A medical implant assembly having at least two bone attachment structures cooperating with a dynamic longitudinal connecting member, the improvement wherein the connecting member includes: a first end, a transition portion, and a second end; a substantially rod portion extending longitudinally from the first end to the transition portion, and including a longitudinal axis and a substantially rigid core running substantially parallel with the longitudinal axis; a substantially cord portion joined with the rod portion and extending from the transition portion to the second end; and a substantially flexible jacket portion covering the rod and cord portions.