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Anterior Cruciate Ligament (ACL) Injuries

About ACL Injuries

A torn anterior cruciate ligament (ACL) is a common knee injury, especially for athletes. Ligaments are long, rope-like bands that fasten bones together. The ACL helps give the knee its stability.

Teens who play contact sports (like football) or sports that feature swift, abrupt movements such as pivoting, stopping, or turning on a dime, are most likely to get ACL injuries. People also can tear an ACL when they jump and land with their knees straight or “locked” instead of flexed. This puts pressure on the knee joint and causes the ACL to tear or break apart.

Teen girls are 8 to 10 times more likely than guys to tear an ACL. There are several reasons why, including the fact that girls and guys have different body shapes and limb alignment. Hormones can play a role, too. Female hormones may loosen the ligaments.

ACL injuries can really hurt. They can also lead people to be unsteady on their feet and have difficulty walking. Depending on the person’s age and the severity of the injury, a torn ACL often requires surgery in addition to 6 to 12 months of rehabilitation.

What an ACL Does

The ACL is one of the four main ligaments in the knee joint that connect it to the shinbone (tibia) and thighbone (femur). It’s located deep within the joint, behind the kneecap (patella), above the shinbone, and below the thighbone. torn ACL illustration

The ACL works with the PCL (posterior cruciate ligament), which crosses over it to form an “X.” Together, these two ligaments help keep the knee stable when rotating. The ACL keeps the shinbone in place and prevents it from moving too far forward and away from the knee and thighbone.

Signs That It Might Be an ACL Injury

Here’s what most people notice after tearing an ACL:

  • feeling unstable or “wobbly” when walking
  • inability to put weight on the affected leg
  • swelling in the knee joint within 24 hours of the tear
  • pain, which may be intense

Many people report hearing a “pop” sound when the ligament tears. Others also report the knee feeling looser than it was before. Sometimes, though, people with ACL injuries don’t notice anything different. It all depends on how severe the injury is.

If you injure your knee — whether out on the field or at home — stop all activity to prevent further injury. See a doctor as soon as possible. In the meantime, keep your knee iced and elevated to reduce swelling. Don’t put weight on your knee.

How Do Doctors Diagnose an ACL Injury?

A doctor will examine the knee and probably do imaging tests to see how it might be injured and, if so, how badly.

These tests can help diagnose an ACL injury:

  • Lachman test. During this exam, a person will lie down flat on his or her back with the affected knee lifted and flexed at a 20- to 30-degree angle. The doctor then places one hand on the person’s calf and the other on the top of the thigh, applying pressure to move the shin forward. If it moves too far forward, it can signal a torn ACL.
  • Anterior drawer test. During this test, the hip is flexed at 45 degrees and the knee at 90 degrees. The examiner grasps the back of the shin, just below the knee, placing index fingers on hamstring tendons and thumbs on the side of the kneecaps to feel any shift of the knee joint and surrounding areas while attempting to pull the tibia forward.

Doctors may order X-rays with knee injuries, but that’s usually to see if a bone is fractured, since X-rays only image bone. So some doctors will order an MRI, which images tissue (like ligaments and muscles), to confirm a partial or complete ACL tear.

Treating ACL Injuries

ACL injuries usually need surgery. If a person is still growing, doctors want to be sure the surgery doesn’t affect the growth plates — the tissue on the ends of long bones (like the tibia and femur) that allows people to grow.

When someone stops growing, the growth plates harden (ossify) along with the rest of the bone. Doctors also refer to this as the growth plates “closing.” Girls tend to stop growing earlier than guys; their growth plates usually close around ages 14 to 15, while guys’ growth plates close later, at around ages 16 to 17.

If surgeons think someone is still growing, they will focus on surgeries that don’t interfere with the growth plates. For people who have reached skeletal maturity, the surgeon will drill a small tunnel down through the femur to reach the inside of the knee joint. Surgeons replace the torn ACL with tissue from the patient’s own body (usually bone from the kneecap and shinbone with tendon in between) or with donor tissue (called an allograft) from the Achilles tendon in the foot. The new ACL tissue is fed through the tibia and femur tunnels and is secured in the proper area with screws or other fixtures.

After surgery, patients will need to use crutches, limit physical activity, and wear a full-leg brace for 4 to 6 weeks, depending on what the surgeon advises.

Rehab and Recovery

Recovery from ACL surgery can take from 6 months to a year. Rehabilitation (“rehab”) therapy is needed to help heal the knee and to:

  • restore range of motion
  • regain strength in the knee, thigh, and shin muscles
  • prevent atrophy, the breakdown of muscle tissue
  • reduce pain and swelling
  • improve balance

Most people do rehab at a center three times a week, with daily exercises they practice at home. Accelerated rehab programs can speed up recovery to 4 to 6 months, though they require more frequent therapy.

In the early stages of recovery, you’ll have to wear a leg brace, followed by a knee brace, to reduce the risk of reinjuring the knee. Keeping the knee iced and elevated can help to reduce swelling. Over-the-counter or prescription painkillers and anti-inflammatory medicine can help you deal with the pain and feel more comfortable.

While most sports are off limits — especially the activity that caused the injury in the first place — you might try some low-impact activities like swimming, bike riding, or protected running. Talk to your doctor about what activities might help you; some of these may even count as therapies.

Coping With an ACL Injury

Being told that you can’t do the things you love — like running or playing football, field hockey, or softball — can be frustrating. Recovering from an ACL injury may cause you to feel angry or even depressed, especially if you’re no longer playing team sports with your friends.

But in the meantime, there are ways to still feel like part of the team. Keeping score, being a coach’s assistant, or bringing water to your teammates may help. If you don’t want to do these, start something new, like playing the guitar, painting, drawing, or another activity that won’t put strain on the knee.

In time, you can do the things you love. If you feel like you’re struggling with recovery, consider talking to a counselor for support.

Reviewed by: Alfred Atanda Jr, MD
Date reviewed: October 2012