Management of rotary-translation in complex endodontic anatomies.

Ni-Ti rotary instruments must be so flexible to permit the clinician to use them safely even in those unexpected apical abrupt curvatures. Both cross-sectional area and file design (influencing stress distribution during load) may affect an instrument’s resistance to fracture when subjected to flexural and torsional load [3]. Larger tapered Ni-Ti rotary endodontic files should not be considered to be safer or more resistant during clinical use. In fact, these files, when employed to shape curved root canals, will fracture after fewer stress cycles than the smaller tapered ones. This observation suggests that an early discard of the larger tapered instruments may ensure greater efficiency and safety in clinical procedures [4]. This is due to the fact that when an instrument rotates in a curved root canal, any points within it in the segment subjected to the maximum stress, except those in the center (neutral axis), are subjected to repeated tensile or compressive strains. The farther away from the central axis, the greater the imposed strain at that point. This explains why instruments of a larger diameter are affected by fatigue more than smaller ones.