An analysis of cyclic fatigue resistance of reciprocating instruments in different canal curvatures after immersion in sodium hypochlorite and autoclaving: An in vitro study.

AIM: The aim of this study is to assess the effect of sodium hypochlorite and sterilization on the cyclic fatigue resistance of two reciprocating nickel-titanium files using a rotary and a reciprocating motor.
MATERIALS AND METHODS: Totally, 160 Reciproc and Wave One files were tested. The eighty files of the same brand were assigned into four groups (n = 20). These four groups were further divided into two subgroups, 1a and 1b, 2a and 2b, 3a and 3b, 4a and 4b consisting of ten files in each group. 2.5% NaOCl for 5 min, 2.5% NaOCl for 5 min + 1 cycle of autoclave sterilization, 2.5% NaOCl for 5 min + 5 cycles of autoclave sterilization, no sterilization (control). Groups 1a, 2a, 3a, and 4a were subjected to 30° and Groups 1b, 2b, 3b, and 4b were subjected to 60° canal curvature.
RESULTS: Intergroup comparison reveals that there is an increase in cyclic fatigue resistance from the first cycle to the fifth cycle of autoclave which is statistically significant.
CONCLUSION: Reciproc and Wave One showed reduced cyclic fatigue resistance on immersion in NaOCl for 5 min. Multiple autoclaving cycles significantly increased the cyclic fatigue resistance of Reciproc file system in both the curvatures. There was a decrease in cyclic fatigue resistance for Wave One after multiple autoclaving at 60° curvature which was statistically significant.

INTRODUCTION

Nickel-titanium (NiTi) instruments are commonly used for shaping the root canal system as they are known for its shape memory and superelasticity which enables greater NiTi flexibility and resiliency. Despite these advantages, NiTi instruments appear to have a risk of separation, mainly because of cyclic fatigue and torsional shear.[123] Cyclic fatigue occurs when an instrument is subjected to repeated cycles of compression and tension, as would occur during rotary instrumentation in a curved canal.[4]

One additional factor that potentially limits the resistance to fatigue fracture is corrosion that occurs during chemicomechanical preparation of root canal system, chemical disinfection,[5] and sterilization of endodontic instruments in the presence of NaOCl solution. Active CLO− ions have the potential to increase corrosion.

NiTi shape memory and superelastic properties are strongly dependent on the thermomechanical processing and the additional heat treatment of NiTi instruments during autoclave sterilization.[6] Serene et al. verified that the Vickers microhardness increased approximately 18% for NiTi wires after one and five cycles of sterilization in dry heat or autoclave and hypothesized that the deformation of NiTi files caused during clinical use could possibly be reversed using sterilization temperature above 125°C.[7]

The reciprocating motion of the NiTi rotary instrument has shown to decrease the impact of cyclic fatigue and also has shown to extend the lifespan, in comparison with continuous rotation.[89]

Fracture and overall failure were more frequent in severely curved canals (angle 60 ± 10 and radius 2 ± 1 mm) than in straight and moderately curved canals (30 ± 10) and the same radius.[10] This can be explained by the overloading induced on the instrument by an abrupt change in canal curvature which restrains the rotating instrument, giving rise to multidirectional loading (tension, bending, and torsion) that leads to ductile fracture.[11]

Reciproc and Wave One reciprocating file systems are made up of the same innovative M-Wire NiTi alloy but have different cross sections.[11] Instruments made from an M-Wire NiTi offer greater flexibility and resistance to cyclic fatigue than traditional NiTi.[1213]

The aim of this in vitro study was to assess the effect of sodium hypochlorite and autoclave sterilization after one and five cycles of autoclave sterilization on the cyclic fatigue resistance of two reciprocating NiTi files used in a VDW gold reciprocating motor in canal curvatures of 30° and 60°, respectively.

MATERIALS AND METHODS

A total of 160 NiTi instruments: 80 Reciproc R25 (VDW, Munich, Germany), 80 Wave One primary (Dentsply Maillefer, Ballaigues, Switzerland) no. 25, 0.08 taper, 21 mm were used. Before instrumentation, all the files were inspected using an optical stereomicroscope under ×20 magnification for morphological and dimensional analysis and for any sign of visible deformations. Any file with faults or defects was discarded. Study protocol and the cyclic fatigue testing device used in the present study was based on the method described by Sundaram et al.[11]

The files were randomly divided into four groups (n = 20) and were further divided into two subgroups (n = 10). Ten files from each subgroup were subjected to 30° and 60° canal curvature, respectively.

Group 1: The instruments were dynamically immersed in 2.5% NaOCl for 5 min at room temperature

Group 2: The instruments were immersed in NaOCl, under the same conditions as in Group 1, and then files were instrumented and subjected to one cycle of autoclave sterilization at a temperature of 121°C for 32.5 min

Group 3: The instruments were subjected to five cycles of the same test protocols as in Group 2

Group 4: In the control group, the instruments were not immersed in NaOCl and were not sterilized.

For the dynamic immersion, the endodontic instruments were attached to an endodontic motor and rotated freely at a constant speed (350 rpm) in a small glass container with an amount of the NaOCl solution that was sufficient to be in contact with the instruments but not the shaft. The files were rinsed with distilled water to neutralize the effect of the NaOCl and then dried.

Totally, 240 single-rooted and single-canaled mandibular central incisors extracted for periodontal problems were selected for this study and were decoronated approximately at 1 mm above the level of cementoenamel junction as to standardize the length of 13 mm.

Coronal portion of the canal was prepared with GG drills of size 2, 3, and 4. The working length was obtained by introducing #15 size k file until its tip was just seen at the apical foramen. From this length, 1 mm was reduced to obtain the final working length.

The canals were then instrumented to a size of #20 k files, and the canals were copiously irrigated with 5 ml of 17% EDTA and 5 ml of 3% NaOCl, followed by 10 ml of 0.9% w/v of saline. The teeth were then stored on 0.9% saline until further instrumentation with rotary NiTi endodontic files.

The stored mandibular central incisors were instrumented with the respective files of each group and then subjected to autoclave at 121°C at 15 lb pressure for 15 min. The files were again instrumented and subjected to sterilization after every 2nd, 3rd, 4th, and 5th use.

Custom-made jig was designed such that the depth of each artificial canal was +0.2 mm allowing the instrument to rotate freely with an angle of curvature of 30° and 60° and radii of curvature of 5 mm radius. The center of the curvature was 6 mm from the tip of the instrument, and the curved segment of the canal was approximately 6 mm in length [Figure 1]. To reduce friction of the instruments in metal canal walls, lubricant oil was used.

Figure 1

Cyclic fatigue testing jig

All instruments were rotated at a constant speed of 350 rpm at 30° and 60° canal curvatures using a 6:1 contra angle (Sirona, Bensheim, Germany) powered by a torque-controlled electric motor (Gold, VDW) until their fracture occurred. The time to fracture was recorded visually with a 1/100 s chronometer and was multiplied by the number of rotations per minute to obtain the number of cycles to fracture (NCF).

RESULTS

The mean values and standard deviation expressed as NCF are displayed in Table 1. A higher value of NCF is related to greater resistance to cyclic fatigue fracture of the instruments tested. The three-way analysis of ANOVA using SPSS software (Version 20., SPSS Inc., Chicago, IL, USA) was done which showed a significant difference among the NCF's means of the instruments.

Table 1

Mean and standard deviation of time (min) in four Groups (I, II, III IV), two files (Reciproc and WaveOne) and two curvatures (30° and 60°)

Comparing the results between groups for each type of instrument, by Tukey's multiple post hoc procedures showed that the differences were statistically significant (P < 0.05). Intergroup comparison reveals that there is an increase in cyclic fatigue resistance for Reciproc files from the first cycle to the fifth cycle of autoclave sterilization at both 30° and 60° canal curvatures which is statistically significant but increase in cyclic fatigue resistance for Wave One instruments only at 30° canal curvature as shown in Graph 1.

Graph 1

Mean number of cycles to fracture ± standard deviation for each instrument in each group

Immersion in NaOCl for 5 min decreased the cyclic fatigue resistance which was statistically significant (P < 0.05) for Reciproc but not Wave One.

DISCUSSION

The reciprocating working motion has shown to extend the lifespan of a NiTi instrument, hence resistance to fatigue, in comparison with continuous rotation. M-Wire alloy was used in the development of two new instrumentation systems that are specifically designed to be used with reciprocating motion: The Reciproc (VDW GmbH, Munich, Germany) and Wave One. Wave One has a continuously decreasing taper from its tip to its shaft and is characterized by different cross-sectional designs over the entire length of the working part. Files have radial lands with modified convex triangular cross section at the tip end and a neutral rake angle convex triangular cross section at the coronal end. Reciproc files have a continuous taper over the first 3 mm of their working part followed by a decreasing taper until the shaft. An S-shaped cross section is used for the entire working part of these instruments.

The cyclic fatigue resistance of NiTi rotary instruments has been a subject of great interest in the endodontic literature. The impact of torsional fracture, metal fatigue, or fracture of NiTi rotary instruments caused by a combination of torsional stress and accumulation of fatigue is still debated.[14] It has been suggested that the cyclic fatigue has accounted for 50%–90% of mechanical failures compared to a torsional failure. Most of the fractured instruments were analyzed as a flexural failure, implying cyclic fatigue is the predominant mechanism for material failure.[15]

In this study, canals were simulated with a stainless steel device that guaranteed a fixed radius of curvature along with a fixed angle of curvature. The method used in our study for cyclic fatigue is based on a method described by Li et al.[16] This method accurately describes the root canal curvature based on the angle of curvature and radius of curvature.

To simulate clinical conditions, the test protocol chosen for this study included dynamic immersion in 2.5% NaOCl, maximum of five cycles of autoclave sterilization, and dynamic cyclic fatigue resistance testing. To remain within a realistic time frame of clinical practice and considering the mean life of the control instruments, 5 min was selected as the contact time of the solution with the instrument.[17] The shaft of the instrument was not immersed in the irrigating solution as is the case in root canal instrumentation procedures. This also served the purpose of avoiding galvanic corrosion phenomenon.[18]

The number of times the instrument can be reused is suggested to vary between 1 and 10.[918] (Gambarini et al. and Inan et al.), depending on the canal conditions and the type of instrument usage. Clinicians often recycle NiTi files owing to economic considerations.[19]

A study has reported that the static or dynamic immersion in 5% NaOCl did not significantly reduce the cyclic fatigue resistance of NiTi files.[17] On the contrary, another study showed a reduction in fatigue resistance of NiTi files after immersion in a heated NaOCl solution for 1 h.[20] This study was done as there are no studies regarding the effect of both NaOCl and autoclave sterilization on reciprocating files when used in reciprocating motion in two different canal curvatures.

In the present study, reciprocating dynamic immersion in NaOCl for 1 or 5 min did not reduce significantly the cyclic fatigue resistance of reciprocating NiTi files. Significant differences were found between the different NiTi files. Reciproc R25 was more resistant to cyclic fatigue than Wave One primary. The results are in agreement with recent reports that showed a higher cyclic fatigue resistance for Reciproc than Wave One.[21]

Under the conditions of the present study, five cycles of autoclave sterilization significantly influenced the cyclic fatigue produced by an innovative heat-treated alloy, i.e. Reciproc and Wave One files (M-wire technology). These instruments showed a statistically significant increase in the number of cycles to cause fracture after the sterilization. These results are in contrast with those obtained by Hilfer et al. who reported a significant decrease of the mean NCF after sterilization exhibited by the group of twisted files[20] suggesting that any enthalpy generated during autoclave processing did not provide enough energy to enable a heat treatment effect that could cause a crystalline phase change with these new manufacturing techniques.

On the contrary, the results of the present study suggest that autoclave sterilization might improve mechanical properties of instruments. NiTi is a very sensitive alloy to both thermal and mechanical stress. Thermal treatment of the alloy is known to produce a better arrangement of the crystal structure thus leading to improved flexibility and also changes in the percentage of phases of the alloy, thus leading to improved resistance or plastic behavior.[22]

The present study confirmed that the number of rotation to fracture an instrument largely depends on the degree of curvature with more incidence of breakage at a greater degree of curvature as concluded by other studies. In severely curved canals, the instruments with greater taper generally fracture earlier as compared to 0.02 tapered instruments due to reduced flexibility.

In this study, Reciproc R25 exhibited significantly higher cyclic fatigue resistance than the other instruments in all canals probably because of the NiTi alloy, the instrument geometry, and the reciprocating movement.

There appears to be a risk of corrosion of NiTi rotary files in contact with NaOCl. However, single use of NiTi rotary files may be indicated to reduce the danger of premature fatigue failure after contact with NaOCl. There were also no obvious surface morphological changes of the autoclaved files after instrumentation. This may be due to the fact that the number of autoclaving cycles was restricted to five, whereas other studies have employed up to 10 autoclaving cycles, which in turn, may have produced an increase in the depth of surface irregularities due to the surface corrosion and the cumulative effects of autoclaving.[2023]

CONCLUSION

Within the limitations of the study, it can be concluded that Reciproc and Wave One showed reduced cyclic fatigue resistance on immersion in NaOCl for 5 min as compared to the control group. Multiple autoclaving cycles significantly increased the cyclic fatigue resistance of Reciproc in both the curvatures except for a decrease in resistance for Wave One at 60° curvature which was statistically significant. R25 was the most fatigue resistant compared to Wave One in all the groups and curvatures.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.