13. The improvement of claim 12 wherein the bar is substantially solid.
12. In a medical implant assembly having at least first and second bone anchors cooperating with a longitudinal connecting member, the improvement wherein the connecting member comprises: a) an elongate bar of one-piece construction, the bar attachable to the first and second bone anchors, the bar having a slitted portion; and b) an elastically compressible outer sleeve including opposed flat end surfaces adapted for flush engagement with a side surface of each of the first and second bone attachment structures, and surrounding the slitted portion and being slidable with respect to the slitted portion, the sleeve being positioned entirely between the first and second bone anchors.