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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ACL injury is caused by 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

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 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


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


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—In the first few days, icing 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't heal by itself, but surgery may not be needed if the knee is stable. This option may be best in those who are elderly or less active. Those who are highly active or want 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 who are young and active or those who want to return to intense sports. It may also be needed if other ligaments of the knee are damaged or the knee is 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 sports.

Physical Therapy

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.


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.

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