Influence of axial motion amplitude on stress generation, canal-centering ability, and motor-recorded torque during XP-endo Shaper instrumentation: an in vitro study

The optimal axial motion amplitude during nickel-titanium rotary canal instrumentation has not yet been fully clarified. To evaluate the effects of axial motion amplitude on shaping behavior and motor-recorded torque during root canal instrumentation using the XP-endo Shaper (XPS). Thirty J-shaped resin canal blocks were instrumented with XPS using an automated device at varying axial motion amplitudes: XP-S (apical/coronal; 1 s/0.5 s), XP-M (2 s/1 s), and XP-L (4 s/2 s) ( n = 10 each). Intracanal torque and force (those generated within the model) were recorded using strain gauges and a load cell, respectively, while motor-recorded torque was simultaneously acquired with a data logger. Canal-centering ability was evaluated at five apical levels. Cumulative intracanal torque in XP-L was significantly lower than in the other groups (S vs. L: p < 0.001, M vs. L: p = 0.034). The regression line for XP-S (motor-recorded torque vs. intracanal torque) had a significantly lower slope than those of the other groups (S vs. L: p = 0.025, M vs. L: p = 0.001, analysis of covariance) and intersected them within the measurement range. At 0 and 0.5 mm from the apex, XP-L exhibited superior canal-centering ability compared to XP-S and XP-M, as evidenced by significantly lower positive ratios that indicated reduced outward deviation (at 0 mm, L vs. S: p < 0.001, L vs. M: p = 0.012; at 0.5 mm, L vs. S: p < 0.001, L vs. M: p = 0.006). During XPS instrumentation in curved canals, a greater axial motion amplitude reduced cumulative intracanal torque and was associated with less outward canal deviation and better preservation of the original canal path in the apical region. At a smaller amplitude, motor-recorded torque underestimated intracanal torque more at higher torque levels than in the other groups. Taken together, these findings suggest that larger axial motion amplitudes may offer mechanical advantages during XPS instrumentation, although their clinical relevance requires further investigation.