The anterior cruciate ligament (ACL) is one of four ligaments located in the knee joint, and is responsible for providing stability during walking and other activities. A torn or sprained ACL is a highly common knee injury, which affects more than 200,000 people in the United States each year, out of which approximately half undergo knee surgery. About half of all cases are moreover accompanied by other damages to the structures of the knee.

What Causes a Torn ACL?

ACL injuries most commonly occur in athletes, but can also happen while performing everyday activities, due to falling or tripping, or result from vehicle accidents. Especially, sports such as soccer, football, hockey, rugby, gymnastics, and skiing are associated with relatively high rates of torn ACLs. Tears or sprains are generally caused by (1) overextension or twisting of the knee joint; (2) getting hit hard on the side of the knee, for example during a tackle; (3) quickly turning or stopping while running at a high speed; or (4) landing improperly after a jump.

Interestingly, women are at a higher risk of sustaining ACL injuries than men, owing to slight differences in the female and male physiology, which make the ACL more susceptible to injuries in women. Furthermore, differences in muscular strength and mass, neuromuscular control, physical conditioning, and estrogen levels may also impact the strength or the ligaments in the body, and thereby their risk of getting injured.


The most commonly described symptom of a torn ACL is a loud “popping” sound and sensation in the knee joint at the time when the injury occurs. Immediately after, intense knee pain usually develops, making weight-bearing, and even walking, very difficult; and the knee may become unstable and buckle under your weight as you try to put weight on it. Within approximately six to 24 hours, bleeding within the knee joint will cause the knee to swell, and this swelling along with the accompanied inflammation may further limit movements to the joint and your ability to walk.

While the intense pain and swelling may subside temporarily with time, if a torn ACL is left untreated, the feelings of unsteadiness and discomfort will likely remain, and you may experience recurrent pain and swelling, as well as have your knees give way when walking, especially when going up or down stairs. If the injury is mild, you may only notice that the knee feels unstable or seems to give way when you are using it; however, even mild cases should not be ignored, and if you attempt to go back to sports before seeing a sports medicine or orthopedic trauma expert, there is a high risk that you may cause further damage to the structures of the injured knee.

How are ACL Tears Diagnosed and Treated?

ACL tears and sprains are diagnosed by physical examination, during which the doctor will look particularly for signs of tenderness, swelling, bruising and deformities; and compare the stability between the injured and healthy knee. Next, imaging studies of the knee, using for example x-ray or magnetic resonance imaging, are performed.

The treatment for ACL tears generally includes a combination of rest, physiotherapy, and/or orthopedic surgery. If you suspect that you have a torn ACL, contact your sports medicine or orthopedic trauma specialist immediately in order to ensure quick and accurate diagnosis and treatment.

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