What is an ACL Injury?



Diagram of a torn ACL

Diagram of a torn ACL

A torn ACL or ACL injury is a common occurrence among athletes and others who participate in sports. Females are 2 to 8 times more likely to experience an ACL injury than a male, depending on which sport is involved. Although experts are not certain why females have more ACL injuries, some say the reason is a difference in muscle balance between the sexes, or that estrogen has an effect on the ACL.

A torn ACL usually occurs when the knee is twisted forcefully while the foot is firmly planted on the ground, or it results from a direct blow to the knee, as commonly happens during contact sports such as football and rugby. A torn ACL frequently occurs along with a medial meniscus tear and MCL injury (see “Torn Meniscus” and “MCL”). Less often the posterior cruciate ligament (PCL), which prevents backwards movement of the tibia and works with the ACL to stabilize the knee joint, is injured as well. If the ACL injury also involved impact to the inner surface of the knee joint, which can occur during a football or rugby tackle, the lateral collateral ligament (LCL) may have been damaged too.

Symptoms of an ACL tear include:

  • An audible crack or pop when the injury occurs
  • A feeling of instability immediately after the injury, including a feeling like the knee is “giving out.” This feeling may be masked later when swelling occurs.
  • Extreme pain, especially immediately after the injury occurs
  • Extensive swelling of the knee, which can be minimized with immediate treatment (see “Treatment of ACL Injury”)
  • Restricted movement of the knee, including an inability to fully straighten the leg


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