47. 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) a substantially cylindrical outer member having a helically wound flange form thereon, the flange form having a pitch of between 0.039 and 0.044 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; and b) wherein the splay control ramp oblique angle is greater than seventy degrees.
47. 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) a substantially cylindrical outer member having a helically wound flange form thereon, the flange form having a pitch of between 0.039 and 0.044 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; and b) wherein the splay control ramp oblique angle is greater than seventy degrees. 49. The combination of claim 47 , wherein the splay control ramp is at an obtuse angle with respect to the load flank.