Odontology DOI 10.1007/s10266-015-0199-0
ORIGINAL ARTICLE
Differences in torsional performance of single- and multipleinstrument rotary systems for glide path preparation Ana Arias • Rupinderpal Singh • Ove A. Peters
Received: 9 September 2014 / Accepted: 31 January 2015 Ó The Society of The Nippon Dental University 2015
Abstract A new rotary instrument has been developed to simplify the glide path preparation in root canals before shaping procedures. The purpose of this study was to compare the peak torque and force induced by nickel– titanium PathFile multiple-instrument system and the recently developed M-Wire ProGlider single instrument during glide path preparation of mesial root canals in extracted mandibular molars. Each independent canal of eight mesial roots of mandibular molars was randomly assigned to achieve a reproducible glide path with a new set of either PathFile #1 and #2 or ProGlider after negotiation with a 10 K-file. Tests were run in a standardized fashion using a torque-testing platform. Peak torque (N cm) and force (N) were registered and analysis of variance and Tukey post-hoc tests were applied. Preliminary data for stationary torque at failure were also obtained and compared with peak torque for each instrument. PathFile #1 and #2 instruments showed statistically lower peak torque (p = 0.001) and peak force (p = 0.008) than ProGlider. Torque at failure according to ADA No. 28/ISO 36030-1 was not significantly different from peak torque during glide path preparation for ProGlider instruments while it was significantly higher for PathFile #1 and #2 (p \ 0.001). Under the conditions of this study, PathFile instruments developed significant lower peak torque and force during glide path preparation compared to ProGlider, which is possibly subjected to a greater contact with the canal walls due to the increase in its flute diameter at middle and coronal levels.
A. Arias (&) R. Singh O. A. Peters Department of Endodontics, University of the Pacific, Arthur A. Dugoni School of Dentistry, 155 Fifth Street, San Francisco, CA 94103, USA e-mail: [email protected]
Keywords Torque
M-Wire PathFile ProGlider Glide path
Introduction Nickel–titanium (NiTi) endodontic instruments have become popular in the field of root canal preparation because of their greater flexibility, their cutting ability, and their more rapid and centered root canal shaping [1–3]. However, they tend to unexpectedly break by flexural fatigue or torsional failure. Flexural fatigue is caused by alternating tension–compression cycles to which rotaries are subjected when flexed in the maximum curvature of the canal and rotational and torsional failure occurs when the tip of the instrument binds but the shank of the instrument continues to rotate [4]. Previous studies have reported that the torque induced by a rotary instrument during root canal shaping depends mainly on the amount of contact between the instrument and the canal walls, the apical force applied to the instrument, the diameter of the instrument, and the volume of the canal [5–7]. The shaping technique used might play an important role in preventing torsional stresses. It has been recommended that a glide path be created to avoid the socalled screw-in effect [8] and the torsional breakage of instruments, by reducing the risk of the so-called taper lock that can occur if the canal cross section is smaller than the tip of the instrument [7, 9]. It is commonly advocated to explore and shape a root canal with a #15 or #20 hand instrument before using a rotary NiTi instrument to full working length [10] to create a glide path for the safe advancement of the rotary instrument tip [11, 12]. The creation of a rotary glide path has shown advantages compared to traditional hand file
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Odontology
preparation: better preservation of the canal anatomy and fewer aberrations [13–15], and less incidence of postoperative pain [16]. However, other studies reported that manual instruments did not increase the occurrence of apical transportation [17] or changes in the angle of canal curvature when compared with rotary glide path preparation [18]. In 2009, Dentsply Maillefer (Ballaigues, Switzerland) marketed ‘‘PathFile’’, the first NiTi rotary system specifically designed to simplify the process of glide path preparation. The system consisted of 3 instruments with a square cross-section and a 0.02 taper. PathFile #1 has a 13 tip size; PathFile #2 has a 16 tip size and PathFile #3 with a 19 tip size [19]. Recently, ProGlider has been marketed, a single file glide path instrument made of M-Wire alloy that features a variable progressive taper of 2–8.5 % with a tip size 16.02. The manufacturer advocates that it creates a glide path faster than hand files or any other alternative rotary glide path solutions [20]. M-Wire (Sportswire LLC, Langley, OK, USA) is a more flexible alloy that has been reported to increase resistance to cyclic fatigue [21–24]; however, torsional resistance of M-wire or other thermally treated alloys does not seem to be significantly improved [25–27]. As per directions for use, clinicians should take all PathFile instruments and ProGlider passively to working length after scouting the canal with a #10 hand file [19, 20]. Despite the similarity in the operation of both systems, the different variable taper and maximum flute diameter could affect the stress distribution pattern and the torsional behavior of both systems, being the first rotary instruments contacting the entire length of the root canal wall [28, 29]. Torsional resistance of PathFile instruments was previously reported [30, 31]; however, the torsional performance of glide path rotary instruments when working inside the root canal has not been reported yet. Currently, usage parameters for PathFile and ProGlider instruments are established in manufacturer’s recommendations, which are 300 rpm and torque preset between 2 and 5.2 N cm but independent data in this regard are scarce [20]. For safe and effective clinical application, more specific knowledge of the best torque recommendations is desirable. In fact, currently there are no data available on torque and force during glide path preparation with PF compared to PG during glide path preparation. Therefore, the aim of this study was to compare baseline torque and apically directed force between PF and PG instruments during the preparation of a reproducible glide path in small human root canals.
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Materials and methods Eight mandibular molars with two independent canals in the mesial root and moderate angle of curvature (10°–20°) according to Schneider´s method [32] were selected from the Department’s pool of extracted teeth. Teeth were stored in 0.1 % thymol prior to usage. Periapical radiographs with different angulations were used to standardize the sample in terms of canal curvature and the teeth were decoronated to obtain similar working lengths. Canals were negotiated with a #10 K instrument in the presence of Glyde (Dentsply Maillefer, Ballaigues, Switzerland). When the tip of the instrument was visible through the main foramen, 0.5 mm was subtracted to determine working length. The apical third of the roots was covered with wax and the specimens were mounted on SEM stubs (014001-T, Balzers Union AG, Balzers, Liechtenstein). Each independent canal of the same mandibular molar mesial root was randomly assigned to two different groups. Root canals in group PF were shaped with PathFile 1 and 2 and in group PG with ProGlider to working length. The same endodontist with more than 15 years of experience in rotary canal instrumentation shaped all root canals. Tap water delivered after each instrument through a 27-gauge needle acted as the irrigant. No effort was made to enlarge canal orifices previous to the action of the rotary instruments. Each canal preparation was performed with a new set of instruments. Thus, a total of 8 new sets of PathFile and 8 ProGliders were used. The tests were run in a standardized manual fashion in a torque-testing platform, which has been described in detail earlier [33]. The bench was configured to determine torque and force during canal preparation that was accomplished via four insertions per instrument. Peak torque as well as positive peak force was registered using the custom-made ENDOTEST software package and collected for off-line analysis. For comparison, an initial analysis of the stationary torque (N cm) at failure during clockwise rotation of 5 samples of PathFile #1, #2 and ProGlider was performed according to ANSI/ADA specification No. 28 (ISO3630-1) using the same torque-testing device. Data for peak torque as well as peak force was found to be compatible with normal distribution and standard deviations of subgroups were similar. Results were analyzed with ANOVA and when appropriate, Tukey post hoc tests were used to compare subgroups. One-sample t test was also used to compare peak torque during glide path preparation with stationary torque at failure for each individual instrument.
Odontology
Results Mean values of peak torque, force, and standard deviations for each instrument are shown in Table 1. PathFile #1 and #2 instruments showed statistically lower peak torque (p = 0.001) and peak force (p = 0.008) than ProGlider for the achievement of a glide path in the mesial canals of mandibular molars. No fracture or plastic deformation was observed in any of the instruments used during glide path preparation in the study. Torque at failure according to ADA No. 28/ISO 36030-1 was significantly higher for PathFile #1 and #2 (p \ 0.001) than mean peak torque during glide path preparation. There were no significant differences between stationary torque at failure and the peak torque induced during glide path preparation for ProGlider instruments. Glide path preparation with PathFile #1 and ProGlider resulted in specific patterns, as illustrated in real-time torque and force curves (Figs. 1, 2).
Discussion The present study was designed to assess the peak torque and force induced by two different systems during glide path preparation of small root canals in extracted teeth. Both systems have a square cross-section and both are used with a single length technique strategy; however, while PathFile needs two instruments to achieve a reproducible glide path with a 0.16-mm apical preparation, ProGlider advocates for the achievement of the same apical diameter with the use of a single instrument. Other improvements have been implemented in the design of ProGlider instruments: variable taper along the active cutting blades, longer active surface (18 mm), and M-Wire alloy [20]. Nevertheless, because both are rotary systems of instruments with a small tip diameter, designed to create a glide path when only a negotiating file has reached working length previously, comparable low peak torque and force were expected. A higher peak torque and force was found when root canals were shaped with a single length preparation Table 1 Mean (± standard deviation) of peak torque and maximum force (n = 8 per group) System
Instrument (tip diameter taper)
Torque at failure (N cm)
Peak torque (N cm)
Force (N)
PathFile
13.02 16.02 16.02
0.30 ± 0.15a 0.37 ± 0.26a 0.43 ± 0.24
0.12 ± 0.04 0.2 ± 0.09 0.71 ± 0.49
1.99 ± 0.98 2.99 ± 1.59 6.43 ± 4.25
ProGlider
Comparison to torque during glide path preparation a
Significantly higher
technique due to the greater contact of the instrument with the walls of the root canal [5]. Such high values were not expected in this study where the tested instruments are not responsible for the shaping of the entire root canal, but only to prepare enough space in the last millimeters for the easy advancement of rotary or reciprocating shaping instruments. It is noteworthy how both systems showed low peak torque values (\1 N cm), much lower than those recommended by the manufacturer that varies from 2 to 5.2 N cm, significantly lower for PathFile than for ProGlider though. A strong relationship between torsional strength and factors as instrument diameter, cross-section, and alloy has been shown in the literature [34–36]. A recent study reported that PathFile instruments were the least torque resistant when compared to comparable rotaries (Race ISO 10 and Scout Race) with similar design and made from similar alloy [30]. However, as shown in Table 1, the results in the present study showed a significant higher stationary torque at failure for PathFile #1 and #2 than the peak torque that the instruments were subjected to when working in the root canal. Stationary torque at failure for different shaping rotary instruments has been compared to the torque generated during the simulated preparation of the root canal in a previous report [37], given that torsional fracture was tested at 3 mm from the tip of the instrument and a proper glide path can prevent the file tip of a rotary shaping instrument to have significant frictional wall contact. On the other hand, the results reported in the present study have important implications because the tested instruments work in a previously non-prepared canal with the specific function of preparing the so-called glide path. Importantly, the mean peak torque that PathFile #1 and #2 generated during the glide path preparation was 40 and 54.1 %, respectively, lower than the expected stationary torque at failure for each individual file. Another important finding was the greater force induced by ProGlider during glide path preparation compared to PathFile instruments, as can also be observed in Figs. 1 and 2. A possible explanation for this result is the increased contact of ProGlider instruments with the canal walls due to the increased taper of the cutting blades along the instrument reaching a coronal flute diameter of 0.99 mm, compared to the less than half (0.45 mm) of PathFile #1 and 0.48 mm of PathFile #2. These results raise the concern about if it is worth it to increase the peak torque and force during glide path preparation to reduce the number of instruments. In the present study, PathFile #1 and #2 induced, respectively, 31 and 46.5 % mean force and 16.9 and 29 % mean peak torque than the force and torque induced by ProGlider. The same testing device used in this study has been used to assess dynamic torque and force associated with different
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Fig. 1 Real-time torque and force curves of ProGlider along the active blades. Diameter along the 18-mm-long section of cutting flutes is also represented. Notice that the torque and force represented is the one induced when the instrument reach that length in the root canal
shaping instruments in plastic blocks [5, 37, 38]. Rotary systems with a variety cross-sections have also been tested with the same device in human dental roots: the radial landed Profile instruments [33] and the non-radial landed ProTaper Universal [7] and ProTaper Next [39]. Of note, the force induced by ProGlider was similar to values found when ProFile and ProTaper Universal [7] shaping instruments were tested in natural teeth [33]. This was in spite of the different function and wall contact that shaping instruments are expected to have during root canal preparation. A glide path preparation reduces the contact area between the shaping instrument and the canal and, therefore, the
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torsional loads induced by shaping rotary instruments [9]; however, low peak torque and force were expected to be induced by the instruments responsible of a glide path preparation that works in an almost intact root canal. These findings suggest that a limitation in the length and diameter of the cutting flutes may be beneficial. It may reduce the force induced in the root canal wall during glide path preparation, considering the higher force found when using ProGlider (similar to the one needed by shaping rotary instruments); this in turn could be excessive and clinically relevant for glide path preparation of certain root canal anatomies. At the same time, while instruments for glide
Odontology
Fig. 2 Real-time torque and force curves of PathFile #1 along the active blades. Diameter along the 16-mm-long section of cutting flutes is also represented. Note that the torque and force represented
are those values induced when the instrument reaches the respective length in the root canal
path preparation are fairly fatigue resistant due to the thin core diameter [40], the higher force needed for ProGlider may increase the chances of torsional fracture. In vitro studies of the torsional profiles of instruments with innovative characteristics are recommended to suggest more accurate directions for use. This study showed torsional profiles of ProGlider and PathFile instruments during glide path preparation in mesial roots in mandibular human molars after the negotiation of root canals with a hand #10 file in an attempt of reproducing a normal clinical situation. Within the limitations of this in vitro study, the results suggest that multiple-instrument systems for glide
preparation showed significant lower peak torque and peak force than single-instrument systems; the latter are subjected to a greater force and torque due to the amount of contact between the instrument with a larger flute diameter at middle and coronal levels and the canal walls. Further studies are needed to compare peak torque and force induced by shaping rotary and reciprocating systems after different glide path preparations. Conflict of interest Drs. Ana Arias and Rupinderpal Singh deny any conflict of interest related to this study. Dr. Ove A. Peters serves as a consultant to Dentsply Maillefer, Ballaigues, Switzerland and Dentsply Tulsa Dental, Tulsa OK.
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References 1. 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. 2. Glossen CR, Haller RH, Dove SB, del Rio CE. A comparison of root canal preparations using Ni–Ti hand, Ni–Ti engine-driven, and K-Flex endodontic instruments. J Endod. 1995;21:146–51. 3. Short JA, Morgan LA, Baumgartner JC. A comparison of canal centering ability of four instrumentation techniques. J Endod. 1997;23:503–7. 4. Sattapan B, Nervo G, Palamara J, Messer H. Defects in rotary nickel–titanium files after clinical use. J Endod. 2000;26:161–5. 5. Schrader C, Peters OA. Analysis of torque and force with differently tapered rotary endodontic instruments in vitro. J Endod. 2005;31:120–3. 6. Bahia MGA, Melo MCC, Buono VTL. Influence of simulated clinical use on the torsional behavior of nickel–titanium rotary endodontic instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2006;101:675–80. 7. Peters OA, Peters CI, Scho¨nenberger K, Barbakow F. ProTaper rotary root canal preparation: assessment of torque and force in relation to canal anatomy. Int Endod J. 2003;36:93–9. 8. Ha JH, Park SS. Influence of glide path on the screw-in effect and torque of nickel–titanium rotary files in simulated resin root canals. Restor Dent Endod. 2012;37:215–9. 9. Blum JY, Machtou P, Micallef JP. Location of contact areas on rotary Profile instruments in relationship to the forces developed during mechanical preparation on extracted teeth. Int Endod J. 1999;32:108–14. 10. Tan BT, Messer HH. The quality of apical canal preparation using hand and rotary instruments with specific criteria for enlargement based on initial apical file size. J Endod. 2002;28: 658–64. 11. Roland DD, Andelin WE, Browning DF, Hsu G-HR, Torabinejad M. The effect of preflaring on the rates of separation for 0.04 taper nickel–titanium rotary instruments. J Endod. 2002;28: 543–5. 12. Patin˜o PV, Biedma BM, Lie´bana CR, Cantatore G, Bahillo JG. The influence of a manual glide path on the separation rate of NiTi rotary instruments. J Endod. 2005;31:114–6. 13. Berutti E, Cantatore G, Castellucci A, Chiandussi G, Pera F, Migliaretti G, et al. Use of nickel–titanium rotary PathFile to create the glide path: comparison with manual preflaring in simulated root canals. J Endod. 2009;35:408–12. 14. Pasqualini D, Bianchi CC, Paolino DS, Mancini L, Cemenasco A, Cantatore G, et al. Computed micro-tomographic evaluation of glide path with nickel–titanium rotary PathFile in maxillary first molars curved canals. J Endod. 2012;38:389–93. 15. Ajuz NCC, Armada L, Gonc¸alves LS, Debelian G, Siqueira JF. Glide path preparation in S-shaped canals with rotary pathfinding nickel–titanium instruments. J Endod. 2013;39:534–7. 16. Pasqualini D, Mollo L, Scotti N, Cantatore G, Castellucci A, Migliaretti G, et al. Postoperative pain after manual and mechanical glide path: a randomized clinical trial. J Endod. 2012;1–5. 17. de Oliveira Alves V, da Silveira Bueno CE, Cunha RS, Pinheiro SL, Fontana CE, de Martin AS. Comparison among manual instruments and PathFile and Mtwo rotary instruments to create a glide path in the root canal preparation of curved canals. J Endod. 2012;38:117–20. 18. D0 Amario M, Baldi M, Petricca R, De Angelis F, El Abed R, D0 Arcangelo C. Evaluation of a new nickel–titanium system to create the glide path in root canal preparation of curved canals. J Endod. 2013;39:1581–4.
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19. Tulsa Dental Specialties. PathFile. Directions per use. http:// www.tulsadentalspecialties.com/default/endodontics/glidepath/ PathFile.aspx. Accessed 13 Feb 2015. 20. Tulsa Dental Specialties. ProGlider. Directions per use. http:// www.tulsadentalspecialties.com/default/endodontics/glidepath/ ProGlider.aspx. Accessed 13 Feb 2015. 21. Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickel–titanium alloy and 508 Nitinol on the cyclic fatigue life of ProFile 25/.04 rotary instruments. J Endod. 2008;34:1406–9. 22. Ye J, Gao Y. Metallurgical characterization of M-Wire nickel– titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod. 2012;38:105–7. 23. Arias A, Perez-Higueras JJ, de la Macorra JC. Influence of clinical usage of GT and GTX files on cyclic fatigue resistance. Int Endod J. 2014;47:257–63. 24. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Effect of environment on fatigue failure of controlled memory wire nickel– titanium rotary instruments. J Endod. 2012;38:376–80. 25. Kramkowski TR, Bahcall J. An in vitro comparison of torsional stress and cyclic fatigue resistance of ProFile GT and ProFile GT Series X rotary nickel–titanium files. J Endod. 2009;35:404–7. 26. Kell T, Azarpazhooh A, Peters OA, El-Mowafy O, Tompson B, Basrani B. Torsional profiles of new and used 20/.06 GT Series X and GT rotary endodontic instruments. J Endod. 2009;35: 1278–81. 27. Wycoff RC, Berzins DW. An in vitro comparison of torsional stress properties of three different rotary nickel–titanium files with a similar cross-sectional design. J Endod. 2012;38:1118–20. 28. Turpin YL, Chagneau F, Vulcain JM. Impact of two theoretical cross-sections on torsional and bending stresses of nickel–titanium root canal instrument models. J Endod. 2000;26:414–7. 29. Melo MCC, Pereira ESJ, Viana ACD, Fonseca AMA, Buono VTL, Bahia MGA. Dimensional characterization and mechanical behaviour of K3 rotary instruments. Int Endod J. 2008;41: 329–38. 30. Nakagawa RKL, Alves JL, Buono VTL, Bahia MGA. Flexibility and torsional behaviour of rotary nickel–titanium PathFile, RaCe ISO 10, Scout RaCe and stainless steel K-File hand instruments. Int Endod J. 2014;47:290–7. 31. Gambarini G, Plotino G, Sannino G, Grande NM, Giansiracusa A, Piasecki L, et al. Cyclic fatigue of instruments for endodontic glide path. Odontology. 2013;7 (Epub ahead of print). 32. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg Oral Med Oral Pathol. 1971;32:271–5. 33. Peters OA, Barbakow F. Dynamic torque and apical forces of ProFile. 04 rotary instruments during preparation of curved canals. Int Endod J. 2002;35:379–89. 34. Scha¨fer E, Dzepina A, Danesh G. Bending properties of rotary nickel–titanium instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;96:757–63. 35. Miyai K, Ebihara A, Suda H, Yoneyama T, Hayashi Y. Influence of phase transformation on the torsional and bending properties of nickel–titanium rotary endodontic instruments. Int Endod. 2006;39:119–26. 36. Viana ACD. Chaves Craveiro de Melo M, Guiomar de Azevedo Bahia M, Lopes Buono VT. Relationship between flexibility and physical, chemical, and geometric characteristics of rotary nickel–titanium instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2010;110:527–33. 37. Pereira ESJ, Singh R, Arias A, Peters OA. In Vitro Assessment of Torque and Force Generated by Novel ProTaper Next Instruments during Simulated Canal Preparation. J Endod. 2013;39: 1615–9.
Odontology 38. Peters OA, Gluskin AK, Weiss RA, Han JT. An in vitro assessment of the physical properties of novel Hyflex nickel–titanium rotary instruments. Int Endod J. 2012;45:1027–34. 39. Arias A, Singh R, Peters OA. Torque and force induced by ProTaper Universal and ProTaper Next during shaping of large and small root canals in extracted teeth. J Endod. 2014;40:973–6.
40. Sung SY, Ha JH, Kwak SW, Abed RE, Byeon K, Kim HC. Torsional and cyclic fatigue resistances of glide path preparation instruments: G-file and PathFile. Scanning (Internet). 2014;36: 500–6.
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ORIGINAL ARTICLE
Differences in torsional performance of single- and multipleinstrument rotary systems for glide path preparation Ana Arias • Rupinderpal Singh • Ove A. Peters
Received: 9 September 2014 / Accepted: 31 January 2015 Ó The Society of The Nippon Dental University 2015
Abstract A new rotary instrument has been developed to simplify the glide path preparation in root canals before shaping procedures. The purpose of this study was to compare the peak torque and force induced by nickel– titanium PathFile multiple-instrument system and the recently developed M-Wire ProGlider single instrument during glide path preparation of mesial root canals in extracted mandibular molars. Each independent canal of eight mesial roots of mandibular molars was randomly assigned to achieve a reproducible glide path with a new set of either PathFile #1 and #2 or ProGlider after negotiation with a 10 K-file. Tests were run in a standardized fashion using a torque-testing platform. Peak torque (N cm) and force (N) were registered and analysis of variance and Tukey post-hoc tests were applied. Preliminary data for stationary torque at failure were also obtained and compared with peak torque for each instrument. PathFile #1 and #2 instruments showed statistically lower peak torque (p = 0.001) and peak force (p = 0.008) than ProGlider. Torque at failure according to ADA No. 28/ISO 36030-1 was not significantly different from peak torque during glide path preparation for ProGlider instruments while it was significantly higher for PathFile #1 and #2 (p \ 0.001). Under the conditions of this study, PathFile instruments developed significant lower peak torque and force during glide path preparation compared to ProGlider, which is possibly subjected to a greater contact with the canal walls due to the increase in its flute diameter at middle and coronal levels.
A. Arias (&) R. Singh O. A. Peters Department of Endodontics, University of the Pacific, Arthur A. Dugoni School of Dentistry, 155 Fifth Street, San Francisco, CA 94103, USA e-mail: [email protected]
Keywords Torque
M-Wire PathFile ProGlider Glide path
Introduction Nickel–titanium (NiTi) endodontic instruments have become popular in the field of root canal preparation because of their greater flexibility, their cutting ability, and their more rapid and centered root canal shaping [1–3]. However, they tend to unexpectedly break by flexural fatigue or torsional failure. Flexural fatigue is caused by alternating tension–compression cycles to which rotaries are subjected when flexed in the maximum curvature of the canal and rotational and torsional failure occurs when the tip of the instrument binds but the shank of the instrument continues to rotate [4]. Previous studies have reported that the torque induced by a rotary instrument during root canal shaping depends mainly on the amount of contact between the instrument and the canal walls, the apical force applied to the instrument, the diameter of the instrument, and the volume of the canal [5–7]. The shaping technique used might play an important role in preventing torsional stresses. It has been recommended that a glide path be created to avoid the socalled screw-in effect [8] and the torsional breakage of instruments, by reducing the risk of the so-called taper lock that can occur if the canal cross section is smaller than the tip of the instrument [7, 9]. It is commonly advocated to explore and shape a root canal with a #15 or #20 hand instrument before using a rotary NiTi instrument to full working length [10] to create a glide path for the safe advancement of the rotary instrument tip [11, 12]. The creation of a rotary glide path has shown advantages compared to traditional hand file
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preparation: better preservation of the canal anatomy and fewer aberrations [13–15], and less incidence of postoperative pain [16]. However, other studies reported that manual instruments did not increase the occurrence of apical transportation [17] or changes in the angle of canal curvature when compared with rotary glide path preparation [18]. In 2009, Dentsply Maillefer (Ballaigues, Switzerland) marketed ‘‘PathFile’’, the first NiTi rotary system specifically designed to simplify the process of glide path preparation. The system consisted of 3 instruments with a square cross-section and a 0.02 taper. PathFile #1 has a 13 tip size; PathFile #2 has a 16 tip size and PathFile #3 with a 19 tip size [19]. Recently, ProGlider has been marketed, a single file glide path instrument made of M-Wire alloy that features a variable progressive taper of 2–8.5 % with a tip size 16.02. The manufacturer advocates that it creates a glide path faster than hand files or any other alternative rotary glide path solutions [20]. M-Wire (Sportswire LLC, Langley, OK, USA) is a more flexible alloy that has been reported to increase resistance to cyclic fatigue [21–24]; however, torsional resistance of M-wire or other thermally treated alloys does not seem to be significantly improved [25–27]. As per directions for use, clinicians should take all PathFile instruments and ProGlider passively to working length after scouting the canal with a #10 hand file [19, 20]. Despite the similarity in the operation of both systems, the different variable taper and maximum flute diameter could affect the stress distribution pattern and the torsional behavior of both systems, being the first rotary instruments contacting the entire length of the root canal wall [28, 29]. Torsional resistance of PathFile instruments was previously reported [30, 31]; however, the torsional performance of glide path rotary instruments when working inside the root canal has not been reported yet. Currently, usage parameters for PathFile and ProGlider instruments are established in manufacturer’s recommendations, which are 300 rpm and torque preset between 2 and 5.2 N cm but independent data in this regard are scarce [20]. For safe and effective clinical application, more specific knowledge of the best torque recommendations is desirable. In fact, currently there are no data available on torque and force during glide path preparation with PF compared to PG during glide path preparation. Therefore, the aim of this study was to compare baseline torque and apically directed force between PF and PG instruments during the preparation of a reproducible glide path in small human root canals.
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Materials and methods Eight mandibular molars with two independent canals in the mesial root and moderate angle of curvature (10°–20°) according to Schneider´s method [32] were selected from the Department’s pool of extracted teeth. Teeth were stored in 0.1 % thymol prior to usage. Periapical radiographs with different angulations were used to standardize the sample in terms of canal curvature and the teeth were decoronated to obtain similar working lengths. Canals were negotiated with a #10 K instrument in the presence of Glyde (Dentsply Maillefer, Ballaigues, Switzerland). When the tip of the instrument was visible through the main foramen, 0.5 mm was subtracted to determine working length. The apical third of the roots was covered with wax and the specimens were mounted on SEM stubs (014001-T, Balzers Union AG, Balzers, Liechtenstein). Each independent canal of the same mandibular molar mesial root was randomly assigned to two different groups. Root canals in group PF were shaped with PathFile 1 and 2 and in group PG with ProGlider to working length. The same endodontist with more than 15 years of experience in rotary canal instrumentation shaped all root canals. Tap water delivered after each instrument through a 27-gauge needle acted as the irrigant. No effort was made to enlarge canal orifices previous to the action of the rotary instruments. Each canal preparation was performed with a new set of instruments. Thus, a total of 8 new sets of PathFile and 8 ProGliders were used. The tests were run in a standardized manual fashion in a torque-testing platform, which has been described in detail earlier [33]. The bench was configured to determine torque and force during canal preparation that was accomplished via four insertions per instrument. Peak torque as well as positive peak force was registered using the custom-made ENDOTEST software package and collected for off-line analysis. For comparison, an initial analysis of the stationary torque (N cm) at failure during clockwise rotation of 5 samples of PathFile #1, #2 and ProGlider was performed according to ANSI/ADA specification No. 28 (ISO3630-1) using the same torque-testing device. Data for peak torque as well as peak force was found to be compatible with normal distribution and standard deviations of subgroups were similar. Results were analyzed with ANOVA and when appropriate, Tukey post hoc tests were used to compare subgroups. One-sample t test was also used to compare peak torque during glide path preparation with stationary torque at failure for each individual instrument.
Odontology
Results Mean values of peak torque, force, and standard deviations for each instrument are shown in Table 1. PathFile #1 and #2 instruments showed statistically lower peak torque (p = 0.001) and peak force (p = 0.008) than ProGlider for the achievement of a glide path in the mesial canals of mandibular molars. No fracture or plastic deformation was observed in any of the instruments used during glide path preparation in the study. Torque at failure according to ADA No. 28/ISO 36030-1 was significantly higher for PathFile #1 and #2 (p \ 0.001) than mean peak torque during glide path preparation. There were no significant differences between stationary torque at failure and the peak torque induced during glide path preparation for ProGlider instruments. Glide path preparation with PathFile #1 and ProGlider resulted in specific patterns, as illustrated in real-time torque and force curves (Figs. 1, 2).
Discussion The present study was designed to assess the peak torque and force induced by two different systems during glide path preparation of small root canals in extracted teeth. Both systems have a square cross-section and both are used with a single length technique strategy; however, while PathFile needs two instruments to achieve a reproducible glide path with a 0.16-mm apical preparation, ProGlider advocates for the achievement of the same apical diameter with the use of a single instrument. Other improvements have been implemented in the design of ProGlider instruments: variable taper along the active cutting blades, longer active surface (18 mm), and M-Wire alloy [20]. Nevertheless, because both are rotary systems of instruments with a small tip diameter, designed to create a glide path when only a negotiating file has reached working length previously, comparable low peak torque and force were expected. A higher peak torque and force was found when root canals were shaped with a single length preparation Table 1 Mean (± standard deviation) of peak torque and maximum force (n = 8 per group) System
Instrument (tip diameter taper)
Torque at failure (N cm)
Peak torque (N cm)
Force (N)
PathFile
13.02 16.02 16.02
0.30 ± 0.15a 0.37 ± 0.26a 0.43 ± 0.24
0.12 ± 0.04 0.2 ± 0.09 0.71 ± 0.49
1.99 ± 0.98 2.99 ± 1.59 6.43 ± 4.25
ProGlider
Comparison to torque during glide path preparation a
Significantly higher
technique due to the greater contact of the instrument with the walls of the root canal [5]. Such high values were not expected in this study where the tested instruments are not responsible for the shaping of the entire root canal, but only to prepare enough space in the last millimeters for the easy advancement of rotary or reciprocating shaping instruments. It is noteworthy how both systems showed low peak torque values (\1 N cm), much lower than those recommended by the manufacturer that varies from 2 to 5.2 N cm, significantly lower for PathFile than for ProGlider though. A strong relationship between torsional strength and factors as instrument diameter, cross-section, and alloy has been shown in the literature [34–36]. A recent study reported that PathFile instruments were the least torque resistant when compared to comparable rotaries (Race ISO 10 and Scout Race) with similar design and made from similar alloy [30]. However, as shown in Table 1, the results in the present study showed a significant higher stationary torque at failure for PathFile #1 and #2 than the peak torque that the instruments were subjected to when working in the root canal. Stationary torque at failure for different shaping rotary instruments has been compared to the torque generated during the simulated preparation of the root canal in a previous report [37], given that torsional fracture was tested at 3 mm from the tip of the instrument and a proper glide path can prevent the file tip of a rotary shaping instrument to have significant frictional wall contact. On the other hand, the results reported in the present study have important implications because the tested instruments work in a previously non-prepared canal with the specific function of preparing the so-called glide path. Importantly, the mean peak torque that PathFile #1 and #2 generated during the glide path preparation was 40 and 54.1 %, respectively, lower than the expected stationary torque at failure for each individual file. Another important finding was the greater force induced by ProGlider during glide path preparation compared to PathFile instruments, as can also be observed in Figs. 1 and 2. A possible explanation for this result is the increased contact of ProGlider instruments with the canal walls due to the increased taper of the cutting blades along the instrument reaching a coronal flute diameter of 0.99 mm, compared to the less than half (0.45 mm) of PathFile #1 and 0.48 mm of PathFile #2. These results raise the concern about if it is worth it to increase the peak torque and force during glide path preparation to reduce the number of instruments. In the present study, PathFile #1 and #2 induced, respectively, 31 and 46.5 % mean force and 16.9 and 29 % mean peak torque than the force and torque induced by ProGlider. The same testing device used in this study has been used to assess dynamic torque and force associated with different
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Fig. 1 Real-time torque and force curves of ProGlider along the active blades. Diameter along the 18-mm-long section of cutting flutes is also represented. Notice that the torque and force represented is the one induced when the instrument reach that length in the root canal
shaping instruments in plastic blocks [5, 37, 38]. Rotary systems with a variety cross-sections have also been tested with the same device in human dental roots: the radial landed Profile instruments [33] and the non-radial landed ProTaper Universal [7] and ProTaper Next [39]. Of note, the force induced by ProGlider was similar to values found when ProFile and ProTaper Universal [7] shaping instruments were tested in natural teeth [33]. This was in spite of the different function and wall contact that shaping instruments are expected to have during root canal preparation. A glide path preparation reduces the contact area between the shaping instrument and the canal and, therefore, the
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torsional loads induced by shaping rotary instruments [9]; however, low peak torque and force were expected to be induced by the instruments responsible of a glide path preparation that works in an almost intact root canal. These findings suggest that a limitation in the length and diameter of the cutting flutes may be beneficial. It may reduce the force induced in the root canal wall during glide path preparation, considering the higher force found when using ProGlider (similar to the one needed by shaping rotary instruments); this in turn could be excessive and clinically relevant for glide path preparation of certain root canal anatomies. At the same time, while instruments for glide
Odontology
Fig. 2 Real-time torque and force curves of PathFile #1 along the active blades. Diameter along the 16-mm-long section of cutting flutes is also represented. Note that the torque and force represented
are those values induced when the instrument reaches the respective length in the root canal
path preparation are fairly fatigue resistant due to the thin core diameter [40], the higher force needed for ProGlider may increase the chances of torsional fracture. In vitro studies of the torsional profiles of instruments with innovative characteristics are recommended to suggest more accurate directions for use. This study showed torsional profiles of ProGlider and PathFile instruments during glide path preparation in mesial roots in mandibular human molars after the negotiation of root canals with a hand #10 file in an attempt of reproducing a normal clinical situation. Within the limitations of this in vitro study, the results suggest that multiple-instrument systems for glide
preparation showed significant lower peak torque and peak force than single-instrument systems; the latter are subjected to a greater force and torque due to the amount of contact between the instrument with a larger flute diameter at middle and coronal levels and the canal walls. Further studies are needed to compare peak torque and force induced by shaping rotary and reciprocating systems after different glide path preparations. Conflict of interest Drs. Ana Arias and Rupinderpal Singh deny any conflict of interest related to this study. Dr. Ove A. Peters serves as a consultant to Dentsply Maillefer, Ballaigues, Switzerland and Dentsply Tulsa Dental, Tulsa OK.
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