Finite element design optimisation by DOE technique of the torsional stress behaviour for NiTi rotary endodontic instruments

The study investigates the impact of design parameters on the torsional stress behaviour of nickel-titanium (NiTi) rotary endodontic instruments, focusing on optimising the structural performance under clinically relevant loading conditions. Using finite element analysis (FEA), five NiTi endodontic file designs with variations in cross-sectional geometry, taper, and pitch were analysed. A custom simulation model was developed to replicate clinical canal anatomy by inserting each instrument into a curved conical canal. Mechanical responses—including von Mises equivalent stress, equivalent strain, and resistant torque—were analysed across these configurations to assess critical performance metrics. The rectangular cross-section design demonstrated the highest torsional rigidity, with a resistant torque higher than 4 Nmm, while the triangular cross-section achieved greater flexibility, accommodating up to 360° of rotation without failure. The study then evaluated the influence of each design parameter on critical mechanical properties, including von Mises equivalent stress, strain distribution, and resistance to torsional failure. Results indicate that taper and pitch also play essential roles, with higher taper values increasing stiffness and reducing flexibility, while larger pitch values decrease stress and strain without significantly impacting resistance torque. Design parameters were further optimised through a Design of Experiments (DOE) approach, revealing that a taper of 5% and a pitch of 20 mm minimised stress and strain distributions, producing von Mises stress below 1000 MPa and maintaining strains within an acceptable threshold. These findings underscore the essential role of cross-sectional shape, taper, and pitch in balancing flexibility and torsional strength, offering valuable insights for designing safer, more efficient endodontic instruments that minimise fracture risk during clinical application.