Splay control closure for open bone anchor

A spinal fixation structure having a bone anchor and a closure, the bone anchor including a receiver with a first arm and a second arm forming a channel, the closure rotatable for guided advancement into the channel to capture a longitudinal connecting member, the spinal fixation structure comprising:

• a closure guide and advancement flange form extending helically along the closure about a central axis, the closure guide and advancement flange form having a root and a crest, the closure guide and advancement flange form having at least one start structure, the at least one start structure having a leading including at least one curvate surface having both a convex and concave surface;
• a first portion of the closure guide and advancement flange form disposed adjacent to the root and extending radially outwards in a direction away from the central axis, the first portion having a closure load flank;
• a second portion of the closure guide and advancement flange form extending from a termination of the closure load flank to the crest, the second portion having a closure splay control ramp and a toe, the toe spaced from the closure load flank both radially and axially;
• a discontinuous receiver guide and advancement flange form extending helically along an inner surface of the first arm and the second arm, the discontinuous receiver guide and advancement flange form having a receiver load flank;
• and
• a receiver splay control ramp;
• wherein the spinal fixation structure generates greater thrust between the closure and the receiver for a given torque during a loading of the closure into the receiver, the receiver splay control ramp engagable with the closure splay control ramp so as to control an outward splay of the first arm and the second arm with respect to each other and the closure.