An anterior cruciate ligament injury is extreme stretching or tearing of the anterior cruciate ligament (ACL) in the knee. A tear may be partial or complete.

The knee is essentially a modified hinge joint located where the end of the thigh bone (femur) meets the top of the shin bone (tibia). Four main ligaments connect these two bones:

• Medial collateral ligament (MCL) -- runs along the inner part of the knee and prevents the knee from bending inward.
• Lateral collateral ligament (LCL) -- runs along the outer part of the knee and prevents the knee from bending outward.
• Anterior cruciate ligament (ACL) -- lies in the middle of the knee. It prevents the tibia from sliding out in front of the femur, and provides rotational stability to the knee.
• Posterior cruciate ligament (PCL) -- works with the ACL. It prevents the tibia from sliding backwards under the femur.

The ACL and PCL cross each other inside the knee, forming an "X." This is why they are called the “cruciate” (cross-like) ligaments.

ACL injuries are often associated with other injuries. The classic example is when the ACL is torn at the same time as both the MCL and medial meniscus (one of the shock-absorbing cartilages in the knee). This type of injury often occurs in football players and skiers.

Women are more likely to have an ACL tear than men. The cause for this is not completely understood, but it may be due to differences in anatomy and muscle function.

Adults usually tear their ACL in the middle of the ligament or pull the ligament off the femur bone. These injuries do not heal by themselves. Children are more likely to pull off their ACL with a piece of bone still attached. These injuries may heal on their own, or they may require an operation to fix the bone.

When your doctor suspects an ACL tear, an MRI may help confirm the diagnosis. This test may also help evaluate other knee injuries, such as to the other ligaments or cartilage.

Some people are able to live and function normally with a torn ACL. However, most people complain that their knee is unstable and may "give out" with attempted physical activity. Unrepaired ACL tears may also lead to early arthritis in the affected knee.

ACL tears may be due to contact or non-contact injuries. A blow to the side of the knee, which can occur during a football tackle, may result in an ACL tear.

Coming to a quick stop, combined with a direction change while running, pivoting, landing from a jump, or overextending the knee joint (called hyperextended knee), also can cause injury to the ACL.

Basketball, football, soccer, and skiing are common causes of ACL tears.

Early symptoms:

• A "popping" sound at the time of injury
• Knee swelling within 6 hours of injury
• Severe pain

Late symptoms:

• Arthritis
• Knee joint instability

An ACL injury should be treated with:

• A splint
• Elevating the joint (above the level of the heart)
• Ice
• Pain relievers such as nonsteroidal anti-inflammatory drugs (like ibuprofen)

Do not play again until you have been evaluated and treated.

Some people may need crutches to walk until the swelling and pain have improved. Physical therapy may help you regain joint motion and leg strength.

If you still have instability after your leg strength and knee motion have returned, your doctor may recommend a reconstruction of the ACL. The old ligament cannot be fixed, so a new one needs to be constructed. Common grafts used for reconstruction include:

• A piece of the tendon that connects the kneecap to the tibia (patellar tendon)
• Hamstring tendon
• Human cadaver graft

• In the case of a serious knee injury, do not try to move the joint. Use a splint to keep the knee straight until a trained medical professional has evaluated it.
• Do not return to play until you have been evaluated and treated.

Anyone with a serious knee injury should seek medical attention for x-rays and evaluation.

If the foot is cool and blue after a knee injury, the knee may be dislocated and blood vessels to the foot may be injured. This is a true medical emergency that requires immediate professional help.

Use proper techniques when playing sports or exercising. Several women's college sports programs have reduced ACL tears through a training program that teaches athletes how to minimize the stress they place on their ACL.

Although the issue is controversial, the use of knee braces during aggressive athletic activity (such as football) has not been shown to decrease the incidence of knee injuries and may give the player a false sense of security.

Renstrom P, Ljungqvist A, Arendt E, Beynnon B, Fukubayashi T, Garrett W, et al. Non-contact ACL injuries in female athletes: an International Olympic Committee current concepts statement. Br J Sports Med. 2008;42:394-412.

Birmingham TB, Bryant DM, Giffin JR, Litchfield RB, Kramer JF, Donner A, Fowler PJ. A randomized controlled trial comparing the effectiveness of functional knee brace and neoprene sleeve use after anterior cruciate ligament reconstruction. Am J Sports Med. 2008;36:648-655.