36. The improvement of claim 23 , wherein the closure flange form is a first flange form having a first start and further comprising a second flange form having a second start, the first and second starts being directly opposed along a length of the closure.
23. 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 0.039 and 0.044 inches, and 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 an elevated toe, the toe being spaced from the load flank both radially and extending axially, the first and second portion forming an angle at their junction defined by an intersection of a radius running from the central axis and perpendicular thereto with a tangent along a substantial portion of the splay control ramp, wherein the angle is oblique 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; and c) a first distance defining a first thickness of the closure flange form first portion measured radially from the root to the termination of the load flank ranges between about forty percent to about sixty percent of an entire thickness of the closure flange form measured radially from the root to the crest.