A quasi-hyperbolic endodontic instrument having a cylindrical, elongated shaft with a radius that varies as a smooth, continuous curve along the length of the shaft and is larger near the distal portion of the file than near the proximal end of the file. The distal radius may be 10% or more larger than the proximal radius. This design provides a flexible file that minimizes the possibility of breaking, and ensures that if breakage does occur, it will occur near the handle, allowing the broken bit to be easily removed from the tooth canal. The instrument may further, or instead, have a metal cable connecting the cutting head to the handle to help reduce metal fatigue.
