## 1 Introduction  
Endodontic instruments have undergone multiple changes in design and metallurgy in the last decades. With specific heat treatment procedures, manufacturers have produced increasingly flexible alloys and claimed to reduce the incidence of instrument fracture during root canal preparation [1]. Fracture of nickel-titanium (NiTi) endodontic instruments as an airtogenic complication during root canal treatment or retreatment is undesired by both patients and clinicians, as it may negatively influence endodontic treatment outcome by preventing a proper chemo-mechanical disinfection of the entire root canal system, i.e., inaccessible areas beyond the obstruction [2]. The incidence of NiTi file separation is highly variable (0.14-5%) and depends on numerous factors such as study design, number and kind of teeth included, clinician's experience, instrument type, single- or multi-use of instruments, and metallurgy [3; 4; 5]. Whereas clinicians can influence the longevity of instruments as a function of the rotational speed [6; 7; 8], the kind of motion [9], the pre-flaring procedures [10], glide path preparation [11], the number of uses [12], and sterilization cycles [13], patient-related factors such as root canal anatomy with its inherent angle and radius of the curvature [7; 14] are immutable. Instrument parameters are also decisive,whereby a bigger instrument diameter leads to a shorter lifetime span in cyclic fatigue tests [15]. Additionally, the core mass is decisive for stress generation and fracture susceptibility [16]. However, the most recent studies showed a markedly reduced incidence when using modern endodontic NiTi instruments [5; 17; 18]. Instruments may fracture due to cyclic fatigue, torsional stress, or a combination of both [19]. Cyclic fatigue is caused by the continuous changes in compression and tension at the inner and outer curvatures depending on the rotational speed, the angle, and the radius of the curvature [20]. Torsional fracture occurs when the tip region of the instrument is locked by engaging into the dental walls and the coronal portion, and when the shaft continues rotating [21]. The properties (i.e., fracture resistance, flexibility) of the instruments are becoming more and more important as the number of instruments used for the entire enlargement of the root canals is minimized when using modern systems with a reduced sequence of instruments or even single-file systems. Thus, the number of cycles and the level of stress onto the instruments are significantly increased. Cyclic fatigue may represent a clinically relevant parameter that can provide evidence about the lifetime of instruments in the root canal, by testing a single parameter under standardized laboratory conditions [22]. However, cyclic fatigue testing is not included in the ISO standard for endodontic instruments. The ISO 3630:1 specification (current version 2019-08) addresses only resistance to fracture by twisting and angular deflection in chapter 7.4, and it recommends standard atmosphere conditions at 23 \({}^{\circ}\)C (\(\pm\)2 \({}^{\circ}\)C) [23]. Numerous setups and devices for cyclic fatigue testing have been proposed and implemented. This fully explains the finding of a recent review elucidating that results in different studies investigating identical instruments differed by up to more than 300% [24]. Hence, results were not comparable. Actually, there is no consensus among scientists concerning crucial parameters such as the angle and radius of curvature and the location of the center of the curvature, despite the influence of these parameters being scientifically proven [25]. The same applies to the vertical amplitude during testing. Although, in 1986, [PERSON] and [PERSON] already presented a dynamic testing device that allowed investigating the cyclic fatigue resistance of endodontic rotary instruments by implementing a vertical amplitude (pure axial motion) [26], in the past, most tests were performed statically [24].