Cyclic fatigue resistance of OneShape, Reciproc, and WaveOne: An in vitro comparative study

Abstract

Aims:

The aim of this in vitro study was to evaluate the cyclic fatigue resistance of three single-use nickel–titanium (NiTi) instruments.

Materials and Methods:

Forty files each of OneShape (OS), Reciproc R25 (R25), WaveOne Primary (WO) file, and ProTaper (PT) F2 (as control) were tested in four curved artificial canals with different angles and radii of curvature. The number of cycles to fracture (NCF) was determined and the data were compared for differences by using two-way analysis of variance (P < 0.05).

Results:

In general, all single-use instruments were more resistant than traditional rotary instrument used as control. R25 showed the highest cyclic fatigue resistance. One Shape and WO files showed similar cyclic fatigue resistance values, higher than PT F2.

Conclusions:

Within the limitations of this study, it can be concluded that R25 was the most fatigue resistant. OS rotary instruments seem to have good mechanical resistance, similar to NiTi files developed for reciprocating motion.

INTRODUCTION

Nickel–titanium (NiTi) instruments are commonly used for endodontic practice nowadays.[1] They offer many advantages over conventional stainless steel files because they are more flexible and have increased cutting efficiency.[2,3] The superelasticity of NiTi rotary files allows to produce the desirable tapered root canal form with a reduced tendency to canal transportation.[2,3,4] Despite these advantages, NiTi instruments appear to have a high risk of separation,[5,6] mainly because of fatigue and torsional shear stresses.[7] Torsional fatigue occurs when the tip of the instrument binds in the root canal while the file continues turning. Cyclic fatigue failure is reported to occur unexpectedly without any sign of previous permanent deformation.[8,9] This happens when the instrument rotates inside a curved root canal and is subjected to an excessive number of tension-compression strain cycles in the region of maximum root canal curvature.[10] Many variables such as the rotational speed, the metal surface treatments, multiple autoclaving, and the metallurgic characterization of the NiTi alloys that could possibly influence the fatigue resistance of NiTi rotary files have been investigated.[11,12]

Canal curvature is suspected to be the predominant risk factor for instrument failure because of flexural stresses and cyclic fatigue[2,13,14] that involve the NiTi core of the files while shaping the root canal walls. Thereby, even if it is possible to try to maintain the original canal configuration and optimal form while shaping,[15] little can be done to prevent or reduce these stresses on the files.[16] In the past years, the reciprocating motion of the NiTi rotary instrument has been shown to decrease the impact of cyclic fatigue compared with rotational motion.[9,17] Therefore, it has been recently proposed that the single-file shaping technique may simplify instrumentation protocols and avoid the risk of cross-contamination.[16,17]

Endodontic files in a reciprocating motion along with new alloys and new manufacturing processes have been introduced recently to improve fracture resistance.[11,18] M-wire is an NiTi alloy prepared by a special thermal process that is claimed to increase flexibility and resistance to cyclic fatigue.[19] Reciprocation was shown to extend the life-span of a NiTi instrument, hence resistance to fatigue, in comparison with continuous rotation.[18,20] Recently, two M-wire NiTi endodontic file systems were introduced for use in reciprocating motion: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland).[21] These instruments are single-use files, avoiding metal weakening through prolonged clinical use; however, single-use means that the same instrument can be used in 3-4 root canals, which could be complex and tortuous.[22] The reciprocating working motion consists of a counterclockwise (cutting direction) and a clockwise motion (release of the instrument), whereas the angle of the counterclockwise cutting direction is greater than the angle of the reverse direction. As the counterclockwise angle is greater than the clockwise one, it is claimed that the instrument continuously progresses toward the terminus of the root canal. Reciproc and WaveOne have been evaluated in preliminary studies[7,17,21,22,23] and the results regarding lifespan, cyclic fatigue resistance, shaping ability, and cleaning efficiency were satisfactory.

A new concept of single-use instrumentation is that a single file is to be used in a full clockwise rotation. OneShape (OS) (Micro Mega, Besancon, France) belongs to this group of single-file systems. This instrument is made of a conventional austenite 55-NiTi alloy. In the tip region, the cross section represents three cutting edges while in the middle of the cross-sectional design progressively changes from a three-cutting-edge design to two cutting edges. At the shank, the S-shaped cross section shows two cutting edges, resembling the cross-sectional design of Reciproc instruments. This design is alleged to eliminate threading and binding of the instrument in continuous rotation.[24]

The purpose of this study is to evaluate in vitro the cyclic fatigue resistance of these single-use Ni-Ti instruments and to compare them with a traditional NiTi instrument (ProTaper (PT) F2). The null hypothesis is that the cyclic fatigue resistance of the different instruments is similar when evaluated in the same canal.

MATERIALS AND METHODS

Grouping

Group 1 (n = 40)

OS (Micro Mega, Besancon, France), constant 0.06 taper, 25-mm length, and tip diameter 25.

Group 2 (n = 40)

Reciproc R25 (R25) (VDW, Munich, Germany), 0.08 taper in the apical part that decreases to the shank, 25-mm length, and tip diameter 25.

Group 3 (n = 40)

WaveOne Primary (WO) (Dentsply Maillefer, Ballaigues, Switzerland), 0.08 taper in the apical part that decreases to the shank, 25-mm length, and tip diameter 25.

Group 4 (n = 40)

PT F2 (Dentsply Maillefer, Ballaigues, Switzerland) as control, 0.08 taper in the apical part that decreases to the shank, 25-mm length, and tip diameter 25.

Cyclic fatigue testing

According to Castello-Escriva et al.,[25] the fatigue testing device consists of four curved stainless steel canals with different angles and radii of curvature, which are stuck on an acrylic surface where the handpiece is also fixed. The four canals are 20 mm in length with a tip of 0.40 mm and 09 taper. Canal 1 has 60° angle and 8-mm radius, canal 2 has 45° angle and 8-mm radius, canal 3 has 60° angle and 5-mm radius, and canal 4 has 45° angle and 5-mm radius.

The diameter of the simulated canals is larger than the instruments, allowing free rotation. To reduce friction as the instruments contacted the metal canal walls, synthetic lubricant oil filled the canal space after each use. Each instrument was used according to manufacturer's instructions. OS was mounted on the Endo Mate DT motor (NSK, Kanuma, Japan) set to 350 rpm and a 4 N/cm torque with a 16:1 contra-angle in clockwise rotation. R25 was mounted on the dedicated reciprocating motor (Silver Reciproc, VDW) used with the manufacturer configuration setup at the preset program “Reciproc ALL” specific to the Reciproc instruments. WO was mounted on the same reciprocating motor (Silver Reciproc, VDW) used with the preset program “WaveOne ALL” specific to the WaveOne instruments. PT F2 was mounted on the Endo Mate DT motor (NSK, Kanuma, Japan) set to 300 rpm and a 2 N/cm torque with a 16:1 contra-angle in clockwise rotation.

Each instrument was allowed to rotate/reciprocate until fracture. The time to fracture was recorded with a 1/100-s chronometer and timing was stopped as fracture is detected visually and/or audibly. The number of cycles to fracture (NCF) was calculated by multiplying the time (seconds) to fracture by the number of rotations or cycles per minute, regardless of the rotation direction.[25] The manufacturers claim that the Reciproc mode has 300 rpm and WaveOne mode has 350 rpm.[22]

Data were analyzed by using two-way analysis of variance in software (Statistical Package for the Social Sciences (SPSS) 15.0, Chicago, IL). Post hoc Bonferroni test was applied to identify the groups that were significantly different from others. Statistical significance was set at P-value less than 5%.

RESULTS

The NCF's means for each file in each canal are presented in Table 1. The means and the standard deviations for each instrument type in each canal are graphically represented in Figure 1. A higher NCF is caused by a higher resistance to cyclic fatigue of the tested instruments. The two-way analysis of variance showed a statistically significant difference among the NCF's means of the instruments when used in the same canal (P < 0.05). Significantly differences between the instruments are presented in Table 1.

Table 1.

Mean and standard deviation of cyclic fatigue of tested NiTi instruments in each canal

Figure 1.

Mean NCF ± standard deviations for each instrument type in each canal. The most difficult curvature (canal 1) generated the least NCF in all the instruments compared; the easiest canal (canal 4) showed the highest NCF in all instruments (OS: OneShape, R25: Reciproc R25, WO: WaveOne Primary, PT: Protaper). NCF: Number of cycles to fracture

The cyclic fatigue resistance was the highest for R25, followed by OS, WO, and PT in that order. Comparing the results between the canals, R25 had the best fatigue resistance in canals 1, 2, 3, and 4 (P < 0.05). WO and OS had similar cyclic fatigue resistance values in all the canals (P > 0,05): OS exhibited the highest mean in canals 1 and 3, WO in canals 2 and 4, although there was no statistical difference between them. PT exhibited the lowest mean NCF in all the canals (P < 0,05).

The results of the present study showed that the most difficult curvature (canal 1) generated the lowest NCF and the easiest curvature (canal 4) showed the highest NCF values in all systems.

DISCUSSION

The aim of the present study was to evaluate the cyclic fatigue resistance of three new different single-use NiTi instruments in four different artificial canals: Reciproc and WaveOne were designed for use in reciprocating motion, OS for use in continuous rotation.

These new single-use systems have been introduced to shape root canals with only one file: this technique simplification allows saving time and cost for endodontic treatment.[21] However, to shape root canals with only one file, the instrument will be subject to a great deal of stresses, both torsional and cyclic (bending) fatigue. The instrument is not used for further canals and does not require thermal sterilization in autoclave, so it will be not exposed to other stresses. But there are varied reports on autoclaving conditions as to either improving or degrading both the performance and physical properties of different rotary NiTi systems. For example, Sundaram et al.,[12] tested cyclic fatigue resistance of NiTi files after autoclaving cycles and concluded that multiple sterilization did not compromise the mechanical behavior of modern rotary NiTi endodontic instruments. Even if autoclaving cycles do not result in deleterious effects that could reduce their mechanical resistance, after multiple usage surface defects like pitting, strip formation, microfractures/microcrack formation, and disruption of cutting edges are evident on the instrument surface and if the instrument is used further, it may result in cyclic fracture.[12]

Reciproc and WaveOne reciprocating file systems are made by the same new M-wire NiTi alloy but have different cross sections, S-shape, and concave triangular shape for Reciproc and WaveOne, respectively.[21] It was reported that the larger cross-sectional area (CSA) would have a higher flexural and torsional stiffness,[26] and thus the file design (cross-sectional shape, diameters of core, etc) would have a significant influence on the torsional and bending (hence, fatigue) resistance.[21] Instruments made from M-wire or R-phase NiTi were reported to offer greater flexibility and resistance to cyclic fatigue than the files from traditional alloys.[19,20,21,22] PT F2 had a similar CSA compared with WaveOne[21] but succumbed at a lower NCF and with a lower torsional strength. This might be related to the mechanical characteristics of the NiTi alloy. In addition, PT F2 has a greater number of spiraling flutes (i.e. smaller pitch) than the other brands. This would have the effect of increasing the stiffness along the shaft and, consequently, resulting in a lower value of torsional strength.[26]

Kim et al.,[21] assessed the cyclic fatigue resistance and torsional resistance of two reciprocating motion systems, R25 and WO file, compared with PT F2 in a continuous rotation. Both reciprocating files demonstrated higher cyclic fatigue and torsional resistance than PT. They observed higher cyclic fatigue but lower torsional resistance in Reciproc than WaveOne. It implies that R25 possesses lower flexural stiffness and smaller polar moment of inertia than WaveOne. In other words, WaveOne had a higher torsional stiffness than Reciproc.

No studies about cyclic fatigue of OS are available. A reciprocating motion may decrease the impact of cyclic fatigue on NiTi rotary instrument life compared with rotational motion.[25] But Oh et al.,[27] verified that the instrument design, particularly the CSA, can also affect the fatigue behavior when subjected to torsion or bending. Sections with a larger CSA are more susceptible to fatigue fracture than smaller sections when rotating at the same curvature. Moreover, an electro-polishing procedure has been attempted during the manufacturing process to reduce the number of machining defects and residual stress of ground NiTi rotary instruments:[27] the grinding of file blanks during the manufacture of NiTi rotary instruments causes many machining defects. This procedure removes the outer layer of a metal, leaving the surface free of contaminants, microcracks, and work-induced residual stress. This means that the resistance to fatigue failure can be enhanced by a smooth defect-free surface.[25]

In the present study, R25 exhibited significantly higher cyclic fatigue resistance than other instruments in all canals, probably because of the NiTi alloy, its geometry, and the reciprocating movement. But all single-use NiTi files were more resistant than traditional rotary instrument used as control. The results obtained for PT were comparable with the previous studies conducted under similar experimental conditions.[21,25] OS showed better results than PT F2: it is made by the same conventional austenite 55-NiTi alloy, but the design is really different. OS is characterized by different cross-sectional designs over the entire length of the working part and its design is alleged to guarantee more flexibility.[24] So, within the limitations of this study, OS instruments showed cyclic fatigue resistance similar to WaveOne used in reciprocating motion, much higher than PT F2. Variable cross section with small residual core and electro-polished surfaces of OS could explain these good results.

CONCLUSION

Within the limitation of this study, it can be concluded that single-use instruments were the most fatigue resistant compared with a conventional instrument. R25, exhibited the better results, but OS showed good mechanical resistance, even if it is developed for use in continuous rotation.