21. The improvement of claim 20 wherein the closure splay control ramp has a frusto-conical surface portion.
20. In a spinal fixation structure having a bone anchor and a closure, the anchor for holding a spinal fixation longitudinal connecting member, the anchor having an open receiver with spaced apart arms defining a longitudinal connecting member receiving channel therebetween and the closure sized for being received within the channel and adapted for rotation and advancement into the channel between the arms to capture a portion of the longitudinal connecting member in the channel, the improvement comprising: a) a closure guide and advancement flange form extending helically along the closure and about a central axis of the closure, the flange form having a pitch of between 0.045 and 0.055 inches, and having a crest portion with a first axial height and a root portion having a second axial height, the first height measured from a top of an upwardly extending toe of the flange form to a stab flank and taken substantially along a crest surface of the flange form, the second height measured from a load flank of the flange form to the stab flank and taken substantially along a root surface of the flange form, the first height being one of slightly less and substantially equal to the second height, the flange form having a splay control ramp surface located between the toe and the load flank running substantially at an oblique angle with respect to a radius of the closure running perpendicular to the closure axis; and b) a discontinuous receiver guide and advancement flange form extending helically about and along an inner surface of each receiver arm, the receiver flang form having a cooperating splay control ramp engaging the closure flange form control ramp during mating of the closure flange form with the receiver flange form, the receiver flange form having a clearance surfaces disposed in close spaced relation to the closure toe, the toe remaining unloaded during mating engagement and torquing of the closure flange form with the receiver flange form.