Figure 9-17 Schematic drawing of an ISO-normed hand instrument size #35. Instrument tip sizing, taper, and handle colors are regulated by the ISO/ANSI/ADA norm.
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Figure 9-18 Increase in tip diameter in absolute figures and in relation to the smaller file size. Note the particularly large increase from size #10 to size #15.
Hedström files are milled from round, stainless steel blanks. They are very efficient for translational strokes,237 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.269
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),101 usually work well for preenlargement of coronal canal areas.77,174 However, when misused, GG drills can dramatically reduce radicular wall thickness.100,132,161
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properties,40 GG drills are side-cutting instruments with safety tips; they can be used to cut dentin as they are withdrawn from the canal (i.e., on the outstroke).227 Used this way, their cutting action can deliberately be directed away from external root concavities in single-rooted and furcated teeth.3 GG instruments should be used only in the straight portions of the canal, and they should be used serially and passively.311
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Figure 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).
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Figure 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.
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 to 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, GG instruments may fracture when used in curved canal areas because of cyclic fatigue, 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 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 LightSpeed (LightSpeed Technologies, San Antonio, TX) and ProFile (Dentsply-Tulsa, Dentsply Maillefer), are still widely used. New designs continually are produced, but the extent to which, if any, clinical outcomes will depend on design characteristics is difficult to forecast.200
Most of the instruments described in this section are manufactured by a grinding process, although some are produced by laser etching. 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.11 Superficial defects such as metal flash and rollover are common in unused NiTi instruments.83,170,336
Attempts have been made to improve surface quality by electropolishing the surface and by coating it with titanium nitride.217,235 The latter process also seems to have a beneficial effect on cutting efficiency.235
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Figure 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).
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.146,199,250,281
Much of what is known about NiTi instruments, including reasons for instrument fracture18 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.198
NiTi rotary instruments have substantially reduced the incidence of several clinical problems, such as blocks, ledges, transportation, and perforation. However, they also have a tendency to fracture more easily than hand instruments. The clinical problems cited above do not by themselves predispose a case to posttreatment disease; rather, they limit the access of disinfecting irrigants to the root canal system, preventing sufficient elimination of microorganisms.115
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).
The LightSpeed file, developed by Dr. Steve Senia and Dr. William Wildey in the early 1990s, was introduced as an instrument different from all others because of its long, thin, noncutting shaft (Fig. 9-23) and 0.25 to 2 mm anterior cutting part. A full set consists of 25 instruments in sizes #20 to #100, including half sizes (e.g., 22.5, 27.5).
The recommended working speed for LightSpeed instruments is 1500 to 2000 rpm, and they should be used with minimal torque.249
The cross sections of the LightSpeed's cutting part show three round excavations, the U-shape design common to many earlier NiTi instruments (Figs. 9-23 and 9-24). Because of the relatively thin noncutting shaft, 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 round tip; tip length increases with instrument size to compensate for decreasing flexibility.
The LightSpeed's predecessor, the Canal Master-U, had the same general design but was used as a hand instrument. LightSpeed's manufacturer still recommends some hand use of its instruments, specifically for determining canal diameter. In general, the LightSpeed system requires a specific instrument sequence to produce a tapered shape that facilitates obturation with a gutta-percha cone or with LightSpeed's proprietary obturation system.
The LightSpeed is a widely researched NiTi rotary instrument, and most reports have found that the system has a low incidence of canal transportation and preparation errors.* Loss of working length was minimal in most of these studies.