JOE JUL 2011 - Correlation between Experimental Cyclic Fatigue Resistance and Numerical Stress Analysis for Nickel-Titanium Rotary Files

Correlation between Experimental Cyclic Fatigue Resistance and Numerical Stress Analysis for Nickel-Titanium Rotary Files

Min-Ho Lee, DDS, MS, PhD,* Antheunis Versluis, PhD,† Byung-Min Kim, PhD,‡ Chan-Joo Lee, BS,‡ Bock Hur, DDS, MS, PhD,* and Hyeon-Cheol Kim, DDS, MS, PhD*

Introduction: 

The aim of this investigation was to study cyclic fatigue resistance of various nickel-titanium (NiTi) rotary files under various root canal curvatures by corre- lating cyclic fatigue fracture tests with finite-element analysis (FEA). 

Methods: 

Four NiTi rotary instruments with different cross-sectional geometries but compa- rable sizes were selected for this study: ProTaper (Dentsply Maillefer, Ballaigues, Switzerland), ProFile (Dentsply Maillefer), HeroShaper (Micromega, Be- sanc ̧on, France), and Mtwo (VDW, Munich, Germany). The ProFile and HeroShaper files were of size 30/.06 taper, the Mtwo was of size 30/.05 taper, and the ProTaper was F3. The cyclic fatigue test was con- ducted in a custom-made device that simulated canals with 25 , 35 , and 45 curvature. For the FEA, the file models were meshed, and 17-mm long curved canals were modeled to have same curvatures as the cyclic fatigue tests. Numerical analysis was performed to determine the stress distributions in the NiTi instru- ments while they rotated in the simulated curved canals. 

Results: 

ProTaper (the stiffest instrument) showed the least cyclic fatigue resistance and highest stress concentration for all tested curvatures, whereas Mtwo showed the best cyclic fatigue resistance. A comparison between the FEA and fatigue results showed that when stresses increased, the number of instrument rotations to fracture decreased. Maximum stresses in the instruments predicted the approximate location of the fatigue fracture. 

Conclusions: 

The stiffer instrument had the highest stress concentration in FEA and the least number of rotations until fracture in the cyclic fatigue test. Increased curvature of the root canal generated higher stresses and shortened the lifetime of NiTi files. Finite-element stress analysis reflected cyclic fatigue fracture resistance. 

(J Endod 2011;37:1152–1157)

Key Words

Cyclic fatigue, finite-element analysis, fracture susceptibility, nickel-titanium file, root canal curvature

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