title: HyFlex EDM: superficial features, metallurgical analysis and fatigue resistance of innovative electro discharge machined NiTi rotary instruments.
abstract: OBJECTIVE
  To evaluate the surface and microstructural alterations of new and used HyFlex EDM prototypes and to test their fatigue resistance.
  
  
  METHODS
  Fifteen HyFlex EDM prototypes were used for in vitro instrumentation of severely curved root canals. Surface and microstructural characteristics of new and used files were compared by ESEM analysis equipped with energy dispersive X-ray spectrophotometry (EDS) and optical metallographic imaging. Usage-induced degradation was assessed. Thirty additional HyFlex EDM prototypes and 20 standard manufactured HyFlex CM files were subjected to cyclic fatigue tests. Time to fracture was recorded, and results were validated using the Kruskal-Wallis test (α-level 0.05). Fatigued files were analysed by ESEM for fractographic evaluation.
  
  
  RESULTS
  Surface and microstructural characterization of EDM prototypes revealed the typical spark-machined surface of a NiTi EDM alloy. No fractures were registered during root canal instrumentation. No evident surface alterations and minor degradation were observed between new and used instruments. The metallographic analysis of new and used files disclosed a homogeneous structure, mostly composed of lenticular martensite grains, and some residual austenite. The cyclic fatigue test showed an increase of fatigue resistance up to 700% on the EDM compared to CM files.
  
  
  CONCLUSIONS
  Spark-machined peculiar surface is the main feature of HyFlex EDM. Low degradation was observed after multiple canal instrumentations. Prototypes exhibited surprising high values of cyclic fatigue resistance and a safe in vitro use in severely curved canals.
authors.affiliation: Department of Biomedical and Neuromotor Sciences (DIBINEM), Endodontic Clinical Section, University of Bologna, Alma Mater Studiorum, Bologna, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Pirani
authors.affiliation: Department of Biomedical and Neuromotor Sciences (DIBINEM), Endodontic Clinical Section, University of Bologna, Alma Mater Studiorum, Bologna, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Iacono
authors.affiliation: Department of Surgery, Medicine, Dentistry and Morphological Sciences with Transplant Surgery, Oncology and Regenerative Medicine Relevance (CHIMOMO), School of Dentistry, Endodontic Section, University of Modena and Reggio Emilia, Modena, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Generali
authors.affiliation: Department of Engineering Enzo Ferrari (DIEF), University of Modena and Reggio Emilia, Modena, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Sassatelli
authors.affiliation: Department of Biomedical and Neuromotor Sciences (DIBINEM), Endodontic Clinical Section, University of Bologna, Alma Mater Studiorum, Bologna, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Nucci
authors.affiliation: Department of Engineering Enzo Ferrari (DIEF), University of Modena and Reggio Emilia, Modena, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Lusvarghi
authors.affiliation: Department of Biomedical and Neuromotor Sciences (DIBINEM), Endodontic Clinical Section, University of Bologna, Alma Mater Studiorum, Bologna, Italy.
authors.forename: M G
authors.initials: MG
authors.lastname: Gandolfi
authors.affiliation: Department of Biomedical and Neuromotor Sciences (DIBINEM), Endodontic Clinical Section, University of Bologna, Alma Mater Studiorum, Bologna, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Prati
mesh_terms: D003722:Dental Alloys; D008669:Metallurgy; D009532:Nickel; D018915:Root Canal Preparation; D014025:Titanium
chemical_list: D003722:Dental Alloys; D009532:Nickel; D014025:Titanium
keywords: CM wire; ESEM analysis; NiTi endodontic instruments; electro discharge machining (EDM); fatigue resistance; usage-induced degradation