## 4 Discussion  
This study analyzed the effect of radial-handed geometry of NiTi rotary instruments on canal-centering ability and torque/force generation, both of which play a crucial role in determining the clinical performance of these instruments [26]. In this study, impact of metallurgic and geometrical factors, except for the presence/absence of a radial land, was made as minimal as possible by using the non-handed JZ as the control. The use of an automated root canal instrumentation device may have contributed to a reproducible assessment while avoiding human intervention. An up-and-down speed of 50 mm/min was set based on a previous study [10]. The results demonstrated that the two instruments did not differ in their canal-centering ability, but JZ exhibited a significantly smaller upward force (screw-in force) and a larger downward force compared with its counterpart (non-handed JZ). Thus, the null hypothesis is partially rejected. Simulated resin canals were employed in the present study to standardize the analysis by excluding morphological differences inherent in natural root canals [27]. In particular, simulated resin canals are more suitable for providing standardized evaluation with direct visual comparison of the centering ratio compared with natural root canals [18; 28]. Thus, many studies have used resin canals as a valid alternative to natural root canals to investigate torque/force generation [29; 30] and canal transportation [31; 32] during root canal instrumentation. However, the use of simulated resin canals has limitations because their physical properties such as hardness that differs from those of real teeth [33]. Resin canals are softer and have a smoother surface texture than dentin and may require less force to cut [27]; nevertheless, instrumentation in resin canals may generate larger stress than in human root dentin because rotary instruments may become entangled in shavings and heat-softened resin [34]. In this regard, future studies using extracted teeth may provide more clinically relevant implications. JZ is made of a heat-treated NiTi alloy, and the manufacturer claims that JZ contains a considerable amount of R-phase at room and body temperatures, although the phase transformation temperature of JZ has not yet been reported. JZ shows improvements in terms of flexibility, cyclic fatigue resistance, and canal-centering ability compared with a non-heat-treated instrument with identical geometry [22], which agrees with the findings of several studies showing that heat treatment improves the mechanical properties of NiTi rotary instruments [15; 16; 17; 18; 35]. Figure 5: Maximum values of upward force (**A**) and downward force (**B**) during root canal instrumentation. Data represent the mean and standard deviation (\(n=10\)). \({}^{*}p<0.05\) within the same instrumentation sequence. Different lowercase letters are significantly different within the same instrument (\(p<0.05\)). Radial lands are believed to prevent deviation of canal preparation from the curvature by their passively cutting nature that helps keeping a rotating instrument centered in the canal [36]. In fact, some studies have demonstrated that landed instruments such as ProFile [20,21] and JZ [22] exhibit a better ability to maintain canal curvature compared with non-landed instruments. This study, however, failed to demonstrate any significant difference between JZ and the non-landed JZ in terms of canal-centering ability, which could be due to the increased flexibility of these instruments resulting from the heat treatment [22]. Since flexible instruments show better ability to follow the canal curvature [18], it seems reasonable to suppose that the flexibility of the heat-treated alloy compensated for the canal-straightening tendency of the non-landed instrument. Thus, in heat-treated flexible instruments, geometric features other than radial lands, such as the taper [31] and core diameter [37], may have greater contribution to the degree of canal straightening.