals in lower molars were shaped to a size #30 .06,¹⁴ K3 had less canal transportation in a modified Bramante model than RaCe but more than ProFile.

FlexMaster

The FlexMaster file system currently is unavailable in the United States. It also features .02, .04, and .06 tapers. The cross sections (Fig. 9-29) have a triangular shape, with sharp cutting edges and no radial lands. This makes for a relatively solid instrument core and excellent cutting ability. The overall manufacturing quality is high, with minimal metal flash and rollover.

FIG. 9-29 Design features of a FlexMaster instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

FlexMaster files have rounded, passive tips; the tip diameters are 0.15 to 0.7 mm for size .02 instruments and 0.15 to 0.4 mm for size .04 and .06 files (see Fig. 9-29). In addition to the standard set, the Intro file, which has a .11 taper and a 9-mm cutting part, is available. The instruments are marked with milled rings on the instrument shaft, and the manufacturer provides a system box that indicates sequences for narrow, medium-size, and wide canals.

Several studies indicate that the FlexMaster allows centered preparations in both constricted and wider canals¹⁸⁸ and that it performed on par with other systems.^189,455 Clinical studies confirmed that the FlexMaster showed superior shaping characteristics compared with K-files.³⁵⁸ Also, novice dental students were able to shape plastic blocks successfully with the FlexMaster after a short training period.^392,393 Tested in a well-described model of simulated canals, FlexMaster instruments led to few aberrations but took longer than preparation with RaCe files.²⁶³ Moreover, FlexMaster appeared to be less effective than RaCe in removing dye from the walls of simulated canals prepared to size #30 but were more effective than ProFile.³⁶²

RaCe, Bio Race

The RaCe has been manufactured since 1999 by FKG and was later distributed in the United States by Brasseler (Savannah, GA). The name, which stands for reamer with alternating cutting edges, describes just one design feature of this instrument (Fig. 9-30). Light microscopic imaging of the file shows flutes and reverse flutes alternating with straight areas; this design is aimed at reducing the tendency to thread the file into the root canal. Cross sections are triangular or square for #.02 instruments with size #15 and #20 tips. The lengths of cutting parts vary from 9 to 16 mm (see Fig. 9-30).

FIG. 9-30 Design features of an RaCe instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

The surface quality of RaCe instruments has been modified by electropolishing, and the two largest files (size #35, #.08 taper and size #40, #.10 taper) are also available in stainless steel. The tips are round and noncutting, and the instruments are marked by color-coded handles and milled rings. RaCe instruments have been marketed in various packages to address small and large canals; recently they are sold as BioRaCe, purportedly to allow preparations two larger sizes, with an emphasis on the use of .02 tapered instruments.

Few results of in vitro experiments comparing RaCe to other contemporary rotary systems are available^359,360: canals in plastic blocks and in extracted teeth were prepared by the RaCe with less transportation from the original curvature than occurred with the ProTaper.³⁵⁹ In a separate study, ProTaper and RaCe performed similarly when canals were prepared to an apical size #30.²⁹³ When preparing to a size #40, RaCe prepared canals rapidly and with few aberrations or instrument deformities.³²⁴ The newer BioRaCe instrument sequences attempt to utilize .02 tapered instruments to promote larger apical sizes; this is also possible in a hybrid technique.

EndoSequence

The Sequence rotary instrument is produced by FKG in Switzerland and marketed in the United States by Brasseler. This is another instrument that adheres to the conventional length of the cutting flutes, 16 mm, and to larger tapers, .04 and .06, to be used in a crown-down approach. The overall design, including the available tapers and cross sections, is thus similar to many other files (Fig. 9-31), but the manufacturer claims that a unique longitudinal design called alternating wall contact points (ACP) reduce torque requirements and keep the file centered in the canal. Another feature of the Sequence design is an electrochemical treatment after manufacturing, similar to RaCe files, that results in a smooth, polished surface. This is believed to promote better fatigue resistance, hence a rotational speed of 600 rpm is recommended for EndoSequence.²¹⁴

FIG. 9-31 Design features of a Sequence instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

Twisted File

In 2008, SybronEndo presented the first fluted NiTi file (Fig. 9-32, A) manufactured by plastic deformation, a process similar to the twisting process that is used to produce stainless steel K-files. According to the manufacturer, a thermal process allows twisting during a phase transformation into the so-called R-phase of nickel-titanium. The instrument is currently available with size #25 tip sizes only, in taper .04 up to .12.

FIG. 9-32 Design features of a Twisted File (TF) instrument. A, Lateral view (scanning electron micrograph [SEM], ×50). B, Cross section (SEM, ×200). C, Lateral view. D, Design specifications.

The unique production process is believed to result in superior physical properties; indeed, early studies suggested significantly better fatigue resistance of size #25 .06 taper Twisted File compared to K3 instruments of the same size and size #20 .06 GTX.¹⁴⁶ Moreover, as determined by bending tests according to the norm for hand instruments, ANSI/ADA No. 28 (ISO 3630), Twisted Files size #25 .06 taper were more flexible than ProFiles of the same size.¹⁴⁵

The manufacturer recommends a conventional crown-down technique after securing a glide path with a size #15 K-file. Specifically, for a “large” canal, tapers .10 to .06 should be used, and in a “small” canal, tapers .08 to .04 are recommended. Although early reports suggest that the Twisted File is clinically resistant to fatigue, there are no reports available at this point that show improved healing outcomes compared to other rotary files.

The preceding descriptions covered only a limited selection of the most popular and widely used rotary instruments on the market. New files are continually added to the armamentarium, and older systems are updated. This is partly the reason for the scarcity of clinical outcome studies at this point.

To summarize, most systems include files with tapers greater than the #.02 stipulated by the ISO norm. The LightSpeed LS1 and LSX are different from all other systems; the ProTaper, RaCe, and Twisted File have some unique features; and most other systems have increased tapers. Minor differences exist in tip designs, cross sections, and manufacturing processes, but the clinical effects of these modifications currently are unknown. Even in vitro, tests have only begun to identify the effect of specific designs on shaping capabilities,^50,192,301 and differences in clinical outcomes in regard to these design variations appear to be minimal.^165,303,358

Physical parameters governing rotary root canal preparation are crucial because NiTi rotary files are felt to have an increased risk of fracture compared with K-files. In a study using plastic blocks, as many as 52 ProFile Series 29 instruments became permanently deformed.⁴¹⁷ Three fractures were reported in a subsequent study on ISO-norm ProFile size #.04 instruments, and three other instruments were distorted.⁷³ An even higher fracture incidence was shown in a study on rotary instruments used in plastic blocks in a specially designed testing machine.⁴¹² These findings were supported by two studies in which high fracture incidences were reported for LightSpeed and Quantec rotary instruments used in a clinical setting.^33,345

On the other hand, a retrospective clinical study suggest similar outcomes with and without retained instrument fragments⁴⁰⁰; moreover, others’ experiences suggest that the number of rotary instrument fractures is lower than previously estimated.^116,213,460 Removal of such fragments is possible in many situations, but there is also the potential for further damage (e.g., perforation) rather than successful removal.^405,453

Consequently, a benefit-versus-risk analysis should be carried out prior to attempts to remove NiTi instrument fragments, addressing the reasons and the clinical consequences of instrument fracture.

Physical and Chemical Properties of NiTi Alloys

During the development of the equiatomic nitinol alloy (55% [by weight] nickel and 45% [by weight] titanium), a shape memory effect was noted; this was attributed to specific thermodynamic properties of the new alloy.⁷⁵ The alloy sparked interest in dental research because of its “shape recovery” property after passage through critical temperatures.¹⁰⁰ Some researchers envisioned the manufacture of nondulling rotary instruments from an alloy called 60-nitinol. However, NiTi wire was found to be difficult to bend into clamp retainers.¹⁰⁰

Subsequently, researchers thought that the superelastic properties of 55-nitinol might prove advantageous in endodontics, and the first hand instruments produced from 55-nitinol were tested (Fig. 9-33).⁴⁴⁵ That study found that size #15 NiTi instruments were two to three times more flexible than stainless steel instruments. Nickel-titanium instruments showed superior resistance to angular deflection; they fractured after full revolutions (900 degrees) compared to 540 degrees for stainless steel instruments (see Fig. 9-33, C).

FIG. 9-33 Stress-strain behavior of nickel-titanium alloy. A, Schematic diagram of linear extension of a NiTi wire. B, Torque to failure test of a size #60, #.04 taper ProFile NiTi instrument. Note the biphasic deformation, indicated by arrows in A-B. C, Comparison of stainless steel and nickel-titanium crystal lattices under load. Hookian elasticity accounts for the elastic behavior (E) of steel, whereas transformation from martensite to austenite and back occurs during the superelastic (SE) behavior of NiTi alloy.

(C modified from Thompson SA: An overview of nickel-titanium alloys used in dentistry. Int Endod J 33:297-310, 2000.)

Furthermore, hardly any plastic deformation of cutting flutes was recorded when an instrument was bent up to 90 degrees,⁴⁴⁵ and forces required to bend endodontic files to 45 degrees were reduced by 50% with NiTi.³⁷⁰ In the latter study, the authors speculated that heat, probably during sterilization cycles, could even restore the molecular structure of used NiTi files, resulting in an increased resistance to fracture.

Specific properties of NiTi can be explained by specific crystal structures of the austenite and martensite phases of the alloy.⁴¹³ Heating the metal above 212° F (100° C) may lead to a phase transition, and the shape memory property forces the instrument back to a preexisting form. Likewise, linear deforming forces are shunted into a stepwise transition from an austenitic to a martensitic lattice, and this behavior leads to a recoverable elastic response of up to 7% (see Fig. 9-33, A).

However, graphs such as those shown in Fig. 9-33, B are generated when larger NiTi instruments are subjected to angular deflection until failure. Such graphs show different results for stainless steel instruments, which produce a relatively steep stress-strain curve with less than 1.3% recoverable deformation.⁴¹³ As stated previously, the superelastic behavior of NiTi also dictates the production of NiTi instruments, which are usually milled.

Similarly to phase transformations induced by strain, heating and cooling NiTi can also result in conformational changes.^179,265 Thermal conditions during the production of the raw wire can be used to modify its properties—most importantly, its flexibility.

Recently, such a thermal process was harnessed to allow twisting of raw NiTi material into the shape of a nonlanded rotary instrument (Twisted File, SybronEndo). This process is believed to respect the grain structure of the material better and does not introduce milling marks or other surface irregularities.

Typically, NiTi instruments may have characteristic imperfections such as milling marks, metal flashes, or rollover.^{125,370,426,445} Some researchers have speculated that fractures in NiTi instruments originate at such surface imperfections.^16,237

Surface irregularities may also provide access to corrosive substances, most notably sodium hypochlorite (NaOCl). Some studies have suggested that chloride corrosion may lead to micropitting³⁴⁴ and possibly subsequent fracture in NiTi instruments.¹⁷⁴ Not only immersion in various disinfecting solutions for extended periods (e.g., overnight) produced corrosion of NiTi instruments and subsequent decreased torsional resistance,^284,394 but for ProTaper,⁵¹ RaCe, and ProFile³⁰⁹ instruments, also short-term immersion. Other authors, however, did not find a corrosion-related effect on K3³⁵ or ProFile²⁵⁷ instruments. Regular cleaning and sterilization procedures do not seem to affect NiTi rotary instruments.^231,266,406 In one study, only limited material loss occurred when NiTi LightSpeed instruments were immersed in 1% and 5% NaOCl for 30 to 60 minutes.⁷⁸ Corrosion of NiTi instruments used in the clinical setting, therefore, might not significantly contribute to fracture except when the instruments are immersed in warmed NaOCl for longer than 60 minutes. Although sterilization procedures per se do not have an impact on NiTi integrity,^{184,266,326,377,442,489} there is an ongoing discussion over the impact of other aspects of clinical usage on the mechanical properties of NiTi rotaries. Most likely, clinical usage leads to some changes in the alloy, potentially work-hardening,¹⁵ depending on the amount of torsional load the instrument was subjected to.

Over the last 4 years, several manufacturers have begun to utilize electropolishing, a process that removes surface irregularities such as flash and burr marks. It is believed to improve material properties, specifically fatigue and corrosion resistance; however, the evidence for both these claims is mixed. One study²⁴ found an extension of fatigue life for electropolished instruments, others found no improvement of fatigue resistance of electropolished instruments.^76,94,182 Still other researchers⁵⁹ suggested a change in cutting behavior with an increase of torsional load after electropolishing. Corrosion resistance of electropolished NiTi rotaries is also controversial. One study⁶² found superior corrosion resistance for electropolished RaCe instruments, whereas another study³⁰⁹ found similar corrosion susceptibility for RaCe and non-electropolished ProFile instruments. A recent review³⁷⁴ points out difficulties in assessing NiTi properties by conventional corrosion tests, since they do not take deformation and consequent surface deformation into account.

Fracture Mechanisms

In general, instruments used in rotary motion break in two distinct modes, torsional and flexural.^302,345,433 Torsional fracture occurs when an instrument tip is locked in a canal while the shank continues to rotate, thereby exerting enough torque to fracture the tip. This also may occur when instrument rotation is sufficiently slowed in relation to the cross-sectional diameter. In contrast, flexural fracture occurs when the cyclic loading leads to metal fatigue. This problem precludes the manufacture of continuously rotating stainless steel endodontic instruments, because steel develops fatal fatigue after only a few cycles.³⁷⁰ NiTi instruments can withstand several hundred flexural cycles before they fracture,^{173,227,321,433,475} but they still can fracture in the endodontic setting after a low (i.e., below 10,000) number of cycles.⁹²

Repeated loading and cyclic fatigue tests for endodontic instruments are not described in pertinent norms. Initially, rotary instruments such as Gates-Glidden burs and Peeso reamers were tested with a superimposed bending deflection.⁶⁶ In GG burs, a 2-mm deflection of the instrument tip resulted in fatigue lifespans ranging from 21,000 revolutions (size #1 burs) to 400 revolutions (size #6 burs).⁶⁶ In another study, stainless steel and NiTi hand files were rotated to failure in steel tubes with an acute 90-degree bend and an unspecified radius.³⁷⁰ Under these conditions, size #40 stainless steel instruments fractured after fewer than 20 rotations, whereas various NiTi files of the same size withstood up to 450 rotations.

Cyclic fatigue was also evaluated for ProFile size #.06 instruments using a similar device.^474,475 The number of rotations to failure for unused control instruments ranged from 1260 (size #15 files) to 900 (size #40 files). These scores did not change when the instruments were tested under simulated clinical conditions such as repeated sterilization and contact with 2.5% NaOCl. Subsequently, control instruments were compared with a group of instruments used in the clinical setting in five molar cases⁴⁷⁴; again, no significant differences were found in resistance to cyclic fatigue.

One study¹⁷³ used a different testing method involving tempered metal cylinders with radii of 5 mm and 10 mm that produced a 90-degree curve. They reported fatigue fractures for size #15, #.04 taper ProFile instruments after about 2800 cycles with the 10 mm cylinders. In size #40, #.04 taper ProFile instruments, fractures occurred after about 500 cycles with the 5-mm cylinders. In comparison, size #15, #.06 taper ProFile instruments also failed after about 2800 revolutions with the 10-mm cylinders, but failure occurred in size #40, #.06 taper ProFile specimens after only 223 cycles with the 5-mm cylinders.

Rotary NiTi instruments with larger tapers and sizes consistently fractured after fewer rotations,³¹³ and although the radius of the curves was halved, fatigue-life was reduced by 400%. Another investigation¹⁷³ reported similar results for selected HERO instruments, and their findings were confirmed by other tests on GT rotary instruments. Size #20, #.06 taper GT files failed after 530 rotations in a 90-degree curve with a 5-mm radius; size #20, #.12 taper GT files failed after 56 rotations under the same conditions.³⁰⁶

Reuse of rotary instruments depends on safety, specifically on assessment of fatigue and also the potential to properly clean NiTi surfaces.^{34,61,284,289,376,394,396,427} Specific instruments perform differently in this regard, since fatigue depends more on the amount of metal in cross section at the point of stress concentration^159,425 than on the specifics of instrument design.⁹³

On the other hand, manufacturers claim that their instrument has been equipped with design elements that render it more fatigue resistant. For example, LightSpeed LSX is manufactured without a milling process. However, no data have been published regarding its fatigue resistance. GTX is manufactured from a novel NiTi alloy, M-Wire, to increase its fatigue resistance.²⁰³ However, investigators²¹⁶ could not confirm these findings. Similarly, another study²²³ did not find the Twisted File, which is not milled and hence believed to be fatigue resistant,¹⁴⁶ to perform better than conventionally manufactured ProFile rotaries. Another feature, electropolishing (see earlier) does not appear to confer a significantly increased fatigue resistance to EndoSequence^223,328 and RaCe.^425,427,471 One possible reason for these variable outcomes are the different testing environments used in vitro⁹⁵; clinically, even greater variability is to be expected.

Attempts have been made to use tests according to norms and specifications described for stainless steel hand instruments such as K-files and Hedström files,¹¹⁷ since no comparable norms exist for instruments used in continuous rotary motion. Consequently, a number of models have been devised to assess specific properties of NiTi rotary instruments, including torque at failure, resistance against cyclic fatigue, and others (Fig. 9-34). These systems can simultaneously assess torque at failure, working torque axial force, and cyclic fatigue (Fig. 9-35).

FIG. 9-34 Testing platform for analysis of various factors during simulated canal preparation with rotary endodontic instruments. Labeled components are a force transducer (A), a torque sensor (B), a direct-drive motor (C), and an automated feed device (D). For specific tests, a cyclic fatigue phantom or a brass mount compliant with ISO No. 3630-1 (inserts) may be attached.

FIG. 9-35 Physical factors (torque, axial force, and insertion depth) that affect root canal instrumentation documented with a torque-testing platform. A, ProFile size #45, #.04 taper used in a mildly curved canal of a single-rooted tooth, step-back after apical preparation to size #40. B, FlexMaster size #35, #.06 taper used in a curved distobuccal canal of a maxillary first molar, crown-down during the initial phase of canal preparation.

According to the norms mentioned previously, torque at failure is recorded with the apical 3 mm of the instrument firmly held in the testing device while the instrument’s handle is rotated. A wide variety of rotary NiTi endodontic instruments have been tested in this way. For example, ProFile NiTi rotary files in ISO sizes #25, #30, and #35 (#.04 taper) fractured at 0.78, 1.06, and 1.47 Ncm, respectively.⁴⁰⁶

Investigators⁴⁰⁷ reported similar scores when instruments were forced to fracture in plastic blocks with simulated curved canals. In a different setup, GT rotary instruments (size #20, #.06 taper to size #20, #.12 taper) fractured at 0.51 and 1.2 Ncm, respectively.³⁰⁶ These values are somewhat lower than recent data obtained from the same but slightly modified torque bench,²⁰⁹ pointing towards the importance of experimental conditions for torque and fatigue measurements.

Compared with NiTi instruments with tapered flutes, LightSpeed instruments had lower torques to fracture (0.23 to 2 Ncm²⁵⁴). No such data are currently available for Lightspeed LSX.

When analyzing clinical factors involved in instrument fracture, one must consider both torsional load and cyclic fatigue³⁴⁵ (Fig. 9-36). However, these are not separate entities, especially in curved canals.⁶³ Working an instrument with high torque may lower resistance to cyclic fatigue.¹⁴² Conversely, cyclic prestressing has been shown to reduce the torsional resistance of ProTaper finishing files,⁴³³ as well as K3³⁰ and MTwo³¹³ (DENTSPLY VDW, Munich, Germany). Also, cyclic fatigue occurs not only in the lateral aspect when an instrument rotates in a curved canal but also axially when an instrument is bound and released by canal irregularities.⁵⁴

FIG. 9-36 Scanning electron micrographs of deformed or separated nickel-titanium rotary instruments. A, Lateral view of a ProTaper F3 instrument after application of torsional load (×25). B, Lateral view of a size #35, #.04 taper FlexMaster instrument after more than 500 rotations in a 90-degree curve with a 5-mm radius (see Figure 9-31) (×30). C, Cross section of the ProTaper instrument in A. Note signs of ductile fracture near center of the instrument core (×140). D, Cross section of FlexMaster instrument in B (×100).

The torque generated during canal preparation depends on a variety of factors, and an important one is the contact area.⁵⁷ The size of the surface area contacted by an endodontic instrument is influenced by the instrumentation sequence or by the use of instruments with different tapers.³⁶⁴ A crown-down approach is recommended to reduce torsional loads (and thus the risk of fracture) by preventing a large portion of the tapered rotating instrument from engaging root dentin (known as taper lock).^57,476

The clinician can further modify torque by varying axial pressure, because these two factors are related³⁶⁴ (see Fig. 9-35). In fact, a light touch is recommended for all current NiTi instruments to avoid forcing the instrument into taper lock. The same effect might occur in certain anatomic situations, such as when canals merge, dilacerate, and divide.

The torsional behavior of NiTi rotary endodontic instruments cannot be described properly without advanced measurement systems and a new set of norms. However, the clinician must be able to interpret correctly the stress-strain curves for all rotary NiTi instruments used in the clinical setting to be able to choose an appropriate working torque and axial force.

Sodium Hypochlorite

NaOCl encompasses many desirable properties of a main root canal irrigant and has therefore been described as the most ideal of all available rinsing agents.^268,481 NaOCl has been in use for almost a century.^111,446 A 0.5% solution of NaOCl was used effectively during World War I to clean contaminated wounds.¹¹¹ In the endodontic field, NaOCl possesses a broad-spectrum antimicrobial activity against endodontic microorganisms and biofilms (Table 9-1), including microbiota difficult to eradicate from root canals, such as Enterococcus, Actinomyces, and Candida organisms.^{175,320,338,481}

TABLE 9-1 Efficacy of Various Irrigants in Deactivating Microorganisms*

NaOCl dissolves organic material such as pulp tissue and collagen. If the organic portion of a smear layer is dissolved in NaOCl, and bacteria inside the main root canal, lateral canals, and dentinal tubules—if in direct contact with the irrigant—are destroyed, then to a minor degree, endotoxins may be eliminated.^256,444

During endodontic therapy, NaOCl solutions are used at concentrations ranging from 0.5% to 6%. In infected dentin blocks, a 0.25% solution of NaOCl was sufficient to kill Enterococcus faecalis in 15 minutes; a concentration of 1% NaOCl required 1 hour to kill Candida albicans.³⁶⁸ In infected extracted teeth, Ruff et al.³³⁸ found that a 1-minute application of 6% NaOCl and 2% chlorhexidine were equally effective in eliminating microorganisms and statistically significantly superior to MTAD and 17% EDTA in eliminating Candida albicans infections.

Lower concentrations (e.g., 0.5% or 1%) dissolve mainly necrotic tissue.⁴⁸² Higher concentrations allow better tissue dissolution but dissolve both necrotic and vital tissue, which is not always a desirable effect. In some cases, full-strength NaOCl (6%) may be indicated, but although higher concentrations may increase antibacterial effects in vitro,⁴⁷⁷ enhanced clinical effectiveness has not been demonstrated conclusively for concentrations stronger than 1%.³⁹⁹

Commercially available household bleach (Clorox, The Clorox Company, Oakland, CA) contains 6.15% NaOCl, has an alkaline pH of 11.4, and is hypertonic.^399,482 Some authors recommend dilution of commercially available NaOCl with 1% bicarbonate instead of water to adjust the pH to a lower level.^111,399 Others do not see any reduction of the aggressiveness on fresh tissue by buffering NaOCl and recommend diluting solutions of NaOCl with water to obtain less concentrated irrigation solutions.^383,482

NaOCl only minimally removes dentin debris or smear layers (Fig. 9-41). Therefore, some authors recommend concurrent use of demineralizing agents to rid the root canal surface of a postinstrumentation smear layer and thus enhance cleaning of difficult-to-reach areas, such as dentinal tubules and lateral canals.^81,283 When using NaOCl over extended periods of time during treatment, it should be mentioned that NaOCl seems to have an undesired side effect on the flexural strength of dentin. One study²⁵² investigated the influence of irrigants on flexural strength of dentin bars and concluded that a 24-minute exposure time to a 2.5% hypochlorite solution caused a significant drop in flexural strength, while the modulus of elasticity was not altered during this time. Other authors discovered a lowering of both flexural and elastic strength after 2-hour submersion of dentin bars in NaOCl.^11,164 The loss of calcium ions appears to be both dependent on NaOCl concentration (5% showing the greatest amount of decalcification) and time of exposure.³⁵¹

FIG. 9-41 Surface textures of an unprepared root canal at various levels. A, Ground section of a mandibular premolar. Areas viewed by scanning electron microscopy (SEM) are indicated by black lines. B, Canal surface at middle section, showing open dentinal tubules and typical calcospherites (×500). C-E, Coronal, middle, and apical areas as compound scanning electron micrographs. Note the numerous open tubules in C-D, whereas fewer tubules are visible in E (×200).

Chlorhexidine

Chlorhexidine (CHX) is a broad-spectrum antimicrobial agent effective against gram-negative and gram-positive bacteria (see Table 9-1). It has a cationic molecular component that attaches to negatively charged cell membrane areas, causing cell lysis.^26,156 CHX as a mouth rinse and periodontal irrigant has been used in periodontal therapy, implantology, and cariology for many years to control dental plaque.^130,232 Its use as an endodontic irrigant^{40,96,131,181,454} is based on its substantivity and long-lasting antimicrobial effect, which arises from binding to hydroxyapatite. However, it has not been shown to have more superior clinical properties than NaOCl. In fact, an in situ disinfection study suggests no additive effect on typical endodontic flora.²⁵⁹

When compared to CHX as an irrigant, it was found that hypochlorite significantly more often achieved negative cultures than CHX.³³³ When used as combination, NaOCl and CHX did not improve the antimicrobial activity of CHX against tested microorganisms.⁴⁴³

Some researchers found that CHX had significantly better antibacterial effects than Ca(OH)₂ when tested on cultures.²³⁰ Combinations of CHX and Ca(OH)₂ are available and show antimicrobial activity against obligate anaerobes, the combination in some studies augmenting the antibacterial effect of either medicament on certain species.^315,385 However, 2% CHX gel alone was more effective than its combination with Ca(OH)₂ against several tested microorganisms in other investigations.^155,248 Medicaments containing 2% CHX have the ability to diffuse through dentin and display antimicrobial action on the outer root surface. The addition of CHX or iodine potassium iodide to an intracanal dressing of Ca(OH)₂ in vitro did not affect the alkalinity (and hence the efficacy) of the Ca(OH)₂ suspensions.³⁸⁵ Chlorhexidine (2%) has been advocated as a final rinse irrigant owing to its substantivity, which allows binding to dentin and sustained antimicrobial activity, especially in endodontic retreatment.⁴⁸⁰

Iodine Potassium Iodide

Iodine potassium iodide (IKI) is a traditional root canal disinfectant and is used in concentrations ranging from 2% to 5%. IKI kills a wide spectrum of microorganisms found in root canals (see Table 9-1) but shows relatively low toxicity in experiments using tissue cultures.³⁹⁸ Iodine acts as an oxidizing agent by reacting with free sulfhydryl groups of bacterial enzymes, cleaving disulfide bonds. E. faecalis often is associated with therapy-resistant periapical infections (see Chapter 15), and combinations of IKI and CHX may be able to kill Ca(OH)₂-resistant bacteria more efficiently. One study³⁸⁵ evaluated the antibacterial activity of a combination of Ca(OH)₂ with IKI or CHX in infected bovine dentin blocks. Although Ca(OH)₂ alone was unable to destroy E. faecalis inside dentinal tubules, Ca(OH)₂ mixed with either IKI or CHX effectively disinfected dentin. Others³¹ demonstrated that IKI was able to eliminate E. faecalis from bovine root dentin when used with a 15-minute contact time. An obvious disadvantage of iodine is a possible allergic reaction in some patients.

MTAD

BioPure (DENTSPLY Tulsa Dental), also known as MTAD (mixture of tetracycline, acid, and detergent), is an irrigation solution that contains doxycycline, citric acid, and a surface-active detergent (Tween 80).⁴²³ Chemicals and their combinations as root canal irrigants are developed constantly, including solutions based on antibiotics. However, doxycycline and other locally applied antibiotics have been unable to destroy microbiota organized in biofilms. One research group²⁸² investigated the effect of five antibiotics on a mature biofilm after 8 days of growth on dentin; in their experiment, none of the topically applied substances could eradicate the biofilm. Use of these irrigants is also controversial because of the emergence of increasingly resistant strains of bacteria (e.g., therapy-resistant enterococci), which may be due to overprescription of antibiotics in general. The increased risk of host sensitization by local antibiotics can be circumvented to some degree by using the antibiotic as a dressing. Because exposure to vital tissues is limited, higher microbicidal concentrations may be used.²⁶⁹ A number of antibiotics, including erythromycin, chloramphenicol, tetracycline, and vancomycin, have been tested successfully against enterococci. In one study, investigators evaluated microbial susceptibility to different antibiotics in vitro and found that enterococcal isolates were resistant to benzylpenicillin, ampicillin, clindamycin, metronidazole, and tetracycline but sensitive to erythromycin and vancomycin.¹¹⁰ Another investigation²⁰² used extracted teeth infected with E. faecalis and compared the efficacy of NaOCl and EDTA versus NaOCl and MTAD. They concluded that although the NaOCl/EDTA combination consistently disinfected the test specimens, almost half of the teeth rinsed with NaOCl/BioPure MTAD remained contaminated.²⁰² In a recent study,³⁷¹ doxycycline was replaced with CHX in one of the test groups of E. faecalis–infected teeth. Whereas no specimens that were cleaned with MTAD or MTAD + CHX showed the presence of residual bacteria, 70% of the samples rinsed with CHX as a substitute for doxycycline demonstrated growth of E. faecalis.

The citric acid component in MTAD effectively removed a smear layer.^44,372,421 Under these conditions, BioPure MTAD was more aggressive in eroding dentin than EDTA. In another investigation,⁴¹⁰ however, the addition of NaOCl was necessary to achieve dissolution of organic matter.

Ethylenediamine Tetra-Acetic Acid

EDTA came into use in endodontics in 1957²⁸³; chelators such as EDTA create a stable calcium complex with dentin mud, smear layers, or calcific deposits along the canal walls. This may help prevent apical blockage (Fig. 9-42) and aid disinfection by improving access of solutions through removal of the smear layer.

FIG. 9-42 Presence of dentin dust as a possible source of microbial irritation. Tooth #18 underwent root canal therapy. The clinician noted an apical blockage but was unable to bypass it. Unfortunately, intense pain persisted and at the patient’s request, the tooth was extracted a week later. A, Mesial root of tooth #18; mesial dentin has been removed. B, Magnified view (×125) of rectangle in A shows an apical block (gradation of ruler is 0.5 mm).

Neutral EDTA (17% concentration) showed a higher degree of decalcification of dentin surfaces than RC-Prep (a gel-type preparation of EDTA), although its effect was reduced in apical regions.⁴⁴¹ Similar to MTAD, RC-Prep did not erode the surface dentin layer.⁴²¹

The effect of chelators in negotiating narrow, tortuous, calcified canals to establish patency depends on both canal width and the amount of active substance available, since the demineralization process continues until all chelators have formed complexes with calcium.^191,485 Calcium binding results in the release of protons, and EDTA loses its efficiency in an acidic environment. Thus the action of EDTA is thought to be self-limiting.³⁶⁷ In one study, demineralization up to a depth of 50 µm into dentin was demonstrated for EDTA solutions¹⁹¹; however, other reports demonstrated significant erosion after irrigation with EDTA.⁴²¹

A comparison of bacterial growth inhibition showed that the antibacterial effects of EDTA were stronger than citric acid and 0.5% NaOCl but weaker than 2.5% NaOCl and 0.2% CHX.³⁷⁹ EDTA had a significantly better antimicrobial effect than saline solution. It exerts it strongest effect when used synergistically with NaOCl, although no disinfecting effect on colonized dentin could be demonstrated.¹⁸¹

Recent reports have indicated that several disinfecting agents such as Ca(OH)₂, IKI, and CHX are inhibited in the presence of dentin.^169,317,318 Moreover, chemical analyses indicated that chlorine, the active agent in NaOCl, is inactivated by EDTA.^161,483 In light of these facts, in addition to the unproven effect of lubricants containing EDTA on rotary-instrument torque, use of these solutions probably should be limited to hand instrumentation early in a procedure. Moreover, an EDTA solution preferably is used at the end of a procedure to remove the smear layer but does not prevent future bacterial penetration between root canal fillings and canal walls.^343,470,483 Clean, smear layer–free canal walls, tubulus openings, and entrances into lateral canals and isthmus areas, taken together with sufficient volume of NaOCl, ensures high disinfecting efficacy by enabling NaOCl to penetrate even into deeper dentin layers (Fig. 9-43).

FIG. 9-43 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.

Calcium Hydroxide

Ca(OH)₂ via its alkaline pH is generally very effective at eradicating intraradicular bacteria, with the exception of E. faecalis.¹³³ Increased effectiveness was observed when Ca(OH)₂ was mixed with some common irrigating solutions. Although additive effects could not be confirmed, and a reduction in the antimicrobial action of CHX was detected,¹⁷⁶ it appears that Ca(OH)₂ mixed with IKI or CHX may be able to kill Ca(OH)₂-resistant bacteria³⁸⁵ (Box 9-4).

Unfortunately, it is not as effective when used short term and is not recommended as an irrigant but rather as an interappointment dressing.³⁸⁸ To achieve optimal antimicrobial activity, therefore, it requires prolonged exposure³⁶ or higher temperatures for use as an endodontic irrigant.¹³¹

Other Irrigants

Electrochemically activated water (also known as oxidative potential water) recently was tested as a potential irrigant.^166,250,391 Although this solution is active against bacteria¹⁶⁶ and removes the smear layer,²⁴⁹ no assessment of its clinical potential is available, and in vitro research indicates that NaOCl is a superior disinfectant.¹⁶⁶

Hydrogen peroxide traditionally has been used as an irrigant in conjunction with NaOCl; however, no additional benefit beyond NaOCl was registered.¹⁸¹

Some authors have advocated the use of 0.2% or 0.5% CHX in addition to NaOCl,^169,181 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 medicaments^169,318 and can optimize their antimicrobial properties against certain resistant bacteria and yeasts.^449,451

Recently, one group²³³ compared NaOCl with a final 17% EDTA rinse with an equal mixture of 2% NaOCl and 18% etidronic acid during and after instrumentation and a protocol involving 1% NaOCl during preparation and 2.25% peracetic acid after instrumentation. The results indicated that both acids had a similar effect on a smear layer as EDTA but with less demineralization of intratubular dentin. Strong demineralization has been shown to have a negative influence on canal sealability.¹¹⁵

Lubricants

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 wax-based RC-Prep, which contains EDTA and urea peroxide, and 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% NaOCl solutions increased cutting efficiency compared with dry conditions.⁴⁷⁸ The authors of this study cited the ability of a lubricant to remove debris as the factor for the increased efficiency. Similarly, a reduction of torque scores was found when canals in normed dentin disks were prepared with ProFile and ProTaper instruments under irrigation, but the use of a gel-type lubricant resulted in similar torques as in dry, nonlubricated canals.^60,304

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 for maximum cleaning efficiency. This strategy also minimizes inactivation of NaOCl by chemical interactions.^161,481

Standardized Technique

The standardized technique adopts the same working length definition for all instruments introduced into a root canal and therefore relies on the inherent shape of the instruments to impart the final shape to the canal. Negotiation of fine canals is initiated with lubricated fine files in a so-called watch-winding movement. These files are advanced to working length and worked either in the same hand movement or with “quarter-turn-and-pull” until a next larger instrument may be used. Conceptually, the final shape should be predicted by the last-used instrument. A single matching gutta-percha point may then be used for root canal filling. In reality, this concept is often violated: curved canals shaped with the standardized technique will be wider than the last used instrument,¹⁹ exacerbated by the pulling portion of the hand movement. Moreover, adequate compaction of gutta percha in such small a taper (~.02) is difficult or impossible¹⁸ (see Chapter 11).

Step-Back Technique

Realizing the importance of a shape larger than that produced with the standardized approach, one investigator⁴⁵⁷ suggested the step-back technique, incorporating a stepwise reduction of the working length for larger files, typically in 1-mm or 0.5-mm steps, resulting in flared shapes with 0.05 and 0.10 taper, respectively. Incrementally reducing the working length when using larger and stiffer instruments also reduced the incidence of preparation errors, in particular in curved canals. This concept appeared to be clinically very effective.²⁷³

Although the step-back technique was primarily designed to avoid preparation errors in curved canals, it applies to the preparation of apparently^103,356 straight canals as well. Several modifications of the step-back technique have been described over the years. Another investigator⁴²⁰ advocated the insertion of progressively larger hand instruments as deep as they would passively go in order to explore and provide some enlargement prior to reaching the working length.

Step-Down Technique

Other investigators¹⁵⁰ described a different approach. They advocated shaping the coronal aspect of a root canal first before apical instrumentation commenced. This technique is intended to minimize or eliminate the amount of necrotic debris that could be extruded through the apical foramen during instrumentation¹³⁴; moreover, by first flaring the coronal two-thirds of the canal, apical instruments are unimpeded through most of their length. This in turn may facilitate greater control and less chance of zipping near the apical constriction.²²⁵

Crown-Down Technique

Numerous modifications of the original step-down technique have been introduced, including the description of the crown-down technique.^136,253,348 The more typical step-down technique includes the use of a stainless steel K-file exploring the apical constriction and establishing working length. In contrast, a crown-down technique relies more on coronal flaring and then determination of the working length later in the procedure.

To ensure penetration during step-down, one may have to enlarge the coronal third of the canal with progressively smaller GG drills or with other rotary instruments. Irrigation should follow the use of each instrument and recapitulation after every other instrument. To properly enlarge the apical third and to round out ovoid shape and lateral canal orifices, a reverse order of instruments may be used starting with a size #20 (for example) and enlarging this region to a size #40 or #50 (for example). The tapered shape can be improved by stepping back up the canal with larger instruments, bearing in mind all the time the importance of irrigation and recapitulation.

The more typical crown-down, or double-flare technique¹³⁶ consisted of an exploratory action with a small file, a crown-down portion with K-files of descending sizes, and an apical enlargement to size #40 or similar. The original technique included stepping back in 1-mm increments with ascending file sizes and frequent recapitulations with a small K-file and copious irrigation. It is further emphasized that significant wall contact should be avoided in the crown-down phase to reduce hydrostatic pressure and the possibility of blockage. Several studies^347,348 demonstrated more centered preparations in teeth with curved root canals shaped with a modified double-flare technique and Flex-R files compared to shapes prepared with K-files and step-back technique. A double-flare technique was also suggested for ProFile rotary instruments.³⁶³

Balanced Force Technique

Regarding hand movements, a general agreement exists that the so-called balanced force technique creates the least canal aberrations with K-files. Researchers described this technique as a series of rotational movements for Flex-R files, but it can also be used for K-files and other hand instruments such as GT hand files. Many different explanations have been offered for the obvious and undisputed efficacy of the balanced force approach,^91,221,336 but general agreement exists that it provides excellent canal-centering ability, superior to other techniques with hand instruments.^29,69,226

The balanced force technique involves three or four steps.³³⁴ The first step (after passive insertion of an instrument into the canal) is a passive clockwise rotation of about 90 degrees to engage dentin (Fig. 9-55). In the second step, the instrument is held in the canal with adequate axial force and rotated counterclockwise to break loose the engaged dentin chips from the canal wall; this produces a characteristic clicking sound. Classically, in the third step, the file is removed with a clockwise rotation to be cleaned; however, because files used with the balanced force technique are not precurved, every linear outward stroke essentially is a filing stroke and may lead to some straightening of the canal path. Therefore, in many cases, the clinician may advance farther apically rather than withdrawing the file, depending on the grade of difficulty.