Mh2 500n

The same instruments as in the bending test were evaluated. A self-made cyclic fatigue tester with a movable test stand (MH2-500N; IMADA, Aichi, Japan) and the X-Smart Plus endodontic motor (Dentsply Sirona) were used [23 (link)] (Figure 2). An artificial root canal made of stainless steel and designed with a 1.5 mm diameter, a 60° curvature, 3 mm radius of curvature, and the center of the curvature at 5 mm from the tip of the instrument was used [24 (link)]. The instruments were rotated as per the manufacturers’ recommendations (500 rpm, 1.5 N·cm in TRN; 400 rpm, 2.4 N·cm in HEDM; 300 rpm, 2.0 N·cm in PTN; and the WaveOne Gold setting in WOG), while moving the handpiece with an axial up-and-down motion of 2 mm amplitude at 300 mm/min. The canal was lubricated with silicon oil (KF-96-100CS, Shin-Etsu Chemical, Tokyo, Japan). The length of time to fracture was measured, and the number of cycles to failure (NCF) was determined as the number of revolutions (rpm) × time to fracture (seconds). All experiments were performed at room temperature.

The dynamic cyclic fatigue test [16 (link)] was conducted at RT (25 ± 1 °C) or BT (37 ± 1 °C) in an incubator made of a plastic box and a temperature controller (E5C4, OMRON Corporation, Kyoto, Japan). The testing device consisted of a test stand with a movable stage (MH2-500N, IMADA, Aichi, Japan), to which the handpiece of an endodontic motor (Tri Auto ZX2, J. Morita, Kyoto, Japan) was attached. A 17-mm-long stainless steel artificial canal with a 1.5-mm diameter, 60° angle of curvature, and 3.0-mm radius of curvature was used, and the environmental temperature of the canal was checked with a thermocouple (HOBO UX120-014 M, Onset Computer Corporation, Bourne, MA, USA). The center of the canal curvature was located 5 mm from the tip of the instrument. The instruments (n = 10, each) were fixed in the canal at 13.6 mm and rotated in combination with a 2-mm back-and-forth motion at 5 mm/s. Silicone oil (KF-96-100CS, Shin-Etsu Chemical, Tokyo, Japan) was used as lubricant. The rotational speed was set to 300 rpm for Mtwo and 500 rpm for EDM, CM, VB, RE, and JIZAI, according to the manufactures’ recommendation. The time to fracture was recorded using a stopwatch, and the number of cycles to failure (NCF) was calculated as rpm × time to failure (min).