Phase Transformations in NiTi Endodontic Files and Fatigue Resistance

Abstract
NiTi endodontic files allow for clinical enlargement of harshly curved root canals. However, files rotated under severe flexion might fracture unexpectedly, remain inside teeth and thus, hinder treatment outcome. ProFile .06(35) are among the most commonly used rotary files. In the present study, 10 of these brand new instruments were used to perform DSC on their cutting regions and determine the amount of R-phase present at 10, 20, 37, and 65°C, both during cooling and heating programmed cycles. Another 48 new ProFile .06(35) instruments underwent rotation/flexion assays at 300 rpm and 10 mm radius of curvature. Files were divided into four groups of 12 instruments each. Each group was tested at 10, 20, 37, and 65°C. The number of cycles to fracture (N_f) and the length of the broken segment (L_f) were determined for each tested file. Results show no statistically significance (α =0,05) as for L_f in all groups. On the contrary, N_f depends highly on testing temperature: as temperature increases from 10 to 37 °C, N_f strikingly decreases; from 37 and 65 °C, no significant changes in N_f occur. As for DSC results, it is clearly shown that R-phase is most abundant at 10 °C and consistently decreases up to 37°C, both during heating and cooling; from 37 to 65 °C, the quantity of R-phase in the alloy is negligible. In conclusion, it is shown that fatigue resistance in ProFile .06(35) instruments is not only highly enhanced but also proportional to the amount of R-phase in the alloy.