Effect of several nickel-titanium rotary systems on stress distribution in mandibular molars under occlusal forces: a finite element analysis

This study assessed the stress distribution under occlusal forces in mandibular molars after utilizing several nickel-titanium rotary systems and identified potential root fracture patterns through finite element analysis (FEA). Five three-dimensionally printed mandibular molars were used, with one tooth left unshaped and the remaining four shaped using ProTaper Gold (Dentsply, Tulsa Dental Specialties, Tulsa, OK), Reciproc Blue (VDW, Munich, Germany), XP-endo Shaper (FKG Dentaire, La Chaux-de-Fonds, Switzerland), and Hyflex EDM (Coltene/Whaledent, Altst€atten, Switzerland) rotary systems. Subsequently, micro-CT scans were performed on the teeth, and representative FEA models were generated. Two distinct loadings, vertical and oblique, were applied, and stress parameters including von Mises stress, maximum principal stress, and minimum principal stress were recorded and compared across groups. Although stress values in both loading conditions correlated with volume increases post-shaping, the stress distribution patterns indicative of potential fractures were comparable across groups. Stresses under oblique loads were observed to be higher than those under vertical loads. Several rotary systems, based on the volumetric changes they induce in dental hard tissues, may elevate stress values throughout the tooth, leading to root fractures in regions where stress concentration occurs. Conservative root canal shaping leads to a lower overall stress concentration. In mandibular molars, oblique forces have a more destructive effect compared to vertical forces.