The knee is essentially a hinge joint located where the end of the femur (thighbone) meets the top of the tibia (shinbone). There are four main ligaments connecting these two bones: 

• The MCL (medial collateral ligament) runs along the inner part of the knee and prevents the knee from bending inward. 
• The LCL (lateral collateral ligament) runs along the outer part of the knee and prevents the knee from bending outward.
• The ACL (anterior cruciate ligament) 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. 
• The PCL (posterior cruciate ligament) 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” ligaments.

ACL injuries are often associated with other injuries.  The “unhappy triad” is a classic example, where the ACL is torn at the same time as the MCL and the lateral 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 much more likely to suffer an ACL tear than men are. The cause for this is not completely understood.

Adults who tear their ACL usually do so in the middle of the ligament or pull the ligament off the bone. These injuries do not heal by themselves. But children are more likely to pull off their ACL with a piece of bone still attached, and these may heal on their own.

An MRI is the only imaging study that can demonstrate an ACL tear.

Some people are able to live and function normally with a torn ACL. However, most people complain that their knee is unstable and causes them to stumble. Unrepaired ACL tears may also lead to early arthritis in the affected knee.

ACL tears are usually non-contact injuries. 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. This injury occurs in about 4 out of 1,000 people.

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 Motrin).

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.

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.