Abstract : Purpose : The aim of this study was to evaluate and compare the cyclic fatigue resistance of nickel-titanium rotary files in continuous rotation and reciprocating motion in artificial metal canals.

 Methods : A total of 44 new rotary BioRaCe files (ISO size=25, taper=0.06, length=25 mm) were selected and randomly divided into four groups (n=11 each) : CR (BioRaCe NiTi files in continuous rotation with up-and-down), C (BioRace NiTi files in continuous rotation without up-and-down), Re (BioRace NiTi files in reciprocating motion with up-and-down), and Re-C (BioRace NiTi files in reciprocating motion without up-and-down). A cyclic fatigue testing device was fabricated with a 45° angle of curvature in artificial metal blocks.

 A continuous up-and-down (4 mm in each direction at 0.5 second) pecking movement was incorporated to simulate the pecking motion in a real clinical situation. All instruments were rotated or reciprocated until fracture occurred. The time taken for each instrument to fracture and the lengths of the broken fragments were recorded. All the fractured files were analyzed under a scanning electron microscope to detect the mode of fracture. Data were analyzed by one-way analysis of variance, followed by a Tukey-Kramer multiple comparison test.

 Results : The time taken for the instruments of the Re groups to fail under cyclic loading was significantly longer compared with the CR groups. The Re groups showed a significant difference in the time of cyclic fatigue fracture compared with the Re-C and C groups (p<0.01). The lengths of the fractured segments were between 3 and 5 mm, which was not statistically significant among the experimental groups. Scanning electron microscope observations showed craterlike formations along with numerous dimples and micro-bubbles.

 Conclusion : Reciprocating motion showed a significantly higher cyclic fatigue resistance than continuous rotation.

Abstract : The objective of this study was to assess the effect of smear layer removal and dentin (dentinal tubules) erosion using EDTA agent (MORHONINE^® for dental liquid, Showa Yakuhin Kako Co., Ltd.) with two root canal irrigation methods (syringe irrigation and passive ultrasonic irrigation), and to determine the effective duration of action.

 Twelve extracted human permanent teeth were used, with the crowns cut at the cement-enamel junction. A Gates Glidden drill was used for coronal flaring, followed by root canal preparation up to #35 with a K-file. Then, using a 30-second, 60-second or 90-second duration of action of EDTA agent, we prepared specimens for a syringe irrigation group and a passive ultrasonic irrigation group. A scanning electron microscope was used to observe the root canal wall surface of the specimens at 5000×magnification. The resulting images were assessed by five dentists unaware of the objective of the study. The Kruskal-Wallis and Steel-Dwass multiple comparison tests were used to test for statistically significant differences.

 The results suggested that the desirable duration of action is 60 seconds with the syringe irrigation group and 30-60 seconds with the passive ultrasonic irrigation group.