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). There are four main ligaments connecting 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 “unhappy triad” is a classic example, in which where the ACL is torn at the same time as the MCL and the medial meniscus (one of the shock-absorbing cartilages in the knee). This type of injury is most often seen 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 may have to do with differences in anatomy as well as muscular functioning.

Adults who tear their ACL usually do so 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 may heal on their own, or may require the bone to be fixed.

In cases of suspected ACL tear, an MRI may help to confirm the diagnosis, and to evaluate other injuries to the knee, 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, such as may 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, can cause injury to the ACL.

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

Early symptoms:

• A "pop" 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 have been regained, 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 importantly, 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 and professional help must be sought immediately.

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.