Improvement of the fatigue resistance of NiTi endodontic files by surface and bulk modifications.
OBJECTIVE
To assess the failure mechanism of rotary NiTi instruments by chemical, structural and morphological analyses to provide a rational explanation of the effects of surface and bulk treatments on their resistance to fatigue fracture.


METHODS
Thermal treatment (350-500 °C) was performed on electropolished (EP) and non-electropolished (Non-EP) NiTi endodontic instruments. Bulk and surface chemical composition and crystallographic structures were determined by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) to evaluate the effects of thermal treatment and electropolishing on the NiTi alloy. Fatigue tests of all instruments were performed. Surface morphology before and after the tests, and fractured section were analysed using scanning electron microscopy to determine crack extensions. Results were analysed statistically using analysis of variance (anova) and post hoc Student-Newman-Keuls test.


RESULTS
Before thermal treatment, significant differences (P < 0.05) in fatigue resistance between EP and Non-EP instruments (the number of revolutions to failure, N(f) , was 385 and 160, respectively) were attributed to differences in the surface morphology of the instruments. SEM analysis of the fracture surfaces indicated that flexural fatigue fractures occurred in two steps: first by a slow growth of initial cracks and then rapid rupture of the remaining material. Thermal treatment did not affect the surface morphology but resulted in significant changes in the instrument bulk with the appearance of an R-phase and an improved fatigue resistance; indeed after treatment at 500 °C, N(f) increased up to 829 and 474 for EP and Non-EP instruments, respectively.


CONCLUSIONS
Both thermal treatment and electropolishing improved the resistance of NiTi rotary instruments against fatigue fracture.
Condorelli
G G
GG
Department of Chemical Science, University of Catania, Catania, Italy Private Practice, Catania, Italy.
Bonaccorso
A
A
Smecca
E
E
Schäfer
E
E
Cantatore
G
G
Tripi
T R
TR
D002244:Carbon / Q000032:analysis; D003300:Copper / Q000032:analysis; D003461:Crystallography; D003722:Dental Alloys / Q000032:analysis / Q000737:chemistry*; D003775:Dental Polishing / Q000379:methods; D004560:Electricity; D004577:Electron Probe Microanalysis; D004867:Equipment Design; D004868:Equipment Failure; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000032:analysis / Q000737:chemistry*; D010100:Oxygen / Q000032:analysis; D018915:Root Canal Preparation / Q000295:instrumentation*; D012825:Silicon / Q000032:analysis; D013052:Spectrometry, X-Ray Emission; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000032:analysis / Q000737:chemistry*; D014961:X-Ray Diffraction
D003722:Dental Alloys; C040654:titanium nickelide; D002244:Carbon; D003300:Copper; D009532:Nickel; D014025:Titanium; D010100:Oxygen; D012825:Silicon