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.
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