A spinal fixation device combines an open-headed anchor member, such as a bone screw or a hook with spaced apart arms forming a rod receiving channel. The arms have break-off arm extensions that are broken-off or separated after the rod is clamped. The closure and inner surfaces of the arms and tabs have mating helical anti-splay guide and advancement flanges formed thereon which radially interlock and mechanically cooperate to prevent splaying the arms and extensions as the closure is advanced into the rod receiving channel.
