In the continuous search for improvement of endodontic instruments, the greatest innovation introduced in the last decade was the development of nickel-titanium (Ni-Ti) rotary files. The Ni-Ti rotary files exhibit super-elasticity and can prepare root canals with a suitable taper and flow compared to stainless steel hand files and with less apical aberrations.1-3) Despite the evident advantages, the Ni-Ti rotary files can undergo unexpected fracture as a result of shear or fatigue failure in curved root canals, with the latter being implicated in more than one-third of those instruments fractured clinically.4-6) Many factors have been implicated in the fatigue fracture of the Ni-Ti rotary files. Of these, the instrument designs and the cross-sectional area can affect the magnitudes of stress and strain when subject to torsion or bending.7-9) Recently, Ni-Ti rotary instruments are available in a variety of cross-sectional shapes such as a triangle (ProFile, K3, and HeroShaper), modified rectangle (NRT and Mtwo), and pentagon (Alpha System). To date, there has been few studies on how Ni-Ti rotary instruments with different cross-sectional shapes behave when subjected to rotational-bending, a deformation mode similar to an engine-file rotating in a curved root canal, but the effect of cross-sectional shapes on the cyclic fatigue fracture is still controversial.7-12) Several studies have been performed to evaluate the cyclic fatigue in a dynamic model incorporating axial oscillating motion.13,14) This not only provides a better clinical simulation than the static model, but these studies also demonstrated that the axial (pecking) motion significantly lengthens the life span of the rotary files.15) Fractography can be broadly defined as the science of observing, measuring and interpreting a fractured surface topography. When material failure involves actual breakage, fractography can be used to identify the fracture origin, the direction of crack propagation, the failure mechanism, materials defects, and the nature of the stresses.16)

Therefore, this in vitro study aimed to assess the influence of different cross-sectional shapes of Ni-Ti rotary files on the flexural fatigue fracture in a dynamic model and to determine the fracture patterns of broken file surfaces through a fractographic analysis using a scanning electron microscope.

Six experimental groups were allocated according to 6 brands of Ni-Ti rotary instruments of 10 each. The instruments tested were Alpha System (KOMET, Culver, Germany), HeroShaper (Micro-Mega, Besançon, France), K3 (SybronEndo, CA, USA), Mtwo (VDW, Munich, Germany), NRT (MANI, Utsunomiya, Japan), and ProFile (Dentsply, Ballaigues, Switzerland). All the instruments were #30/.04 in size and 25 mm in length. Fatigue testing was carried out in a fatigue tester (Denbotix, Bucheon, Korea) that allows the files to rotate freely inside an artificial stainless-steel canal and also allows the handpiece to move vertically (Figure 1). Each file was mounted on an electric motor (Aseptico, WA, USA) using a 1:20 reduction contra-angle handpiece. The instrument was rotated at 300rpm inside an artificial canal. An artificial canal was made out of stainless steel with an inner diameter of 1.05 mm, 10 mm in length, angle of 60°, and a radius of curvature of 5 mm. A continuous, 6 mm axial oscillating motion was applied at 0.5 cycles per second to simulate a clinical pecking motion. The apparatus can control the pecking distance, oscillation speed and set starting and ending point. The starting point was set at the apical end of the canal.

During the test, the artificial canal was filled with RC Prep (Premier Dental Co., PA, USA) to reduce the instrument's friction against the canal wall and heat generation. Instrument fracture was visually detected and the time until fracture was recorded using a digital stop watch. The mean and standard deviation values of the fracture time were calculated for each group. Using the SPSS statistical package version 12 (SPSS Inc., Chicago, IL, USA), the difference among the instruments as to their fatigue resistance was analyzed using one way analysis of the variance (ANOVA). Tukey post hoc analysis was used to identify the specific groups that showed significant difference. The level of significance was set at 5%.

For fractographic analysis, all fractured files from each group were photographed using a scanning electron microscope (JSM840A, JOEL, Tokyo, Japan) to determine the modes and patterns of fracture at magnifications of ×200, ×500, and ×1000. In addition, the cross-sectional area of 6 brands of Ni-Ti instruments was determined using SEM photographs as follows. Three new Ni-Ti instruments for each group was embedded in resin and cut at a 3.0 mm from the tip with an ISOmet 11-1180 low-speed saw (Buehler, Lake Bluff, Ill, USA). Then, the specimens were cleaned in an ultrasonic water bath for 15 min. The sections were examined by SEM at a magnification of ×200. The mean cross-sectional area of each file was calculated using Image-Pro Plus software (Imagej 1.34n, NIH, Bethesda, MD, USA). The correlation between the mean cross sectional area and the mean time to fracture of each group was examined using linear regression analysis with statistical significance set to α=0.05.

Many manufacturers have developed and designed new Ni-Ti rotary instruments that have different features, such as various cross-sectional shapes, pitches, helix, and tips. The fatigue resistance of endodontic files made of same material and same dimension (size and taper) is entirely dependent on instrument designs (cross-sectional area, pitch, helix, radial land, and/or spirals) and the cross-sectional area is closely associated with file flexibility (stiffness).12) Therefore, this study compared the cyclic fatigue resistance for 6 Ni-Ti rotary instruments with different cross-sectional configurations. A previous study by Schäfer et al.17) demonstrated that Ni-Ti rotary instruments with small cross-sectional area showed better results in terms of fatigue resistance. Turpin et al7) calculated the cross-sectional areas of triple-helix (Hero) and triple-U files (ProFile) and compared bending stresses in these 2 instruments. For identical working diameters, the cross-sectional area of the triple-helix (Hero) was found to be approximately 30% greater than that of the triple-U shaped ProFile. The greater cross-sectional area determined a lower flexibility of the Hero file compared to ProFile, which can explain the poor fatigue resistance in their study. Likewise, Alpha System was more vulnerable to cyclic fatigue due to its large cross-sectional area. Alpha System has five-edged (pentagon) cross-section and its cross-sectional area was the largest among 6 experimental groups. The present result, however, was not consistent with that of Turpin et al.7) and Schäfer et al.17) In other words, although Mtwo has the smallest cross-sectional area among the 6 Ni-Ti files tested, Mtwo showed the lowest value of fatigue resistance. A possible explanation might be that Mtwo instrument has a lower number of spirals per unit length. The fewer the spirals a flute has per unit length around the shaft of a file, the more it resists deformation but at the same time the more rigid it becomes.18)

On the contrary, Yao et al19) showed that K3 file was more resistant to cyclic fatigue than ProFile although K3 has larger cross-sectional area than ProFile. The authors explained that it may be related to the overall design of the K3 file, that is, the variable core diameter. Tripi et al.20) also demonstrated in their study that the variable core diameter and the presence of radial lands could explain the high fatigue resistance of K3 instruments. However, the present study showed that the fatigue resistance of K3 instrument was lower than that of ProFile which has a smaller cross-sectional area. This result is consistent with those of the previous studies.11,12) Therefore, more controlled studies regarding the effect of instrument designs on the fatigue fracture are necessary.

An interesting finding in the present study was that NRT file showed the best value of fatigue resistance although the cross-sectional area was not the smallest. This finding is in agreement with the finding of other authors that Ni-Ti instruments with a triangular-based cross-sectional shapes (ProFile) displayed a higher bending load than those that rectangular-based cross-sectional shapes (NRT).21) The authors contributed the high flexibility of NRT to the additional heat treatment during the manufacturing process. A previous study also suggested that rectangular shape can allow improvement of file flexibility.22)

In conclusion, the cross-sectional area is not a determining factor affecting the fatigue resistance of Ni-Ti rotary file. In addition, NRT and ProFile system can be useful for safe instrumentation in curved root canals in the aspects of fatigue resistance.