###### Abstract  
Background: To evaluate the cyclic fatigue resistance of rotary endodontic nickel-titanium instruments using both experimental and finite element methods. Methodology: Four different proprietary rotary endodontic instruments (PTG, PTU, VB, and TRN) were subjected to cyclic fatigue tests through a custom-built fixture (n = 30 for each group) and tested until failure according to the manufacturer's recommended settings, and the number of cycles to failure (NCF) was measured. Separated fragments were measured to determine the location of the fracture event. Finite element modeling replicated this experiment in silico via a two-step method, modeling the dynamic motion via an initial insertion step, then followed by rotation. Results: TRN showed the highest NCF (2027.94 +- 452.50) with the lowest values for strain (4.49% \(\pm\) 0.23%) and fragment lengths (3.26 mm \(\pm\) 0.47 mm), whereas PTU showed the lowest NCF (600.90 \(\pm\) 80.86) and highest values for strain (6.29% \(\pm\) 0.47%) and fragment lengths (5.36 mm \(\pm\) 0.79 mm). All instruments showed an increase in von Mises and maximum principal stress during the insertion step but showed no changes to the maximum stress values during the rotational step. Conclusions: The finite element analysis approach used provided useful insights into the stress evolution and stress distribution of instruments during cyclic fatigue but lacked the ability with existing inputs to predict failure from cyclic fatigue.  
studies on the mechanical behavior of rotary endodontic instruments has also increased significantly in the last decade [7,8].  
The finite element method (FEM) is particularly useful in observing stress behaviors that are not measurable in physical experiments. In short, FEM begins by defining the problem, including the relevant geometry, properties, loads and boundary conditions. As real structures and components are generally very complex, reduction and compromise are normally required to create a manageable geometry [9]. The geometrical model is \"meshed\" by discretizing the body into an optimal number of arbitrary 2D or 3D elements (\"finite elements\"), resulting in matrix equations relating the input at specified points in the elements (nodes) to the output at these same points [9,10]. The sum of these matrix equations from the nodes results in a global matrix equation and can be used to solve equations over larger regions [10]. Once solved, the results are then interpreted and validated.  
With the advent of improved computer processing power and finite element software development, complex matrix equations that were historically solved manually can now be calculated with increased accuracy and efficiency. Particularly in the case of complex geometrical structures, such as a rotary endodontic instrument, using finite element models is appropriate. The task of the user is to ensure the problem is well defined, with a geometrical approximation that accurately reflects the true geometry of the instrument.  
Despite the recent surge in publications of FEA of NiTi instruments, cyclic fatigue is currently not well studied when compared to static bending and torsion [8]. To date, only one study has reproduced the number of cycles to failure (NCF) data on FEA [11]. Given this background, the present study aims to compare experimental cyclic fatigue data from a proposed standardized fatigue device with a two-step routine in FEA, in which both the instruments and fatigue device are modeled closely in terms of geometry and boundary conditions.