Odontology DOI 10.1007/s10266-013-0138-x
ORIGINAL ARTICLE
Cyclic fatigue of instruments for endodontic glide path Gianluca Gambarini • Gianluca Plotino • GianPaolo Sannino • Nicola Maria Grande • Alessio Giansiracusa • Lucila Piasecki • Ulisses Xavier da Silva Neto • Dina Al-Sudani • Luca Testarelli
Received: 29 July 2013 / Accepted: 2 October 2013 Ó The Society of The Nippon Dental University 2013
Abstract Endodontic glide path is the creation of a smooth patency from canal orifice to apex, which can be performed manually or with small tapered NiTi rotary instruments. The use of stainless steel (SS) hand K-files inserted in a reciprocating handpiece can be a possible alternative to create a mechanical glide path. The aim of this study was to compare the cyclic fatigue resistance between SS K-files used in a reciprocating motion and NiTi rotary instruments in artificial curved canals. Ten SS size 15 K-files used with the M4 handpiece (SybronEndo, Glendora, CA, USA) and ten PathFiles (Maillefer-Dentsply, Ballaigues, CH, Switzerland) NiTi rotary instruments size 16, 0.02 taper were tested for resistance to cyclic fatigue. The time to fracture inside an artificial curved canal was recorded for each instrument. Data were analyzed by one-way ANOVA and Tukey HSD test. Mean time (and SD) to failure was 464 s (±40.4) for the Group PF (NiTi rotary PathFile), and 1049 s (±24.8) for the Group M4 (SS K-files reciprocating) with a statistically significant difference between the two groups (p = 0.033). The SS 15 K-files used with the M4 handpiece showed a significant greater resistance to cyclic fatigue when compared to the NiTi rotary PathFiles. Therefore, the use of small size SS files in a reciprocating motion might be a
G. Gambarini G. Plotino (&) G. Sannino N. M. Grande A. Giansiracusa L. Testarelli ‘‘Sapienza’’ - University of Rome, Via Caserta 6, 00187 Rome, Italy e-mail: [email protected] L. Piasecki U. X. da Silva Neto PUCPR, Curitiba, Brazil D. Al-Sudani King Saud University, Riyadh, Saudi Arabia
rational choice for the creation of a mechanical endodontic glide path in curved root canals. Keywords Continuous rotation Cyclic fatigue Glide path M4 safety handpiece Reciprocating motion
Introduction There are currently many controversial issues about endodontic glide path, whether it is really necessary or not, which amount of canal enlargement or preflaring should be achieved, and which could be the best technique (manual versus mechanical instrumentation) to perform it [1–4]. For many clinicians an endodontic glide path is the creation of a smooth patency from canal orifice to apex, in which a size manual size 15 K-file fits loose [5]. This procedure is usually performed before the intracanal use of any nickel-titanium rotary instrument of greater tapers (NTRIGT), aiming to improve efficacy and safety of shaping procedures. It has been shown that the creation of a glide path reduces the frictional forces applied to the NTRIGTs, because the canal diameter becomes at least equal to the file tip used for shaping [3]. Moreover, it allows the apical end of the NTRIGT to act as a passive pilot, reducing the torsional stress, binding, structural fatigue and failure rate [4, 6]. On the contrary, other clinicians believe than when you are able to negotiate a root canal with a manual, small (size 08 or 10) K-file, a natural glide path is already existing [7, 8]. Considering this, an innovative instrumentation technique based on reciprocation has been proposed, thus allowing a single NTRIGT to progress easily to full working length with minimal risks of iatrogenic errors [7].
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Fig. 1 The testing apparatus for cyclic fatigue
The basic concept here lies on improving of safety, because reciprocating motion reduces metal fatigue and torsional stress, which are key factors in determining intracanal fracture of NTRIGT when used in continuous rotation [7, 9–11]. Nevertheless, according to other authors, an endodontic glide path is still preferable before using reciprocating NTRIGT [12, 13]. For those who decide to perform an endodontic glide path, a second question arises: whether to choose manual stainless steel (SS) or small tapered (0.02) nickel–titanium (NiTi) rotary instruments, in ISO sizes ranging from 10 to 20 [1]. The manual glide path with SS instruments enables to understand the original anatomy of the root canals and to eliminate potential anatomical problems and interferences; however, it is time consuming and technically demanding [3, 4]. It has been shown that the creation of a glide path with 0.02 tapered NiTi rotary instruments is faster and presents better maintenance of the original canal anatomy, less modification of canal curvature, fewer canal aberrations and lower prevalence/severity of postoperative pain, compared with manual glide path performed with SS K-files [2, 14]. Moreover, rotary glide path is less affected by the clinician’s expertise; under experimental conditions, an inexpert clinician using NiTi rotary PathFiles (Dentsply Maillefer, Ballaigues, Switzerland) produced more conservative shaping than an expert endodontist with manual preflaring [2]. On the other hand, as NiTi rotary pathfinding instruments present reduced resistance to buckling, they might not be able to advance in the apical direction beyond constrictions and anatomic impediments, during exploration of narrow curved canals [15]. A possible third alternative could be the use of SS hand K-files inserted in reciprocating handpieces, which allow
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movements than can be considered the mechanical expression of the balanced force motion [16]. The M4 safety handpiece (SybronEndo, Glendora, CA, USA) was developed for such purpose. This handpiece features a 4:1 gear reduction, and oscillates 30° in both clockwise (CW) and counterclockwise (CCW) directions. According to the manufacturer, this watch-winding motion keeps the file loose inside the canal, reduces torsional stress and metal fatigue, and permits safe negotiation while the operator controls the apical pressure. While reciprocation with NiTi instruments has become very popular in recent years, with a significant number of research and published articles [7, 9, 17], only a few have been written in the last decades about reciprocation with SS instruments [18–21]. Moreover, none of these articles was specifically designed to evaluate the possible use of reciprocation and SS K-files in the creation of an endodontic glide path. Since recent studies have shown that reciprocation can improve fatigue resistance of NiTi instruments [9–11, 22, 23], it is interesting to evaluate if, and to which extent, also SS instruments can benefit from a reciprocating motion while creating an endodontic glide path. Therefore, the aim of this in vitro study was to compare cyclic fatigue resistance of NiTi rotary instruments (size 16, 0.02 taper), used in an endodontic motor with continuous rotation, with SS K-files (ISO size 15), used in a M4 reciprocating handpiece.
Materials and methods Ten new PathFiles NiTi Rotary instruments size 16, .02 taper (Maillefer-Dentsply, Ballaigues, CH, Switzerland) and ten new SS manual K-files ISO size 15 (MailleferDentsply, Ballaigues, CH, Switzerland) were selected for this study. Instruments were visually examined under a stereomicroscope to discard any defective instruments, before being tested for resistance to cyclic fatigue. The cyclic fatigue testing device, used in the present study, has been used in many published researches on cyclic fatigue resistance performed by the authors [24, 25]. The device consists of a mainframe to which a mobile plastic support for the electric handpiece is connected and a stainless steel block containing the artificial canals (Fig. 1). The electric handpiece was mounted on a mobile device to allow the precise and reproducible placement of each instrument inside the artificial canal. This placement ensured three-dimensional alignment and the positioning of the instruments to the same depth. The artificial canal was manufactured by reproducing an instrument’s size and taper, thus providing the instrument with a suitable trajectory that respects the parameters of the curvature
Odontology
Discussion
Fig. 2 Resistance to cyclic fatigue of PF (NiTi rotary Pathfiles) and M4 (SS size 15 K files) groups: mean values and SD for time to fracture
chosen. A simulated root canal with a 60° angle of curvature and 5-mm radius of curvature was constructed for each instrument type. The center of the curvature was 5 mm from the tip of the instrument, and the curved segment of the canal was approximately 5 mm in length. In Group PF, the NiTi instruments were rotated at 300 rpm inside the artificial canals using an endodontic motor (ASEPTICO, Woodinville, WA, USA) and an 8:1 reduction handpiece (ASEPTICO, Woodinville, WA, USA), until fracture occurred. In Group M4, SS manual instruments were reciprocated with a M4 handpiece (4:1 reduction, 30°CW/30°CCW), mounted on the same motor set on the M4 option and a speed of 1250 rpm (ASEPTICO, Woodinville, WA, USA). For each instrument, the time to fracture in seconds (s), was recorded by the same operator with a chronometer to an accuracy of 0.1 s. After positioning the instrument into the artificial canal, as soon as rotation or reciprocation started, timing was initiated. Timing stopped when instrument breakage was observed (Fig. 2). Mean values and standard deviation (SD) were then calculated for each group. Cyclic fatigue data were analyzed by one-way ANOVA and Tukey HSD test to determine any statistical difference between groups; the significance was determined at the 95 % confidence level.
Results Mean time (and SD) to failure was 464 s (±40.4) for the Group PF (NiTi rotary PathFile) and 1049 s (±24.8) for the Group M4 (reciprocating SS K-files). Statistical analysis showed a significant difference between the two groups (p = 0.033), with the Group M4 showing an increased time to fracture, which means greater resistance to cyclic fatigue [24].
The results of the present study showed that despite the more favorable mechanical properties of NiTi alloys, and the recent improvement in manufacturing of NiTi instruments for endodontic use [24, 26, 27], the reciprocating motion plays a significant role in determining resistance to metal fatigue of endodontic instruments [17, 22]. The SS K-files used with the M4 handpiece showed a significant greater resistance to cyclic fatigue compared to the 0.02 tapered NiTi instruments used with continuous rotation. The two tested instruments present similar design features, square cross-section and constant 0.02 taper, although the K-files presents size 15, and the PathFile instrument, size 16. This slight difference in the file dimension might not explain the significant difference in the instruments’ behavior under the experimental conditions of this study. In the endodontic literature, there is still controversy regarding the role of the cross-section and the file design in determining in vitro resistance to cyclic fatigue [17, 28, 29]. The findings of the present study might have a significant clinical impact. It is well known that SS alloy is more susceptible than NiTi to metal fatigue [26] with a much higher risk of intracanal separation if a SS file is rotated inside a curved canal [15]. This is the main reason why NiTi, and not SS, rotary instruments have become so popular in the last decades. However, the present results demonstrate that with a specific reciprocating motion, small size SS instruments could be an alternative method for the creation of a glide path, and might be safer than small tapered NiTi rotary instruments of the same dimensions. Therefore, the clinical use of the M4 handpiece in the creation of a mechanical endodontic glide path could be a valid choice, overcoming some critical disadvantages of the SS alloy in the small, narrow and curved canals. Stainless steel alloy is harder than NiTi, and permits a more efficient cutting action [30] and improved torsional resistance. Previous studies have shown that values of maximum torque to fracture are higher for SS when compared to NiTi [15, 31]. Improved cutting efficiency and higher torsional resistance are important properties for a glide path instrument, which is meant to negotiate and pre-enlarge mainly small, narrow and curved canals. One possible disadvantage of SS K-files could be the inherent rigidity of the alloy [15, 31] with a possible risk of straightening, aberrations and canal transportation [18, 20, 32, 33]. However, the flexibility of the endodontic instruments is mainly dependent on their dimensions and metal mass (cross-section) [34]; research data clearly demonstrated that small SS K-files are usually more flexible than
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NTRIGT [26]. Since only small SS K-files (up to ISO size 20) are used for the creation of a glide path, the rigidity of SS alloy might be a negligible disadvantage. Alves et al. [1] reported no occurrence of apical transportation or aberration, after creation of glide path in curved root canals, using either manual SS files or small tapered rotary NiTi. Furthermore, the watch-winding movement provided by the M4 handpiece, should allow easier negotiation of curved canals by reducing the risk of tip blockage. Although the angular deflection (the amount of rotation that a file can withstand before breakage) is better for NiTi than SS alloy [15, 27], the small reciprocation angles allow SS instruments to be used in a safe range, despite its lower mechanical properties [34]. In earlier studies, it was demonstrated that Giromatic handpiece (MicroMega, Besancon, France), which performed reciprocation 90°CW/ 90°CCW, was able to negotiate narrow canals and retain original shape during the enlargement process up to a size 25 [35–37]. In the present study, the positive results for the SS instruments might be assigned to the very small reciprocating angles (30°CW/30°CCW) of the M4 handpiece. Such a constant and predictable 1/12-turn motion is very difficult to achieve manually, and this could be a reason to prefer reciprocation over hand filing. There is also some difference in speed between the two tested instruments, with the PathFile instruments rotated at 300 rpm and K-file at a higher speed. It has been shown that in cyclic fatigue tests, when speed is increased, the time to failure is usually reduced [38]. In the present study, it was not possible to compare or to define speed with the same parameters (i.e., rpm, which indicates number of rotations per minute) due to the different motions: one group (PF) fully rotated, while the other group (M4), did not. Therefore, results were expressed in time to failure, which is a parameter more similar to performance during intracanal use. Cyclic fatigue resistance is one of the most important mechanical properties for the clinical use of endodontic instruments in curved canals. Under the experimental conditions of the present study, reciprocating SS K-files were significantly more resistant than small tapered NiTi rotary instruments. Hence, we may conclude that SS K-files used in a reciprocating motion, might represent a valid alternative technique for the creation of a mechanical endodontic glide path. Conflict of interest of interest.
The authors declare that they have no conflict
References 1. Alves VDO, Eduardo C, Cunha RS, Fontana CE, Martin AS. Comparison among manual instruments and Pathfile and Mtwo
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comparative analysis of dentinal removal during root canal preparation by different operators. Braz Dent J. 2006;17:10–4. You S-Y, Kim H-C, Bae K-S, Baek S-H, Kum K-Y, Lee W. Shaping ability of reciprocating motion in curved root canals: a comparative study with micro-computed tomography. J Endod. 2011;37:1296–300. Gambarini G, Gergi R, Naaman A, Osta N, Sudani D Al. Cyclic fatigue analysis of twisted file rotary NiTi instruments used in reciprocating motion. Int Endod J. 2012;45:802–6. Gambarini G, Grande NM, Plotino G, Somma F, Garala M, De Luca M, Testarelli L. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod. 2008;34:1003–5. Al-Sudani D, Grande NM, Plotino G, Pompa G, Di Carlo S, Testarelli L, Gambarini G. Cyclic fatigue of nickel-titanium rotary instruments in a double (S-shaped) simulated curvature. J Endod. 2012;38:987–9. Testarelli L, Plotino G, Al-Sudani D, Vincenzi V, Giansiracusa A, Grande NM, Gambarini G. Bending properties of a new nickel-titanium alloy with a lower percent by weight of nickel. J Endod. 2011;37:1293–5. Walia HM, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endod. 1988;14:346–51. Grande NM, Plotino G, Pecci R, Bedini R, Malagnino VA, Somma F. Cyclic fatigue resistance and three- dimensional analysis of instruments from two nickel– titanium rotary systems. Int Endod J. 2006;39:755–63. Cheung GSP, Darvell BW. Low cycle fatigue of NiTi rotary instruments of various cross-sectional shape. Int Endod J. 2007;40:626–32.
30. Tepel J, Scha¨fer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency. J Endod. 1995;21:418–21. 31. Tepel J, Scha¨fer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 3. Resistance to bending and fracture. J Endod. 1997;23:141–5. 32. Kosa DA, Marshall G, Baumgartner JC. An analysis of canal centering using mechanical instrumentation techniques. J Endod. 1999;25:441–5. 33. Lloyd A, Jaunberzins A, Dhopatikar A, Bryant S, Dummer PMH. Shaping ability of the M4 handpiece and Safety Hedstrom Files in simulated root canals. Int Endod J. 1997;30:16–24. 34. Krupp JD, Brantley WA, Gerstein H. An investigation of the torsional and bending properties of seven brands of endodontic files. J Endod. 1984;10:372–80. 35. Hu¨lsmann M, Stryga F. Comparison of root canal preparation using different automated devices and hand instrumentation. J Endod. 1993;19:141–5. 36. Klayman SM, Brilliant JD. A comparison of the efficacy of serial preparation versus Giromatic preparation. J Endod. 1975;1:334–7. 37. Weine FS, Kelly RF, Bray KE. Effect of preparation with endodontic handpieces on original canal shape. J Endod. 1976;2:298–303. 38. Plotino G, Grande NM, Cordaro M, Testarelli L, Gambarini G. A review of cyclic fatigue testing of nickel-titanium rotary instruments. J Endod. 2009;35:1469–76.
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ORIGINAL ARTICLE
Cyclic fatigue of instruments for endodontic glide path Gianluca Gambarini • Gianluca Plotino • GianPaolo Sannino • Nicola Maria Grande • Alessio Giansiracusa • Lucila Piasecki • Ulisses Xavier da Silva Neto • Dina Al-Sudani • Luca Testarelli
Received: 29 July 2013 / Accepted: 2 October 2013 Ó The Society of The Nippon Dental University 2013
Abstract Endodontic glide path is the creation of a smooth patency from canal orifice to apex, which can be performed manually or with small tapered NiTi rotary instruments. The use of stainless steel (SS) hand K-files inserted in a reciprocating handpiece can be a possible alternative to create a mechanical glide path. The aim of this study was to compare the cyclic fatigue resistance between SS K-files used in a reciprocating motion and NiTi rotary instruments in artificial curved canals. Ten SS size 15 K-files used with the M4 handpiece (SybronEndo, Glendora, CA, USA) and ten PathFiles (Maillefer-Dentsply, Ballaigues, CH, Switzerland) NiTi rotary instruments size 16, 0.02 taper were tested for resistance to cyclic fatigue. The time to fracture inside an artificial curved canal was recorded for each instrument. Data were analyzed by one-way ANOVA and Tukey HSD test. Mean time (and SD) to failure was 464 s (±40.4) for the Group PF (NiTi rotary PathFile), and 1049 s (±24.8) for the Group M4 (SS K-files reciprocating) with a statistically significant difference between the two groups (p = 0.033). The SS 15 K-files used with the M4 handpiece showed a significant greater resistance to cyclic fatigue when compared to the NiTi rotary PathFiles. Therefore, the use of small size SS files in a reciprocating motion might be a
G. Gambarini G. Plotino (&) G. Sannino N. M. Grande A. Giansiracusa L. Testarelli ‘‘Sapienza’’ - University of Rome, Via Caserta 6, 00187 Rome, Italy e-mail: [email protected] L. Piasecki U. X. da Silva Neto PUCPR, Curitiba, Brazil D. Al-Sudani King Saud University, Riyadh, Saudi Arabia
rational choice for the creation of a mechanical endodontic glide path in curved root canals. Keywords Continuous rotation Cyclic fatigue Glide path M4 safety handpiece Reciprocating motion
Introduction There are currently many controversial issues about endodontic glide path, whether it is really necessary or not, which amount of canal enlargement or preflaring should be achieved, and which could be the best technique (manual versus mechanical instrumentation) to perform it [1–4]. For many clinicians an endodontic glide path is the creation of a smooth patency from canal orifice to apex, in which a size manual size 15 K-file fits loose [5]. This procedure is usually performed before the intracanal use of any nickel-titanium rotary instrument of greater tapers (NTRIGT), aiming to improve efficacy and safety of shaping procedures. It has been shown that the creation of a glide path reduces the frictional forces applied to the NTRIGTs, because the canal diameter becomes at least equal to the file tip used for shaping [3]. Moreover, it allows the apical end of the NTRIGT to act as a passive pilot, reducing the torsional stress, binding, structural fatigue and failure rate [4, 6]. On the contrary, other clinicians believe than when you are able to negotiate a root canal with a manual, small (size 08 or 10) K-file, a natural glide path is already existing [7, 8]. Considering this, an innovative instrumentation technique based on reciprocation has been proposed, thus allowing a single NTRIGT to progress easily to full working length with minimal risks of iatrogenic errors [7].
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Fig. 1 The testing apparatus for cyclic fatigue
The basic concept here lies on improving of safety, because reciprocating motion reduces metal fatigue and torsional stress, which are key factors in determining intracanal fracture of NTRIGT when used in continuous rotation [7, 9–11]. Nevertheless, according to other authors, an endodontic glide path is still preferable before using reciprocating NTRIGT [12, 13]. For those who decide to perform an endodontic glide path, a second question arises: whether to choose manual stainless steel (SS) or small tapered (0.02) nickel–titanium (NiTi) rotary instruments, in ISO sizes ranging from 10 to 20 [1]. The manual glide path with SS instruments enables to understand the original anatomy of the root canals and to eliminate potential anatomical problems and interferences; however, it is time consuming and technically demanding [3, 4]. It has been shown that the creation of a glide path with 0.02 tapered NiTi rotary instruments is faster and presents better maintenance of the original canal anatomy, less modification of canal curvature, fewer canal aberrations and lower prevalence/severity of postoperative pain, compared with manual glide path performed with SS K-files [2, 14]. Moreover, rotary glide path is less affected by the clinician’s expertise; under experimental conditions, an inexpert clinician using NiTi rotary PathFiles (Dentsply Maillefer, Ballaigues, Switzerland) produced more conservative shaping than an expert endodontist with manual preflaring [2]. On the other hand, as NiTi rotary pathfinding instruments present reduced resistance to buckling, they might not be able to advance in the apical direction beyond constrictions and anatomic impediments, during exploration of narrow curved canals [15]. A possible third alternative could be the use of SS hand K-files inserted in reciprocating handpieces, which allow
123
movements than can be considered the mechanical expression of the balanced force motion [16]. The M4 safety handpiece (SybronEndo, Glendora, CA, USA) was developed for such purpose. This handpiece features a 4:1 gear reduction, and oscillates 30° in both clockwise (CW) and counterclockwise (CCW) directions. According to the manufacturer, this watch-winding motion keeps the file loose inside the canal, reduces torsional stress and metal fatigue, and permits safe negotiation while the operator controls the apical pressure. While reciprocation with NiTi instruments has become very popular in recent years, with a significant number of research and published articles [7, 9, 17], only a few have been written in the last decades about reciprocation with SS instruments [18–21]. Moreover, none of these articles was specifically designed to evaluate the possible use of reciprocation and SS K-files in the creation of an endodontic glide path. Since recent studies have shown that reciprocation can improve fatigue resistance of NiTi instruments [9–11, 22, 23], it is interesting to evaluate if, and to which extent, also SS instruments can benefit from a reciprocating motion while creating an endodontic glide path. Therefore, the aim of this in vitro study was to compare cyclic fatigue resistance of NiTi rotary instruments (size 16, 0.02 taper), used in an endodontic motor with continuous rotation, with SS K-files (ISO size 15), used in a M4 reciprocating handpiece.
Materials and methods Ten new PathFiles NiTi Rotary instruments size 16, .02 taper (Maillefer-Dentsply, Ballaigues, CH, Switzerland) and ten new SS manual K-files ISO size 15 (MailleferDentsply, Ballaigues, CH, Switzerland) were selected for this study. Instruments were visually examined under a stereomicroscope to discard any defective instruments, before being tested for resistance to cyclic fatigue. The cyclic fatigue testing device, used in the present study, has been used in many published researches on cyclic fatigue resistance performed by the authors [24, 25]. The device consists of a mainframe to which a mobile plastic support for the electric handpiece is connected and a stainless steel block containing the artificial canals (Fig. 1). The electric handpiece was mounted on a mobile device to allow the precise and reproducible placement of each instrument inside the artificial canal. This placement ensured three-dimensional alignment and the positioning of the instruments to the same depth. The artificial canal was manufactured by reproducing an instrument’s size and taper, thus providing the instrument with a suitable trajectory that respects the parameters of the curvature
Odontology
Discussion
Fig. 2 Resistance to cyclic fatigue of PF (NiTi rotary Pathfiles) and M4 (SS size 15 K files) groups: mean values and SD for time to fracture
chosen. A simulated root canal with a 60° angle of curvature and 5-mm radius of curvature was constructed for each instrument type. The center of the curvature was 5 mm from the tip of the instrument, and the curved segment of the canal was approximately 5 mm in length. In Group PF, the NiTi instruments were rotated at 300 rpm inside the artificial canals using an endodontic motor (ASEPTICO, Woodinville, WA, USA) and an 8:1 reduction handpiece (ASEPTICO, Woodinville, WA, USA), until fracture occurred. In Group M4, SS manual instruments were reciprocated with a M4 handpiece (4:1 reduction, 30°CW/30°CCW), mounted on the same motor set on the M4 option and a speed of 1250 rpm (ASEPTICO, Woodinville, WA, USA). For each instrument, the time to fracture in seconds (s), was recorded by the same operator with a chronometer to an accuracy of 0.1 s. After positioning the instrument into the artificial canal, as soon as rotation or reciprocation started, timing was initiated. Timing stopped when instrument breakage was observed (Fig. 2). Mean values and standard deviation (SD) were then calculated for each group. Cyclic fatigue data were analyzed by one-way ANOVA and Tukey HSD test to determine any statistical difference between groups; the significance was determined at the 95 % confidence level.
Results Mean time (and SD) to failure was 464 s (±40.4) for the Group PF (NiTi rotary PathFile) and 1049 s (±24.8) for the Group M4 (reciprocating SS K-files). Statistical analysis showed a significant difference between the two groups (p = 0.033), with the Group M4 showing an increased time to fracture, which means greater resistance to cyclic fatigue [24].
The results of the present study showed that despite the more favorable mechanical properties of NiTi alloys, and the recent improvement in manufacturing of NiTi instruments for endodontic use [24, 26, 27], the reciprocating motion plays a significant role in determining resistance to metal fatigue of endodontic instruments [17, 22]. The SS K-files used with the M4 handpiece showed a significant greater resistance to cyclic fatigue compared to the 0.02 tapered NiTi instruments used with continuous rotation. The two tested instruments present similar design features, square cross-section and constant 0.02 taper, although the K-files presents size 15, and the PathFile instrument, size 16. This slight difference in the file dimension might not explain the significant difference in the instruments’ behavior under the experimental conditions of this study. In the endodontic literature, there is still controversy regarding the role of the cross-section and the file design in determining in vitro resistance to cyclic fatigue [17, 28, 29]. The findings of the present study might have a significant clinical impact. It is well known that SS alloy is more susceptible than NiTi to metal fatigue [26] with a much higher risk of intracanal separation if a SS file is rotated inside a curved canal [15]. This is the main reason why NiTi, and not SS, rotary instruments have become so popular in the last decades. However, the present results demonstrate that with a specific reciprocating motion, small size SS instruments could be an alternative method for the creation of a glide path, and might be safer than small tapered NiTi rotary instruments of the same dimensions. Therefore, the clinical use of the M4 handpiece in the creation of a mechanical endodontic glide path could be a valid choice, overcoming some critical disadvantages of the SS alloy in the small, narrow and curved canals. Stainless steel alloy is harder than NiTi, and permits a more efficient cutting action [30] and improved torsional resistance. Previous studies have shown that values of maximum torque to fracture are higher for SS when compared to NiTi [15, 31]. Improved cutting efficiency and higher torsional resistance are important properties for a glide path instrument, which is meant to negotiate and pre-enlarge mainly small, narrow and curved canals. One possible disadvantage of SS K-files could be the inherent rigidity of the alloy [15, 31] with a possible risk of straightening, aberrations and canal transportation [18, 20, 32, 33]. However, the flexibility of the endodontic instruments is mainly dependent on their dimensions and metal mass (cross-section) [34]; research data clearly demonstrated that small SS K-files are usually more flexible than
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NTRIGT [26]. Since only small SS K-files (up to ISO size 20) are used for the creation of a glide path, the rigidity of SS alloy might be a negligible disadvantage. Alves et al. [1] reported no occurrence of apical transportation or aberration, after creation of glide path in curved root canals, using either manual SS files or small tapered rotary NiTi. Furthermore, the watch-winding movement provided by the M4 handpiece, should allow easier negotiation of curved canals by reducing the risk of tip blockage. Although the angular deflection (the amount of rotation that a file can withstand before breakage) is better for NiTi than SS alloy [15, 27], the small reciprocation angles allow SS instruments to be used in a safe range, despite its lower mechanical properties [34]. In earlier studies, it was demonstrated that Giromatic handpiece (MicroMega, Besancon, France), which performed reciprocation 90°CW/ 90°CCW, was able to negotiate narrow canals and retain original shape during the enlargement process up to a size 25 [35–37]. In the present study, the positive results for the SS instruments might be assigned to the very small reciprocating angles (30°CW/30°CCW) of the M4 handpiece. Such a constant and predictable 1/12-turn motion is very difficult to achieve manually, and this could be a reason to prefer reciprocation over hand filing. There is also some difference in speed between the two tested instruments, with the PathFile instruments rotated at 300 rpm and K-file at a higher speed. It has been shown that in cyclic fatigue tests, when speed is increased, the time to failure is usually reduced [38]. In the present study, it was not possible to compare or to define speed with the same parameters (i.e., rpm, which indicates number of rotations per minute) due to the different motions: one group (PF) fully rotated, while the other group (M4), did not. Therefore, results were expressed in time to failure, which is a parameter more similar to performance during intracanal use. Cyclic fatigue resistance is one of the most important mechanical properties for the clinical use of endodontic instruments in curved canals. Under the experimental conditions of the present study, reciprocating SS K-files were significantly more resistant than small tapered NiTi rotary instruments. Hence, we may conclude that SS K-files used in a reciprocating motion, might represent a valid alternative technique for the creation of a mechanical endodontic glide path. Conflict of interest of interest.
The authors declare that they have no conflict
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