Behaviour of Hero NiTi instruments used by an experienced operator under access limitations

The purpose of this study was to evaluate the failure incidence of Hero 642 nickel-titanium (NiTi) rotary instruments (HRI) when used by an experienced operator in conjunction with different motors and under limitations of access. HRI were used by an experienced operator in a crown-down technique. The teeth were mounted in resin arches of a phantom head to reproduce limitations of access. In Groups 1, 2, 3 and 4, air, high torque control, low torque control and very low torque control motors were used respectively. Each group included 30 canals in extracted human molars. One set of HRI was used for each canal; the HRI were sterilised before each case. A 2.5% NaOCl solution was used as an irrigant. An experienced operator performed all the cleaning and shaping procedures after application of a rubber dam. The opening of the mouth in the phantom head was standardised for the four groups. Instrument fracture did not occur in any of the groups. The air, high torque control and low torque control motors were as safe as the very low torque control motor when HRI were used by an experienced operator in a crown-down manner at 300 rpm.