Stainless-steel prototypes characterized by five different cross-sectional shapes (square, triangular, rhomboidal, "S"-shaped, and the cross-sectional design of H-type files) and three different number of flutes (16, 24, and 32) were used for investigation of the relationship between design features and the resistance to bending and fracture of root canal instruments. Both resistance to bending (bending moment) and resistance to fracture (angular deflection and torque) were determined in accordance to ISO 3630-1. Numbers 15, 25, and 35 prototypes were tested with a sample size of 10 instruments each. Whereas prototypes with the rhombus-shaped cross-sectional design displayed the lowest resistance to bending, those of the square cross-section showed the greatest. In general, S-shaped prototypes and those with the H-type cross-section achieved lower angular deflection than all other prototypes, whereas those with the triangular cross-sectional shape and 32 flutes reached the greatest angular deflection. Overall, rhombus-shaped prototypes showed lower torque values than all other prototypes. Results indicate that bending and torsional properties of endodontic instruments are mainly influenced by their cross-sectional design. Moreover, these properties are also affected by the number of flutes and the manufacturing process (twisted or milled) of endodontic instruments.
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