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

ABSTRACT

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 degrees 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.