Actually, not a whole lot has changed with ACL replacement in the last decade. The remains of the torn ACL are cleaned out, and then another one (usually from a cadaver) is threaded in in its place. A new ACL grows around the cadaver ligament. For the first few months, or so, the new ligament actually gets weaker, as the body rids itself of the graft, while the new one grows. It's usually a few months or more before someone can run. Rushing rehab can really mess up the new ACL.