## 1 Introduction  
Nickel-titanium (NiTi) endodontic rotary instruments and the subsequent development of mechanical preparation has improved the prognosis of root canal treatment, as their increased taper and automated motion enable more effective cleaning and shaping of the root canal system [1]. Many advances have been made in the last few years, including innovations in instrument design, new NiTi alloys, thermal treating of NiTi alloys, and the addition of new movements to instrumentation systems. The changes produced in the martensitic phase of the NiTi alloy, led with little or no memory, and the decrease in the tendency of the file to straighten during use, results in a more flexible file [2] with greater resistance to both cyclic fatigue as a torsional fracture [3]. However, the possible separation of instruments remains a major concern during clinical use of NiTi files. Failure due to cyclic fatigue or torsional fracture occurs unexpectedly, without any sign of previous permanent plastic deformation. It is caused by alternating tension-compression cycles, which are generated in the instrument when it is turned in a curved channel. Many variables that can influence resistance to fracture due to cyclic fatigue and torsional stress of NiTi rotary files have been investigated, including operating speed, instrument design, metal surface treatments, effect of irrigation solutions, and the sterilization cycles to which rotary files are subjected [4]. NiTi rotary files are often reused in clinical practice for cost-saving reasons. The sterilization procedure follows the steps of disinfection, cleaning, washing, drying, packaging, and heat sterilization [5]. The disinfection and cleaning steps reduce the bacterial load and remove debris from the blades of the instrument, and the sterilization step kills any form of microorganism, including spores [6]. The most widely used method for sterilization in the dental field described in the scientific literature is heat sterilization [7]. Repeated autoclave sterilization is necessary to prevent cross-contamination between patients [8]. The heat used during sterilization procedures may affect the mechanical and physical properties of these files [9]. Previous research has found that autoclave sterilization resulted in an increase in the surface roughness of NiTi rotary files, which affects the mechanical properties [10; 11]. These changes can affect the external surfaces in the form of micropitting and/or corrosion [12], reduction in cutting capacity, and/or by impairing resistance to cyclic fatigue or torsional fatigue [4]. Other studies hypothesize that after repeated clinical uses and sterilizations, there is a change in the austenite finishing temperature closest to the clinical operating temperatures. This would alter the proportions of austenite and microstructural phases of NiTi of martensite, ultimately affecting the mechanical properties of the files [2; 3; 13]. On the other hand, other studies suggest that autoclaving could improve the mechanical properties of instruments manufactured using a type of heat treatment, although this does not necessarily apply to other heat treatment methods. NiTi as an alloy is very \"sensitive\" to both thermal and mechanical (machining) tension that may occur during the raw material manufacturing processes and subsequent use of endodontic instruments. Adequate control of transition temperatures is essential to ensure optimum super-elastic properties. In addition, any other machining process will affect transition temperatures. This may explain why the performance of NiTi rotary instruments is affected by manufacturing quality processes and different thermal treatments [4; 14]. Therefore, the most recent scientific literature does not always agree on the effects of sterilization. These differences may be due to the heterogeneity of the instruments researched.