Effects of simultaneous liquid or gel sodium hypochlorite irrigation on cyclic fatigue of two single-file nickel titanium systems at room and body temperature

Aim: To evaluate the effects of simultaneous liquid or gel sodium hypochlorite (NaOCl) irrigation on cyclic fatigue of F6 SkyTaper (F6ST; Komet Dental, Gebr. Brasseler, Lemgo, Germany) and OneCurve (OC; Micro Méga, Besançon, France) single files at room (20 °C±1°C) and body (37°C±1°C) temperature. Methods: One hundred and eighty new 25/.06 F6ST and OC files were randomly allocated into 6 groups (n = 15) for each brand. Groups 1 and 4 included new files that were not exposed to NaOCl at 20°C and 37°C, respectively. Groups 2 and 5 included instruments activated with liquid NaOCl at 20°C and 37°C, respectively. Groups 3 and 6 consisted of instruments tested with NaOCl gel at 20°C and 37°C, respectively. A customized static cyclic fatigue testing device was used. The device allowed the standardized instrument insertion thanks to a fixed block and the possibility to test the files at different inclinations. For this study, all instruments were tested at the standard position which corresponds to a 0° inclination. In addition, a simulated 16-mm-long ceramic artificial was employed (60° angle and 5-mm radius of curvature) by reproducing the instrument’s size and taper. The simultaneous irrigation was ensured by a cover formed by the superimposition of two Plexiglas slides in which a channel and a small circular hole were built inside to permit the irrigation flow and outflow trough aspiration system, respectively. A thermostat associated to the customized device allowed the temperature adjustment in the artificial canal. Fifteen instruments of each system were activated in continuous rotation (300 rpm) following the manufacturer’s instructions and torque fixed at the maximum value provided. For control groups (without NaOCl), the friction of the file on the artificial canal walls was reduced by a special highflow synthetic oil (Super Oil; Singer Co Ltd, Elizabeth, NJ) designed for lubrication of mechanical parts while experimental protocols (for liquid and gel solutions) included a continuous and constant injection of irrigant for 10 seconds every 30 seconds of rotation of the instruments. For each instrument, the time to fracture in seconds (TtF) from the start of the test until the moment breakage was detected visually and/or audibly was recorded with a chronometer to an accuracy of 0.1 second and the length of the fractured file tip was measured by using a digital microcaliper (Mitutoyo Italiana srl, Lainate, Italy). The surface of fractured instruments was studied by fieldemission scanning electron microscopy (SEM). The means and standard deviations of TtF were calculated and statistically analyzed (P<0.05, Two-way ANOVA, Tukey Test). Results: TtF of all tested instruments decreased at body temperature (P<0.05). At 20°C, NaOCl enhanced TtF of F6ST and OC (P<0.05). Instruments irrigated with NaOCl liquid had higher TtF in comparison with gel for F6ST (P<0.05); no difference was observed between the two formulations for OC. At 37 °C, both NaOCl formulations had no significant influence on TtF for F6ST while improved cyclic fatigue resistance of OC (P<0.05). Independently from the temperature and irrigant use, OC reported significant higher TtF than F6ST (P<0.05). The length of the fractured file fragments was not statistically different among the tested instruments (5.1 mm) (P>0.05). Moreover, SEM analysis of the fractured surfaces showed the typical features of cyclic fatigue failure for both instruments. Conclusion: Within the limitations of the present in vitro study, NaOCl (liquid or gel) improved cyclic fatigue resistance of OC, independently from the temperature, while for F6ST the negative impact of higher temperature reduced the irrigant benefits.