Because an acl (anterior cruciate ligament) cannot heal itself on its own, an acl is "reconstructed" by placing a graft tendon substitute (from your own or donor cadaver tendon) placed into tunnels made in the original acl position, and fixed in place with screws, buttons, or other fixation device. The body then replaces the cells and structure of the graft over 6-8 months, recreating a new acl.