Cyclic Fatigue Resistance and Phase Transformation Behavior of SlimShaper and SlimShaper PRO NiTi Instruments: A Mechanical and Thermal Analysis

Objectives: This study compared the cyclic fatigue resistance and the mechanical, thermal, and metallurgical characteristics of SlimShaper® and SlimShaper PRO® instruments. Both sequences include three instruments (ZS1–ZS3) with identical geometries, although SlimShaper PRO features an apically modified thermal treatment. Methods: Cyclic fatigue tests were performed using a standardized metallic guide with a 45◦ curvature, on six instruments of each sequence type. Fractured segments were measured, and fracture surfaces were analyzed using optical microscopy and Scanning Electron Microscopy (SEM). Elemental composition was assessed by Energy-dispersive Spectroscopy (EDS), while Differential Scanning Calorimetry (DSC) was used to determine transformation temperatures and enthalpy. Results: ZS1 and ZS1 PRO exhibited comparable cyclic fatigue resistance, whereas ZS2 and ZS3 showed significantly higher resistance than their PRO counterparts. SlimShaper PRO® instruments fractured with segments approximately 0.5–1 mm longer. EDS confirmed that both instruments were made of NiTi, with minor differences in surface composition, while DSC demonstrated similar enthalpy values but distinct transformation ranges. At room temperature (27 ◦C), SlimShaper® ZS2 remained predominantly martensitic, whereas ZS2 PRO could be partially austenitic, explaining its observed reduced fatigue resistance. The results of the DSC allowed to deduce the microstructure and thus the fatigue behavior at the temperature of the oral cavity. Conclusions: SlimShaper ZS2 and ZS3 showed significantly greater cyclic fatigue resistance than their PRO counterparts, while DSC analysis revealed distinct differences in phase transformation behavior that explain their mechanical performance.