For decades there have been unsuccessful efforts to repair the anterior cruciate ligament (ACL) after rupture. Why? Because the forces required to tear the complex weave of the ACL’s collagen fibers involve both tearing and pulling apart. Think of a climbing rope: When overstretched the rope fails within its sheath, not just at a single clean edge. Here is why the ruptured anterior cruciate ligament is most often replaced (reconstructed) rather than repaired.