Elements for Understanding the Bending and Cyclic Fatigue Behavior of Endodontic Instruments

NiTi endodontic instruments are used for root canal treatment, but can sometimes fracture due to bending cyclic fatigue. Three comparable instruments (X2 Protaper Next®, Race® and F6 Skytaper®) were tested under bending (at a 45° angle) and cyclic rotary bending fatigue (at 30°, 45° and 60° angles). Experimental (differential scanning calorimetry (DSC) and scanning electronic microscopy) and numerical approaches were used to investigate the parameters determining their bending behavior and their cyclic fatigue fracture. The results showed that Race® was the stiffer instrument under bending and presented a significant difference compared to the two other instruments at 60° fatigue test. There was a significant difference among all instruments at 45°. These results could be partly explained by the geometry of the instruments and the phase-transition temperatures (DSC). The microscopic observations showed that, in the case of Protaper Next® and F6 Skytaper® instruments, the fracture could come from surface defects resulting from their manufacturing process. This was confirmed by the finite-element analysis on these instruments which revealed a mismatch between the simulated area of maximum bending stress and the experimental fracture zone. Thus, the geometry, alloy and surface condition have an influence on the bending behavior and cyclic fatigue life.