Effect of asymmetry on the behavior of prototype rotary triple helix root canal instruments

Two factors affect the choice of instruments for root canal preparation: its ability to achieve the canal's shaping and its safety. These factors depend on the profile of the instrument and the design of its active part. In this study, we tried to assess the role of an asymmetric cross-section on the behavior of the instrument by measuring the stresses generated during the preparation of a simulated canal.

Three 25-mm-long, 0.06 taper, ISO size #30 helical nickel-titanium instruments were tested. The first HeroShaper (Micro-Mega, Besançon, France) used had a symmetric triple helix section (H0). The others, based on the HeroShaper design, had a constant asymmetry of 4/100 mm (H4) or 6/100 mm (H6). Six canals were prepared using each instrument, and the experimental conditions (ie, speed and movement) were the same for each sample. A dynamometer with a sensitivity of 0.1 N recorded the stresses transmitted by the instruments.

Torque and apical force increased proportionally with instrument penetration. At the end of the preparations, the axial stress averaged 7.39 N for the symmetric instrument and 5.92 and 5.15 N for the asymmetric instruments, which indicated a significant statistical analysis of variance (P < .001). The average torque was low (1.05-1.13 N.cm), which indicated a nonsignificant statistical analysis of variance (P = .2385).

In this study, axial stresses decreased, but torque did not change with an asymmetric triple helix cross-section.