doi:10.1111/iej.12233
Fatigue life enhancement of NiTi rotary endodontic instruments by progressive reciprocating operation
C. S. Shin1, Y. H. Huang1, C. W. Chi2 & C. P. Lin2 1
Department of Mechanical Engineering, National Taiwan University; and 2Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University and National Taiwan University Hospital, Taipei, Taiwan
Abstract Shin CS, Huang YH, Chi CW, Lin CP. Fatigue life enhancement of NiTi rotary endodontic instruments by progressive reciprocating operation. International Endodontic Journal, 47, 882–888, 2014.
Aim To evaluate the effect of reciprocating amplitude and progressive angular increment on fatigue life enhancement of NiTi rotary endodontic instruments. Methodology ProTaper F2 instruments were operated in steel artificial canals with both stationary reciprocating (SR) and progressive reciprocating (PR) motions. The SR motions involved symmetric to and fro reciprocation of 180o, 135o, 90o, 60o and 45o. The PR motions were 45o stationary motion superimposed with angular increments of 7o, 11o, 22.5o or 31o whenever an instrument completed 1, 10 or 30 reciprocating cycles (rc). The fatigue lives were compared with those under continuous rotation (CR) and a reciprocating operation with a forward 144o and backward 72o motion proposed by Yared (2008). The statistical significance of these operating modes on fatigue life was examined using one way ANOVA and post hoc Tukey’s tests at P = 0.05. Fractographic analysis was also applied to probe the fracture mechanisms of different rotation motions.
Results Fatigue life increased with decreasing reciprocating amplitude. Operating in the SR increased fatigue life by 355% over that in the CR. Except for the 22.5o increment, all PR motions yielded longer fatigue lives than the SR motion. A progressive reciprocating operation with a 45o reciprocating amplitude and a + 7o progressive angular increment every 10 reciprocating cycles (45o/10rc/+7o) increased fatigue life by 990% over that in the CR motion. In terms of life enhancement over the CR motion, the larger the curvature the less are the differences between different movements. Single crack initiation sites were found in the CR and SR motions, while three crack initiation sites were typical in the 45o/ 10rc/+7o motion. Conclusions Fatigue life increased with decreasing reciprocating amplitude in stationary reciprocation. A progressive reciprocating operation with 45o/10rc/ +7o motion led to significant fatigue life enhancement and multiple fatigue crack initiation in NiTi endodontic instruments. Keywords: cyclic fatigue failure, fatigue life enhancement, fractographic analysis, NiTi endodontic instrument, progressive reciprocation, stationary reciprocation. Received 6 August 2013; accepted 16 December 2013
Introduction Correspondence: Chun-Pin Lin, School of Dentistry, National Taiwan University and National Taiwan University Hospital, No.1, Chang-Te Street, Taipei 10048, Taiwan (Tel.: +886 2 23562148; fax: +886 2 2383 1346; e-mail: pinlin@ntu. edu.tw).
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NiTi rotary instruments have been used in root canal treatment because of their superelasticity. Root canal preparation with NiTi rotary instruments could maintain the appropriate canal centrality and provide more predictable outcomes than stainless steel files
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
(Sch€ afer et al. 2004). However, intracanal NiTi instrument fracture can jeopardize the success of treatment and be a major concern (Souter & Messer 2005, Spili et al. 2005). In fact, fatigue fracture is one of the predominant intracanal failure modes (Parashos et al. 2004), and it arises because the instruments suffer alternating tensile and compressive stresses induced by rotating inside curved canals. Various techniques have been proposed that may alleviate instrument fracture (Di Fiore 2007). Instead of the conventional continuous rotation motion (CR), Yared (2008) proposed a single instrument technique that employed a reciprocating operation with a forward 144o and backward 72o reciprocating motion (YR). This technique was found to be more cost-effective and time-saving (You et al. 2010, Paque et al. 2011) with the shaping outcome being similar (De-Deus et al. 2010a, Paque et al. 2011) or even better (Franco et al. 2011). Moreover, the reciprocating operation extended the fatigue life of the instrument when compared with conventional rotary movement (De-Deus et al. 2010b, VarelaPati~ no et al. 2010, Gavini et al. 2012, Lopes et al. 2013, Pedull a et al. 2013). Previous research used reciprocating movement produced by commercial handpieces and motors with rotating kinematics set by the manufacturer. There was little flexibility to change the forward and reverse rotational angles in the reciprocating action, and so the effect of changing the reciprocating amplitude was seldom investigated. Recently, it was reported that reducing the reciprocating angle increased fatigue life (Gambarini et al. 2012). During continuous rotation, an arbitrary point on an instrument will be moving along a circle (shown schematically in Fig. 1). During a stationary reciprocating motion (SR), the reference point will traverse the arc
Figure 1 Path of a reference point during continuous rotation and reciprocating operations, illustrating the reciprocating amplitude /a and angular increment /inc.
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
AB, starting from A, stop momentarily at B and traverse back to A. The angle subtended by the arc AB will be referred to as the reciprocating amplitude (/a in Fig. 1). To avoid fatigue damage localization, the traversing path may be shifted forward (e.g. from along AB to along CD) by an angular increment (/inc in Fig. 1). The periodic forward shifting of the reciprocating path will be referred to as progressive reciprocating motion (PR). The magnitude and the frequency of occurrence of the angular increment govern when the reference point goes past point A again. When the latter happens, the instrument completes a full revolution and is subjected to a straining cycle of the largest severity dictated by the canal curvature. This large straining cycle and the small straining cycles from the partial revolutions during reciprocation have different impacts on fatigue damage. Previous studies have not distinguished these two effects and implemented an angular increment after every reciprocating cycle. The two effects have been studied separately in this study by controlling the reciprocating amplitude and the magnitude of angular increment independently. It is hoped that this can help to cast light on the fracture mechanisms of CR, SR and PR, as well as on the development of operating mode that can significantly prolong fatigue life and alleviate unexpected clinical fracture of rotary endodontic instruments.
Materials and methods Cyclic fatigue testing in artificial canals was carried out using ProTaperâ F2 instruments (Dentsply Maillefer, Ballaigues, Switzerland) with an effective length of 21 mm. The artificial canals were formed by a steel cylinder of radius R and an exterior steel block with a matching circular arc, similar to that employed by Gambarini (2001). Three radii R (5, 7.5 and 10 mm) were used, and the depth of instrument insertion was varied to produce angles of curvature h of 40o and 60o. The specimens were video-taped during testing. The exact time of fracture could then be accurately determined through playing back the video. The instruments were operated by a DC servo motor (CS60-150C5AE, CSIM Inc., Taipei, Taiwan) driven by a servo driver (CSBL900, CSIM Inc., Taipei, Taiwan). The driver was controlled by a motor controller (PCI-7390, National Instrument, Austin, TX, USA) through a program written in Labview (National Instrument, Austin, TX, USA) in which the rotational speed, forward and reverse rotation angles of any cycle could be specified independently. Two
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categories of reciprocating motions were examined. The first category was SR with reciprocating amplitudes of 180o, 135o, 90o, 60o and 45o. The second category was PR that involved 45o SR motion superimposed with a forward angular increment of 7o, 11o, 22.5o or 31o whenever an instrument completed 1, 10 or 30 reciprocating cycles (rc). A PR motion was designated in a three-number code such as 45o/10rc/+7o, meaning a stationary reciprocation of 45o with an angular increment of +7o on completion of 10 reciprocating cycles. The CR and YR motions were used as benchmarks to gauge the fatigue life performances of the above operating modes. The instrument bending condition of R7.5h40o was mainly employed. Other combinations of R and h were used to check the performance of the operating mode that gave the best enhancement of fatigue life. Figure 2 summarizes the test matrix employed. For each condition, five instruments were tested. The fracture surfaces of some of the instruments were examined with a scanning electron microscope (SEM, JSM 5610, Jeol, Tokyo, Japan). The average rotational speeds were set to be equivalent to 450 rpm in the CR. The number of reciprocating cycle per minute was confirmed by a stroboscope (PKDS-112, Pokai technology, Taipei, Taiwan). Fatigue life of an instrument was quantified by the total angular distance travelled before fracture, measured in terms of equivalent revolutions. Statistical significance of fatigue life difference under different operating modes was checked for using one way ANOVA and post hoc Tukey’s test at P = 0.05 with a statistical analysis software (SPSS 16 for Windows, Chicago, IL, USA).
Results As shown in Fig. 3, operating in the SR increased fatigue life by ~50% (in 180o) to 355% (in 45o) over that in the CR. Fatigue life increased with a
decrease in reciprocating amplitude. Except for the 180o and YR, all other reciprocating motions had significantly longer lives than the CR motion. Life under the 45o motion was significantly longer than that under all other motions (P < 0.001). Life under the 60o motion came next, but its difference from that under 90o was not significant. The life under YR lay between that for the 180o and 135o motions. The differences between these three motions were not significant. Figure 4 compares the fatigue lives among 45o/ 1rc/+7o, 45o/10rc/+7o and 45o/30rc/+7o. These were significantly different with P < 0.005. 45o/ 10rc/+7o led to more life enhancement than the other two motions. 45o/1rc/+7o gave a comparable fatigue life to the stationary 45o motion. The effect of applying different angular increment in the PR motions is shown in Fig. 5. Progressive angular increment was made whenever an instrument completed 10 reciprocating cycles. Except for the 22.5o increment, all PR motions yielded longer fatigue lives than the SR motion. The progressive increment of 7o prolonged life significantly above that of 11o and 31o (P < 0.005), and the corresponding fatigue life enhancement amounted to ~990% above that under the CR motion. As shown in Fig. 6, the beneficial effect of the SR and PR operations on fatigue life is a general phenomenon occurring in different canal geometries. For each of the four combinations of R and h tested, the 45o/10rc/+7o motion gave the longest fatigue life, followed by the 45o SR motion. YR came next and always gave a longer life than the CR motion. However, for the R7.5h40o and R10h60o canals, the differences between the last two motions were not statistically significant. Figure 7 shows typical low magnification SEM views of instrument sections fractured under the CR and the 45o/10rc/+7o motions. White arrows in the figures point to crack initiation locations where diver-
Figure 2 Test matrix showing the combination of reciprocation amplitudes (/a), magnitude and frequency of angular increment (/inc) employed. rc: reciprocating cycle; CR: continuous rotation motion; YR: a forward 144o and reciprocating backward 72o motion proposed by Yared.
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© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
Figure 3 Fatigue lives of instruments in a R7.5h40o canal under various stationary reciprocating (SR) amplitudes compared with that under the continuous rotation motion (CR) and a forward 144o and backward 72o reciprocating motion proposed by Yared (YR). The same alphabet above the error bar denotes data groups with P > 0.05.
Figure 5 Effect of different periodic angular increment on fatigue lives of the instruments in a R7.5h40o canal for progressive reciprocation with amplitude of 45o. The same alphabet above the error bar denotes data groups with P > 0.05. CR: continuous rotation motion; YR: a forward 144o and backward 72o reciprocating motion proposed by Yared; 0o corresponds to stationary reciprocation.
Figure 4 Effect of different reciprocating cycles (rc) for periodic angular increment on fatigue lives of the instruments.
gent radial lines originated. Higher magnification of the radial line region revealed striation markings characteristic of fatigue crack propagation. Beyond the dotted boundaries were dimple structures representing the final fracture stage. The fracture surfaces resulting from the CR and SR motions all showed a single crack initiation site (Fig. 7a). The 45o/10rc/ +7o fracture surfaces all exhibited 3 crack initiation sites (Fig. 7b).
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Figure 6 Comparison of fatigue lives in 4 different canal geometries under stationary reciprocation of 45o, progressive 45o/10rc/+7o mode, a forward 144o and backward 72o reciprocating motion proposed by Yared (YR) and the continuous rotation motion (CR).
Discussion Analysis of the rotation mechanics shows that by completing a full revolution, an instrument undergoes a reverse bending and suffers a tension–com-
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(a)
(b)
Figure 7 SEM fractographs of instruments operated under (a) continuous rotation motion; (b) progressive 45o/10rc/ +7o. White arrows indicate crack nucleation sites. Dotted lines demarcate boundaries between fatigue striation region and dimple structure region.
pression straining cycle of the largest severity dictated by the canal curvature. If it only rotates through part of a revolution, the straining cycle suffered is less severe. Fatigue life (Nf) follows a power law relation with the strain amplitude ea in the form (Bannantine et al. 1990): ð1Þ ea Nfn ¼ k; where k and n are material constants. Typical values of n reported for NiTi wire were 0.235 (Tobushi et al. 1997) and 0.28 (Matsui et al. 2004). With n < 0.3, Eqn. 1 implies that the damage caused by a large amplitude cycle is much more serious than a number of smaller amplitude cycles. This explains the general observation that reciprocating operations lead to prolonged fatigue life compared with the CR motion
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(De-Deus et al. 2010b, Varela-Pati~ no et al. 2010, Gavini et al. 2012, Lopes et al. 2013, Pedull a et al. 2013). It also explains the fatigue life increase with decreasing reciprocating amplitude observed in this study (Fig. 3) and by Gambarini et al. (2012). The SR motions in this work differ from the reciprocating operations in the literature in two ways: (i) an instrument will never complete a full revolution, and so there will only be smaller amplitude cycles, and (ii) instead of every point facing the same strain history in turn, the most seriously strained points are restricted to some fixed locations. The first phenomenon decreases fatigue damage per cycle, but the second concentrates cyclic damage accumulation at localized positions. The results from Fig. 3 show that the beneficial effect of reducing strain amplitude dominates over the detrimental localization of damage. In the PR motions, the most critically strained locations move forward to new locations during the periodic angular increment instead of remaining stationary. This effectively distributes fatigue damage to different points on the circumference, alleviating the detrimental effect of localized damage concentration. It leads to further improvement in fatigue life as shown in Fig. 5. There are two additional parameters to control besides the reciprocating amplitude in the PR motion, namely when to make the angular increment and by how much it should be. As pointed out before, completing a full revolution in a curved canal will result in a large amplitude cycle that causes more fatigue damage. A smaller progressive increment will suppress a fast completion of a full revolution and so should be better than a large one. The 7o increment was indeed more effective in enhancing fatigue life than the larger increments (Fig. 5). Unlike the other three increments, the 22.5o increment did not give additional life enhancement over the stationary 45o motion. This is because 360 is completely divisible by 22.5. As a result, the most seriously strained points will land on the same locations over and over again on completing a full revolution, concentrating the damage to these positions. Regarding the YR (Yared 2008), the instrument turns clockwise 144o and returns 72o, which means for every five reciprocating movements, the instrument completes one entire revolution (360o) and lands on the same location again. This may be the main reason why YR gave the lowest fatigue life among other PR motions in Fig. 6. From the above observation, the progressive increment should preferably be small and should not completely divide 360 to be effective.
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
There are two contrasting effects regarding the reciprocating cycle interval between which to make the progressive angular increment. A PR with very short interval approaches the CR motion while that with a very long interval approaches the SR motion. An optimum interval should lie between these two extremes. Figure 4 shows that both the 1-rc interval and the 30-rc interval are inferior to the 10-rc interval, which probably represents a more balanced compromise between the above two extreme situations. Reciprocating motions used in previous studies (Yared 2008, Gambarini et al. 2012, Kim et al. 2012, Lopes et al. 2013, Pedulla et al. 2013) may essentially be viewed as PR with rather large angular increments (e.g. 72o in Yared (2008) and 120o in Lopes et al. 2013) made at a 1-rc interval. Moreover, the angular increments often completely divide 360. These settings are not optimum parameters for fatigue life enhancement. Fatigue life of an instrument generally decreases when the canal curvature increases (Fig. 6). In terms of fatigue life enhancement over the CR motion, the larger the curvature, the less are the differences between the different movements. Nevertheless, the 45o/10rc/+7o motion still extended life significantly over the YR and the CR in the most difficult curvature tested. Under the CR motion, fatigue damage per cycle is heavy, crack initiation and propagation to fracture is rapid. The chance of nucleating a second crack is small. In the SR motions, although fatigue damage is lighter, it is localized and likely to lead to a single crack initiation. In the PR motions, the light fatigue damage is periodically relocated to new positions. This both delays crack nucleation and promotes the chance of multiple crack nucleation. The observation of multiple initiation sites (Fig. 7b) in all instruments fractured under the 45o/10rc/+7o motion is consistent with the above explanation. Regarding the shaping capability of the current progressive reciprocating operation, B€ urklein et al. (2012) showed that the use of Reciproc (VDW, Munich, Germany) decreased the preparation time by up to 60% compared with Mtwo (VDW) or ProTaper (Dentsply Maillefer, Ballaigues, Switzerland). Franco et al. (2011) reported that reciprocating movement shaped a preparation in a more uniform manner centred on the original canal. You et al. (2011) demonstrated that reciprocating motion did not cause excessive aberration even in the apical area of the curved canal when compared with CR motion. In
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
addition, the reciprocating motion also produces a beneficial shaping result by reducing the screw-in effect. It may be worthwhile to develop handpiece driving motors that can exercise the above PR motions to evaluate the shaping capability and fatigue life in artificial as well as real tooth canals for further assessment.
Conclusions Fatigue life increased with decreasing reciprocating amplitude in stationary reciprocation. A progressive reciprocating operation with a 45o reciprocating amplitude, and a + 7o progressive angular increment every 10 reciprocating cycles led to significant fatigue life enhancement and multiple fatigue crack initiation in NiTi endodontic instruments.
Acknowledgement The authors wish to thank the support of National Science Council, R.O.C. through the project NSC 96-2628-E-002 -223 -MY3.
References Bannantine JA, Comer JJ, Handrock JL (1990) Fundamentals of metal fatigue analysis. New Jersey: Prentice-Hall. B€ urklein S, Hinschitza K, Dammaschke T, Sch€ afer E (2012) Shaping ability and cleaning effectiveness of two single-file systems in severely curved root canals of extracted teeth: Reciproc and WaveOne versus Mtwo and ProTaper. International Endodontic Journal 45, 449–61. De-Deus G, Brandao MC, Barino B, Di Giorgi K, Fidel RA, Luna AS (2010a) Assessment of apically extruded debris produced by the single-file ProTaper F2 technique under reciprocating movement. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontology 110, 390–4. De-Deus G, Moreira EJ, Lopes HP, Elias CN (2010b) Extended cyclic fatigue life of F2 ProTaper instruments used in reciprocating movement. International Endodontic Journal 43, 1063–8. Di Fiore PM (2007) A dozen ways to prevent nickel-titanium rotary instrument fracture. The Journal of the American Dental Association 138, 196–201. Franco V, Fabiani C, Taschieri S, Malentacca A, Bortolin M, Del Fabbro M (2011) Investigation on the shaping ability of nickel-titanium files when used with a reciprocating motion. Journal of Endodontic 37, 1398–401. Gambarini G (2001) Cyclic fatigue of ProFile rotary instruments after prolonged clinical use. International Endodontic Journal 34, 386–9.
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Gambarini G, Rubini AG, Al Sudani D et al. (2012) Influence of different angles of reciprocation on the cyclic fatigue of nickel-titanium endodontic instruments. Journal of Endodontics 38, 1408–11. Gavini G, Caldeira CL, Akisue E, de Miranda Candeiro GT, Kawakami DAS (2012) Resistance to flexural fatigue of Reciproc R25 files under continuous rotation and reciprocating movement. Journal of Endodontics 38, 684–7. Kim HC, Kwak SW, Cheung GSP, Ko DH, Lee WC (2012) Cyclic fatigue and torsional resistance of two new nickeltitanium instruments used in reciprocation motion: Reciproc versus WaveOne. Journal of Endodontics 38, 541–4. Lopes HP, Elias CN, Vieira MVB et al. (2013) Fatigue Life of Reciproc and Mtwo instruments subjected to static and dynamic tests. Journal of Endodontics 39, 693–6. Matsui R, Tobushi H, Furuichi Y, Horikawa H (2004) Tensile deformation and rotating-bending fatigue properties of a highelastic thin wire, a superelastic thin wire, and a superelastic thin tube of NiTi alloys. Journal of Engineering Materials and Technology 126, 384–91. Paque F, Zehnder M, De-Deus G (2011) Microtomographybased Comparison of Reciprocating Single-File F2 ProTaper Technique versus Rotary Full Sequence. Journal of Endodontics 37, 1394–7. Parashos P, Gordon I, Messer HH (2004) Factors influencing defects of rotary nickel- titanium endodontic instruments after clinical use. Journal of Endodontics 30, 722–5. Pedull a E, Grande NM, Plotino G, Gambarini G, Rapisarda E (2013) Influence of continuous or reciprocating motion on
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cyclic fatigue resistance of 4 different nickel-titanium rotary instruments. Journal of Endodontics 39, 258–61. Sch€ afer E, Schulz-Bongert U, Tulus G (2004) Comparison of hand stainless steel and nickel titanium rotary instrumentation: a clinical study. Journal of Endodontics 30, 432–5. Souter NJ, Messer HH (2005) Complications associated with fractured file removal using an ultrasonic technique. Journal of Endodontics 31, 450–2. Spili P, Parashos P, Messer HH (2005) The impact of instrument fracture on outcome of endodontic treatment. Journal of Endodontics 31, 845–50. Tobushi H, Hachisuka T, Yamada S, Lin PH (1997) Rotating-bending fatigue of a TiNi shape-memory alloy wire. Mechanics of Materials 26, 35–42. Varela-Pati~ no P, Iba~ nez-P arraga A, Rivas-Mundi~ na B, Cantatore G, Otero XL, Martin-Biedma B (2010) Alternating versus continuous rotation: a comparative study of the effect on instrument life. Journal of Endodontics 36, 157–9. Yared G (2008) Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. International Endodontic Journal 41, 339–44. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W (2010) Lifespan of one nickel–titanium rotary file with reciprocating motion in curved root canals. Journal of Endodontics 36, 1991–4. You SY, Kim HC, Bae KS, Baek SH, Kum KY, Lee W (2011) Shaping ability of reciprocating motion in curved root canals: a comparative study with micro–computed tomography. Journal of Endodontics 37, 1296–300.
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Fatigue life enhancement of NiTi rotary endodontic instruments by progressive reciprocating operation
C. S. Shin1, Y. H. Huang1, C. W. Chi2 & C. P. Lin2 1
Department of Mechanical Engineering, National Taiwan University; and 2Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University and National Taiwan University Hospital, Taipei, Taiwan
Abstract Shin CS, Huang YH, Chi CW, Lin CP. Fatigue life enhancement of NiTi rotary endodontic instruments by progressive reciprocating operation. International Endodontic Journal, 47, 882–888, 2014.
Aim To evaluate the effect of reciprocating amplitude and progressive angular increment on fatigue life enhancement of NiTi rotary endodontic instruments. Methodology ProTaper F2 instruments were operated in steel artificial canals with both stationary reciprocating (SR) and progressive reciprocating (PR) motions. The SR motions involved symmetric to and fro reciprocation of 180o, 135o, 90o, 60o and 45o. The PR motions were 45o stationary motion superimposed with angular increments of 7o, 11o, 22.5o or 31o whenever an instrument completed 1, 10 or 30 reciprocating cycles (rc). The fatigue lives were compared with those under continuous rotation (CR) and a reciprocating operation with a forward 144o and backward 72o motion proposed by Yared (2008). The statistical significance of these operating modes on fatigue life was examined using one way ANOVA and post hoc Tukey’s tests at P = 0.05. Fractographic analysis was also applied to probe the fracture mechanisms of different rotation motions.
Results Fatigue life increased with decreasing reciprocating amplitude. Operating in the SR increased fatigue life by 355% over that in the CR. Except for the 22.5o increment, all PR motions yielded longer fatigue lives than the SR motion. A progressive reciprocating operation with a 45o reciprocating amplitude and a + 7o progressive angular increment every 10 reciprocating cycles (45o/10rc/+7o) increased fatigue life by 990% over that in the CR motion. In terms of life enhancement over the CR motion, the larger the curvature the less are the differences between different movements. Single crack initiation sites were found in the CR and SR motions, while three crack initiation sites were typical in the 45o/ 10rc/+7o motion. Conclusions Fatigue life increased with decreasing reciprocating amplitude in stationary reciprocation. A progressive reciprocating operation with 45o/10rc/ +7o motion led to significant fatigue life enhancement and multiple fatigue crack initiation in NiTi endodontic instruments. Keywords: cyclic fatigue failure, fatigue life enhancement, fractographic analysis, NiTi endodontic instrument, progressive reciprocation, stationary reciprocation. Received 6 August 2013; accepted 16 December 2013
Introduction Correspondence: Chun-Pin Lin, School of Dentistry, National Taiwan University and National Taiwan University Hospital, No.1, Chang-Te Street, Taipei 10048, Taiwan (Tel.: +886 2 23562148; fax: +886 2 2383 1346; e-mail: pinlin@ntu. edu.tw).
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International Endodontic Journal, 47, 882–888, 2014
NiTi rotary instruments have been used in root canal treatment because of their superelasticity. Root canal preparation with NiTi rotary instruments could maintain the appropriate canal centrality and provide more predictable outcomes than stainless steel files
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
(Sch€ afer et al. 2004). However, intracanal NiTi instrument fracture can jeopardize the success of treatment and be a major concern (Souter & Messer 2005, Spili et al. 2005). In fact, fatigue fracture is one of the predominant intracanal failure modes (Parashos et al. 2004), and it arises because the instruments suffer alternating tensile and compressive stresses induced by rotating inside curved canals. Various techniques have been proposed that may alleviate instrument fracture (Di Fiore 2007). Instead of the conventional continuous rotation motion (CR), Yared (2008) proposed a single instrument technique that employed a reciprocating operation with a forward 144o and backward 72o reciprocating motion (YR). This technique was found to be more cost-effective and time-saving (You et al. 2010, Paque et al. 2011) with the shaping outcome being similar (De-Deus et al. 2010a, Paque et al. 2011) or even better (Franco et al. 2011). Moreover, the reciprocating operation extended the fatigue life of the instrument when compared with conventional rotary movement (De-Deus et al. 2010b, VarelaPati~ no et al. 2010, Gavini et al. 2012, Lopes et al. 2013, Pedull a et al. 2013). Previous research used reciprocating movement produced by commercial handpieces and motors with rotating kinematics set by the manufacturer. There was little flexibility to change the forward and reverse rotational angles in the reciprocating action, and so the effect of changing the reciprocating amplitude was seldom investigated. Recently, it was reported that reducing the reciprocating angle increased fatigue life (Gambarini et al. 2012). During continuous rotation, an arbitrary point on an instrument will be moving along a circle (shown schematically in Fig. 1). During a stationary reciprocating motion (SR), the reference point will traverse the arc
Figure 1 Path of a reference point during continuous rotation and reciprocating operations, illustrating the reciprocating amplitude /a and angular increment /inc.
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
AB, starting from A, stop momentarily at B and traverse back to A. The angle subtended by the arc AB will be referred to as the reciprocating amplitude (/a in Fig. 1). To avoid fatigue damage localization, the traversing path may be shifted forward (e.g. from along AB to along CD) by an angular increment (/inc in Fig. 1). The periodic forward shifting of the reciprocating path will be referred to as progressive reciprocating motion (PR). The magnitude and the frequency of occurrence of the angular increment govern when the reference point goes past point A again. When the latter happens, the instrument completes a full revolution and is subjected to a straining cycle of the largest severity dictated by the canal curvature. This large straining cycle and the small straining cycles from the partial revolutions during reciprocation have different impacts on fatigue damage. Previous studies have not distinguished these two effects and implemented an angular increment after every reciprocating cycle. The two effects have been studied separately in this study by controlling the reciprocating amplitude and the magnitude of angular increment independently. It is hoped that this can help to cast light on the fracture mechanisms of CR, SR and PR, as well as on the development of operating mode that can significantly prolong fatigue life and alleviate unexpected clinical fracture of rotary endodontic instruments.
Materials and methods Cyclic fatigue testing in artificial canals was carried out using ProTaperâ F2 instruments (Dentsply Maillefer, Ballaigues, Switzerland) with an effective length of 21 mm. The artificial canals were formed by a steel cylinder of radius R and an exterior steel block with a matching circular arc, similar to that employed by Gambarini (2001). Three radii R (5, 7.5 and 10 mm) were used, and the depth of instrument insertion was varied to produce angles of curvature h of 40o and 60o. The specimens were video-taped during testing. The exact time of fracture could then be accurately determined through playing back the video. The instruments were operated by a DC servo motor (CS60-150C5AE, CSIM Inc., Taipei, Taiwan) driven by a servo driver (CSBL900, CSIM Inc., Taipei, Taiwan). The driver was controlled by a motor controller (PCI-7390, National Instrument, Austin, TX, USA) through a program written in Labview (National Instrument, Austin, TX, USA) in which the rotational speed, forward and reverse rotation angles of any cycle could be specified independently. Two
International Endodontic Journal, 47, 882–888, 2014
883
Fatigue life enhancement by progressive reciprocation Shin et al.
categories of reciprocating motions were examined. The first category was SR with reciprocating amplitudes of 180o, 135o, 90o, 60o and 45o. The second category was PR that involved 45o SR motion superimposed with a forward angular increment of 7o, 11o, 22.5o or 31o whenever an instrument completed 1, 10 or 30 reciprocating cycles (rc). A PR motion was designated in a three-number code such as 45o/10rc/+7o, meaning a stationary reciprocation of 45o with an angular increment of +7o on completion of 10 reciprocating cycles. The CR and YR motions were used as benchmarks to gauge the fatigue life performances of the above operating modes. The instrument bending condition of R7.5h40o was mainly employed. Other combinations of R and h were used to check the performance of the operating mode that gave the best enhancement of fatigue life. Figure 2 summarizes the test matrix employed. For each condition, five instruments were tested. The fracture surfaces of some of the instruments were examined with a scanning electron microscope (SEM, JSM 5610, Jeol, Tokyo, Japan). The average rotational speeds were set to be equivalent to 450 rpm in the CR. The number of reciprocating cycle per minute was confirmed by a stroboscope (PKDS-112, Pokai technology, Taipei, Taiwan). Fatigue life of an instrument was quantified by the total angular distance travelled before fracture, measured in terms of equivalent revolutions. Statistical significance of fatigue life difference under different operating modes was checked for using one way ANOVA and post hoc Tukey’s test at P = 0.05 with a statistical analysis software (SPSS 16 for Windows, Chicago, IL, USA).
Results As shown in Fig. 3, operating in the SR increased fatigue life by ~50% (in 180o) to 355% (in 45o) over that in the CR. Fatigue life increased with a
decrease in reciprocating amplitude. Except for the 180o and YR, all other reciprocating motions had significantly longer lives than the CR motion. Life under the 45o motion was significantly longer than that under all other motions (P < 0.001). Life under the 60o motion came next, but its difference from that under 90o was not significant. The life under YR lay between that for the 180o and 135o motions. The differences between these three motions were not significant. Figure 4 compares the fatigue lives among 45o/ 1rc/+7o, 45o/10rc/+7o and 45o/30rc/+7o. These were significantly different with P < 0.005. 45o/ 10rc/+7o led to more life enhancement than the other two motions. 45o/1rc/+7o gave a comparable fatigue life to the stationary 45o motion. The effect of applying different angular increment in the PR motions is shown in Fig. 5. Progressive angular increment was made whenever an instrument completed 10 reciprocating cycles. Except for the 22.5o increment, all PR motions yielded longer fatigue lives than the SR motion. The progressive increment of 7o prolonged life significantly above that of 11o and 31o (P < 0.005), and the corresponding fatigue life enhancement amounted to ~990% above that under the CR motion. As shown in Fig. 6, the beneficial effect of the SR and PR operations on fatigue life is a general phenomenon occurring in different canal geometries. For each of the four combinations of R and h tested, the 45o/10rc/+7o motion gave the longest fatigue life, followed by the 45o SR motion. YR came next and always gave a longer life than the CR motion. However, for the R7.5h40o and R10h60o canals, the differences between the last two motions were not statistically significant. Figure 7 shows typical low magnification SEM views of instrument sections fractured under the CR and the 45o/10rc/+7o motions. White arrows in the figures point to crack initiation locations where diver-
Figure 2 Test matrix showing the combination of reciprocation amplitudes (/a), magnitude and frequency of angular increment (/inc) employed. rc: reciprocating cycle; CR: continuous rotation motion; YR: a forward 144o and reciprocating backward 72o motion proposed by Yared.
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© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
Figure 3 Fatigue lives of instruments in a R7.5h40o canal under various stationary reciprocating (SR) amplitudes compared with that under the continuous rotation motion (CR) and a forward 144o and backward 72o reciprocating motion proposed by Yared (YR). The same alphabet above the error bar denotes data groups with P > 0.05.
Figure 5 Effect of different periodic angular increment on fatigue lives of the instruments in a R7.5h40o canal for progressive reciprocation with amplitude of 45o. The same alphabet above the error bar denotes data groups with P > 0.05. CR: continuous rotation motion; YR: a forward 144o and backward 72o reciprocating motion proposed by Yared; 0o corresponds to stationary reciprocation.
Figure 4 Effect of different reciprocating cycles (rc) for periodic angular increment on fatigue lives of the instruments.
gent radial lines originated. Higher magnification of the radial line region revealed striation markings characteristic of fatigue crack propagation. Beyond the dotted boundaries were dimple structures representing the final fracture stage. The fracture surfaces resulting from the CR and SR motions all showed a single crack initiation site (Fig. 7a). The 45o/10rc/ +7o fracture surfaces all exhibited 3 crack initiation sites (Fig. 7b).
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Figure 6 Comparison of fatigue lives in 4 different canal geometries under stationary reciprocation of 45o, progressive 45o/10rc/+7o mode, a forward 144o and backward 72o reciprocating motion proposed by Yared (YR) and the continuous rotation motion (CR).
Discussion Analysis of the rotation mechanics shows that by completing a full revolution, an instrument undergoes a reverse bending and suffers a tension–com-
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(a)
(b)
Figure 7 SEM fractographs of instruments operated under (a) continuous rotation motion; (b) progressive 45o/10rc/ +7o. White arrows indicate crack nucleation sites. Dotted lines demarcate boundaries between fatigue striation region and dimple structure region.
pression straining cycle of the largest severity dictated by the canal curvature. If it only rotates through part of a revolution, the straining cycle suffered is less severe. Fatigue life (Nf) follows a power law relation with the strain amplitude ea in the form (Bannantine et al. 1990): ð1Þ ea Nfn ¼ k; where k and n are material constants. Typical values of n reported for NiTi wire were 0.235 (Tobushi et al. 1997) and 0.28 (Matsui et al. 2004). With n < 0.3, Eqn. 1 implies that the damage caused by a large amplitude cycle is much more serious than a number of smaller amplitude cycles. This explains the general observation that reciprocating operations lead to prolonged fatigue life compared with the CR motion
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(De-Deus et al. 2010b, Varela-Pati~ no et al. 2010, Gavini et al. 2012, Lopes et al. 2013, Pedull a et al. 2013). It also explains the fatigue life increase with decreasing reciprocating amplitude observed in this study (Fig. 3) and by Gambarini et al. (2012). The SR motions in this work differ from the reciprocating operations in the literature in two ways: (i) an instrument will never complete a full revolution, and so there will only be smaller amplitude cycles, and (ii) instead of every point facing the same strain history in turn, the most seriously strained points are restricted to some fixed locations. The first phenomenon decreases fatigue damage per cycle, but the second concentrates cyclic damage accumulation at localized positions. The results from Fig. 3 show that the beneficial effect of reducing strain amplitude dominates over the detrimental localization of damage. In the PR motions, the most critically strained locations move forward to new locations during the periodic angular increment instead of remaining stationary. This effectively distributes fatigue damage to different points on the circumference, alleviating the detrimental effect of localized damage concentration. It leads to further improvement in fatigue life as shown in Fig. 5. There are two additional parameters to control besides the reciprocating amplitude in the PR motion, namely when to make the angular increment and by how much it should be. As pointed out before, completing a full revolution in a curved canal will result in a large amplitude cycle that causes more fatigue damage. A smaller progressive increment will suppress a fast completion of a full revolution and so should be better than a large one. The 7o increment was indeed more effective in enhancing fatigue life than the larger increments (Fig. 5). Unlike the other three increments, the 22.5o increment did not give additional life enhancement over the stationary 45o motion. This is because 360 is completely divisible by 22.5. As a result, the most seriously strained points will land on the same locations over and over again on completing a full revolution, concentrating the damage to these positions. Regarding the YR (Yared 2008), the instrument turns clockwise 144o and returns 72o, which means for every five reciprocating movements, the instrument completes one entire revolution (360o) and lands on the same location again. This may be the main reason why YR gave the lowest fatigue life among other PR motions in Fig. 6. From the above observation, the progressive increment should preferably be small and should not completely divide 360 to be effective.
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
Shin et al. Fatigue life enhancement by progressive reciprocation
There are two contrasting effects regarding the reciprocating cycle interval between which to make the progressive angular increment. A PR with very short interval approaches the CR motion while that with a very long interval approaches the SR motion. An optimum interval should lie between these two extremes. Figure 4 shows that both the 1-rc interval and the 30-rc interval are inferior to the 10-rc interval, which probably represents a more balanced compromise between the above two extreme situations. Reciprocating motions used in previous studies (Yared 2008, Gambarini et al. 2012, Kim et al. 2012, Lopes et al. 2013, Pedulla et al. 2013) may essentially be viewed as PR with rather large angular increments (e.g. 72o in Yared (2008) and 120o in Lopes et al. 2013) made at a 1-rc interval. Moreover, the angular increments often completely divide 360. These settings are not optimum parameters for fatigue life enhancement. Fatigue life of an instrument generally decreases when the canal curvature increases (Fig. 6). In terms of fatigue life enhancement over the CR motion, the larger the curvature, the less are the differences between the different movements. Nevertheless, the 45o/10rc/+7o motion still extended life significantly over the YR and the CR in the most difficult curvature tested. Under the CR motion, fatigue damage per cycle is heavy, crack initiation and propagation to fracture is rapid. The chance of nucleating a second crack is small. In the SR motions, although fatigue damage is lighter, it is localized and likely to lead to a single crack initiation. In the PR motions, the light fatigue damage is periodically relocated to new positions. This both delays crack nucleation and promotes the chance of multiple crack nucleation. The observation of multiple initiation sites (Fig. 7b) in all instruments fractured under the 45o/10rc/+7o motion is consistent with the above explanation. Regarding the shaping capability of the current progressive reciprocating operation, B€ urklein et al. (2012) showed that the use of Reciproc (VDW, Munich, Germany) decreased the preparation time by up to 60% compared with Mtwo (VDW) or ProTaper (Dentsply Maillefer, Ballaigues, Switzerland). Franco et al. (2011) reported that reciprocating movement shaped a preparation in a more uniform manner centred on the original canal. You et al. (2011) demonstrated that reciprocating motion did not cause excessive aberration even in the apical area of the curved canal when compared with CR motion. In
© 2013 International Endodontic Journal. Published by John Wiley & Sons Ltd
addition, the reciprocating motion also produces a beneficial shaping result by reducing the screw-in effect. It may be worthwhile to develop handpiece driving motors that can exercise the above PR motions to evaluate the shaping capability and fatigue life in artificial as well as real tooth canals for further assessment.
Conclusions Fatigue life increased with decreasing reciprocating amplitude in stationary reciprocation. A progressive reciprocating operation with a 45o reciprocating amplitude, and a + 7o progressive angular increment every 10 reciprocating cycles led to significant fatigue life enhancement and multiple fatigue crack initiation in NiTi endodontic instruments.
Acknowledgement The authors wish to thank the support of National Science Council, R.O.C. through the project NSC 96-2628-E-002 -223 -MY3.
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