Relationship between file size and stiffness of nickel titanium instruments.

The purpose of this study was to evaluate the relationship existing between file size and stiffness for 3 endodontic files made of nickel titanium. Three groups of instruments with different cross-sections were tested: a triangular cross-section, a square cross-section and a modified triangular cross-section. The instruments were tested from size 15 to size 40 or 60 according to ANSI/ADA specification no. 28 for binding moment evaluation. There was a statistically significant difference between the 3 groups: the square cross-section K files presented a larger bending moment than the triangular cross-section K files, which presented a larger bending moment than the modified cross-section K files. Like the stainless steel instruments, there was an exponential relationship between file size and bending moment for triangular and square cross-section K files, but a linear relationship between file size and bending moment for the files with a modified triangular cross-section.