Curved molar root canal preparations using Hero 642 rotary nickel-titanium instruments

Aim The Bramante method was used to study how the natural curvature of root canals was modified when using the Hero 642 nickel-titanium rotary system. Methodology Twenty-four canals were selected in freshly extracted human mandibular molars. Every root canal had an angle of curvature greater than 20 degrees. Roots were embedded in clear casting resin using a plaster mould. Cross-sectional cuts were made in the apical, middle and coronal thirds of each root. A first group (control) of 12 curved root canals was prepared using stainless steel hand instruments and the step-back technique. A second group of 12 curved root canals was instrumented using the Hero sequence according to the manufacturer's instructions. For each sectioning level, the amount of dentine removed, the change of shape factor and the canal centre displacement were calculated with the aid of a digital image analysis system. The results were subjected to the Mann-Whitney U-test. Results No significant difference occurred in the amount of dentine removed, change of shape factor or canal centre displacement in the coronal and middle thirds of the curved canals. In the apical third, transportation was significantly higher (P = 0.0171) with stainless steel hand instruments than with the Hero system. Conclusions The original canal shape was maintained better in the apical third of curved canals when using the Hero system in a crown-down technique when compared to a hand preparation technique with stainless steel instruments.