Ligaments are not elastic like muscles. When they tear, it's like a broken rubber band. It doesn't grow back together. In order to fix a ruptured anterior cruciate ligament (ACL), surgeons take a piece of tendon tissue from some other place and transfer or graft it into place.
The two most common ACL repairs are done with a bone-patellar tendon-bone (BPTB) graft or a hamstring tendon (HT) graft. The patellar tendon is in front of the knee just below the kneecap. A small piece of bone is taken along with the tendon tissue.
The hamstring tendon is attached to the large muscle behind the thigh. The tendon is harvested and folded in half to increase the strength of the graft. The goal is to use a graft material that is easy to remove, gives a strong repair, and results in no problems afterwards. The long-term goal is to restore function and allow the patient to get back to a preinjury level of activity.