HyFlex EDM: innovative electro discharge machined rotary instruments. Fatigue testing, superficial and microstructural characterization

Aim To test the fatigue resistance and to evaluate the surface and microstructural alterations of new and in vitro used HyFlex EDM NiTi rotary prototypes. Methodology Thirty electro discharge machined HyFlex EDM (Colt ene/Whaledent, Switzerland) prototypes, and 20 HyFlex CM (Colt ene/Whaledent, Switzerland) files were subjected to cyclic fatigue test in a standardized 70° artificial metal canal. Results were validated using Mann-Whitney tests (a-level 0.05). Fractographic analysis on broken samples was conducted using environmental scanning electron microscopy (ESEM) imaging. Fifteen new HyFlex EDM prototypes were used for in vitro instrumenta- tion of severely curved root canals of extracted multi-rooted teeth. Surface and microstructural characteristics of files were analyzed by ESEM equipped with energy dispersive x-ray spectrophotometry (EDS) and optical metallographic imaging. The same analysis was conducted on new and used instruments to assess the usage- induced degradation. Results The cyclic fatigue test revealed an increase of fatigue resistance up to 700% of EDM files compared to CM (P = 0.0001). Fractographic analysis of fatigued EDM instruments disclosed multiple crack origins in correspondence of the cutting edges, with a limited fatigue striation zone and a noteworthy dimpled area. Surface and microstructural characterization of new instruments revealed the typical features of a NiTi ED-Machined alloy with a ‘crater-like’ surface. No fractures were registered during instrumentation of curved canals. Surface and microstructural characterization of used files revealed no wear and no degradation of the size 25, .12 taper and size 25, .08 taper files. The tip segment was confirmed as the most mechanically stressed portion of size 10, .05 taper files. The metallographic inspection on the cross section of brand new HyFlex EDM files showed an homoge- neous martensitic phase. The microstructure appeared uniform from the surface to the bulk, and no microcracks or defect were identified, even at high optical magnification (1000X). Conclusions HyFlex EDM exhibited high values of cyclic fatigue resistance and a safe in vitro use in severely curved canals. Unaltered spark-machined surface and low microstructural degradation are the main features of HyFlex EDM. Acknowledgements The authors deny any conflict of interest related to this study and would thank Coltene Whaledent for providing the test material.