A spinal fixation device combines an open-headed anchor member, such as a bone screw or a hook, with a closure member to thereby clamp a spinal fixation rod to the anchor member. The anchor member has spaced apart arms forming a rod receiving channel. The closure member and inner surfaces of the arms and tabs have helical anti-splay guide and advancement interlocking flanges formed thereon which cooperate to prevent splaying the arms and extensions as the closure member is advanced into the rod receiving channel.
