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Anterior Cruciate Ligament Injury

(ACL Injury)

Pronounced: an-TEER-ee-or KROO-shee-ate ligament


An anterior cruciate ligament (ACL) injury is a tear in a ligament of the knee. The ACL is a tough band of fiber in the middle of the knee joint. It connects the lower leg bone to the thigh bone. The ACL keeps the knee stable during movement by keeping the lower leg bone from sliding too far forward. An injury to this ligament can make the knee unstable. The injury may be partly torn or a complete tear.

Anterior Cruciate Ligament Injury

ACL injury
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Causes    TOP

ACL injury is caused be excess force on the knee. It occurs most often when your knee gets twisted or during a hard landing from a jump. It can also happen with:

  • Sudden stops or changes in direction
  • Sidestepping or pivoting
  • Direct contact

Risk Factors    TOP

Factors that increase your chance of ACL injury include:

  • Weak knee structure
  • Muscle strength imbalance between the quadriceps and hamstrings
  • Playing sports that require sudden changes of direction and deceleration
  • Use of incorrect technique for cutting, planting, pivoting, or jumping
  • Previous injury or reconstructive ACL surgery

Symptoms    TOP

Symptoms may include:

  • 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
  • Difficulty walking

Diagnosis    TOP

You will be asked about your symptoms and how you injured your knee. A physical exam will be done and your doctor will test your knee's strength and stability.

The doctor may do further tests to see if there is any other damage to the joint. This may be done with:

Ligament sprains are graded according to their severity:

  • Grade 1—Mild ligament damage.
  • Grade 2—Partial tearing of the ligament.
  • Grade 3—Complete tearing of the ligament.

Treatment    TOP

Talk with your doctor about the best treatment plan for you. Recovery time will depend on the severity of the injury. Immediate care includes:

  • Rest—Avoiding activities that cause pain.
  • Ice—Regular icing in first few days may help slow swelling.
  • Compression—Compression bandages can provide gentle pressure to help move fluids out of the area.
  • Elevation—Keeping the knee elevated at rest can help fluids drain out of the area.

Nonsurgical Care

The ACL can not heal by itself but if the knee is fairly stable surgery may not be needed. This option may be best in those that are less active or elderly. Those that are highly active or wish to return to sports will probably need surgery.

A knee brace can help keep the knee stable. Crutches may also be helpful in the beginning.

Over-the-counter or prescription medications may be used to reduce pain and swelling.


Surgery may be needed for those that are young and active or those that need to return to intense sports. It may also be needed if other ligaments of the knee are damaged or the knee is very unstable.

The surgery will use tissue from another part of the body to make a new ligament. It can take several months for the graft to become strong enough to return to sport activity.

Physical Therapy    TOP

The physical therapist can test your knee movement and develop a recovery plan. Therapy can include exercise and stretching programs to help balance the muscles of your legs. This can help stabilize the knee and decrease the risk of further injury.

Prevention    TOP

To reduce your chance of injuring the ACL:

  • Try plyometrics, a form of exercise that uses explosive movements.
  • When jumping and landing or turning and pivoting, your hips and knees should be bent, not straight.
  • Strengthen and stretch the muscles of your legs.
  • Maintain proper technique when exercising or playing sports.


OrthoInfo—American Academy of Orthopaedic Surgeons
Sports Med—American Orthopaedic Society for Sports Medicine


Canadian Orthopaedic Association
Canadian Orthopaedic Foundation


Anterior cruciate ligament (ACL) injury. EBSCO DynaMed Plus website. Available at: http://www.dynamed.... Updated August 18, 2015. Accessed September 30, 2016.
Anterior cruciate ligament injuries. Ortho Info—American Academy of Orthopaedic Surgeons website. Available at:
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Updated March 2014. Accessed February 29, 2016.
Anterior cruciate ligament injuries: treatment and rehabilitation. Encyclopedia of Sports Medicine and Science website. Available at:
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Updated April 18, 1998. Accessed February 29, 2016.
ACL Injury: Does It Require Surgery? American Academy of Orthopaedic Surgeons website. Available at:
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Updated September 2009. Accessed February 29, 2016.
Griffin LY, Agel J, et al. Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. J Am Acad Orthop Surg. 2000;8:141-150.
Knee sprains and meniscal injuries. Merck Manual for Health Care Professionals. Available at:
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Updated December 2014. Accessed February 29, 2016.
Ligament injuries to the knee. John Hopkins Medicine website. Available at.
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Accessed February 29, 2016.
7/6/2009 DynaMed Plus Systematic Literature Surveillance http://www.dynamed...: Prodromos CC, Han Y, et al. A meta-analysis of the incidence of anterior cruciate ligament tears as a function of gender, sport, and a knee injury-reduction regimen. Arthroscopy. 2007;23:1320-1325.
5/12/2014 DynaMed Plus Systematic Literature Surveillance http://www.dynamed...: Anterior cruciate ligament injuries: Diagnosis, treatment, and prevention. Pediatrics. 2014 Apr [Epub ahead of print].
Last reviewed March 2017 by EBSCO Medical Review BoardTeresa Briedwell, PT, DPT
Last Updated: 3/15/2017

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