A torn anterior cruciate ligament (ACL) is a serious 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 get ACL injuries usually play contact sports (like football) or sports that feature swift, abrupt movements such as pivoting, stopping, or turning on a dime. People also can tear an ACL when they do movements they’re used to doing all the time, like jumping and landing hard on the feet. If the quadriceps muscles aren’t strong enough, a movement a player is used to doing can suddenly put too much pressure on the knee joint and cause the ACL to tear or break apart.
Teen girls are 2 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 during sports. Depending on the person's age and the severity of the injury, a torn ACL usually heals best with surgery in addition to 6 to 12 months of rehabilitation. Getting surgery can prevent a player from getting arthritis or other knee damage later on.
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.
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.
Here's what most people notice after tearing an ACL:
pain, which may be intense
inability to put weight on the affected leg
swelling in the knee joint within 24 hours of the tear
Many people report hearing a "pop" sound when the injury happens. This is the sound of the shinbone popping out and back into place 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. If it feels painful, 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.
Doctors highly recommend surgery for teens with ACL injuries. If a person is still growing, surgeons make sure not to touch 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 (tissue from the main patellar tendon or the hamstring) or with tissue donated from someone else (called an allograft). The new ACL tissue is fed through the tunnels created in the tibia and femur 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
Most people do 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 doctor may recommend a leg brace or knee brace. Keeping the knee iced and elevated can help to reduce swelling. Doctors usually prescribe painkillers and anti-inflammatory medicine to help you deal with the pain and feel more comfortable. In some cases, doctors may give you 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 — 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.
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.