## 1 Introduction  
NiTi rotary instruments have enabled efficient and faster canal shaping with less transportation than hand instruments [1]. However, the separation of NiTi rotary files remains a concern for clinicians. Manufacturers have developed new NiTi files with heat treatment and variations in design to reduce the risk of file fracture [2, 3].  
The controlled memory (CM) wire, introduced in 2010, was the first thermomechanically treated NiTi endodontic alloy with reduced super-elasticity [4]. NiTi files with CM wire contain martensite, whereas conventional NiTi files contain austenite at body temperature [5]. CM wire instruments have a lower tendency to straighten curved root canals during preparation, thereby resulting in less transportation [6]. In 2015, another proprietary thermomechanically treated NiTi alloy, martensite-austenite-electropolis-file X (Max-Wire), was introduced [7]. Max-Wire NiTi files exist in a martensitic state at room temperature; at body temperature, phase transformation to an austenitic state occurs, and these wires acquire a memorized curved shape [7].  
Recently, many developments and advances have been made, not only in terms of heat treatment but also geometric designs. NiTi files with a small taper and slim wire allowed sufficient enlargement of the apical portion of the canal without undue file stress or compression of debris [8; 9]. The XP-endo Shaper (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and TruNatomy (Dentsply Sirona, Maillefer, Ballaigues, Switzerland) systems were recently introduced as a new generation of NiTi file systems designed to respect the original canal anatomy and preserve the peri-cervical dentin [9; 10]. These files are shape-conforming and can easily take the original shape of a canal [11]. The XP-endo Shaper is a single-file system made of Max-Wire with a 1% taper [7]. Due to the geometric design, the XP-endo Shaper offers minimal stress to the dentine wall, excellent debris removal, remarkable flexibility, and cyclic fatigue resistance [8; 12; 13]. The TruNatomy file system is produced via special heat treatment using a 0.8 mm wire, which has an off-centered parallelogram cross-section [9; 14]. TruNatomy has been reported to have enhanced cyclic fatigue resistance and cause less extrusion of debris [9; 14].  
Another novel file is the Spring Endo file (Denflex, Seoul, Republic of Korea), which has an elastic spring on the shaft of the NiTi rotary instrument [15]. According to its manufacturer, the elastic spring on the shaft of the NiTi instrument can make its insertion in the root canals of posterior teeth easy and reduce the risk of fracture [16]. The manufacturer also suggests that the spring structure buffers the overload applied to the instrument, resulting in an improved resistance to cyclic fatigue. Previous studies showed that the spring part of the shaft of NiTi rotary instruments improved the torsional resistance, bending flexibility, and cyclic fatigue resistance [3; 15]. Two types of Spring Endo Files are produced: unheated and heated.  
To the best of our knowledge, few studies have compared the mechanical properties of the newly introduced NiTi rotary systems, which have been fabricated using novel designs and heat treatments. Therefore, the purpose of this study was to evaluate the mechanical properties and phase transformation behavior of the following systems: TruNatomy Prime, XP-endo Shaper, Spring Endo file with an unheated finish, and Spring Endo file with a heated finish. Additionally, their cutting ability and the maximum torque and screw-in force generated during canal shaping were compared by simulating canal shaping in resin blocks.