Quantitative Analysis of the Fatigue Fracture of Ni-TI Rotary Instruments

Objectives: On rotation under bending, the endodontic rotary instruments are subjected to cyclic fatigue fracture. The purpose of this study was to evaluate the correlation between fatigue life and cyclic strain amplitude by theoretical finite element analysis as well as experimental cyclic fatigue tests. Methods: Cyclic fatigue testing was carried out on ProFile-#25 and ProTaper-F2 rotary endodontic instruments in planar canals with radii of curvature from 5mm to 10mm and extent of insertion from 20o to 60o. The experimental results have been examined against a parallel finite element analysis, which provided the magnitudes and locations of the maximum bending strain in different canals. Results: When correlated in terms of the corresponding maximum bending strain, the fatigue lives of the ProFile-#25 and ProTaper-F2 fall onto the same scatter band, which is close to the smooth Ni-Ti wire fatigue results as well. Fatigue cracks initiated from one of the cutting tips and propagated before final fracture occurred. Conclusion: The predictable locations of maximum strain agreed well with the experimental fracture positions. Fractography of the broken instruments suggested that a considerable fraction of the fatigue lives were spent on crack initiation.