Cyclic fatigue resistance evaluation of heat-treated rotary files after immersion in NaOCl and EDTA: an in vitro study

Introduction
Rotary file fracture during its rotation inside root canal is considered an important clinical problem. Corrosion is one of the factors that affect the instrument’s life-time, and decreases fracture resistance.

Objectives
This study assessed the effect of immersion in different irrigation solutions on cyclic fatigue resistance using two types of nickel-titanium rotary files.

Material and methods
Sixty new nickel-titanium rotary files (30 AF files, 30 Plex-V files) were chosen. Each type of files were divided into three groups: control, NaOCl, and EDTA. Instruments were immersed in solutions for 15 min at room temperature. After immersion, cyclic fatigue of files was tested by rotating the files in an artificial stainless-steal canal, with a 120-degree angle and 2.5 mm radius of curvature. The instruments rotated until a fracture occurred, and time to fracture was recorded in seconds. NCF was also calculated, and data were statistically analyzed with one-sample Kolmogorov-Smirnov, Mann-Whitney, and Kruskal-Wallis H tests. A p-value of less than 0.05 was considered statistically significant.

Results
Cyclic fatigue results showed a statistically significant difference between Plex-V files, which had the highest resistance to fatigue compared with AF files (p < 0.001). Statistical analysis was performed to determine which irrigation solutions had the most effect on resistance in each brand.

Conclusions
Sodium hypochlorite solution had the most effect on the cyclic fatigue resistance of AF files, while EDTA solution had the strongest effect on reducing fatigue resistance of Plex-V files. However, Plex-V instruments demonstrated better fatigue resistance compared with AF instruments.