## 1 Introduction  
Nickel-titanium (NiTi) rotary instruments are considered suitable for preparing curved root canals because of their higher flexibility [1] and superior canal-centering ability [2] compared with stainless steel files. However, intracanal fracture of NiTi rotary instruments is a problem that has not yet been fully resolved [3]. Tight binding of a rotating instrument with the canal wall may induce torsional overload and, thus, is considered as a cause of the fracture [4]. Moreover, the binding may produce a pulling force toward the root apex on the rotating instrument due to the helical design of the flutes. This phenomenon, termed the screw-in effect, may cause instantaneous engagement of the instrument giving rise to sudden torsional fracture [5]. The screw-in effect may also cause over-instrumentation, which could induce postoperative pain [6] and increase the risk of apical crack formation [7]. The screw-in tendency of NiTi rotary instruments is associated with their geometry; instruments with an active cutting edge and short pitch length may thread into the canal wall leading to a greater tendency to screw-in [8, 9]. In addition, pecking motion with higher pecking speed [10] and shorter pecking depth [11], and reciprocal rotation [12, 13] reduce the screw-in force. Heat treatment of NiTi alloys changes their phase transformation temperatures, leading to generation of soft and ductile R-phase and martensite phase at room or body temperature [14, 15]. Thus, contemporary heat-treated NiTi instruments exhibit a significant improvement in flexibility, cyclic fatigue resistance [16, 17] and canal-centering ability [18] compared with conventional NiTi instruments. The heat-treated instruments are reported to produce less screw-in forces compared with those manufactured from conventional alloys [13]. Radial land is a smooth cutting surface between the flutes of a rotary instrument. Radial land scrapes the canal wall with a plaining action and is believed to allow the instrument to remain centered in the canal space, leading to safer and less deviated instrumentation [19]. A radial-handed design was incorporated in several early generations of NiTi rotary instruments, such as ProFile (Dentsply Sirona, Ballaigues, Switzerland), which has three landed areas in a cross-section. ProFile produces significantly less transportation compared with non-handed instruments with actively cutting blades [20, 21] and generates smaller screw-in force compared with non-handed instruments [9]. Nevertheless, later, NiTi rotary systems were designed usually with actively cut blades without radial lands to increase the cutting efficiency [19], although this could give rise to a certain risk of preparation errors such as canal straightening [20, 21]. JIZAI (MANI, Utsunomiya, Japan; JZ) is a recently developed NiTi rotary system fabricated from a proprietary heat-treated NiTi alloy. JZ has a unique quasi-rectangular cross-sectional shape in which a radial land is provided on one of the short sides (Figure 1A). JZ (#25/.06 taper) is reported to generate a smaller screw-in force and shows better canal-centering ability compared with HyFlex EDM OneFile (#25/.08 taper at the tip; Coltene-Whaledent, Allstatten, Switzerland) [22]. The one-side radial-handed cross-sectional design of JZ can be assumed as a factor impacting these findings. Information on the effect of design features of NiTi rotary instruments on their canal-shaping ability [23] and screw-in force generation [9, 13, 22] has been gained through numerous laboratory studies using commercially available instruments. However, most of these studies have a major drawback, which is the inability to eliminate several confounding parameters other than a single variable of interest, resulting from the difference in multiple parameters among the brands being tested. Thus, some studies have compared instruments with the same geometry but different heat-treatment [22, 24, 25].