The objective of this study was to assess the cyclic fatigue resistance of a single-file system, specifically the Hyflex EDM OneFile, under a new kinematics (i.e., ReFlex Dynamic motion) and compared it with continuous rotation, with and without simultaneous irrigation.

In recent years, the performance of NiTi instruments has undergone significant modifications to enhance their safety and efficiency in clinical use. The mechanical NiTi files experiment cyclic fatigue when rotating within a curved space, with repetitive mechanical cycles leading to structural changes and eventual file separation [4]. Factors such as file geometry directly influence an instrument’s resistance to cyclic fatigue and torsional stress [20, 21]. To eliminate these variables, the same NiTi files were used and the temperature was set to simulate clinical conditions at 37 °C [4]. Specifically, HEDM files were selected for their widespread clinical use, high cyclic fatigue resistance, and flexibility as heat-treated instruments [22]. Although the novel motion was originally designed for a different type of instrument, it was applied here to evaluate its performance with flexible, heat-treated files, providing valuable insights into its clinical application.

A customized machine ensured the reproducibility of the tests, using a zirconium artificial canal to avoid galvanic phenomena under NaOCl irrigation [10]. The correct instrument placement in the artificial canal was confirmed by the similar length of fractured segments.

Natural extracted teeth were not used due to difficulties in achieving standardization and ensuring suitable clinical conditions [4]. Furthermore, it is notable that only dynamic devices can replicate the pecking motion carried out by the operator [9]. The dynamic test, involving axial movements, closely replicates real clinical conditions. Axial motion, mimicking a pecking motion, prolongs the instrument’s lifespan by distributing stress, unlike the static cyclic fatigue resistance test that concentrates stress in a single area [18].

ReFlex uses an advanced motion system that transitions from rotary to reciprocating motion upon detecting resistance, reducing torsional stress on the file. It operates in two modes: ReFlex Smart, which continuously adapts to torque resistance to minimize file stress, making it suitable for complex canals, and ReFlex Dynamic, which operates at a higher rotation speed until torque resistance is detected, at which point it switches to a reciprocating motion [23]. The ReFlex Dynamic motion was selected due to its innovative mechanics and the lack of studies comparing it with continuous rotation. While only three studies [9, 14, 24] in literature deal with ReFlex motion and cyclic fatigue, they are not directly comparable with our results due to the different files and kinematics.

The NCF values of instruments in continuous rotation were significantly higher than those observed with ReFlex Dynamic motion; thus, the first null hypothesis can be rejected. This difference could be attributed to the fact that, in reciprocating motion, the CW and CCW angles are specifically tailored to endodontic reciprocating systems. To prevent exceeding elastic deformation, the CCW angle must remain smaller than the elastic limit of the system material [9]. Supporting this, the study by Zubizarreta-Macho et al. (2021) compared ‘Smart’ and ‘Dynamic’ ReFlex motions with a reciprocal alternative movement [9]. Their findings revealed that the ReFlex Dynamic motion was more effective than ReFlex Smart and reciprocation when stiffer instruments were used. However, with heat-treated instruments or those designed to enhance flexibility, the kinematics showed less effective performance [9, 25]. These observations align with the study by Generali et al. [25], which demonstrated that the time to fracture increased for instruments operating with ReFlex Dynamic motion when non-heat-treated and stiffer instruments were used. Despite these findings, further research is necessary to validate these interpretations.

Irrigation with NaOCl significantly increased the NCF values of the files tested in ReFlex Dynamic motion; for this reason, the second hypothesis is only partially rejected. This effect may be explained by the continuous irrigation method employed in our study, which likely provided a lubricating effect, reducing friction and heat generation. These findings differ from those of Huang et al. [26], who reported no statistically significant differences among instruments tested with 5.25% NaOCl and water. The discrepancy may be attributed to methodological differences, as Huang et al. used a pre-immersion approach, whereas our study applied continuous irrigation during the test. This effect was observed only for the ReFlex Dynamic motion, probably because instruments under this motion experience greater flexural stress compared to continuous rotation, making them more sensitive to the lubricating effect of NaOCl.

The scanning electron microscopic examination revealed a characteristic fractographic pattern consistent with cyclic fatigue fracture, with comparable visual results across the groups evaluated. The images highlighted crack initiation zones, while the fractured surfaces displayed multiple dimples.

The study holds clinical significance as it was carried out in rigorous laboratory conditions designed to mimic in vivo scenarios. This methodology enables the extrapolation of valuable insights into how the cyclic fatigue resistance of flexible instruments is influenced by the novel hybrid kinematic and the environmental conditions. However, considering the limitations of in vitro studies, further research considering anatomical root variability and operator variables is needed. Validation of these in vitro findings and additional examinations with various irrigants and instruments are recommended. A comparison with other ReFlex motions (i.e., Reflex Smart) could also be explored.