Abstract: An improved fastener is provided for use in retaining vertebrae in a desired spatial relationship. The fastener has a mounting section with a first external thread to engage a vertebra. The fastener has a retaining section with a second external thread to engage an internal thread on a retaining element, such as a nut. A pair of parallel flats are formed on opposite sides of the retaining section. The flats are spaced apart by a distance which is substantially the same as or greater than a root diameter of the external thread on the retaining section. A driving recess in an axially outer end of the retaining section has arcuate corner portions which receive force to effect rotation of the fastener relative to a vertebra engaged by the mounting section. The arcuate corner portions of the driving recess are offset from a plane containing the longitudinal central axis of the retaining section and extending perpendicular to the flats.