From Jonathan Cluett, M.D.,
Your Guide to Orthopedics.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

What is an ACL injury?
One of the most common problems involving the knee joint is an anterior cruciate ligament tear. The anterior cruciate ligament (also called the ACL) is one of four ligaments that are critical to the stability of the knee joint. A ligament is made of tough fibrous material and functions to control excessive motion by limiting joint mobility. Of the four major ligaments of the knee, the ACL injury is the most common knee ligament injury.

What is the normal function of the ACL?
The anterior cruciate ligament is the primary restraint to forward motion of the shin bone (tibia). The anatomy of the knee joint is critical to understanding this relationship. Essentially, the femur (thigh bone) sits on top of the tibia (shin bone), and the knee joint allows movement at the junction of these bones. Without ligaments to stabilize the knee, the joint would be unstable and prone to dislocation. The ACL prevents the tibia from sliding too far forward.

The ACL also contributes stability to other movements at the joint including the angulation and rotation at the knee joint. The ACL performs these functions by attaching to the femur on one end, and to the tibia on the other. The other major ligaments of the knee are the posterior cruciate ligament (PCL), and the medial and lateral collateral ligaments (MCL and LCL, respectively).

What happens when there is an ACL injury?
When an ACL injury occurs, the knee becomes less stable. The ACL injury is a problem because this instability can make sudden, pivoting movements difficult, and it may make the knee more prone to developing arthritis and cartilage tears.

Why are ACL tears such a big problem?
When the knee is unstable, patients often complain of a sensation that the knee will 'give out' from under them. When this giving way sensation is because of an ACL injury, the knee joint is sliding too much. This can be a problem because each episode of instability (the 'giving way' sensation) can cause damage to the knee cartilage. Therefore an ACL injury makes patients more prone to developing arthritis and meniscus tears.

Athletes often have particular difficulty once they have sustained an ACL injury. Many sports require a functioning ACL to perform common maneuvers such as cutting, pivoting, and sudden turns. These high demand sports sports include, but are not limited to:

• Football
• Soccer
• Basketball
• Skiing
• Gymnastics
• Hockey (Ice and Field)
• Wrestling
• Lacrosse
• Rugby

Patients may be able to function in their normal daily activities without a normal ACL, but these high-demand sports may prove difficult. Therefore, athletes are often faced with the decision to undergo surgery in order to return to their previous level of competition.

What factors contribute to ACL injuries?
ACL injuries can strike anyone, but there are certainly some individuals more prone to ACL injury. Sports listed above can cause high forces to be placed on the ACL. Participants in these sports are especially prone to ACL injury. Another factor that contributes to ACL injuries is the gender of the patient. In college basketball, women players may be up to 8 times more likely than their male counterparts to sustain an ACL injury. There are different theories as to why women are especially susceptible to ACL injury.

For more information about the symptoms and diagnosis of ACL tears, read on...