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. It keeps 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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ACL injury is caused by excess force on the knee. It occurs most often with:

  • Twisting at knee
  • Hard landing from a jump
  • Sudden stops or changes in direction
  • Sidestepping or pivoting
  • Direct hit to knee

Risk Factors

An ACL injury may be more likely with:

  • Weak knee structure
  • Muscle strength imbalance between muscles in front and back of the leg
  • Sports that require sudden changes of direction and sudden stopping or slowing
  • Poor technique when cutting, planting, pivoting, or jumping
  • Earlier ACL injury or surgery


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
  • Difficulty walking


You will be asked about your symptoms. The doctor will want to know how the knee was injured. A physical exam will be done with tests for knee strength and stability.

The doctor may suspect an ACL injury based on symptoms. Tests may be done if a severe injury is possible or other areas of the joint may be damaged. Tests may include:


Treatment will depend on how severe the injury is and your overall health. Recovery time will vary.

First steps will help to reduce pain and swelling. It will include self care such as rest, ice, and keeping leg elevated when resting. It may be at least 2 to 4 weeks before surgery can be done. Medicine may also be needed to ease pain and swelling.

Nonsurgical Care

The ACL can't heal. The tear will remain but surgery may not be needed if the knee is stable. This may work for those that are less active or those who can adjust their activity.


Surgery may be needed:

  • For people who are young and active
  • For people who want to return to intense sports
  • If other ligaments of the knee are damaged
  • The knee is unstable and makes basic movement difficult

The ACL will be remade with tissue from other areas of the body. A piece of tendon from another area of the body may be used. Some may use a piece of tissue from a donor. It can take several months for the graft to become strong enough to return to sports.

Physical Therapy

Physical therapy may be done for those with or without surgery. Knee movement can be tested to make a plan for recovery. Therapy can include exercise and stretching to help balance the muscles of the legs. This can help stabilize the knee and lower the risk of more injury.


To reduce your chance of ACL injuries:

  • 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.
  • Look for programs that help strengthen knees and reduce injury.

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