Anterior Cruciate Ligament Tear (Torn ACL)

  • Medical Author:
    Benjamin Wedro, MD, FACEP, FAAEM

    Dr. Ben Wedro practices emergency medicine at Gundersen Clinic, a regional trauma center in La Crosse, Wisconsin. His background includes undergraduate and medical studies at the University of Alberta, a Family Practice internship at Queen's University in Kingston, Ontario and residency training in Emergency Medicine at the University of Oklahoma Health Sciences Center.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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What is a torn anterior cruciate ligament (ACL)?

A torn anterior cruciate ligament (ACL) is a second- or third-degree sprain of the ACL. The ACL arises from the front of the medial femoral condyle and passes through the middle of the knee to attach between the bony outcroppings (called the tibia spine) that are located between the tibia plateaus. It is a small structure, less than 1½ inches long and ½ inch wide. The anterior cruciate ligament is vital in preventing the thighbone (femur) from sliding backward on the tibia (or, from the other point of view, the tibia sliding forward under the femur). The ACL also stabilizes the knee from rotating, the motion that occurs when the foot is planted and the leg pivots.

Without a normal ACL, the knee becomes unstable and can buckle, especially when the leg is planted and attempts are made to stop or turn quickly.

What causes a torn ACL?

Most anterior cruciate ligament injuries occur due to injury, usually in a sport or fitness activity. The ligament gets stretched or tears when the foot is firmly planted and the knee locks and twists or pivots at the same time. This commonly occurs in basketball, football, soccer, and gymnastics, where a sudden change in direction stresses and damages the ligament. These injuries are usually noncontact, occur at low speed, and occur as the body is decelerating.

ACL injuries may also occur when the tibia is pushed forward in relation to the femur. This is the mechanism of injury that occurs because of a fall when skiing, from a direct blow to the front of the knee (such as in football) when the foot is planted on the ground, or in a car accident.

Risk factors for ACL injury in women

Women are more prone to ACL injuries than men. Women have slightly different anatomy that may put them at higher risk for ACL injuries:

  • The intercondylar notch at the end of the femur is narrower in women than men. When the knee moves, this narrower space can pinch and weaken the ACL.
  • Women have a wider pelvis than men, and this causes the femur to meet the tibia at a greater angle (called the Q angle). This increases the force that the ACL has to withstand with any twisting motion, increasing the risk of damage.
  • Genetic differences may put the female ACL at risk. Female muscles tend to be more elastic and decrease the protection that the hamstring muscles can provide to the ACL. Hormonal changes during menses may also affect elasticity. Moreover, female hamstrings react and contract a millisecond slower than in a male, increasing the risk of ACL damage when landing from a jump.
Medically Reviewed by a Doctor on 3/18/2016
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