Anterior Cruciate Ligament (ACL) Injury

What is an ACL injury?

An ACL injury is a common knee injury that happens when the anterior cruciate ligament (ACL) is sprained or torn, typically during a sporting event or practice.

The ACL is band of tissue that connects the shinbone (tibia) to the thighbone (femur) and provides stability to the knee joint. The ligament is flexible up to a point, but can be injured if it is stretched too far. This typically happens as a result of twisting, cutting, and decelerating during sports.

Meet Sophia

She loves racing downhill at top speed. But after two ACL injuries, she had to recover both physically and emotionally.

The most common type of ACL injury in children is a complete ACL tear. This often happens in combination with other injuries, such as a torn meniscus. Usually, surgery is recommended to repair the knee. The type of surgical procedure depends on the patient’s age and their stage of growth.

Patients who have torn an ACL are at high risk of repeat injury. The injury increases the risk of osteoarthritis in early adulthood. You can reduce your risk of an ACL injury or re-injury by developing your overall fitness, especially leg strength and balance. Proper athletic technique, such as positioning and landing with bent knees, can also reduce your risk.

Where is the ACL located in the anatomy of the knee?

A look at the anatomy of the anterior cruciate ligament, or the ACL.

The ACL is one of four main ligaments in the knee that attach the thigh bone (femur) to the shin bone (tibia). The kneecap (patella) located in the front of the knee, protects the ACL and other knee ligaments.

The ACL and another ligament called the posterior cruciate ligament (PCL) run through the center of the knee. These ligaments prevent the shin bone from sliding too far forward or backward under the thigh bone.

Two other ligaments run along either side of the knee, the medial collateral ligament (MCL) and lateral collateral ligament (LCL). These ligaments prevent the knee from bending too far to either side of the leg.

The meniscus provides padding and shock absorption for the knee. There are two menisci in each knee. Without these wedge-shaped pieces of cartilage, the thigh bone and shin bone would rub painfully against each other.

What are the symptoms of an ACL injury?

If you have injured your ACL, you probably heard a popping noise at the moment the ligament was sprained or torn.

Other symptoms of an ACL injury include:

  • sudden instability and feeling that the knee cannot support your weight
  • knee pain so severe you can’t continue playing
  • swelling around the knee within 24 hours
  • reduced range of motion
  • joint tenderness
  • bruising around the knee

What causes ACL injuries?

ACL injuries typically happen while playing sports. Athletic moves that strain the knee include:

  • Changing direction rapidly: A rapid turn or “cut,” common games like soccer, football, field hockey, lacrosse, and rugby, can put extreme pressure on the ACL and other ligaments in the knee.
  • Slowing down or stopping suddenly: Sprinting, then slowing down or coming to a complete stop can put a twisting force (torque) on the knee.
  • Jumping and landing: In any activity that involves jumping, a hard or awkward landing can damage the ACL. This includes sports like volleyball, basketball, figure skating, and gymnastics.
  • Contact and collisions: The ACL is at risk when an athlete’s knee is hit by or collides with another player or object. This is why ACL tears are common in contact sports

A physician examines a young patient’s knee for a possible ACL injury.

Who is at risk of a torn ACL?

Teenage athletes have the highest rate of ACL injuries, largely because they are the most physically active age group. Girls are five to eight times more likely to tear an ACL than boys. There are several possible reasons that ACL injuries affect more girls than boys:

  • Unless they target their calves as part of their training, girls don’t tend to develop as much lower leg strength as boys as they grow.
  • Girls tend to have more strength in the muscles in the front of their thighs (quadriceps) than in the muscles in the back of their thighs (hamstrings). This imbalance puts stress on their knees.
  • Girls tend to land with straighter knees than boys, which reduces the effectiveness of their muscles as shock absorbers.
  • Girls often let their knees drop inward during maneuvers that involve cutting, pivoting, or landing, which can put more stress on the ligaments in their knees.

With focused training, any athlete can increase the stability of their knees and reduce their risk of ACL injury.

How serious are ACL injuries?

An ACL tear is a serious injury that can end an athlete’s season. Fortunately, treatment of ACL injuries has improved, along with the rate of recovery.

Most patients start to use an exercise bike right after surgery, begin physical therapy two weeks after surgery, and begin jogging three months after surgery. More than 90 percent of injured athletes are cleared to return to sports after nine months, often with a custom knee brace. The brace is usually worn for one to two years after surgery.

How we care for ACL injuries

The ACL Program at Boston Children’s Hospital combines expertise in ACL repair and recovery to help injured athletes recover. Our team of orthopedic surgeons and sports medicine specialists are experts in caring for athletes of all ages with ACL injuries and ACL tears, from aspiring novices to Olympic competitors. We work as a team with patients and families to help athletes come back as strong, or stronger, than they were before their injury.