Phase transformation behaviour and bending properties of hybrid nickel-titanium rotary endodontic instruments

Aim To investigate the bending properties of hybrid rotary nickel-titanium endodontic instruments in relation to their transformation behaviour. Methodology Four types of nickel-titanium rotary endodontic instruments with different cross-sectional shapes (triangular-based and rectangular-based) and different heat treatment conditions (super-elastic type and hybrid type with shape memory effect) were selected to investigate bending properties and phase transformation behaviour. Bending load of the instruments was measured in a cantilever-bending test at 37°C with the maximum deflection of 3.0 mm. A commercial rotary instrument, ProFile (PF; Dentsply Maillefer, Ballaigues, Switzerland) was used as a reference for the bending test. Phase transformation temperatures were calculated from the diagrams obtained from differential scanning calorimetry. Data were analysed by ANOVA and Scheffe's test. Results The bending load values of the hybrid type that had undergone additional heat treatment at the tip were significantly lower (P < 0.05) than those of the super-elastic type with no additional heat treatment. The bending load values of rectangular-based cross-sectional shaped instruments were significantly lower (P < 0.05) than those of triangular-based cross-sectional shaped instruments. Phase transformation temperatures (M_s and A_f points) of the hybrid type were significantly higher (P < 0.05) than the super-elastic type. The M_f and A_s points of the tip part were significantly higher (P < 0.05) than those of the whole part of the hybrid instrument. Conclusions Additional heat treatment of hybrid nickel-titanium instruments may be effective in increasing the flexibility of nickel-titanium rotary instruments. < 2007 International Endodontic Journal.