1. 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 first portion located adjacent a root of the form and extending radially outwardly therefrom in a direction away from the central axis, the first portion having a first load flank, the flange form having a second portion extending radially outwardly from a termination of the load flank to a crest of the flange form, the second portion having a first splay control ramp and a rounded toe, the toe being spaced from the load flank both radially and axially, the closure flange form further comprising a depth and an axial height, the depth measured radially from the root to the crest and the axial height measured from the load flank to a stab flank and taken substantially along the root surface of the flange form, and wherein a ratio of the depth over the axial height is a value ranging from less than 1 to about 1; and b) a discontinuous receiver guide and advancement flange form extending helically about and along an inner surface of each receiver arm, the receiver flange form having a second load flank and a second splay control ramp respectively engaging the first load flank and the first splay control ramp during mating of the closure flange form with the receiver flange form, the receiver flange form having clearance surfaces disposed in close spaced relation to a remainder of the closure flange form.
1. 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 first portion located adjacent a root of the form and extending radially outwardly therefrom in a direction away from the central axis, the first portion having a first load flank, the flange form having a second portion extending radially outwardly from a termination of the load flank to a crest of the flange form, the second portion having a first splay control ramp and a rounded toe, the toe being spaced from the load flank both radially and axially, the closure flange form further comprising a depth and an axial height, the depth measured radially from the root to the crest and the axial height measured from the load flank to a stab flank and taken substantially along the root surface of the flange form, and wherein a ratio of the depth over the axial height is a value ranging from less than 1 to about 1; and b) a discontinuous receiver guide and advancement flange form extending helically about and along an inner surface of each receiver arm, the receiver flange form having a second load flank and a second splay control ramp respectively engaging the first load flank and the first splay control ramp during mating of the closure flange form with the receiver flange form, the receiver flange form having clearance surfaces disposed in close spaced relation to a remainder of the closure flange form. 10. The improvement of claim 1 wherein an angle defined by a radius running perpendicular from the closure central axis and one of a substantial portion of the splay control ramp and a tangent thereto is greater than seventy degrees.