Fractographic examination of broken protaper S1 instruments after clinical use

Objectives: To investigate the mode of failure of a brand of NiTi rotary instruments fractured during clinical use, by detailed examination of the fracture surface. Methods: A total of 122 ProTaper S1 instruments were discarded from an endodontic clinic at a stomatological school in China over a period of 17 months; 28 had fractured. These separated files were ultrasonically cleaned, autoclaved and then examined under SEM. From the lateral view, the fracture was classified into "torsional" or "flexural". The specimens were then re-mounted and the presence of characteristics of shear failure and fatigue striations were recorded under high-power view of the fracture surface. Results: 27 separated instruments were available for analysis. Under low-power magnification, only two fell into the category of "torsional" failure in lateral view; the others appeared to be "flexural". Close examination of the fracture surface revealed the presence of fatigue striations in 18 specimens. Nine instruments (including the two putative "torsional" failures above) fell into the shear-fracture group, in which fatigue striations were absent or characteristics of shear failure of the material were found. The mean length of broken fragments due to fatigue failure (4.3±1.9 mm) was significantly greater than that for shear failure (2.5±0.8 mm) (P<0.001). Conclusions: Examination of the fracture surface at high magnification is essential to reveal features on the fracture surface that may indicate the possible origin of cracks and mode of material failure. Macroscopic or lateral examination of separated instruments would fail to reveal the true mechanism of failure. Fatigue seems to be an important reason for the separation of rotary instruments during clinical use.