Injury to the anterior cruciate ligament (ACL) in the knee is common. While this condition can be very painful and may require surgery, 95% of people who suffer ACL injuries return to their previous level of activity after recovery.

The knee is a hinge joint held together by four ligaments. The two ligaments deep inside the knee that cross each other are the anterior and posterior cruciate ligaments. During activity, the ACL controls how far forward the tibia (shin bone) can slide relative to the femur (thigh bone). It essentially acts to prevent too much forward movement, which might damage structures in the knee.

The ACL can be injured or torn in a number of ways, such as by a sudden pivoting or cutting maneuver during sports. A "pop" can sometimes be felt or heard, and the amount of pain can be severe. An ACL tear can be diagnosed by physical examination. An X-ray may be taken to look for a fracture; an MRI might also be done when the diagnosis is uncertain.

Treatment options depend on your age, activity level, and whether other structures within the knee have been injured. Surgery is often recommended for young, active patients and for those who have additional injuries. This will restore stability to the knee and protect the cartilage from further damage, which can lead to arthritis. An ACL left untreated will lead to further damage to meniscal cartilage with each episode of instability. 

The most common procedure uses an arthroscopic technique, which involves two small incisions and the placement of screws or staples inside the knee. Since the ACL cannot be repaired, it must be reconstructed with tissue from other parts of the knee, usually the patella tendon or hamstrings. Ultimately, treatment is based more on patient expectations than on age.

Nonsurgical treatment is often recommended for older, more sedentary patients. This consists of physical therapy, activity modification, and use of a brace. The goal is to strengthen the muscles around the knee to compensate for the ACL. This conservative form of treatment can be successful, and sports such as jogging or cycling can often be resumed without difficulty.

Rehabilitation involves physical therapy several times a week at first, then less frequently. The patient may be on crutches for a few weeks, progressing gradually to full weight bearing. Within four months, full muscle strength is usually recovered, and by eight months most people return to full activity.