Effect of autoclave sterilization on the cyclic fatigue resistance of thermally treated Nickel Titanium instruments

Transcription

1 doi: /iej Effect of autoclave sterilization on the cyclic fatigue resistance of thermally treated Nickel Titanium instruments D. Zhao 1, Y. Shen 2,3, B. Peng 1 & M. Haapasalo 2 1 Department of Operative Dentistry and Endodontics, School and Hospital of Stomatology, Wuhan University, Wuhan, China; 2 Division of Endodontics, Department of Oral Biological & Medical Sciences, Faculty of Dentistry, The University of British Columbia, Vancouver, BC; and 3 Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada Abstract Zhao D, Shen Y, Peng B, Haapasalo M. Effect of autoclave sterilization on the cyclic fatigue resistance of thermally treated Nickel Titanium instruments. International Endodontic Journal, 49, , Aim To compare the cyclic fatigue resistance of HyFlex CM, Twisted Files (TF), K3XF, Race, and K3, and evaluate the effect of autoclave sterilization on the cyclic fatigue resistance of these instruments both before and after the files were cycled. Methodology Five types of NiTi instruments with similar size 30,.06 taper were selected: HyFlex CM, TF, K3XF, Race and K3. Files were tested in a simulated canal with a curvature of 60 and a radius of 3 mm. The number of cycles to failure of each instrument was determined to evaluate cyclic fatigue resistance. Each type of instruments was randomly divided into four experimental groups: group 1 (n = 20), unsterilized instruments; group 2 (n = 20), pre-sterilized instruments subjected to 10 cycles of autoclave sterilization; group 3 (n = 20), instruments tested were sterilized at 25%, 50% and 75% of the mean cycles to failure as determined in group 1, and then cycled to failure; group 4 (n = 20), instruments cycled in the same manner as group 3 but without sterilization. The fracture surfaces of instruments were examined by scanning electron microscopy (SEM). Results HyFlex CM, TF and K3XF had significantly higher cyclic fatigue resistance than Race and K3 in the unsterilized group 1 (P < 0.05). Autoclave sterilization significantly increased the MCF of HyFlex CM and K3XF (P < 0.05) both before and after the files were cycled. SEM examination revealed a typical pattern of cyclic fatigue fracture in all instruments. Conclusions HyFlex CM, TF and K3XF instruments composed of new thermal-treated alloy were more resistant to fatigue failure than Race and K3. Autoclaving extended the cyclic fatigue life of HyFlex CM and K3XF. Keywords: autoclave sterilization, cyclic fatigue, NiTi instrument, thermal treated. Received 14 November 2014; accepted 9 September 2015 Introduction The fracture of instruments used in rotary motion occurs in two ways: torsion and flexural cyclic fatigue Correspondence: Bin Peng, State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory of Oral Biomedicine Ministry of Education, School & Hospital of Stomatology, Wuhan University, 237 Luoyu Road, Wuhan , P.R. China (Tel: ; Fax: ; phs301@vip.163.com). (Sattapan et al. 2000, Plotino et al. 2009, Shen & Cheung 2013). Cyclic fatigue fracture is caused by repeated tension/compression cycles accumulated at the point of maximum flexure in a curved canal. To reduce the risk of intracanal fracture caused by cyclic fatigue, new file designs and manufacturing methods have been developed. Thermal processing procedures which optimize the microstructure of NiTi alloys are one of the most fundamental approaches that affect the fatigue resistance of NiTi endodontic files (Shen et al. 2012, 2013a,b,c). CM Wire (DS Dental, Johnson 990 International Endodontic Journal, 49, , International Endodontic Journal. Published by John Wiley & Sons Ltd

2 Zhao et al. Cyclic fatigue resistance of thermally treated NiTi instruments City, TN, USA) is a novel NiTi alloy introduced in HyFlex CM (HyFlex; Coltene Whaledent, Cuyahoga Falls, OH, USA) files made from CM wire have been manufactured using a special thermomechanical process that controls the memory of the material, making the files extremely flexible and highly resistant to cyclic fatigue (Shen et al. 2011, Zhao et al. 2013, Plotino et al. 2014). R-phase wire (SybronEndo, Orange, CA, USA) is another form of thermal-treated NiTi alloy. Twisted Files (TF; SybronEndo) made from R-phase wire with twisting (instead of grinding) was introduced in The manufacturer claims that their heat treatment of R-phase combined with proprietary twisting process provided superior flexibility and resistance to fatigue. K3XF (SybronEndo) is manufactured using the basic features of the conventional K3 NiTi rotary instruments (SybronEndo) plus the proprietary R-phase technology. To pursue a safe and efficient use of NiTi instruments, manufacturers, scientists and clinicians have attempted several ways to evaluate their resistance to cyclic fatigue. An ideal model to test cyclic fatigue resistance would involve instrumentation of curved canals in natural teeth. However, in such tests, a tooth can only be used once and the shape of the root canal will change during instrumentation, which makes it impossible to standardize experimental conditions. As a result, several devices such as glass or metal tubes have been used testing (Plotino et al. 2009). However, there has been no international laboratory standard used to investigate the cyclic fatigue fracture resistance of NiTi rotary endodontic instruments. NiTi instruments are usually reused in clinical practice for economic reasons, which leads to their repeated autoclave sterilization. NiTi shape memory and superelastic properties are considered to be highly dependent on the thermomechanical processing history of the manufactured product (Zinelis et al. 2007). Yahata et al. (2009) suggested additional heat retreatment of NiTi instruments during autoclave sterilization may increase this flexibility. Silvaggio & Hicks (1997) demonstrated that 10 cycles of heat sterilization of ProFile instruments (Dentsply Maillefer, Ballaigues, Switzerland) did not increase likelihood of fracture. Viana et al. (2006) reported that ProFile NiTi rotary files had a higher mean number of cycles to failure (NCF) when exposed to both dry heat and autoclave sterilization. However, Mize et al. (1998) reported that exposure to heat during autoclave sterilization did not increase the cyclic fatigue resistance of Lightspeed instruments (Lightspeed Inc., San Antonio, TX, USA). Thus, this study aimed to compare the cyclic fatigue resistance between thermally treated and traditional NiTi instruments, as well as to evaluate the effect of autoclave sterilization on the cyclic fatigue resistance of these instruments. The null hypothesis is that there is no difference in fatigue resistance between thermally treated and conventional NiTi files, and autoclave sterilization does not affect their fatigue resistance. Materials and methods Three NiTi rotary endodontic instruments made from thermal-treated NiTi alloy [HyFlex CM (Coltene Whaledent), TF and K3XF (SybronEndo)] and two NiTi rotary endodontic instruments made from traditional NiTi alloy [Race (FKG Dentaire, La-Chaux-de- Fonds, Switzerland) and K3 (SybronEndo)] with same size 30,.06 taper were selected. The effect of autoclave sterilization on the NCF was tested both before and after the files were cycled. Each type of instruments was randomly divided into four experimental groups. For group 1, files (n = 20) in each group were tested without sterilization to establish baseline mean cycles to failure (MCF). For group 2, files (n = 20) in each group were tested after 10 cycles of autoclave sterilization (Melag Autoklav Vacuklav 23 B+, Melag ohg, Germany). Each cycle was performed including sterilization at 134 C, with a pressure of 30 psi, for 20 min and then dying for 15 min (Plotino et al. 2012). For group 3, files (n = 20) in each group were autoclaved at 25%, 50% and 75% of the mean cycles to failure (MCF) determined in group 1. All remaining files were then cycled to failure (Hilfer et al. 2011). The autoclave sterilization was performed at 134 C, with a pressure of 30 psi, for 5 min (Mize et al. 1998). For group 4, files (n = 20) in each group were cycled in the same manner as group 3 without sterilization. All the files were subjected to cyclic fatigue tests in a metal block with a 1.5-mm-wide simulated canal with 60 curve and a 3 mm radius (Fig. 1). The block with dimensions of 36.8 mm mm mm was covered with a swivelling acrylic face to observe the files rotating in the canal and to remove broken files during the test. The canal space was filled with synthetic oil (VV Lubricant No. 56; Whip Mix Corporation, Louisville, KY, USA) to reduce friction. All files were placed in the simulated canal to 19 mm. A 1:16 reduction contra-angle handpiece (ATR Tecnika Vision, Dentsply Tulsa Dental, Tulsa, OK, USA) was mounted on an Instron Testing Machine 2015 International Endodontic Journal. Published by John Wiley & Sons Ltd International Endodontic Journal, 49, ,

3 Cyclic fatigue resistance of thermally treated NiTi instruments Zhao et al. (a) (b) Figure 1 (a) Simulated canals in a metal block, at 60 and 3 mm curvature radius, and (b) fatigue test of TF in the block. (Mechanical Tester 8841; Instron Corp., Canton, MA, USA), allowing a precise and reproducible placement of each instrument in the canal. The instruments were rotated until failure occurred at the speed and torque recommended by the manufacturers. HyFlex CM and TF were rotated at 500 rpm, whereas the other files were rotated at 300 rpm. Instrument fracture was detected visually. The time to failure was measured to the 1/100 s using a digital stopwatch (Timex, Middlebury, CT, USA). For each instrument, the time to failure was multiplied by the number of rotations per minute to obtain the NCF. After cyclic fatigue tests were performed, all of the broken instruments were ultrasonically cleaned in acetone for 5 min to remove oil and then treated with absolute alcohol for 5 min to remove water. The fractured surfaces of the instruments were observed under scanning electron microscopy (SEM) (FEI Sirion FEG, Eindhoven, the Netherlands) to evaluate the topographic characteristics of the fractured surfaces. Statistical analysis The number of cycles to failure for the various groups was analysed by ANOVA in SPSS 11.0 (SPSS Inc., Chicago, IL, USA). Means and standard deviations of NCF were calculated for each system. Tukey s post hoc analysis was applied to determine significant differences between groups. Statistical significance was set at a confidence level of 95%. P < 0.05 was considered to be statistically significant. Results The number of instruments sterilized at 25%, 50% and 75% of the mean cycles to failure (MCF) is shown in Table 1. Some instruments fractured during the process of pre-cycling to 50% and 75% of MCF (Table 1). The data of cycles to failure for the instruments in the four groups are summarized in Table 2. Unsterilized HyFlex CM, TF and K3XF had significantly higher cyclic fatigue resistance than Race and K3 (group 1) (P < 0.05) (Table 1). A significant difference was observed between unsterilized (group 1) and pre-sterilized (group 2) files for HyFlex CM and K3XF (P < 0.05), whilst other instruments were not significantly different (P > 0.05) for mean NCF (Table 2). Furthermore, a significantly higher mean NCF (P < 0.05) was observed for HyFlex CM and K3XF files sterilized before use and after the three period (25%, 50% and 75%) of cycling (group 3) than for the same files sterilized only after the periods of cyclic stress (group 4). SEM images of the fracture surfaces with crack initiation and dimple regions for each type of instrument are shown in Fig. 2. Discussion The two scenarios, multiple sterilizations of unused instruments and repeated sterilization of files after being challenged by stepwise increased amount of cyclic stress (fatigue), were examined in the current study. Sterilization of rotary NiTi files must be done before 992 International Endodontic Journal, 49, , International Endodontic Journal. Published by John Wiley & Sons Ltd

4 Zhao et al. Cyclic fatigue resistance of thermally treated NiTi instruments Table 1 Baseline scores of fatigue lifespan [the mean cycles to failure (MCF)] for all NiTi instruments, as well as the number of instruments sterilized at 25%, 50% and 75% of the MCF HyFlex CM Twisted Files K3XF K3 Race MCF N(G3) N(G4) MCF N(G3) N(G4) MCF N(G3) N(G4) MCF N(G3) N(G4) MCF N(G3) N(G4) No sterilization a a a b b 25% of MCF % of MCF % of MCF G3: group 3; G4: group 4; N: the number of instruments. Different superscript letters indicate statistically differences between groups (P < 0.05). use (except for pre-sterilized files), and this is repeated if the files are reused and/or carried forward between appointments of the same patient. Many practitioners prefer having pre-arranged sets of selected files, all of which may not be used during a single treatment. As a result, unused rotary files are also subjected to multiple autoclave cycles. Most of the previous studies focused on evaluating the effect of repeated sterilization on unused instruments (Mize et al. 1998, Viana et al. 2006, Valois et al. 2008, Plotino et al. 2012). Only one study (Hilfer et al. 2011) observed the effects of multiple sterilization on cyclic fatigue of thermally treated NiTi instruments (TF and GTX), which were exposed to defined controlled stresses. A metal block with simulated canals (Fig. 1) was used to evaluate fatigue resistance. The canal had two straight portions with a 60 o curve with a small radius between them. Whilst the simplified model in the metal groove cannot represent complicated clinical situations, it has been used in studies to standardize the experimental conditions for cyclic fatigue testing and allows comparisons with previous studies. In the present study, the fatigue resistance of thermally treated, unsterilized NiTi instruments (HyFlex CM, K3XF and TF) was higher than that of superelastic NiTi instruments (K3 and Race). This is in accordance with several recent studies (Larsen et al. 2009, Gambarini et al. 2011, Bouska et al. 2012, Plotino et al. 2012, Shen et al. 2013b). Interestingly, in the present study, the three thermally treated NiTi instruments without sterilization were equally resistant to cyclic fatigue. Resistance to cyclic fatigue of both HyFlex CM and K3XF instruments that had not undergone cyclic stress increased significantly after sterilization (10 ster. cycles). No corresponding increase was detected with the TF files and the superelastic NiTi instruments in the present study. The literature of the impact of sterilization on conventional superelastic NiTi instruments appears contradictory. Viana et al. (2006) reported that five autoclave cycles significantly increased the cyclic fatigue resistance of ProFile instruments. Later, Zinelis et al. (2007) reported that sterilization in an autoclave below 170 C did not provide enough energy to enable a change in the crystalline phase. Plotino et al. (2012) reported that repeated cycles (10 cycles) of autoclave sterilization (134 C for 35 min) did not influence the mechanical properties of K3, Mtwo (VDW, Munich, Germany) and Vortex (Dentsply Tulsa Dental Specialties) NiTi instruments. However, the K3XF prototypes demonstrated a significant increase in 2015 International Endodontic Journal. Published by John Wiley & Sons Ltd International Endodontic Journal, 49, ,

5 Cyclic fatigue resistance of thermally treated NiTi instruments Zhao et al. Table 2 The number of cycles to failure for instruments that were not sterilized at any point of the experiment (group 1) instruments sterilized 10 times before any use (group 2), instruments sterilized at 0%, 25%, 50% and 75% of the MCF (group 3) and instruments sterilized at 25%, 50% and 75% of the mean cycles to failure (group 4) (Mean SD) HyFlex CM e Twisted Files e K3XF e K3 f Race f No sterilization (group 1) a,d a a b b Sterilization 10 times before cyclic stress (group 2) c a c b b Sterilized at start and after each cyclic stress (group 3) d a d b b No sterilization at start, only after each cyclic stress (group 4) a a a b b Different superscript letters indicate statistically significant differences between groups (P < 0.05). (a) (b) (c) (d) (e) Figure 2 Scanning electron micrographs of the fracture surfaces of (a) TF (9200), (b) Race (9150), (c) K3XF (9150), (d) K3 (9150), (e) HyFlex CM (9150) and instruments with the region of fatigue crack propagation and dimple area outlined (dotted line) with crack origins (arrows). cyclic fatigue resistance after autoclaving in the same study, which was also confirmed by the present study. Although R-phase is found on both K3XF and TF files, only the TF instruments undergo a twisting process during their manufacture. It is thus possible that the subsurface microstructure of the thermally treated NiTi instruments manufactured by grinding (vs. twisting) before the cyclic stress may be altered by the sterilization process. In the present study, autoclave sterilization after cyclic stress significantly improved the cyclic fatigue behaviour of HyFlex CM and K3XF files, whereas TF, K3 and Race were not affected (size 30,.06 taper). Hilfer et al. (2011) reported a significant decrease of fatigue resistance after autoclaving at 25%, 50% and 75% of the established mean cycles to failure for TF (size 25,.06 taper). However, size 25,.04 taper TF instruments in the same study were not significantly 994 International Endodontic Journal, 49, , International Endodontic Journal. Published by John Wiley & Sons Ltd

6 Zhao et al. Cyclic fatigue resistance of thermally treated NiTi instruments affected by the same conditions (Hilfer et al. 2011). The different results between the present study and that of Hilfer et al. (2011) may be explained by different instrument size, sterilization condition (132 C for 6 min vs. 134 C for 5 min) and fatigue test set-up (90 and a 5 mm curvature radius vs. 60 and a 3 mm curvature radius). The instruments were submitted to standardized fatigue testing only in the current study. In a clinical situation, the instrument may be subjected to repeated, reverse torsional loads and cyclic fatigue stresses inside the canal at the same time. The data obtained here cannot be directly extrapolated to clinical conditions, and any conclusions from the present study must be drawn with caution. Conclusions Autoclave sterilization before use of HyFlex CM, TF and K3XF, significantly increased their resistance to cyclic fatigue, whilst no effect was detected with Race and K3. The autoclave sterilization of HyFlex CM and K3XF files after being challenged by cyclic fatigue significantly extended their remaining cyclic fatigue lifetime. Acknowledgement This study was financially supported by grants from Hubei Natural Science Foundation (no. 2014CFB468) and the Fundamental Research Funds for the Central Universities (no kf0103) of China. References Bouska J, Justman B, Williamson A et al. (2012) Resistance to cyclic fatigue failure of a new endodontic rotary file. Journal of Endodontics 38, Gambarini G, Plotino G, Grande NM et al. (2011) Mechanical properties of nickel titanium rotary instruments produced with a new manufacturing technique. International Endodontic Journal 44, Hilfer PB, Bergeron BE, Mayerchak MJ et al. (2011) Multiple antoclave cycle effects on cyclic fatigue of nickel-titanium rotary files produced by new manufacturing methods. Journal of Endodontics 37, Larsen CM, Watanabe I, Glickman GN, He J (2009) Cyclic fatigue analysis of a new generation of nickel titanium rotary instruments. Journal of Endodontics 35, Mize SB, Clement DJ, Pruett JP et al. (1998) Effect of sterilization on cyclic fatigue of rotary nickel-titanium endodontic instruments. Journal of Endodontics 24, Plotino G, Grande NM, Cordaro M et al. (2009) A review of cyclic fatigue testing of nickel-titanium rotary. Journal of Endodontics 35, Plotino G, Costanzo A, Grande NM et al. (2012) Experimental evaluation on the influence of autoclave sterilization on the cyclic fatigue of new nickel-titanium rotary instruments. Journal of Endodontics 38, Plotino G, Testarelli L, Al-Sudani D et al. (2014) Fatigue resistance of rotary instruments manufactured using different nickel-titanium alloys: a comparative study. Odontology 102, Sattapan B, Nervo GJ, Palamara JE et al. (2000) Defects in rotary nickel-titanium files after clinical use. Journal of Endodontics 26, Shen Y, Cheung GS (2013) Methods and models to study nickel-titanium instruments. Endodontic Topics 29, Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M (2011) Fatigue testing of controlled memory wire nickel titanium rotary instruments. Journal of Endodontics 37, Shen Y, Qian W, Abtin H et al. (2012) Effect of environment on fatigue failure of controlled memory wire nickel-titanium rotary instruments. Journal of Endodontics 38, Shen Y, Zhou HM, Zheng YF et al. (2013a) Current challenges and concepts of the thermomechanical treatment of nickeltitanium instruments. Journal of Endodontics 39, Shen Y, Zhou HM, Wang Z et al. (2013b) Phase transformation behavior and mechanical properties of thermomechanically treated K3XF nickel-titanium instruments. Journal of Endodontics 39, Shen Y, Coil JM, Zhou H et al. (2013c) HyFlex nickel-titanium rotary instruments after clinical use: metallurgical properties. International Endodontic Journal 46, Silvaggio J, Hicks ML (1997) Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files. Journal of Endodontics 23, Valois CR, Silva LP, Azevedo RB (2008) Multiple autoclave cycles affect the surface of rotary nickel-titanium files: an atomic force microscopy study. Journal of Endodontics 34, Viana AC, Gonzalez BM, Buono VT et al. (2006) Influence of sterilization on mechanical properties and fatigue resistance of nickel-titanium rotary endodontic instruments. International Endodontic Journal 39, Yahata Y, Yoneyama T, Hayashi Y et al. (2009) Effect of heat treatment on transformation temperatures and bending properties of nickel-titanium endodontic instruments. International Endodontic Journal 42, Zhao D, Shen Y, Peng B et al. (2013) Micro-computed tomography evaluation of the preparation of mesiobuccal root canals in maxillary first molars with Hyflex CM, Twisted Files, and K3 instruments. Journal of Endodontics 39, Zinelis S, Darabara M, Takase T et al. (2007) The effect of thermal treatment on the resistance of nickel-titanium rotary files in cyclic fatigue. Oral Surgery, Oral Medicine, Oral Pathology 103, International Endodontic Journal. Published by John Wiley & Sons Ltd International Endodontic Journal, 49, ,