Since Walia et al. (1) introduced nickel-titanium (NiTi) instruments for root canal therapy in 1988, their properties have improved, with newer rotary systems demonstrating increased flexibility (2), as well as efficiency and cutting abilities (3). Further, the availability of proprietary thermal treatments in gas atmospheres or salt baths (4) has resulted in an apparent improvement in the mechanical properties of shaping instruments (4–6).

Indeed, numerous studies have already detailed ex vivo performance of these instruments. Specifically, torsional or cyclic fatigue (CF) behaviors of instruments were discussed (4, 7). CF resistance of nickel-titanium rotary endodontic instruments have been analyzed as early as 1997 (8). Subsequently, CF resistance of rotary endodontic instruments has been rigorously studied for more than two decades now. A variety of approaches have been deployed in the assessment of fatigue resistance and these have been extensively reviewed (4, 9).

The basic requirements for the testing of endodontic instruments are defined in the revised ADA Specification No. 28 (10) and ISO Specification 3630-1:2008 (11). However, none of these norms describe instruments with tapers >0.02 and neither include a description of a fatigue test. In the absence of one applicable norm fatigue devices have been designed with very different characteristics.

In 2009, Plotino et al. (9) criticized that although many studies at that time (and to this day) used a roughly comparable design, no attention has been paid to the diameter of the tubes or to the fit of the instrument inside the tubes. Nevertheless, a considerable variance in details of study protocols is evident in the literature, making comparisons of results very difficult, if not impossible. Some form of standardization was deemed necessary to obtain comparable results and identify reliable clinical implications.

At the same time, new martensitic alloys have demonstrated superior fatigue behavior (6, 12). Most CF studies of NiTi files have been performed at room temperature, however recent literature confirmed that sodium hypochlorite placed in root canals at room temperature rapidly approached body temperature (13). For this reason, instruments are expected to work at body temperature during clinical use. Importantly, a decrease in CF of martensitic instruments has been reported for increased temperature (14–16).

Differential scanning calorimetry (DSC) analysis of a specific instrument identifies the crystallographic transformation temperatures. This phase transformation determines the temperature at which the instrument would be working in a martensitic or austenitic phase and hence allows to understand the properties of the instrument in clinical conditions. Therefore, any testing device specifically designed to investigate the cyclic fatigue resistance of rotary endodontic instruments should consider the transformation temperatures of the specific alloy included in the study. Moreover, low-cost instruments are now manufactured, and no comprehensive standard has been established to guarantee that instruments have a minimum quality before being marketed.

Therefore, the aim of this study was to validate a new model to evaluate cyclic fatigue resistance of nickel-titanium (NiTi) instruments through a test rig proposed for ISO Specification 3630-1 (11) and to establish a minimum quality standard based on testing conditions and material characteristics.

Mean values and standard deviations for each tested variable and environmental factors at the two different radii and for both instruments are shown in Table 1. VB instruments were statistically more resistant to cyclic fatigue than PF when tested both in a 7.5-mm radius curve (p = 0.001) and 5-mm radius curve simulated root canal (p = 0.002). However, no significant differences were found when the same instrument was compared under the two different radii of curvature settings. Figure 2 shows the relation of surface strain to NCF, indicating the formation of 4 clusters, albeit with considerable variation within each group. Linear regression analysis did not reveal any strong associations between the two variables when viewed separately in the four clusters, neither was there an overall relationship.

Weibull probability plots (reliability vs. time) per group for both tested radii of curvature are shown in Figure 3. Mean life, eta, and beta parameters and their 95% confidence intervals are shown in Table 2.

Vortex Blue instruments significantly outlasted PF instruments for both tested radii of curvature with a probability of 79% when the instruments were tested in a setting with 7.5-mm radius of curvature and 77% when the radius of curvature was set at 5 mm. Neither PF nor VB instruments showed significant differences when tested at the two different radii of curvature settings. On the other hand, the highest beta parameter, indicating the highest predictability, was found for VB when tested at a 7.5 mm radius curvature and PF when tested in a 5 mm radius of curvature simulated root canal. The slope of the line in the figure also confirmed this assumption.

When using the correction factor, the range of rotations needed to pass the test ranged from 109 to 171, with a mean of 143 NCF. Using this criterion for a successful test, all rotaries tested passed the test for both geometrical conditions.

This work describes an initial validation study for a new framework to potentially be added to the current ISO 3630-1 norm. This norm in its current form does not address contemporary rotary instruments that vary considerably from traditional so-called ISO norm hand instruments. Specifically, there is no test described to determine CF resistance of engine-driven rotary instruments, yet this remains a primary source of intracanal fracture. Understandably, no fatigue test model can perfectly recreate all the clinical parameters observed during an endodontic procedure. The implementation of a standardized fatigue testing device would be a valuable tool for manufacturers to quality control prototypes for future novel endodontic instruments, but not necessarily as a standard for comparison between different proprietary instruments, particularly where the geometries and dimensions of the instruments differ.

ProFile Vortex and Vortex Blue were compared in the present study. These instruments were specifically selected for the present study to avoid the influence of instrument design. Both instruments share the exact same design except for the alloy in which they were manufactured. Other authors have previously used PF and VB to isolate the effect of the alloy in CF resistance (5, 17–20). Table 3 presents selected data of those publications that also compared PF and VB instruments regarding their cyclic fatigue resistance.

Expectedly, the results in this study demonstrate a higher cyclic fatigue resistance for VB compared to PF, which is in agreement with the studies detailed in Table 3. The mean NCF values of both the VB and PF instruments appear to have exceeded the minimum mean NCF by over 10-fold, with a pass rate of 100% for every sample tested in each of the 5 and 7.5 mm test rigs. The criterion for passing the benchmark appear to be considerably low when compared to the high NCF values that PF and VB are capable of. One may speculate that that there could potentially be a market for lower-costing NiTi instruments that would pass the minimum requirements. The expectation is that new instrument designs, particularly for novel metallurgical compositions, can also be tested against this benchmark before proceeding with full scale production.

The proposed cyclic fatigue test rig is also validated with regards to strain rates seen in both the 5 and 7.5 mm fixture, with all the test values shown to be between 2 and 8%, which appears to be similar to strain rates expected in clinical use, as judged from canal curvatures. Previous fatigue studies have not discussed whether their test conditions were reasonable and may have either subceeded or exceeded the clinically relevant strain rates.

On the other hand, there certain limitations with the proposed system; for example, the test fixture does not approximate clinical conditions (21). Rather it presents a standardized test for manufacturing standards. Moreover, the mathematical approach taken in Equations 2, 3 was based on empirical findings from currently marketed instruments. It appears that future research is needed to potentially amend and fine-tune the equations that are meant to provide a cut-off for quality assurance.

Recent studies have convincingly demonstrated a higher CF resistance of instruments with new alloys and designs; however, low-cost instruments are being incorporated to the market and a lower reliability has been reported to be expected when using such offerings (12, 22). We suggest that minimum criterion be implemented for manufacturers to demonstrate the reliable use of new instruments in clinical settings, before marketing a new product. In fact, as Hülsmann suggested in recent publications (21, 23), it should be the responsibility of the manufacturers to provide comprehensive data on CF resistance. Indeed, such relevant data should be based on accepted standards across all manufacturers to demonstrate required performance.

Such standards should also evolve continuously at the same rhythm as industry launches new developments. As an example, the proposal in the present study applied a correction factor based on the austenite finish temperature of each alloy included in the study, determined with differential scanning calorimetry, and the testing temperature. This stipulation was not included in any standard before, but literature has thoroughly demonstrated how CF resistance of martensitic alloys is considerably affected by temperature (14–16). Any further development that may affect CF resistance of endodontic rotary instruments would require a new implementation or factor correction in future proposals. In conclusion, the data presented in this study support the findings of previous studies where VB and PF were compared. This study affirms the new test model as a suitable addition to ISO 3630-1.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

OP conceived and designed the study. KA collected data. OP, AA, and JM analyzed the data. PC, OP, and AA wrote the manuscript. OP and PC edited the final version of the text. All authors reviewed and approved the manuscript.

OP is a consultant for Dentsply Sirona. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.