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Anterior cruciate ligament injury

Anterior cruciate ligament injury
Ryan P Friedberg, MD
Section Editor
Karl B Fields, MD
Deputy Editor
Jonathan Grayzel, MD, FAAEM


Last literature review version 19.3: Fri Sep 30 00:00:00 GMT 2011 | This topic last updated: Tue Aug 17 00:00:00 GMT 2010 (More)

ANTERIOR CRUCIATE LIGAMENT INJURY OVERVIEW — The anterior cruciate ligament (ACL) is an important stabilizing ligament in the knee. It is frequently injured by athletes and trauma victims; in the United States alone, there are between 100,000 and 200,000 ACL tears per year.

This topic review will discuss the causes, signs and symptoms, diagnostic tests, and treatment options for ACL injuries.

WHAT IS THE ANTERIOR CRUCIATE LIAGMENT (ACL)? — The knee joint is held tightly together by four ligaments: the inner and outer fan-shaped hinge ligaments (medial and lateral collateral ligaments) and the crossing (cruciate) ligaments, which sit in the middle of the joint (the anterior and posterior cruciate ligaments) (figure 1).

The collateral ligaments are firmly attached to the far end of the femur (thigh bone) and the near end of the tibia and fibula (lower leg bones). The ligaments hold the two bones together and prevent side to side motion. The anterior cruciate ligament prevents forward and backward motion. You can partially or completely tear the ligament(s).

Other structures can be damaged during an acute ACL injury, including:

  • The meniscus
  • Joint capsule (the tissue that surrounds the joint)
  • Articular cartilage (cartilage that covers the ends of bones where they meet in a joint)
  • The ends of the femur or tibia
  • Other ligaments (medial collateral ligament (MCL), lateral collateral ligament (LCL), posterior cruciate ligament (PCL)) (figure 2)

One common injury is called the athlete’s triad, in which the ACL, MCL, and medial meniscus are all torn.

CAUSES OF ANTERIOR CRUCIATE LIGAMENT INJURY — Non-contact ACL injuries typically occur when a person is running or jumping and then suddenly slows and changes direction (eg, cutting) or pivots in a way that involves rotating or bending the knee sideways. Women appear to be at a higher risk of non-contact ACL injuries than men, although the exact reason for this is not clear [1].

Contact-related ACL injuries usually occur from a direct blow causing hyperextension or when the knee is forced inwards towards the other leg. This is often seen in American football when a player’s foot is planted and an opponent strikes him on the outside or front of that thigh.

ACL injuries most commonly occur during the following activities:

  • Noncontact sports, such as downhill skiing, gymnastics, and tennis
  • Certain contact sports, including rugby, American football, soccer, and basketball
  • Motor vehicle collisions

ANTERIOR CRUCIATE LIGAMENT INJURY SYMPTOMS — People who have an ACL injury often complain of feeling a “pop” in their knee at the time of injury and have a feeling the knee is unstable or “giving out.” Within a few hours of the ACL injury, nearly everyone develops swelling in the knee, caused by bleeding from injured blood vessels; this is called an effusion.

After the initial swelling has improved, most people are able to bear weight but feel unsteady on the affected knee. Movements such as squatting, pivoting, and stepping sideways, and activities such as walking down stairs, in which the entire body weight is placed on the affected leg, can cause the feeling of unsteadiness.

ANTERIOR CRUCIATE LIGAMENT INJURY TESTS — Anyone who experiences a knee injury and subsequently has pain, swelling, and/or feels unsteady while standing should be evaluated by a healthcare provider. The provider will perform a physical examination. An imaging test may be recommended to examine the bones and ligaments.

ANTERIOR CRUCIATE LIGAMENT INJURY TREATMENT — ACL injuries are treated with surgery and post-surgical rehabilitation or a non-surgical rehabilitation program. The decision to have surgery is based upon several factors, including the person’s age, level of activity, and the presence of other knee injuries [2].

A person is likely to choose to have surgery if he or she:

  • Participates in high-level sports or has a job that requires a strong and stable knee (eg, requires twisting and pivoting)
  • Is unsteady when standing on the injured knee
  • Has multiple injuries
  • Has completed rehabilitation and still has instability in the knee
  • Is willing to complete the rigorous post-surgical rehabilitation program. Most programs require daily strengthening and stretching exercises and one or more weekly visits with a physical therapist for the first three to six months after surgery. (See ‘Post-surgical rehabilitation’ below.) Failure to follow this program could increase the risk of re-injury, allow scar tissue to develop, and lead to limited movement of the knee.

A person may decide not to have surgery if he or she:

  • Has a small partial tear in the ACL that may heal with rest and rehabilitation
  • Does not participate in sports that require pivoting or stopping quickly, especially if the person is older than 55 years
  • Is willing to complete a non-surgical rehabilitation program to strengthen and stabilize the knee (see ‘Non-surgical rehabilitation’ below)

If the ACL is not reconstructed, there may be an increased risk of future knee problems, including chronic pain, a decreased level of activity, and injury to other parts of the knee (the meniscus) [3].

Presurgical rehabilitation — Surgery is not usually performed immediately after an ACL injury because this could cause excessive scar tissue (arthrofibrosis) to develop, which would limit knee motion. In most cases, surgery is delayed until the swelling has resolved and the person is able to bend and straighten the knee without difficulty. Using ice packs and elevating the knee above the chest can help to reduce swelling. The time between an ACL injury and surgical reconstruction depends upon how quickly the person recovers.

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During the time between the injury and the surgery, many surgeons recommend a “pre-habilitation” exercise program to help reduce pain and swelling, improve range of motion (the ability to flex and extend the knee), and increase strength in the muscles of the thigh, knee, and hip. Walking, bike riding, and swimming (with light kicks and no breast stroke) can be continued, although other sports should be avoided.

An example of a presurgical rehabilitation exercise program is detailed below. (See ‘Non-surgical rehabilitation’ below.)

Surgery — After the ACL is torn, it is not possible to repair the ligament. This is due to several factors, including a damaged blood supply to the ligament (blood vessels damaged during injury) and cells inside the synovial fluid (normal fluid in the knee), that prevent healing. Research is underway to determine how to repair the tendon, but the only way to repair the ACL currently is to reconstruct it.

Surgical reconstruction of the ACL is usually done in a hospital or surgical center. Most people are given general anesthesia to induce sleep and prevent pain. The surgery itself usually takes less than two hours.

To reconstruct the torn ligament, a piece of healthy tendon, called an autograft, is “harvested” from another area in the leg. There are several common autograft sites, including the patellar tendon, hamstring tendon, or rarely the quadriceps tendon (figure 3). Another option is to use a tendon from a deceased donor, called an allograft. No one type of graft has been proven to be better than another. Thus, the type of graft that is used depends upon the surgeon’s preference and experience.

  • Patellar autograft — When harvesting a patellar autograft, an extra incision is made in the skin to remove a strip of tendon with a piece of bone at each end. The graft site usually heals quickly and regains normal strength. Some people have soreness in this area for several months after surgery, especially if pressure is applied to the area (eg, while kneeling).
  • Hamstring autograft — If using a hamstring autograft, there are no extra incisions needed and the pain at the harvest site is usually less than that seen with a patellar autograft. Hamstring strength usually returns to normal within three to six months.
  • Allograft — Allografts do not require any extra incisions, and there is no risk of pain or weakness at the site of graft harvest.

The torn ACL is removed and replaced with the graft using a narrow telescope-like device, called an arthroscope. The scope contains a camera and light source, and can be inserted into the knee joint through a small skin incision. Instruments are inserted into other small incisions, allowing the physician to place the graft with precision. After the graft is secured, the knee is wrapped with sterile dressings and an immobilizer is placed around the knee to allow the person to walk more easily with crutches.

Most people are able to go home after spending several hours in the recovery room; it is not usually necessary to spend the night. A machine that moves the knee through a range of motions, called a continuous passive motion (CPM) machine, will be used immediately after surgery, and then sent home with the patient. CPM helps to prevent the development of scar tissue. A prescription for pain medications is given to relieve pain at home. Most people visit their surgeon one to two weeks after surgery.

During the first few days after surgery, the goal is to control swelling and pain. Elevating the knee above the chest and applying ice to the knee are the best ways to do this. Most people use crutches to assist with walking for the first seven to 10 days after surgery, although most patients are encouraged to begin bearing weight on the affected leg as soon as possible. If more extensive surgery is performed, the surgeon may recommend delaying weight bearing for a longer period. Stretching and strengthening exercises can usually begin within the first few days after surgery.

Potential complications — Most people do well after ACL reconstruction and have no major complications. However, complications occasionally occur during surgery or during the rehabilitation period. The most common complications include:

  • Bleeding into the joint (effusion)
  • Joint infection
  • Blood clot in the deep veins of the leg (deep vein thrombosis)
  • Arthrofibrosis (scar tissue)
  • Loosening of the graft

ANTERIOR CRUCIATE LIGAMENT INJURY REHABILITATION — Rehabilitation is a several month long program that is designed to stretch and strengthen the knee after ACL reconstruction or injury. No one program is best for all people, although the following exercises are one example of a program that may be recommended.

Non-surgical rehabilitation — If surgery is not planned, rehabilitation can help to reduce the risk of further injury. Rehabilitation should begin as soon as swelling and pain begin to improve. Use the stretching and strengthening exercises listed above at least once per day for four to six weeks. These exercises are also recommended as a pre-surgical rehabilitation program.

These exercises may cause some discomfort but should not cause significant pain, especially after the exercise session is over. If pain is severe or continues after resting and icing the knee, contact a healthcare provider.

  • Assisted knee flexion — Sit on the floor with the legs extended in front of the body. Place the hands behind the injured thigh, bend the knee and pull it towards the chest, keeping the back straight (picture 1). Hold for five seconds then straighten leg. Repeat 10 to 15 times (one set). Perform a total of three sets.
  • Quad sets — Sit on the floor with the legs extended in front of the body. Place the hands behind the affected knee. Keep the leg straight and contract the quadriceps muscle (just above the knee), which should cause the knee cap to move towards the body (picture 2). Hold for a count of 10 seconds. Release and rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets.
  • Straight leg raises — Lie on a bed or the floor. Bend the “good” knee and keep the foot on the floor. Keep the injured leg straight. On the injured side, tighten the quadriceps (as above), keep the leg straight, and lift the leg about 18 inches off the floor (picture 3). Slowly lower the leg back to the bed or floor. Rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets.
  • Calf raises — Stand behind a chair, holding onto the chair. Slowly rise up and stand on the balls of the feet and toes (picture 4). Hold for five seconds then slowly roll down onto the entire foot. Rest as needed. Repeat 10 to 15 times (one set). Increase the difficulty of this exercise by rising higher, holding longer, or moving up and down more quickly. Perform a total of three sets.
  • Hip extension — You will need 18 to 24 inches of rubber tubing or an elastic band (eg, Theraband®) to perform these exercises. Secure the tubing around the leg of a heavy piece of furniture or close it in a door. Stand facing the furniture/door and place the injured leg in the loop of the tubing. You should not have any slack in the tubing. Hold the door/furniture and extend the injured leg backwards, stretching the tubing as far as possible (picture 5). Hold for five seconds. Slowly return the leg to the floor. Rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets.
  • Hip abduction — As above, you will need a piece of rubber tubing or elastic band. Stand with the legs shoulder width apart, with the “good” leg closest to the furniture or door. The tubing should loop around the outside of the injured leg. Lift the affected leg to the side, 18 to 24 inches away from the body, stretching the tubing (picture 6). Hold for five seconds, then slowly release. Rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets.
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Stretching and strengthening should then continue as discussed below. (See ‘Second phase’ below.)

Post-surgical rehabilitation — Most people who have ACL reconstruction will be under the care of a surgeon and physical therapist who will work together to design a rehabilitation program. The following rehabilitation schedule is an example of one that may be recommended.

First phase — During the first two weeks after surgery, the goal is to increase range of motion (flexing and extending the knee), maintain strength, minimize the development of scar tissue, and eliminate swelling. Most people begin to walk without crutches by the end of the first week. The knee should be iced and elevated daily to minimize swelling.

Exercises during this phase should include those discussed above. (See ‘Presurgical rehabilitation’ above.)

Second phase — Between the third and twelfth weeks after surgery, the goal is to improve range of motion, strength, walking, and balance. Most people are allowed to walk or use an exercise bike for 15 to 20 minutes per day. When possible, walking or running in a pool with a floating belt can be helpful. The following exercises may also be recommended.

  • Presurgical exercises — (See ‘Non-surgical rehabilitation’ above.)
  • Quarter squats — Stand 18 to 24 inches from a wall. Lean back against the wall. Bend both knees slightly (the buttocks should not be lower than the knees), keeping the back straight (picture 7). Hold for five seconds then slowly stand up straight. Rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets. To increase the difficulty, bend the knees more deeply, hold for a longer time, and increase the speed.

    Alternately, use an exercise ball to perform squats. Stand up straight, holding the ball between your back and the wall. Slowly bend the knees and lower the back (roll the ball down the wall). Hold for a count of five. Stand up. Repeat 10 to 15 times.

  • Bridges — Lie on your back on the floor. Keep the feet on the floor and bend both knees. Place the hands about 12 inches to the side of the body (on the floor). Lift the buttocks six to eight inches off the floor (figure 4). Hold for five seconds, then slowly release. Rest as needed. Repeat 10 to 15 times (one set). Perform a total of three sets.

    To increase the difficulty, keep the right foot on the floor and lift the left foot off the floor, keeping the left leg straight. Raise the buttocks using the right foot to support the lower body. Switch sides. Repeat 10 to 15 times (one set). Perform a total of three sets.

  • Single-leg calf raises — Stand behind a chair, holding onto the chair. Lift the “good” foot off the floor so that you are standing on the injured leg. Slowly rise up and stand on the ball of the foot and toes (picture 8). Hold for five seconds then slowly roll down onto the entire foot. Rest as needed. Repeat 10 to 15 times (one set). Increase the difficulty of this exercise by rising higher, holding longer, or moving up and down more quickly. Perform a total of three sets.
  • Step ups — Use a stair climber or steps, step up first with the injured leg. Continue for 10 to 15 minutes per day.
  • Balance — Use a wobble board or balance disk to improve knee strength and balance ability.

    If a wobble board or balance disk is not available, try balancing on the affected leg while lifting the unaffected leg off the ground; do not hold onto any support (picture 9). Hold this position for a count of five to 10. Rest and repeat 10 to 15 times. To increase the difficulty, raise the unaffected leg into the air.

Phase three — During the fourth to sixth months after surgery, the difficulty and intensity of the exercises described above should be continued. In addition, exercises that include jumping and landing can be started.

  • Lunge — Stand with the feet together. Step the right foot approximately 36 inches in front of the body. The right knee should be over the right ankle and the left calf should be parallel to the floor (picture 10). Hold for five seconds. Step the right foot back so that the feet are together. Rest as needed. Repeat with the left leg. Repeat 10 to 15 times (one set). Perform a total of three sets.

Some activities may be resumed at this point, including jogging in a straight line, swimming (kick lightly), and biking on the road. As strength and ability improve, running and other activities can be restarted as well.

Prognosis — Most people who have surgical reconstruction of the ACL have a good outcome and are usually able to return to all of their previous activities by six months after surgery. Athletes can return to sports once their reconstructed knee demonstrates strength and balance roughly equal to the uninjured knee. This generally occurs within 6 to 12 months, depending upon the sport and the person’s dedication to the rehabilitation program.

There are no studies that address the risk of reinjury after ACL reconstruction. When ACL reconstruction is done properly, there should be no increased risk of ACL reinjury.

ANTERIOR CRUCIATE LIGAMENT INJURY PREVENTION — Numerous organizations, including the American Academy of Orthopaedic Surgeons and the American College of Sports Medicine, agree that programs to prevent ACL injury are beneficial for female athletes [4]. Many experts also believe that any athlete who is at high-risk for an ACL injury (eg, American football players, skiers) should participate in a prevention program.

An analysis of ACL injury prevention programs noted the following:

  • Programs that incorporated high-intensity jumping exercises reduced injury rates.
  • Programs that analyzed athletes’ movements and provided direct feedback about proper positioning and movement reduced injury rates.
  • Programs that incorporated strength training reduced injury rates, although strength training alone did not.
  • Balance training alone is unlikely to reduce injury rates, although it may enhance other prevention techniques.
  • Athletes must participate in prevention training at least two times per week for a minimum of six consecutive weeks to accrue benefit.

Prevention programs are usually tailored to a particular sport and should initially be taught and supervised by a knowledgeable athletic trainer, physical therapist, or comparable professional ( Use of external braces or other devices has not been shown to reduce the risk of ACL tears and is not recommended for prevention.

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient Level Information:

Patient information: Knee pain
Patient information: Total knee replacement (arthroplasty)

Professional Level Information:

Anterior cruciate ligament injury
Evaluation of the adult patient with knee pain
Knee bursitis
Medial collateral ligament injury of the knee
Meniscal injury of the knee
Overview of running injuries of the lower extremity
Patella fractures
Patellofemoral pain syndrome
Proximal tibial fractures in adults
Proximal tibial fractures in children

The following organizations also provide reliable health information.

  • National Library of Medicine

(, available in Spanish)

  • American Academy of Orthopaedic Surgeons


  • American Physical Therapy Association



  1. Prodromos CC, Han Y, Rogowski J, et al. A meta-analysis of the incidence of anterior cruciate ligament tears as a function of gender, sport, and a knee injury-reduction regimen. Arthroscopy 2007; 23:1320.
  2. Linko E, Harilainen A, Malmivaara A, Seitsalo S. Surgical versus conservative interventions for anterior cruciate ligament ruptures in adults. Cochrane Database Syst Rev 2005; :CD001356.
  3. Kostogiannis I, Ageberg E, Neuman P, et al. Activity level and subjective knee function 15 years after anterior cruciate ligament injury: a prospective, longitudinal study of nonreconstructed patients. Am J Sports Med 2007; 35:1135.
  4. Female athlete issues for the team physician – A consensus statement. American Academy of Orthopaedic Surgeons 2003. Available on line at: (Accessed on 4 February 2008).
  5. Benjaminse A, Gokeler A, van der Schans CP. Clinical diagnosis of an anterior cruciate ligament rupture: a meta-analysis. J Orthop Sports Phys Ther 2006; 36:267.