Anterior cruciate ligament (ACL) repairs of the knee are usually done with a piece of tendon from the patellar tendon below the knee or from the hamstring tendon behind the knee. This donor graft may come from your own leg or from an outside donor.
Either way, healing is a step-by-step event. First the graft begins to die, a process called necrosis. This signals the body to send blood to the area to help it out. New cells arrive and new collagen tissue is deposited to form what's called a matrix. Eventually the matrix is remodeled to remove any excess tissue and reshape it.
Grafts can fail to take if they don't get enough blood supply. This can happen when there's too much tension on the graft. Pressure from the bone can also contribute to this problem.
Keeping the leg immobile too long after surgery can have a negative effect. As the old saying goes, Motion is lotion. Keep in mind that not enough exercise or too much exercise can both be detrimental. Follow your rehab program carefully to avoid this problem.
Finally, good nutrition is always important for any injury or postoperative healing process. Getting plenty of fruits and vegetables and keeping up on hydration (drinking water or sports drinks with minimal sugars) is always advised. Avoid tobacco use as nicotine has a known delaying effect on wound healing.