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

The knee is similar to a 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 (side) of the knee and prevents the knee from bending inward.
• Lateral collateral ligament (LCL) -- runs along the outer part (side) 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 often occur 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 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.