13. In a spinal implant having an open head forming a channel for receiving a longitudinal connecting member and a closure member for closing the channel and capturing the longitudinal connecting member in the channel, the closure member having an axis of rotation and a first helical guide and advancement structure and the head having a second guide and advancement structure following a helically wound path, the first guide and advancement structure mating with the second guide and advancement structure under rotation, the improvement wherein: a) the closure member has a substantially cylindrical body having a central axis, a drive feature located one of at and near a top surface of the body and the first guide and advancement structure extends helically about the body from near the top surface to near a bottom surface of the body, the first guide and advancement structure comprising a loading flank having an axially extending lip with a peak surface, the lip receivable in a groove of the second guide and advancement structure and wherein a height of the lip is slightly less than a depth of the groove, such that the peak surface does not engage the second guide and advancement structure; and b) the open head has spaced apart arms defining the channel, each arm having a main portion and an extended portion connected to the main portion by a weakened region, the second guide and advancement structure being located on an inner surface of both the main and extended portion of each arm, the arm extended portions guiding the closure member into the channel during rotation of the closure into the channel and thereafter being separable from the main portions.
13. In a spinal implant having an open head forming a channel for receiving a longitudinal connecting member and a closure member for closing the channel and capturing the longitudinal connecting member in the channel, the closure member having an axis of rotation and a first helical guide and advancement structure and the head having a second guide and advancement structure following a helically wound path, the first guide and advancement structure mating with the second guide and advancement structure under rotation, the improvement wherein: a) the closure member has a substantially cylindrical body having a central axis, a drive feature located one of at and near a top surface of the body and the first guide and advancement structure extends helically about the body from near the top surface to near a bottom surface of the body, the first guide and advancement structure comprising a loading flank having an axially extending lip with a peak surface, the lip receivable in a groove of the second guide and advancement structure and wherein a height of the lip is slightly less than a depth of the groove, such that the peak surface does not engage the second guide and advancement structure; and b) the open head has spaced apart arms defining the channel, each arm having a main portion and an extended portion connected to the main portion by a weakened region, the second guide and advancement structure being located on an inner surface of both the main and extended portion of each arm, the arm extended portions guiding the closure member into the channel during rotation of the closure into the channel and thereafter being separable from the main portions. 16. The improvement of claim 13 wherein the spinal implant is a fixed headed bone screw.