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Knee Pain

ACL Tear

An anterior cruciate ligament (ACL) injury is the over-stretching or tearing of the ACL in the knee. A tear may be partial or complete. The ACL is probably the most commonly injured ligament of the knee. It is usually injured during sports.

The ACL is located in the center of the knee joint where it runs from the backside of the femur (thighbone) to connect to the front of the tibia (shinbone). The ACL is the main controller of how far forward the tibia moves under the femur. If the tibia moves too far, the ACL can rupture. The ACL is also the first ligament that becomes tight when the knee is straightened. If the knee is forced past this point, or hyperextended, the ACL can also be torn.

ACL injuries often occur with other injuries. For example, an ACL tear often occurs along with tears to the medial collateral ligament (MCL) and the shock-absorbing cartilage in the knee (meniscus).


The mechanism of injury for many ACL ruptures is a sudden deceleration, hyperextension, or pivoting in place. Sports-related injuries are the most common.


Basketball, football, soccer, and skiing are common sports linked to ACL tears. Common causes include:

  • Getting hit very hard on the side of your knee (e.g. during a football tackle)

  • Overextending your knee joint

  • Quickly stopping movement and changing direction while running, landing from a jump, or turning