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Anterior Cruciate Ligament (ACL) Injuries

About ACL Injuries

Knee injuries can happen in active kids, especially athletes, and a torn anterior cruciate ligament (ACL) — a ligament that helps give the knee its stability — is one of the most serious types of knee injuries.

Kids who get ACL injuries tend to play contact sports (like football and basketball) or so-called "cutting" sports (like soccer and baseball that feature swift, abrupt movements such as pivoting, stopping, or turning on a dime).

The injury can also happen with certain movements, like when a child jumps and lands hard on his or her feet. If the quadriceps muscles aren’t strong enough, a movement an athlete is used to doing can suddenly put too much pressure on the knee joint, causing the ACL, a rope-like band of tissue, to tear or break apart.

Teenage girls are 2 to 10 times more likely than boys to tear an ACL. That's because girls have different risk factors that make ACL injuries more likely, such as body shape, limb alignment, neuromuscular control, and hormones that might loosen the ligament.

ACL injuries can be very painful, especially during cutting and pivoting movements. Depending on the age of the child and the severity of the injury, a torn ACL often benefits best from surgery as well as 6 to 12 months of rehabilitation. This treatment can help prevent a player from getting arthritis or further knee damage.

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 is located deep within the joint, behind the kneecap (patella), above the shinbone, and below the thighbone. 

Together with the PCL (posterior cruciate ligament), which crosses over it to form an "X," the ACL helps keep the knee stable while rotating.

Specifically, the ACL keeps the shinbone in place and prevents it from moving too far forward and away from the knee and thighbone. It also provides stability when rotating the shinbone.

Signs and Symptoms

Kids with a partially or completely torn ACL will definitely feel pain when the ACL tear happens. Afterward, they may or may not have symptoms, depending on the severity of the injury.

Most will have no problems with walking. But if they return to their sport, they may have some instability, feeling "wobbly" or unable to bear weight on the affected leg. Oftentimes there is pain, which can be very intense, and swelling of the knee joint, which can happen within 24 hours of the tear.

When the injury is happening, many kids report hearing a "pop" sound — the sound of the shinbone popping out of and back into place. Others also report the knee feels "less tight" or less compact than it was before.

A child who has injured a knee — whether out on the field or at home — should stop all activity (to prevent further injury) and seek immediate medical care. In the meantime, keep the area iced — place the ice in a plastic bag, wrap the bag in a cloth, and hold it to the knee for up to 20 minutes at a time. Also, keep the knee elevated as much as possible to reduce swelling. If putting weight on the knee is painful, a child should not bear weight on the knee.


At the doctor's office or emergency room, doctors will perform physical exams and imaging tests to determine if there is a knee injury and, if so, how severe it is. 

These tests can help diagnose an ACL injury:

  • Lachman test. During this exam, a child 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 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. 

To properly diagnose ACL injuries, doctors usually perform these tests in combination with or in addition to other physical exams.

While X-rays might be taken to determine the extent of the injury, they only image bone and, therefore, can only confirm the presence of bone fractures in the knee. An MRI, which images tissue (like ligaments and muscles), can confirm a partial or complete ACL tear so some doctors will order one to confirm a diagnosis.


Treatment  of ACL injuries depends on the age of the child and the type of injury, but most often involves some type of surgery. If a child is still growing, surgeons make sure not to touch the growth plates. Growth plates are the developing tissue on the ends of long bones, like the tibia and femur. Kids who are still growing have "open" growth plates, while those who have reached skeletal maturity have "closed" growth plates.

When a child stops growing, the growth plate hardens (ossifies) along with the rest of the bone. Girls tend to stop growing earlier than boys; their growth plates usually close around ages 14 to 15, while boys' growth plates close later, at around ages 16 to 17.

If a child has 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 tissue from the main patellar tendon or hamstring) or with tissue donated from someone else (called an allograft). The new ACL tissue is put into the body through the tunnels created in the tibia and femur and is secured in the proper area with screws or other fixtures.

If a child is young, surgeons will use a technique that spares the growth plate. One example is iliotibial (IT) band reconstruction. The IT band is a thick band of tissue that spans the leg from the lower pelvis to the bottom of the shinbone. Tightening this tissue over the kneecap helps to stabilize the knee and prevent the tibia from moving too far forward. 

After surgery, a child will need to walk with the assistance of 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 is a lengthy process that 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 (and prevent atrophy, the breakdown of muscle tissue)
  • reduce pain and swelling  
  • improve balance

Most kids undergo rehab at a center three times a week, with daily exercises they practice at home. Accelerated rehab programs involve more frequent therapy, but they won’t necessarily speed recovery.

In the early stages of recovery, a child may receive a leg brace and subsequent knee brace, depending on the surgeon’s recommendations. Keeping the knee iced and elevated can help to reduce swelling. Doctors may prescribe prescription painkillers and anti-inflammatory medicine to help kids deal with the pain and feel more comfortable. In some cases, doctors may give people steroid injections to ease pain and help reduce swelling.

While most sports are off limits — especially the activity that caused the injury in the first place — kids can do some low-impact activities that may be fun and even therapeutic, like swimming, bike riding, or protected running. Talk to your doctor about what activities might benefit your child.

Helping Your Child Cope

Being told that you can't do the things you love — like running or playing football, field hockey, or softball — can be a devastating blow to any child. Kids recovering from an ACL injury may feel angry, frustrated, or even depressed, especially if they're no longer participating in team sports with their 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 the games may help. If your child doesn't want to do these, suggest starting a hobby, like playing the guitar, painting, drawing, or another activity that won't put too much strain on the knee. Low-impact activities, like swimming, are another option.

In time, kids can return to the things they love. But if your child continues to feel angry or depressed during recovery, encourage him or her to talk to a school psychologist or counselor, who can help your child cope and look ahead to better days.

Date reviewed: October 2015