What is the anterior cruciate ligament?

The anterior cruciate ligament (ACL), which is located in the center of the knee, connects the thigh bone (femur) to the shin bone (tibia). (A ligament connects one bone to another.) The main function of the ACL is to stop forward movement and rotation of the shin bone on the thigh bone.

Who is at risk for ACL injuries?

In sports, athletes who play football, basketball, soccer, and volleyball are at higher risk for an ACL injury. Females are 4 times more likely to have an ACL tear than males are.

What causes ACL injuries?

Most ACL tears are non-contact injuries. They can occur in several different ways:

  • Twisting the knee too forcefully.
  • Planting the foot and then suddenly changing direction.
  • Landing “the wrong way” from a jump.
  • Impact to the knee (for example, from a tackle in a football game).

Injuries to the ACL are classified as Grade 1 sprains (slightly stretched), Grade 2 strains (partial tear), and Grade 3 sprains (complete tear).

What are the symptoms of an ACL tear?

When the ACL tears, the person will usually feel a “pop" in the knee, or a sense that the knee has “given out.” Other symptoms include the following:

  • Pain and instability in the knee.
  • Swelling that starts immediately (but can start 2 to 6 hours after the injury) and lasts for 2 to 4 weeks. When the swelling goes away, the knee will still cause the person to feel unsteady.
  • Loss of range of motion in the knee.
  • Tenderness.

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