A radically different way of instrumenting root canals was introduced when clinicians became able to activate files by electromagnetic ultrasonic energy.227 Piezoelectrical ultrasonic units are also available for this purpose. These units activate an oscillating sinusoidal wave in the file with a frequency of about 30 kHz.
Figure 8-50 ENAC piezoelectrical ultrasonic device. The handpiece has adapters for periodontal scaling and endodontic work. Power and water flow can be adjusted. (Courtesy Osada USA, Los Angeles.)
Two types of units, ultrasonic and sonic, are primarily available. Ultrasonic devices, which operate at 25 to 30 kHz, include the magnetostrictive Cavi-Endo (Caulk/Dentsply, Milford, DE), the piezoelectrical ENAC (Osada, Tokyo), and the EMS Piezon Master 400 (Electro Medical Systems [EMS] Vallée de Joux, Switzerland). Sonic devices, which operate at 2 to 3 kHz, include the Sonic Air MM 1500 (Micro Mega, Prodonta, Geneva, Switzerland, the Megasonic 1400 (Megasonic corp, House Springs, MO), and the Endostar (Syntex Dental Products, Valley Forge, PA) (Figs. 8-49 and 8-50). Ultrasonic devices use regular types of instruments (e.g., K-files), whereas sonic devices use special instruments known as Rispi-Sonic, Shaper-Sonic, Trio-Sonic, or Heli-Sonic files.
Although similar in function, piezoelectrical units have some advantages over the magnetostrictive systems. For example, piezoelectrical devices generate little heat; therefore, no cooling is needed for the electrical handpiece. The magnetostrictive system generates considerable heat, and a special cooling system is needed in addition to the irrigation system for the root canal. The piezoelectrical transducer transfers more energy to the file than does the magnetostrictive system, making it more powerful.11
The file in an ultrasonic device vibrates in a sinus wave-like fashion. A standing wave has areas with maximal displacement (i.e., antinodes) and areas with no displacement (i.e., nodes). The tip of the instrument exhibits an antinode. If powered too high, the instrument may break because of the intense vibration. Therefore files must be used only for a short time, and the power must be sent carefully. The frequency of breakage in files used for longer than 10 minutes may be as high as 10%, and the breakage normally occurs at the nodes of vibrations.4
Ultrasonic devices have proved very efficient for irrigating root canal systems. During free ultrasonic vibration in a fluid, two significant physical effects are observed: cavitation and acoustic streaming. During oscillation in a fluid, a positive pressure is followed by a negative pressure. If the fluid's tensile strength is exceeded during this oscillation of pressure gradients, a cavity is formed in the fluid in the negative phase. During the next positive pressure phase, the cavity implodes with great force; this is cavitation. Under normal clinical conditions the power of dental ultrasonic units is too low to create significant cavitation effects on the dentin walls.8,9Acoustic streaming creates small, intense, circular fluid movement (i.e., eddy flow) around the instruments. The eddying occurs closer to the tip than in the coronal end of the file, with an apically directed flow at the tip. Acoustic streaming increases the cleaning effect of the irrigant in the pulp space through hydrodynamic shear stress. The increased amplitude that occurs with the smaller file sizes enhances the acoustic streaming.7 This has proved valuable in the cleaning of root canals because conventional irrigation solutions do not penetrate small spaces well.236,259
Acoustic streaming has little direct antimicrobial effect.5,10 Both cavitation and acoustic streaming are dependent on the free vibration of the file. The limits of the space in a root canal significantly inhibit the practical utility of ultrasonic devices for root canal cleaning. Depending on size and power the file tip may have amplitude of 20 to 140 mm, requiring a canal size of at least a #30 file through a #40 file for free oscillation. Any contact with the root canal walls dampens oscillation. As the contact with the canal wall increases, the oscillation is dampened and becomes too weak to maintain acoustic streaming. Using a small file size with minimal contact to the root canal wall provides optimal cleaning conditions.12
Ultrasonic devices have proved disappointing as instruments for improving the removal of dentin from the root canal walls.193,216 They do improve the ability to clean the pulp space and difficult-to-debride areas through acoustic streaming.17,61,62,317 However, whether this can be achieved during regular preparation when the file is actively dampened and little acoustic streaming takes place is unclear.289,311,312 Cleaning is further enhanced by the excellent irrigation systems some of the devices provide. Application of a freely oscillating file with sodium hypochlorite (NaOCl) irrigation for a couple of minutes to aid pulp space disinfection is believed to be useful after complete biomechanical instrumentation of the pulp space.268
Sonic devices are more useful for true hard-tissue removal during root canal preparation.193,326 Because the files operate like a conventional handpiece, the file vibrations are less likely to be dampened by contact with the root canal walls. Therefore the special files used in these systems are true bulk dentin removers. The Rispi-Sonic file is less aggressive than the Shaper-Sonic file. The instruments come in 17 to 29 mm lengths and in various sizes from #010 up. Because of their rasplike design, these instruments tend to leave a rougher canal surface than many other devices.
The working length and the apical part of the root canal normally are prepared with conventional files, after which the sonic files are used. Both sonic and ultrasonic instruments are prone to causing canal transport if used carelessly.6,152,177 Various and often conflicting techniques for using these instruments have been described; therefore the clinician should take time to become acquainted with the instrument.
National and International Standards for Instruments
As a result of concerns that arose nearly 40 years ago, efforts were made to standardize endodontic files and root filling materials. As mentioned previously, this resulted in an international standard for endodontic files, known in the United States as ANSI standard no. 58 for Hedström files and ANSI standard no. 28 for K-files (Table 8-1). The standards have several similarities, but some important differences exist. Fig. 8-51 shows the important measurements dictated by the standards. The size designation is derived from the projected diameter at the tip of the instrument. This is an imaginary measurement and is not reflected in the real size of the working part of the instrument. The taper of the instruments is prescribed to be 0.02 taper of length, starting at the tip. Thus the working diameter is the product of taper and the length of the tip. Three standard lengths are available at 21 mm, 25 mm, and 31 mm. The working part of the instrument must be at least 16 mm. As stated previously, tapers other than #.02 and working parts of instruments less than 16 mm are now available and outside the standard.
This system of numbering files with at least 15 different sizes replaced the old, somewhat imperfect system that numbered the sizes from 0 through #6. Although the new standard includes many sizes, astute clinicians may include fewer instrument sizes for their special work habits.
Table 8-1. Dimensions of Standardized K-File, H-File, and Gutta-Percha Cones (ANSI No. 28, 58, and 78)*
No color assigned
No color assigned
*Sizes in italics are only for files that are commercially available but that are not covered by American National Standards Institute (ANSI) regulation no. 28 or no. 58. Colors are not required for instrument handles or gutta-percha cones; however, the size must be printed on the handle. Tolerances are ±0.02 ≥30 mm ±2 mm.
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Figure 8-51 Measuring points for American National Standards Institute (ANSI) and American Dental Association (ADA) regulations #28 and #58, which cover K-type and H-type instruments. The measuring point for the diameter of the instrument (size) is imaginary (D0) and projects the taper of the instrument at the tip. Therefore an instrument with a short tip is more true to its size than an instrument with a long tip. D16 represents the diameter at the end of the working part, which must be at least 16 mm long.
In recent years suggestions to change the numbering system for files with different sizes have been implemented by several manufacturers. One system has introduced "half" sizes in the range of #15 through #60, resulting in instruments in sizes #15, #17.5, #20, #22.5, and so on. Considering the fact that most manufacturers already are unable to size their instruments within the accepted range (Fig. 8-52),285 the introduction of half sizes seems unnecessary. However, if standards are strictly adhered to, the use of half sizes seems more reasonable for instrument systems such as the LightSpeed, in which the strength of the instrument is such that full-size increments may generate stresses beyond the tolerance of the instrument. The standards are overdue for reevaluation in light of recent technology changes.
Effectiveness and Wear of Instruments
Although the advertising literature is rich in claims of superiority of various file designs, few of these claims can be verified by well-designed studies in objective endodontic literature. No standards exist for either the cutting or machining effectiveness of endodontic files, nor have clear requirements been established for resistance to wear.
In any study of the effectiveness of an instrument, two factors must be investigated: (1) effectiveness in cutting or breaking loose dentin and (2) effectiveness in machining dentin. These two parameters are radically different. Methods exist for measuring machining, but currently no good method is available for measuring cutting. Some studies have attempted to evaluate cutting, but the methodologies have involved the use of a drilling motion with K-type instruments and at a speed higher than that used for clinical procedures.83,310
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Figure 8-52 Results of a study of the size of several versions of a standardized root canal instrument (#030). Measurements were made 3 mm from the tip; at this point, an instrument that meets the standards should measure 0.36 ± 0.02 mm. The mean is shown in black; shaded areas indicate maximal and minimal values. Few instruments conformed to the standard. For details see Stenman and Spångberg.2901, Antaeos; 2, Hygenic Corp.; 3, Miltex; 4, Dentsply Maillefer 5, J.S. Dental; 6, Union Broach; 7, Brasseler; 8, Antaeos; 9, Miltex, 10, Dentsply Maillefer 11, J.S. Dental; 12, Brasseler; 13, S-File; 14, K-Flex; 15, Flex-R.285
Some studies of machining have evaluated the effectiveness of an instrument when used with a linear movement.* These studies showed that instruments can differ significantly not only when comparing brands and types but also within one brand and type. For K-files, effectiveness varies 2 to 12 times between files of the same brand. The variation for Hedström files is greater, ranging from 2.5 to more than 50 times.192,292 The greater variation among Hedström files is easy to understand because the H-file is the result of more individual grinding during manufacture than the conventional K-file, which is difficult to alter much during the manufacturing process. For example, during the grinding of a Hedström file, the rake angle can be modified to neutral or even slightly positive; this is impossible to achieve with a K-file. The Hedström file, therefore, is approximately 10 times more effective at removing dentin than the K-file (Fig. 8-53).
In the machining process, the rake edge shaves off dentin that accumulates in the grooves between the rake edges. The deeper and larger this space, the longer the stroke can be before the instrument is riding on its own debris, making it ineffective. These design variations and the rake angle of the edges determine the effectiveness of a Hedström file. Of the hybrid files, the K-Flex file, which is a modified K-file, shows variables similar to those of K-files. The Flex-R file, which is a ground instrument, more closely resembles the H-files in its variation in effectiveness. It also is much more effective at substrate removal than the K-files but cannot measure up to the H-files' ability to machine.
Modern endodontic stainless steel instruments are fabricated from excellent metal alloys and have considerable resistance to fracture. The clinician who practices careful application of force and a strict program of discarding instruments after use should have few instrument fractures. Stainless steel files are so inexpensive that adequate cleaning and sterilization for reuse of files in sizes up to #60 may not be cost effective. Therefore files in the range up to #60 should be considered disposable instruments. Fig. 8-54 shows a file setup that provides efficient overview of the instruments. In a disposable file system, the sponge used to hold treatment instruments is disposed of along with the files.