In the past decade, rotary nickel titanium instrumentation, together with development and refinement of the surgical microscope and ultrasonic instrumentation, have helped improve the efficiency and accuracy of endodontic procedures. This literature review summarizes the present body of knowledge pertaining to nickel titanium endodontic instruments. Articles that examine rotary files to investigate other clinical problems were not reviewed. The focus here is on files that are commercially available in the United States.
The emphasis of this review will be on clinical application and will not seek to identify the best files. None of the articles compared all of the files that are available in a double-blind fashion, duplicating the manufacturer’s instructions. In addition, variations in the experimental method make interpretation and comparison of many of the studies very difficult. Some general trends and conclusions are, however, possible.
The superiority of rotary files over hand files in all aspects of canal preparation (ie, speed, canal transportation) cannot be conclusively proven given a survey of the literature. Rotary files have 3 common drawbacks: separation (because of cyclic fatigue and torque failure1); the potential for perforation if the file is advanced beyond the apical foramen; and canal transportation if used inappropriately. Separation of rotary files is a problem as evidenced by a 30% to 60% rate of separation and a 9.4% breakage rate2,3 (Figures 1a and 1b). This finding depends on the file used and its application.
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| Figures 1a and 1b. Separation of rotary nickel titanium files has proven challenging. Their removal is a microsurgical procedure. | |
This article reviews the literature regarding physical properties and considerations in clinical use of nickel titanium endodontic instruments.
CYCLIC FATIGUE
Cyclic fatigue is the result of the cumulative strain from bending an object repeatedly in the same location. For example, in moving through a curve, rotary instruments are subject to tensile forces (at the outside of the curvature) and compressive forces (on the inside of the curvature). These forces alternate on a file as it moves through and up and down a canal (Figure 2).
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| Figure 2. Cyclic fatigue. (Printed with permission of Dr. Chris Lampert.) |
The amount of cyclic fatigue a rotary file experiences is primarily dependent on the instrument size, the angle of canal curvature, and the radius of canal curvature.4 Of these, the radius of canal curvature is the dominant factor causing cyclic fatigue. A small radius of curvature corresponds to a more abrupt curvature, increasing the risk of cyclic fatigue.1 An increasing angle of curvature is correlated with a decreasing number of rotations to cause failure and results in a less favorable canal preparation.4,5 Even though 2 canals may have identical angles of curvature, they may also have a different radius of curvature (ie, one canal may have a more abrupt curvature). Smaller diameter, less tapered instruments are more resistant to cyclic fatigue failure.1 It has been noted that prolonged clinical use of rotary instruments significantly reduces resistance to cyclic fatigue.6 In addition, it has been reported that lower torque motors reduce failure due to cyclic fatigue.7
TORQUE FAILURE
Aside from cyclic fatigue, torque failure is the second significant cause of instrument separation. Such failure results from a torque load in rotation that exceeds the torque limit of the instrument. This can easily occur when an instrument is used with excessive pressure and binds at the tip during apical movement. Torsional strength is directly related to mass, and as such is correlated to the tip size and taper of an instrument.8 Not surprisingly, the cross-sectional diameter of a file determines its resistance to torque-induced separation. One study found that the number of cycles to failure is diminished with an increase in the size of the instrument.4 This was confirmed in a second study, which found that as the diameter of the file increased, the time to fracture decreased.1 Used instruments, as expected, have significantly lower values of torque at fracture than do new instruments.9 This data argues for single use of rotary files, with exceptions being brand-dependent and empirically derived.
To help prevent torque failure, torque-controlled motors have been introduced and will be addressed later in this paper.
ROTATIONAL SPEED
Studies are in agreement that lower rotational speeds (150 to 250 rpm versus 300 rpm and higher) will help avoid instrument separation and deformation.10-13 A report concluded that a speed of 150 rpm was associated with delayed breakage because of cyclic fatigue when compared to 350 rpm.14 Another study concluded that rotational speed was not a significant factor affecting cyclic fatigue failure.4 Confusing this matter, most manufacturers recommend rotational speeds in the range of 150 to 350 rpm.15 The importance of rotational speed in regard to instrument failure requires further study.
EFFECT OF HEAT STERILIZATION ON ROTARY NICKEL TITANIUM INSTRUMENTS
It has been shown that heat sterilization of rotary files up to 10 times does not increase the likelihood of instrument separation.16 In that study, files were sterilized in different autoclaves and tested for torsional strength and rotational flexibility. The files demonstrated a significant increase or no change in strength and flexibility after autoclaving. Furthermore, dry heat sterilization has also been found to increase rotary instrument resistance to cyclic fatigue failure.17 This finding was confirmed in another study18 although contradicted in a third study.19
Although the data noted above may suggest that the effects of heat sterilization might be beneficial, it has also been found that files subjected to more episodes of sterilization exhibit a decrease in cutting efficiency in comparison with the control group. Repeated sterilizations alter the superficial structure of the files.20
EFFECTS OF SODIUM HYPOCHLORITE ON INSTRUMENT SEPARATION
Sodium hypochlorite has no significant effect on rotary files, whether in reference to the torque at failure, maximum angular deflection, maximum bending, or permanent angular deflection. This is true regardless of the time of exposure to the solution.21 In addition, sterilization and clinical use in the presence of sodium hypochlorite has not been shown to lead to a reduction in the number of rotations to breakage.22
EFFECT OF TORQUE CONTROL MOTORS
Studies are contradictory concerning the effect of the type of motor (air-driven, high torque control, no torque control, or very low torque control) on failure of power rotary files.23-26 There is no strong evidence that any motor is superior to any other. It would seem wise to use the lowest possible torque with a given file to maximize cutting performance and minimize the chance of torque-induced failure.
CANAL TRANSPORTATION EXPERIENCED WITH ROTARY FILES
The literature is consistent regarding canal transportation that results from the use of rotary files. This transportation is toward the outer aspect of canal curvature.27-34 These findings underscore the importance of achieving and maintaining apical patency, determining true working length, and instrumenting in a coronal to apical (crown-down) sequence, with consideration of a master apical file of suitable size relative to the canal being prepared. Apical patency is important in that its maintenance can prevent a rotary file from being inserted against an apical blockage that might cause instrument separation or dentin removal (transportation) when not desired. Failure to consider any of the above steps will increase the possibility of canal transportation with these files.
EXPERIENCE AND TRAINING BEFORE CLINICAL ROTARY USE
The data supports that experience and preclinical training with rotary files are desirable to minimize instrument separation.35-37 One study found that the time required for canal preparation was inversely related to operator experience.38 However, in another study, inexperienced dental students were able to prepare curved root canals with rotary files in less time, with less transportation, and with greater conservation of tooth structure when compared to hand instruments.39 Certainly, as the familiarity of the clinician with the handling characteristics of rotary files increases, the chance for adverse sequelae decreases.
ROTARY FILES IN RE-TREATMENT
Rotary files can be used to remove gutta-percha during re-treatment.40-41 The use of 1,500 rpm was found to be associated with more rapid and effective removal of gutta-percha than either 350 or 700 rpm. At this higher rpm, there was also less instrument separation.42 It is important to note that when using rotary files at this speed, the file should have minimal if any engagement of the canal walls and should be placed completely into the gutta-percha. Rotating a nickel titanium file at this speed at a location below the middle third of any canal is to be avoided.
OTHER FINDINGS
1. Stainless steel H and K files as well as rotary files were measured to determine deviations from ISO standards. No file tested complied precisely with the ISO nominal size standard, although all files tested were within ISO tolerance limits. This data suggests that the possibility exists for overlapping between sequential file sizes. This variation can explain how, relative to the last instrument used, a file may not advance apically when it is expected to do so.43
2. In a study comparing various rotary files, it was found that all instrumentation techniques left 35% or more of the canal’s surface area untouched. This finding underscores the importance of irrigation of the root canal system since irrigants aid in the cleansing process and remove pulp tissue not contacted by files.44
3. A laboratory study evaluated the cutting tips of one brand of instruments before and after usage. It was found that all cutting tips had one or more imperfections, even before usage, which likely reflects the difficulty in grinding nickel titanium.45 The clinical importance of manufacturing defects present on rotary instruments, relative to cyclic fatigue and torque failure, deserves study.
References
1. Haikel Y, Serfaty R, Bateman G, et al. Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instruments. J Endod. 1999;25(6):434-440.
2. Szep S, Gerhardt T, Leitzbach C, et al. Preparation of severely curved simulated root canals using engine-driven rotary and conventional hand instruments. Clin Oral Investig. 2001;5(1):17-25.
3. Baumann MA, Roth A. Effect of experience on quality of canal preparation with rotary nickel-titanium files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1999;88(6):714-718.
4. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod. 1997;23(2):77-85.
5. Booth JR, Scheetz JP, Lemons JE, et al. A comparison of torque required to fracture three different nickel-titanium rotary instruments around curves of the same angle but of different radius when bound at the tip. J Endod. 2003;29(1):55-57.
6. Gambarini G. Cyclic fatigue of ProFile rotary instruments after prolonged clinical use. Int Endod J. 2001;34(5):386-389.
7. Gambarini G. Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and high-torque endodontic motors. J Endod. 2001;27(12):772-774.
8. Sattapan B, Palamara JE, Messer HH. Torque during canal instrumentation using rotary nickel-titanium files. J Endod. 2000;26(3):156-160.
9. Yared G, Kulkarni GK. An in vitro study of the torsional properties of new and used rotary nickel-titanium files in plastic blocks. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;96(4):466-471.
10. Dietz DB, Di Fiore PM, Bahcall JK, et al. Effect of rotational speed on the breakage of nickel-titanium rotary files. J Endod. 2000;26(2):68-71.
11. Gabel WP, Hoen M, Steiman HR, et al. Effect of rotational speed on nickel-titanium file distortion. J Endod. 1999;25(11):752-754.
12. Li UM, Lee BS, Shih CT, et al. Cyclic fatigue of endodontic nickel titanium rotary instruments: static and dynamic tests. J Endod. 2002;28(6):448-451.
13. Yared GM, Bou Dagher FE, Machtou P. Influence of rotational speed, torque and operator’s proficiency on ProFile failures. Int Endod J. 2001;34(1):47-53.
14. Martin B, Zelada G, Varela P, et al. Factors influencing the fracture of nickel-titanium rotary instruments. Int Endod J. 2003;36(4):262-266.
15. K3 Technique page. SybronEndo website. Available at:http://www.sybronendo.com/products/k3NiTiFiles/techniqueK3General.cfm. Accessed December 2003.
16. Silvaggio J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files. J Endod. 1997;23(12):731-734.
17. Chaves Craveiro de Melo M, Guiomar de Azevedo Bahia M, Lopes Buono VT. Fatigue resistance of engine-driven rotary nickel-titanium endodontic instruments. J Endod. 2002;28(11):765-769.
18. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after simulated clinical use. Int Endod J. 1999;32(2):115-119.
19. Mize SB, Clement DJ, Pruett JP, et al. Effect of sterilization on cyclic fatigue of rotary nickel-titanium endodontic instruments. J Endod. 1998;24(12):843-847.
20. Rapisarda E, Bonaccorso A, Tripi TR, et al. Effect of sterilization on the cutting efficiency of rotary nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1999;88(3):343-347.
21. Haikel Y, Serfaty R, Wilson P, et al. Mechanical properties of nickel-titanium endodontic instruments and the effect of sodium hypochlorite treatment. J Endod. 1998;24(11):731-735.
22. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after clinical use. Int Endod J. 2000;33(3):204-207.
23. Yared GM, Bou Dagher FE, Machtou P. Failure of ProFile instruments used with high and low torque motors. Int Endod J. 2001;34(6):471-475.
24. Bortnick KL, Steiman HR, Ruskin A. Comparison of nickel-titanium file distortion using electric and air-driven handpieces. J Endod. 2001;27(1):57-59.
25. Yared GM, Kulkarni GK. Failure of ProFile Ni-Ti instruments used by an inexperienced operator under access limitations. Int Endod J. 2002;35(6):536-541.
26. Suffridge CB, Hartwell GR, Walker TL. Cleaning efficiency of nickel-titanium GT and .04 rotary files when used in a torque-controlled rotary handpiece. J Endod. 2003;29(5):346-348.
27. Bryant ST, Thompson SA, al-Omari MA, et al. Shaping ability of ProFile rotary nickel-titanium instruments with ISO sized tips in simulated root canals: Part 2. Int Endod J. 1998;31(4):282-289.
28. Thompson SA, Dummer PM. Shaping ability of Lightspeed rotary nickel-titanium instruments in simulated root canals. Part 2. J Endod. 1997;23(12):742-747.
29. Bryant ST, Dummer PM, Pitoni C, et al. Shaping ability of .04 and .06 taper ProFile rotary nickel-titanium instruments in simulated root canals. Int Endod J. 1999;32(3):155-164.
30. Griffiths IT, Chassot AL, Nascimento MF, et al. Canal shapes produced sequentially during instrumentation with Quantec SC rotary nickel-titanium instruments: a study in simulated canals. Int Endod J. 2001;34(2):107-112.
31. Fabra-Campos H, Rodriguez-Vallejo J. Digitization, analysis and processing of dental images during root canal preparation with Quantec Series 2000 instruments. Int Endod J. 2001;34(1):29-39.
32. Thompson SA, Dummer PM. Shaping ability of Quantec Series 2000 rotary nickel-titanium instruments in simulated root canals: Part 2. Int Endod J. 1998;31(4):268-274.
33. Thompson SA, Dummer PM. Shaping ability of ProFile.04 Taper Series 29 rotary nickel-titanium instruments in simulated root canals. Part 2. Int Endod J. 1997;30(1):8-15.
34. Hata G, Uemura M, Kato AS, et al. A comparison of shaping ability using ProFile, GT file, and Flex-R endodontic instruments in simulated canals. J Endod. 2002;28(4):316-321.
35. Yared GM, Dagher FE, Machtou P, et al. Influence of rotational speed, torque and operator proficiency on failure of Greater Taper files. Int Endod J. 2002;35(1):7-12.
36. Yared G, Bou Dagher F, Kulkarni K. Influence of torque control motors and the operator’s proficiency on ProTaper failures. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;96(2):229-233.
37. Mandel E, Adib-Yazdi M, Benhamou LM, et al. Rotary Ni-Ti profile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. Int Endod J. 1999;32(6):436-443.
38. Gluskin AH, Brown DC, Buchanan LS. A reconstructed computerized tomographic comparison of Ni-Ti rotary GT files versus traditional instruments in canals shaped by novice operators. Int Endod J. 2001;34(6):476-484.
39. Mesgouez C, Rilliard F, Matossian L, et al. Influence of operator experience on canal preparation time when using the rotary Ni-Ti ProFile system in simulated curved canals. Int Endod J. 2003;36(3):161-165.
40. Sae-Lim V, Rajamanickam I, Lim BK, et al. Effectiveness of ProFile .04 taper rotary instruments in endodontic retreatment. J Endod. 2000;
26(2):100-104.
41. Barrieshi-Nusair KM. Gutta-percha retreatment: effectiveness of nickel-titanium rotary instruments versus stainless steel hand files. J Endod. 2002;28(6):454-456.
42. Bramante CM, Betti LV. Efficacy of Quantec rotary instruments for gutta-percha removal. Int Endod J. 2000;33(5):463-467.
43. Zinelis S, Magnissalis EA, Margelos J, et al. Clinical relevance of standardization of endodontic files dimensions according to the ISO 3630-1 specification. J Endod. 2002;28(5):367-370.
44. Peters OA, Schonenberger K, Laib A. Effects of four Ni-Ti preparation techniques on root canal geometry assessed by micro computed tomography. Int Endod J. 2001;34(3):221-230.
45. Eggert C, Peters O, Barbakow F. Wear of nickel-titanium lightspeed instruments evaluated by scanning electron microscopy. J Endod. 1999;25(7):494-497.
Dr. Mounce, an endodontist in Portland, Ore, has published articles in journals and has lectured worldwide on innovations in endodontics. He writes a weekly endodontics newsletter via e-mail, Mounce’s Apex, which is available free of charge. To receive the newsletter, write Dr. Mounce at lineker@aol.com and give your e-mail address.
