Clin Oral Invest DOI 10.1007/s00784-015-1611-4

ORIGINAL ARTICLE

Cyclic fatigue resistance of D-RaCe, ProTaper, and Mtwo nickel–titanium retreatment instruments after immersion in sodium hypochlorite Hüseyin Sinan Topçuoğlu 1 & Kanşad Pala 2 & Ahmet Aktı 1 & Salih Düzgün 1 & Gamze Topçuoğlu 3

Received: 1 December 2014 / Accepted: 21 September 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Objectives The aim of this study was to investigate the effect of immersion in sodium hypochlorite (NaOCl) on cyclic fatigue resistance of three different rotary nickel–titanium (NiTi) retreatment files. Material and methods A total of 90 new ProTaper, D-RaCe, and Mtwo retreatment files were tested. Thirty files of the same brand were randomly assigned to two groups (n=15). Group 1 was no immersion (control group) and 16 mm of group 2 instruments were immersed in 5 % NaOCl at 37 °C for 5 min. All instruments were then tested for cyclic fatigue. Resistance to cyclic fatigue was determined by counting the numbers of cycles to failure in a 60° curve with a 5-mm radius, recording the time in seconds to fracture. Data were analyzed by two-way analysis of variance. Results Resistance to cyclic fatigue decreased significantly for D-RaCe retreatment files after immersion in NaOCl. ProTaper and Mtwo retreatment files were not affected from immersion to NaOCl. D-RaCe retreatment instruments showed better cyclic fatigue resistance than ProTaper and Mtwo retreatment instruments, and the difference was statistically significant (p<0.05). There was no significant difference between ProTaper and Mtwo retreatment instrument groups (p>0.05).

* Hüseyin Sinan Topçuoğlu [email protected] 1

Department of Endodontics, Faculty of Dentistry, Erciyes University, Melikgazi Kayseri 38039, Turkey

2

Department of Restorative Dentistry, Faculty of Dentistry, Erciyes University, Kayseri, Turkey

3

Department of Pedodontics, Faculty of Dentistry, Erciyes University, Kayseri, Turkey

Conclusions D-RaCe retreatment instruments had the highest cyclic fatigue resistance among retreatment files tested in this study, but immersion to NaOCl decreased the cyclic fatigue resistance of D-RaCe retreatment instrument. Clinical relevance It should be considered that cyclic fatigue resistance of D-RaCe can decrease in contact with NaOCl during the removal of canal filling material. Keywords Cyclic fatigue . Retreatment . Rotary instrument . Sodium hypochlorite

Introduction The main goal of non-surgical endodontic retreatment is to achieve the decontamination of the root canal system in order to establish healthy periapical tissues and allow tissue repair. Endodontic retreatment steps consist of the complete removal of previous filling materials, reinstrumentation, and reobturation of the root canal system [1, 2]. Many techniques have been advocated for the removal of gutta-percha from the root canal system. These include endodontic hand files combined with heat or chemical solvents, engine-driven rotary files, ultrasonic instruments, heat-carrying instruments, and lasers [3, 4]. Some rotary nickel–titanium (NiTi) systems have been especially designed for removing root canal filling materials such as the ProTaper Universal retreatment instruments (Dentsply Maillefer, Ballaigues, Switzerland), D-RaCe retreatment instruments (FKG Dentaire, La Chaux-de-Fonds, Switzerland), and Mtwo retreatment instruments (VDW, Munich, Germany). NiTi rotary instruments have a high risk of fracture due to torsional stress or cyclic fatigue [5]. Cyclic fatigue occurs when the NiTi instrument, within its elastic limit, rotates in a curved canal. As the instrument rotates along the curvature,

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cycles of tension/compression are repeatedly generated at the point of maximum flexure until fracture in instrument occurs [6, 7]. One additional factor potentially limiting the resistance to fatigue and torsional fracture is corrosion that may occur in the presence of sodium hypochlorite (NaOCl) solution [8]. NiTi instruments come into contact with NaOCl during root canal disinfection or when the solution is present in the pulp chamber and root canal during instrumentation [9, 10]. If corrosion is present, it may negatively affect the physical properties of NiTi files [11]. As in the canal instrumentation, the working part of retreatment instruments contacts dynamically the NaOCl solution for a few minutes during the removal of root canal filling material. Several authors have studied the effect of NaOCl on cyclic fatigue resistance of NiTi instruments used for canal preparation [12–14]. However, there is no study evaluating the fatigue behavior of retreatment instruments after immersion in NaOCl solution. Therefore, the aim of this study was to evaluate resistance to cyclic fatigue of ProTaper, D-RaCe, and Mtwo retreatment instruments after immersion in NaOCl solutions for 5 min.

Materials and methods A total of 90 new ProTaper (Dentsply Maillefer), DRaCe (FKG Dentaire), and Mtwo (VDW) files designed for removal of gutta-percha were tested in the study. The instruments used at working length were selected from each system ProTaper D3 (size 20, 0.07 taper), D-RaCe DR2 (size 25, 0.04 taper), and Mtwo R2 (size 25, 0.05 taper). Each instrument was inspected for defects or deformities before the experiment with a stereomicroscope (BX60; Olympus, Tokyo, Japan) and none were discarded. The 30 files of the same brand, all from the production lot, were randomly assigned (using a random number table) to two different groups of 15 each. Group 1 (control) was composed of new instruments not immersed in solution. Instruments of group 2 were dynamically immersed in 5 % NaOCl at 37 °C for 16 mm for 5 min. In group 2, all files were placed in small separate glass containers. For the dynamic immersion, the retreatment files were attached to an endodontic motor (X-Smart; Dentsply Maillefer) and rotated freely at constant speed (400 rpm for ProTaper D3, 600 rpm for D-RaCe DR2, 280 rpm for Mtwo R2) in a small glass container with the amount of the NaOCl solution necessary to contact 16 mm of instruments. Immediately after removal from immersion, all files were rinsed by bidistilled water to neutralize the effect of NaOCl, dried, registered with an ID number, and stored in glass vials [13].

The all instruments of the two groups of each brand were then subjected to cyclic fatigue testing using a mechanical device specifically developed for the purpose and already used in previous studies [15–17]. The apparatus was connected to an endodontic motor and enabled the instrument to rotate freely within a stainless steel artificial canal at a constant pressure (Fig. 1). All retreatment files were tested inside a curved artificial canal with a single curvature, with a 60° angle of curvature and 5-mm radius of curvature measured according to the method of Schneider [18]. The center of the curvature was 5 mm from the tip of the instrument, and the curved segment of the canal was approximately 5 mm in length. The diameter of the simulated canal was higher than the instruments allowing free rotation. All instruments were rotated at manufacturer’s recommended speed (ProTaper D3; 400 rpm, D-RaCe DR2; 600 rpm, Mtwo R2, 280 rpm) by using a 16:1 reduction handpiece powered by a torque-controlled electric motor (X-Smart; Dentsply Maillefer). To reduce friction of the file and overheating while rotating inside the artificial canal, a special oil (WD-40 Company, Milton Keynes, England) was used for lubrication. The experimental procedures were performed by a single operator to avoid interoperator variability. The time to fracture was multiplied by the number of rotations per minute (rpm) to obtain the number of cycles to failure (NCF) for each instrument. The length of each fractured tip was also measured with the help of an electronic digital caliper (Teknikel, Istanbul, Turkey). Two-way analysis of variance and Scheffè post hoc tests were used to evaluate the effect of immersion in NaOCl on cyclic fatigue resistance. NCF was dependent variable, whereas file type and immersion conditions

Fig. 1 Cyclic fatigue testing device

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were independent measures. Student’s t test was performed between the data recorded for fragment lengths in different groups. The level of significance was set at p < 0.05. All statistical analyses were performed with SPSS 16.0 software (SPSS Inc, Chicago, IL, USA).

Results The mean NCFs of ProTaper D3, Mtwo R2, and D-RaCe DR2 groups in immersion and non-immersion to NaOCl are shown in Fig. 2. Overall, the D-RaCe DR2 instruments showed better cyclic fatigue resistance than ProTaper D3 and Mtwo R2 instruments, and the differences were statistically significant (p < 0.05). There was no significant difference between ProTaper D3 and Mtwo R2 instruments (p>0.05). Significant differences between the two groups of D-RaCe DR2 occurred when considering the immersion condition as the independent variable (p<0.05). However, immersion in NaOCl had no significant influence in the ProTaper and Mtwo group (p>0.05). Figure 3 shows the mean fracture lengths of ProTaper D3, Mtwo R2, and D-RaCe DR2 groups in immersion and non-immersion to NaOCl. Analysis of the data regarding the length of the fractured segment revealed that there was no statistically significant difference between immersion and non-immersion groups in terms of the mean fracture lengths of the ProTaper D3, Mtwo R2, and D-RaCe DR2 files (p>0.05). Overall, the mean length of fractured segment of ProTaper groups was shorter than D-RaCe and Mtwo groups, and the difference between both ProTaper D3–D-RaCe DR2 and ProTaper D3–Mtwo R2 groups was statistically significant (p<0.05). Fig. 2 Number of cycles to failure (NCF) of retreatment instruments (mean±SD). Different superscript letters indicate statistically significant difference between groups (p<0.05)

Discussion The removal of gutta-percha using hand files, with or without solvent, can be a tedious, time-consuming process, especially when the root filling material is well condensed [19]. The usage of rotary instruments in the retreatment process presents the advantage of reduced clinical time [20]. However, Inan et al. [21] stated that a well-compacted filling material presents resistance to instruments. Therefore, the retreatment files chosen for this procedure must possess greater resistance to cyclic fatigue and torsional forces. Rödig et al. [22] evaluated the efficacy of D-RaCe and ProTaper retreatment NiTi instruments in removing guttapercha from curved root canals and determined that NiTi retreatment instruments removed gutta-percha from the root canal in approximately 4–5 min. Unal et al. [23] compared the efficacy of retreatment instruments in removing gutta-percha root filling in curved root canals and reported that the mean time to remove root canal filling material from curved canals was 6 min using ProTaper retreatment instrument. Zuolo et al. [24] compared the efficacy of reciprocating (Reciproc files) and rotary techniques (Mtwo R files) in removing guttapercha and sealer from straight root canals. They determined that the mean time using Mtwo retreatment files to remove the root filling material was 6 min. To simulate clinical conditions, in the removal of gutta-percha from a curved root canal, the experimental protocol of this study included dynamic immersion of only the working part (16 mm) of three different brands of NiTi retreatment files in a solution of 5 % NaOCl at 37 °C for 5 min. In a difficult scenario where a tooth has severely curved root canals and root canal filling material is well-compacted, the clinical time required to remove guttapercha may be longer than that of straight canals. This study incorporated methodology for a difficult clinical scenario. Retreatment files were immersed in NaOCl for 5 min. The

Clin Oral Invest Fig. 3 Length of fractured fragment (mm) of retreatment instruments during cyclic fatigue testing (mean±SD). Different superscript letters indicate statistically significant difference between groups (p<0.05)

shaft of the retreatment instrument was not immersed in the irrigation solution; this protocol is a characteristic of normal root canal instrumentation procedures and avoids galvanic corrosion [25]. The instruments were rotated according to manufacturers’s instructions, and to reduce the friction of the file, synthetic oil was applied as recommended in a previous study [8]. The point of maximum curvature of the simulated canals in most studies that have evaluated cyclic fatigue resistance of NiTi instruments was 5 mm from the tip of the instrument [15, 26, 27]. The curved segment of the canal in the current study was also set to 5 mm in length. In the present study, retreatment instruments fractured at or just below the center of the curvature. This indicated precise positioning of the retreatment instruments in the test device. It has been stated that the resistance of rotary instruments to cyclic fatigue decreases with increasing instrument diameters and tapers [7, 21]. In the present study, D-RaCe DR2 (size 25, 0.04 taper) showed greater cyclic fatigue resistance than the ProTaper D3 (size 20, 0.07 taper) and Mtwo R2 (size 25, 05 taper). Based on this finding, it can be concluded that the increase in tapers of retreatment instruments decreases the resistance to fracture. Inan et al. [21] evaluated the cyclic fatigue resistance of R-Endo, Mtwo, and ProTaper retreatment instruments in non-immersion conditions, and they found that R-Endo R3 (size 25, 0.04 taper) instrument showed greater cyclic fatigue resistance than the ProTaper and Mtwo retreatment instruments. Also, there was no significant difference between the ProTaper and Mtwo instruments in their study. The findings of the present study are in agreement with those findings. Conflicting reports have been published regarding the effect of instrument design on the cyclic fatigue resistance of NiTi instruments. Several studies [28, 29] have concluded that instrument design has no effect on cyclic fatigue resistance, whereas other studies [30, 31] suggest that the cross-sectional

design is an important factor. In the present study, a statistically significant difference was observed between the D-RaCe DR2 and the other two retreatment instruments (ProTaper D3 and Mtwo R2). Thus, it can be said that the design of a NiTi retreatment instrument might have an influence on cyclic fatigue resistance. Several studies have evaluated cyclic fatigue resistance of different NiTi instruments after immersion in NaOCl [13, 32]. However, the current study evaluated the cyclic fatigue resistance of different retreatment systems after immersion in NaOCl. The results showed that immersion in NaOCl decreased the cyclic fatigue resistance of D-RaCe DR2, but not the ProTaper D3 and Mtwo R2 instruments. In a previous study [13], it was reported that the cyclic fatigue device generates the maximum stress at the center of the simulated curve (approximately 5–6 mm from the tip); if a corrosive zone is present in that area, the instrument could break early. The point of maximum curvature of the artificial canals in most studies was between 5 and 7 mm from the tip of the instrument [8, 33, 34]. In the present study, the fractured length of the instrument was at or just below the center of the curvature, which confirms the positioning of the files in a precise trajectory.

Conclusions Under the limitations of this study, it can be concluded that the D-RaCe DR2 retreatment instruments is more resistant to cyclic fatigue failure than the Mtwo R2 and ProTaper D3 instruments. However, immersion in NaOCl for 5 min reduced the cyclic fatigue resistance of the D-RaCe DR2, but not the other instruments. Further studies are required to evaluate the effects of different variables on the cyclic fatigue resistance of retreatment files.

Clin Oral Invest Compliance with ethical standards This article does not contain any studies with human participants or animals performed by any of the authors.

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Conflict of interest The authors declare that they have no competing interests.

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