Anterior cruciate ligament (ACL) injury
Cruciate ligament injury - anterior; ACL tear; Knee injury - anterior cruciate ligament (ACL)
An anterior cruciate ligament injury is the over-stretching or tearing of the anterior cruciate ligament (ACL) in the knee. A tear may be partial or complete.
The knee joint is located where the end of the thigh bone (femur) meets the top of the shin bone (tibia).
Four main ligaments connect these two bones:
- Medial collateral ligament (MCL) runs along the inside of the knee. It prevents the knee from bending inward.
- Lateral collateral ligament (LCL) runs along the outside of the knee. It prevents the knee from bending outward.
- Anterior cruciate ligament (ACL) is in the middle of the knee. It prevents the shin bone from sliding out in front of the thigh bone.
- Posterior cruciate ligament (PCL) works with the ACL. It prevents the shin bone from sliding backward under the femur.
Women are more likely to have an ACL tear than men.
Let's talk today about ACL injuries. Do you remember that old children's song that goes, The knee bone's connected to the thigh bone, and so on? Well the thigh bone's connected to the shin bone, and one of the four ligaments that connects these two bones is called the anterior cruciate ligament or ACL. The ACL sits in the middle of the knee. It's main job is to prevent the shin bone from sliding in front of the thigh bone. So, how do most ACL injuries occur? Well, getting tackled in football is just one way to hurt your ACL. You could also over extend your knee joint while playing other sports, like basketball or soccer, or while skiing. Or, you could injure your ACL if you quickly change direction while running, or land the wrong way from a jump. Some ACL tears are just partial, while others are complete. The injury that has damaged your ACL can also tear other ligaments or cartilage in your knee. So, what should you do if you think that you might have a torn ACL? Well, if you've injured your knee and it swells up, hurts, or makes a popping sound, or gives way while you walk, you could have an ACL injury. So, stop whatever that you're doing; even if you're just about to score the game-winning touchdown. Try not to move your knee. See your doctor as soon as you can because you may need an MRI or x-ray so your doctor can see exactly what type of injury that you have. You can treat an ACL injury with ice, rest, and pain relievers such as ibuprofen or acetaminophen. But you may also need physical therapy to get your knee moving normally again. If the tear is so bad that you can barely walk on your leg, your doctor may recommend surgery to rebuild your torn ACL. Whenever you run, jump, or play sports, take it easy on your knees. Talk to your doctor or a physical therapist about ways to prevent knee injuries during play. And if you ever hurt your knee so badly that your foot turns blue and feels cool to the touch, get medical help immediately because that's a sign that you've dislocated your knee joint, and you may have injured the blood vessels to your foot.
An ACL injury can occur if you:
- Get hit very hard on the side of your knee, such as during a football tackle
- Overextending your knee joint
- Quickly stop moving and change direction while running, landing from a jump, or turning
Basketball, football, soccer, and skiing are common sports linked to ACL tears.
ACL injuries often occur with other injuries. For example, an ACL tear often occurs along with tears to the MCL and the shock-absorbing cartilage in the knee (meniscus).
Most ACL tears occur in the middle of the ligament, or the ligament is pulled off the thigh bone. These injuries form a gap between the torn edges, and do not heal on their own.
- A "popping" sound at the time of injury
- Obvious knee swelling within 6 hours of injury
- Pain, especially when you try to put weight on the injured leg
- Difficulty in continuing with your sport
- Feeling of instability
Those who have only a mild injury may notice that the knee feels unstable or seems to "give way" when using it.
See your health care provider if you think you have an ACL injury. Do not play sports or other activities until you have seen a provider and have been treated.
Your provider may send you for an MRI of the knee. This can confirm the diagnosis. It may also show other knee injuries.
First aid for an ACL injury may include:
- Raising your leg above the level of the heart
- Putting ice on the knee
- Pain relievers, such as nonsteroidal anti-inflammatory drugs (such as ibuprofen)
You also may need:
- Crutches to walk until the swelling and pain get better
- Brace to give your knee some stability
- Physical therapy to help improve joint motion and leg strength
- Surgery to reconstruct the ACL
Some people can live and function normally with a torn ACL. However, most people complain that their knee is unstable and may "give out" with physical activity. Unstable knee after ACL tears can lead to further knee damage. You are also less likely to return to the same level of sports without the ACL.
- Do NOT move your knee if you have had a serious injury.
- Use a splint to keep the knee straight until you see a doctor.
- Do NOT return to play or other activities until you have been treated.
When to Contact a Medical Professional
Contact your provider right away if you have a serious knee injury.
Get immediate medical attention if the foot is cool and blue after a knee injury. This means that the knee joint may be dislocated, and blood vessels to the foot may be injured. This is a medical emergency.
Use proper techniques when playing sports or exercising. Some college sports programs teach athletes how to reduce stress placed on the ACL. This involves a series of warm up exercises and jumping drills. There are jumping and landing exercises that have been shown to reduce ACL injuries.
The use of knee braces during vigorous athletic activity (such as football) is controversial. It has not been shown to reduce the number of knee injuries, but not specifically ACL injuries.
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Last reviewed on: 6/8/2022
Reviewed by: C. Benjamin Ma, MD, Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery, San Francisco, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.