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 femur (thigh bone) meets the top of the tibia (shin bone). 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 in concert 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 the 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 suffer an ACL tear than men. The cause for this is not completely understood, but it may be due to differences in anatomy and muscular 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.

Alternatively, 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), 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
• Severe pain
• Knee swelling within 6 hours of injury

Late symptoms:

• Knee joint instability
• Arthritis

An ACL injury should be treated with a splint, ice, elevation of the joint (above the level of the heart), and pain relievers such as nonsteroidal anti-inflammatory drugs (like ibuprofen). The patient should not continue to play until evaluation and treatment has taken place.

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

If instability continues even after leg strength and knee motion has returned, most orthopedists will recommend a reconstruction of the ACL. The old ligament cannot be fixed, so a new one needs to be constructed. Usually a piece of the patellar tendon (the tendon connecting the kneecap to the tibia) is used, although the hamstrings can also be effective. Cadaveric grafts may also be used to reconstruct the ACL.

• In the case of a serious knee injury, do not attempt to move the joint. A splint should be used to keep the knee straight until evaluation by trained medical personnel has been performed.
• Do not return to play until proper evaluation and treatment has taken place.

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

More important, if someone's foot is cool and blue after a knee injury, they may have dislocated their knee and injured the blood vessels to the foot. This is a true medical emergency that requires immediate professional help.

Use proper techniques when playing sports or exercising. Several women's collegiate 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.