Knee injuries are common among 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 common types of knee injuries.

Kids who 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) are most likely to get ACL injuries.

The injury also happens when a child jumps and lands on the feet with knees straight or "locked" instead of flexed, putting excessive pressure on the knee joint and causing the ACL, a rope-like band, to tear or break apart.

Teenage girls are four times more likely than boys to tear an ACL, either from hormones that loosen the ligament and make it more susceptible to breaks or a natural inclination to land with straight knees.

ACL injuries can be very painful, causing a child to be unsteady on the feet and have difficulty walking. Depending on the age of the child 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 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 prevent the overextension of the knee joint from front-to-back or side-to-side.

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 may or may not have symptoms, depending on the severity of the injury.

Most will have some instability when walking, 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.

Many kids, especially those who are familiar with the injury or have torn that very same ligament before, report hearing a "pop" sound — the sound of the ligament tearing. Others also report the knee feels "less tight" or less compact than it was before.

an illustration of a knee with the ACL in between the top and bottom bones and the two parts of the ACL separating when a tear occurs

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 and elevated as much as possible to reduce swelling. Do not let the child 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.

In addition to tearing the ACL partially or completely, some kids (depending on their age and the rigidity of the bones) may tear off a portion of the top of the tibia along with the ACL.

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 back of the shin 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.
  • Pivot-shift test. As in the Lachman test, a child lies down flat, but instead of flexing the knee, flexes the hip at a 30-degree angle so the leg is pointed up, straight, and outward. The examiner grasps the ankle and underneath the shin bone and attempts to rotate the tibia to see if there is any interior rotation beyond the normal limit.
  • 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 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. Age is important because it will determine whether a child has intra-articular reconstructive surgery (which takes place inside the knee joint) or extra-articular surgery (which takes place outside the joint).

Two different approaches are needed because most kids who are still growing should not have intra-articular surgery due to the potential harm to 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.

A child who has reached skeletal maturity will probably undergo intra-articular reconstructive surgery. This procedure involves drilling a small tunnel down through the femur to reach the inside of the knee joint. This is the only way to get to the center of the knee, behind the kneecap. A second tunnel is made just below the knee, up through the tibia.

Surgeons replace the torn ACL with tissue from the patient's own body (usually a kneecap or hamstring tendon) 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.

Extra-articular surgery does not seek to replace a torn ACL (thus, it doesn't require tunneling of the tibia or femur), and instead focuses on tightening the iliotibial tract, 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 the severity of the injury.

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 require more frequent therapy and speed up recovery to 4 to 6 months.

In the early stages of recovery, a child will 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 kids 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 — 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 sedentary activity that won't put too much strain on the knee. Low-impact activities, like swimming, are another option.

In time, your child can again do the things he or she loves. But if, during recovery, your child continues to feel angry or depressed, consider talking to a school psychologist or counselor, who may be able to help your child cope and look ahead to better days.

Reviewed by: Kenneth Rogers, PhD, ATC | Date reviewed: June 2009