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 arms have arm extensions or tabs connected to main portions of the arms by weakened regions to enable the tabs to be broken-off or separated after the rod is clamped. 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. The flanges have anti-splay contours which can be formed on load flanks or stab flanks of the flanges. The load flanks can be oriented in such a manner as to aid in the anti-splay characteristics of the flanges or to control the proportioning of axial stresses between the flanges.