1. A medical implant assembly comprising: a) first and second bone anchors each having a bone attachment structure on one end and a channel at an opposite end; b) an elongate core sized and shaped to be received in the bone anchor channels and extend therebetween; c) closures to secure the core in the bone anchor channels; and wherein d) the core is pretensioned between the bone anchors so as to elastically elongate while remaining in tension between the bone anchors during usage.
5. The assembly according to claim 1 including an elastic sleeve that surrounds the core that is positioned between the bone anchors and compressively engages both of the bone anchors.