Influence of different angles of file access on cyclic fatigue resistance of 2shape rotary instruments in different artificial canals

Aim: Several parameters including extent of the curvature and angulated insertion of file into the canal could influence cyclic fatigue of nickel-titanium files. The purpose of this study was to compare the influence of different access angles associated to different curvature radii on cyclic fatigue resistance of 2Shape (TS; Micro-Mega, Besancon, France) rotary instruments. Methods: 60 instruments were tested in two 16mmstainless steel artificial canals with angle of curvature of 60° and 5mm or 3mm radius of curvature respectively. 2Shape TS2 #25.06 and 2Shape TS1 #25.04 were evaluated at 0°, 10° and 20° insertion angles (n=10 for each angle of access). Cyclic fatigue resistance was determined by Number of Cycles to Failure (NCF) using a new testing device machine. This device consists of a platform composed of a block containing the electric handpiece and a mobile support on rails for insertion/disconnection of the file. Moreover, a metal mobile platform is connected to the mobile support. This second platform with the artificial canal produces the different inclination of insertion of tested file maintaining the entrance of instrument perpendicular to the canal. Each instrument was tested in continuous rotation (300 rpm) following the manufacturer’s instructions. Fracture surface was examined with a scanning electron microscope. Data were statistically analyzed using 2-way analysis of variance and the Bonferroni multiple comparison post-hoc test (Prism 7.0; GraphPad Software, Inc, La Jolla, CA) with a significance level of P<0.05. Results: TS1 (#25.04) exhibited higher cyclic fatigue resistance than TS2 (#25.06) for each angle and radius tested (P<0.05). When files were tested inside 3mmradius canal, independently from taper, they recorded lower cyclic fatigue resistance than in 5mm-radius one (P<0.05). Considering canal with 5mm-radius of curvature, no significant cyclic fatigue resistance reduction was observed for .06 taper-instruments for each angle tested (P>0.05), while files with .04 taper exhibited significant lower NCF when tested at 20° (P<0.05). In canals with 3mm radius of curvature, TS2 (#25.06) showed no significant differences for each angle tested (P>0.05). TS1 (#25.04) exhibited significant resistance reduction between 0° and 10° as well as between 0° and 20° (P<0.05), with no difference between 10° and 20° (P>0.05). Conclusion: Inclined angle of file access into the canals decreased cyclic fatigue resistance of #25.04 files especially when the radius of curvature is reduced. Cyclic fatigue resistance of #25.04 instruments was higher than #25.06 files made with the same thermal treatment. Independently from taper, each file tested in 3mm-radius canal showed lower cyclic fatigue resistance than instruments tested in 5mm-radius canal.