Anterior Cruciate Ligament Injury (ACL)



An anterior cruciate ligament (ACL) injury is a partial or full tear of the tough band of fibers that connects the lower leg bone to the thigh bone.

Anterior Cruciate Ligament Injury
ACL injury
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ACL injury is caused by excess force on the knee. This may be from:

  • Twisting the knee
  • Landing hard after jumping
  • Sudden stops or changes in direction
  • A direct blow to the knee

Risk Factors

This problem in more common in young athletes. It is also more common in athletes who are female. Some sports that may raise the risk are skiing, snowboarding, soccer, and basketball.

Other things that may raise the risk are:

  • Abnormal foot structure
  • Poor flexibility and strength
  • Slow reaction time
  • Strength, flexibility, and coordination differences between the right and left leg
  • Playing on uneven or wet surfaces
  • Wearing shoes that are not right for the playing surface



An injury to the ACL may cause:

  • A popping sound at the time of the injury
  • Pain and swelling in the knee
  • Loss of full range of motion
  • Weakness or instability in the knee
  • Problems walking


The doctor will ask about your symptoms, health history, and how the injury happened. A physical exam will be done. It will focus on the knee.

The doctor may suspect an ACL injury based on symptoms. Images may be done to confirm it. This can be done with:

  • X-ray
  • MRI scan
  • Ultrasound



The ACL does not heal on its own. The first steps will be to ease pain and swelling. This can be done with rest, ice, medicine, and elevating the leg.

Active people under 35 years of age may need surgery. It will remake the ACL with tissue from other areas of the body or from donor tissue.

Older adults who are not as active may not need surgery, especially if the knee is stable. Exercises may be given to strengthen the muscles around the knee.


The risk of an ACL injury may be lowered by:

  • Stretching and strengthening the muscles around the knee
  • Wearing proper footwear

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.

Edits to original content made by Denver Health.

a (ACL Injury)


OrthoInfo—American Academy of Orthopaedic Surgeons 

Sports Med—American Orthopaedic Society for Sports Medicine 


Canadian Orthopaedic Association 

Canadian Orthopaedic Foundation 


American Academy of Orthopaedic Surgeons (AAOS). Evidence-based clinical practice guideline for management of anterior cruciate ligament injuries. AAOS 2014 Sep 5.

Anterior cruciate ligament (ACL) injury. EBSCO DynaMed website. Available at:  . Updated June 26, 2017. Accessed March 27, 2020.

Anterior cruciate ligament injuries. Ortho Info—American Academy of Orthopaedic Surgeons website. Available at: Updated March 2014. Accessed March 27, 2020.

ACL Injury: Does It Require Surgery? American Academy of Orthopaedic Surgeons website. Available at: Updated September 2009. Accessed March 27, 2020.