Influence of previous angular deformation on flexural fatigue resistance of K3 nickel-titanium rotary instruments.

Several studies have evaluated the influence of torsion and bending loads on the mechanical resistance of nickel-titanium instruments. The aim of the present study was to analyze the influence of previous angular deformation on K3 files resistance to flexural fatigue. New files and files previously submitted to different angular deformation were tested for flexural fatigue, and the number of cycles to fracture was compared. The results indicated that as the angular deformation increases, the number of cycles attained under flexural fatigue condition decreases. A reduction of fatigue resistance was registered even when a significant plastic deformation was not imposed. From the obtained results, it may be suggested that the sequential loading, including torsion and flexural fatigue, can be responsible for instrument separation during clinical practice. Successive torsion overloads, occurring together with flexural fatigue, reduce the mechanical resistance of nickel-titanium files.