Mayo Clinic Health Library

ACL injury

Updated: 01-04-2013

Definition

An ACL injury is the tearing of the anterior cruciate (KROO-she-ate) ligament, or ACL, inside your knee joint. An ACL injury most commonly occurs during sports that involve sudden stops and changes in direction — such as basketball, soccer, tennis and volleyball.

Immediately after an ACL injury, your knee may swell, feel unstable and become too painful to bear weight. Many people hear or feel a "pop" in their knee when an ACL injury occurs.

Depending on the severity of your ACL injury, treatment may include surgery to replace the torn ligament followed by rehabilitation exercises to help you regain strength and stability. If your favorite sport involves pivoting or jumping, a proper training program may help reduce your chances of an ACL injury.

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Symptoms

At the time of an ACL injury, signs and symptoms may include:

  • A loud "pop" sound
  • Severe pain and inability to continue activity
  • Knee swelling that usually worsens for hours after the injury occurs
  • A feeling of instability or "giving way" with weight bearing

When to see a doctor
Most people seek immediate medical attention after an ACL injury.

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Causes

Ligaments are strong bands of tissue that connect one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.

Most ACL injuries happen during sports and fitness activities. The ligament may tear when you slow down suddenly to change direction or pivot with your foot firmly planted, twisting or hyperextending your knee.

Landing awkwardly from a jump can also injure your ACL, as can falls during downhill skiing. A football tackle or motor vehicle accident also can cause an ACL injury. However, most ACL injuries occur without such contact.

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Risk factors

Women are significantly more likely to have an ACL tear than are men participating in the same sports. Women tend to have a strength imbalance, with the muscles at the front of the thigh (quadriceps) being stronger than the muscles at the back of the thigh (hamstrings). The hamstrings help prevent the shinbone from moving too far forward during activities. When landing from a jump, some women may land in a position that increases stress on their ACL.

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Complications

People who experience an ACL injury are at higher risk of developing knee osteoarthritis, in which joint cartilage deteriorates and its smooth surface roughens. Arthritis may occur even if you have surgery to reconstruct the ligament.

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Preparing for your appointment

The pain and disability associated with an ACL injury prompt many people to seek immediate medical attention. Others may make an appointment with their family doctors. Depending upon the severity of your injury, you may be referred to a doctor specializing in sports medicine or orthopedic surgery.

What you can do
Before the appointment, you may want to write down the answers to the following questions:

  • When did the injury occur?
  • What were you doing at the time?
  • Did you initially hear a loud "pop" sound?
  • Was there much swelling afterward?

What to expect from your doctor
Your doctor may ask some of the following questions:

  • Have your symptoms been continuous or occasional?
  • Do any specific movements seem to improve or worsen your symptoms?
  • Does your knee ever "lock" or feel blocked when you're trying to move it?
  • Do you ever feel that your knee is unstable or unable to support your weight?
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Tests and diagnosis

During the physical exam, your doctor will check your knee for swelling and tenderness — comparing your injured knee to your uninjured knee. He or she also may move your knee into a variety of positions, to help determine if your ACL is torn.

Often the diagnosis can be made on the basis of the physical exam alone, but you may need tests to rule out other causes and to determine the severity of the injury. These tests may include:

  • X-rays. X-rays may be needed to rule out a bone fracture. However, X-rays can't visualize soft tissues such as ligaments and tendons.
  • Magnetic resonance imaging (MRI). An MRI uses radio waves and a strong magnetic field to create images of both hard and soft tissues within your body. An MRI can show the extent of an ACL injury and whether other knee ligaments or joint cartilage also are injured.
  • Ultrasound. Using sound waves to visualize internal structures, ultrasound may be used to check for injuries in the ligaments, tendons and muscles of the knee.
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Treatments and drugs

Initial treatment for an ACL injury aims to reduce pain and swelling in your knee, regain normal joint movement and strengthen the muscles around your knee.

You and your doctor will then decide if you need surgery plus rehabilitation, or rehabilitation alone. The choice depends on several factors, including the extent of damage to your knee and your willingness to modify your activities.

Athletes who wish to return to sports involving cutting, pivoting or jumping usually pursue surgical reconstruction to prevent episodes of instability. More-sedentary people without significant injury to the cartilage or other ligaments usually can maintain knee stability with rehabilitation alone.

Therapy
Whether or not you have surgery on your knee, you'll need rehabilitation. Therapy will include:

  • The use of crutches and, possibly, a knee brace
  • Range-of-motion exercises to regain your full knee motion
  • Muscle-strengthening and stability exercises

Surgery
A torn ACL can't be successfully sewn back together, so the ligament is usually replaced with a piece of tendon from another part of your knee or leg. A tendon graft from a deceased donor also may be an option. This surgery is usually performed through small incisions around your knee joint. A narrow, fiber-optic viewing scope is used to guide the placement of the ACL graft.

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Lifestyle and home remedies

If you choose nonsurgical treatment for your ACL injury, you should avoid future participation in sports activities that involve a lot of cutting, pivoting or jumping.

Self-care treatments you can do at home include:

  • Rest. Use crutches to avoid full weight bearing on your knee after your ACL injury.
  • Ice. When you're awake, try to ice your knee at least every two hours for 20 minutes at a time.
  • Compression. Wrap an elastic bandage or compression wrap around your knee.
  • Elevation. Lie down with your knee propped up on pillows.
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Prevention

To reduce your chance of an ACL injury, follow these tips:

  • Improve your conditioning. Training programs that have been shown to be effective in helping to reduce the risk of ACL injuries typically include strengthening and stability exercises, aerobic conditioning, plyometric exercises, "jump training," and risk-awareness training. Exercises that improve balance also can help when done in conjunction with other training exercises.
  • Strengthen your hamstrings. Women athletes in particular should make sure to strengthen their hamstring muscles as well as their quadriceps.
  • Use proper techniques. If your sport involves jumping, learn how to land safely. Studies have shown that if your knee collapses inward when you land from a jump, you are more likely to sustain an ACL injury. Technique training along with strengthening of some of the hip muscles can help to reduce this risk.
  • Check your gear. In downhill skiing, make sure your ski bindings are adjusted correctly by a trained professional so that your skis will release appropriately when you fall.

Wearing a knee brace has not been found to be helpful in preventing ACL injuries.

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