Constant Insertion Rate Methodology for Measuring Torque and Apical Force in 3 Nickel-Titanium Instruments with Different Cross-sectional Designs

• The cutting efficiency of 3 types of endodontic instruments was evaluated in constant insertion rate experiments.
• Torque and apical force measurements indicated that Mtwo has higher cutting capacity than RaCe and PTU F1.
• The cross-sectional design was a differential factor for determining the instrument's cutting action.

IntroductionThe aim of this study was to evaluate the cutting characteristics of 3 nickel-titanium instruments with different cross-sectional designs using a recently developed methodology.MethodsInsertion tests at a constant rate of 5.4 mm/min were performed on a bench test device by measuring the torque and apical force required for penetrating prefabricated acrylic blocks containing an artificial canal preflared with #10 and #15 K-files. Size 20/.06 Mtwo (VDW, Munich, Germany) and RaCe (FKG, La-Chaux De Fonds, Switzerland) instruments, together with ProTaper Universal (PTU) F1 instruments (Dentsply Maillefer, Ballaigues, Switzerland) (10 instruments of each type), were tested using an endodontic motor set at 300 rpm and 5 N·cm. The instruments were characterized with respect to the pitch length, rake angle, and cross-sectional area. Statistical analysis was performed with analysis of variance (α = 0.05).ResultsThe torque was significantly lower for Mtwo compared with the other 2 groups (P < .05). It was also lower for RaCe in comparison with PTU F1 (P < .05). Regarding the apical force, the RaCe instruments had lower values when compared with the Mtwo and PTU F1 instruments (P < .05). There was no statistically significant difference between the PTU F1 and Mtwo instruments (P > .05).ConclusionsThe methodology allowed the cutting properties of the instruments to be assessed in terms of their different geometric characteristics. The cross-sectional design, especially in Mtwo instruments with 2 sharp edges and great chip removal capability, was a differential factor for their higher cutting action.