ABSTRACT
<p><b>OBJECTIVE</b>To investigate the shaping effects of three nickel-titanium rotary instruments on preparing curved canals in posterior teeth.</p><p><b>METHODS</b>64 curved mesial canals of extracted lower molars, embedded in the Branmante models, were randomly assigned to four groups: (1) Flexofile; (2) LightSpeed; (3) ProFile; (4) Qantec SC. Apical preparation was carried out till size 30. The area of dentine removal, least remaining dentine thickness and transportation of canal center were measured.</p><p><b>RESULTS</b>Flexofile resulted in more dentine removal and canal center transportation than LightSpeed and ProFile (P < 0.05) at the apical and mid-root levels. In the mid-root sections, Flexofile left the thinnest dental wall in the distal aspect; 87% of the canal centers in the Flexofile group were transported in a distal direction as compared with one third in other groups.</p><p><b>CONCLUSIONS</b>LightSpeed and ProFile exhibited better shaping ability than Flexofile in preparing curved canals.</p>