Effect of repeated use of different heat-treated file systems on surface roughness and cyclic fatigue resistance

Objective: The aim of this study was to comparatively evaluate the effects of different heat treatment protocols on the cyclic fatigue resistance and surface roughness of two nickel-titanium (NiTi) rotary file systems: Scope UniGold and Scope UniBlue. Additionally, the influence of the number of uses and sterilization procedures on these parameters was investigated. Materials and Methods: A total of 120 files were divided into four groups (n=12) (unused, used 1, 2 and 3 times), 96 of which belonged to Scope UniGold and Scope UniBlue systems. The file systems were subjected to cyclic fatigue testing at 37°C in 90° inclined stainless steel channels. In addition, the remaining 24 were divided into four groups (n=12) for surface roughness evaluation and SA (arithmetic mean roughness) and SZ (maximum peak-trough height) measurements were made with a contact profilometer. ANOVA, Mann-Whitney U and Kruskal-Wallis tests were used for statistical analysis. Results: The Scope UniBlue files exhibited significantly higher resistance to cyclic fatigue compared to the Scope UniGold files (p<0.001). An increase in the number of uses led to a notable decrease in fatigue resistance in both systems. Surface roughness evaluations revealed that repeated use increased surface irregularities, with Scope UniGold showing greater changes in Sa and Sz values after the second and third uses. Conclusion: Scope UniBlue files demonstrated superior performance in terms of cyclic fatigue resistance, likely due to their higher martensitic phase content and advanced heat treatment technology. Additionally, Scope UniBlue maintained better surface integrity over repeated uses. These findings may guide clinicians in selecting more durable and reliable file systems for safe endodontic treatment.