25. In combination a spinal implant anchor adapted for connection to a bone fixation structural member and a closure for capturing the structural member in the anchor, the closure comprising: a substantially cylindrical outer member having a helically wound flange form thereon, the flange form having a pitch of between 0.045 and 0.055 inches, and having a load flank adjacent a splay control ramp, a substantial portion of the splay control ramp being at an oblique angle with respect to the load flank, the splay control ramp being in spaced relation to both a root and a crest of the flange form, the splay control ramp terminating at a rounded surface that remains unloaded during use wherein the splay control ramp oblique angle is greater than seventy degrees.
25. In combination a spinal implant anchor adapted for connection to a bone fixation structural member and a closure for capturing the structural member in the anchor, the closure comprising: a substantially cylindrical outer member having a helically wound flange form thereon, the flange form having a pitch of between 0.045 and 0.055 inches, and having a load flank adjacent a splay control ramp, a substantial portion of the splay control ramp being at an oblique angle with respect to the load flank, the splay control ramp being in spaced relation to both a root and a crest of the flange form, the splay control ramp terminating at a rounded surface that remains unloaded during use wherein the splay control ramp oblique angle is greater than seventy degrees. 27. The combination of claim 25 wherein the splay control ramp is at an obtuse angle with respect to the load flank.