Effect of pitch length on the behavior of rotary triple helix root canal instruments

The behavior of nickel-titanium instruments depends largely on the cross-section of the working part. The effect of pitch length was evaluated using two instruments with the same cross-section (triple helix; 0.6% taper). One had a short pitch (0.5 mm at the tip to 0.9 mm at 16 mm at the end of the working part). The other had a long pitch (1.18 mm/2 mm). A dynamometer recorded tensional stress (MHz) and the tendency to screw in (Fz) during simulated canal preparations. Rotational speed was kept constant, and a continuous pecking movement was used. At the end of the preparations, the average Fz was 5.02 N for the short-pitch instrument and 1.47 N for the long-pitch instrument (p < 0.001). The average Mz varied from 0.88 N (short pitch) to 0.35 N (long pitch). Increasing the pitch decreased torsional load sharing and the tendency to screw in.