Children's Hospital Colorado

Anterior Cruciate Ligament (ACL) Injury

What is an anterior cruciate ligament (ACL), and where is it located?

The anterior cruciate ligament (ACL) is a commonly injured ligament in the knee. Ligaments are tough bands of tissue that connect the ends of bones together.

The ACL is located in the center of the knee joint, where it runs from the backside of the femur (thigh bone) to connect to the front of the tibia (shin bone). It has two main functions:

  • To prevent the tibia from sliding forward on the femur
  • To prevent the tibia and femur from rotating out of position during various sports movements, such as cutting, twisting and landing activities

What is an ACL tear?

The ACL plays an especially important role when performing activities that require quickly changing direction, such as soccer, football and basketball. If the knee receives too much stress, the ACL could be strained or completely torn.

Athletes who tear their ACL sometimes report a sensation of their knee "giving out." Short term, this makes it difficult to play sports. Long term, it is believed that this "giving out" causes damage to other structures in the knee, including the cartilage, which can cause arthritis.

Orthopedic doctors at the Orthopedics Institute at Children's Hospital Colorado believe that surgical reconstruction is the best treatment for long-term preservation of the knee and for young athletes to make an early return to sports.

Are girls more likely to tear their ACL than boys?

Anyone can suffer an ACL injury; however, proper training can help athletes minimize their risk. Females are approximately six times more likely to tear their ACL than males, due to reasons ranging from differences in bone structure to muscle imbalances.

Have ACL tears become more common?

Research has shown that about one in every 60 young athletes will experience an ACL tear at some point during their athletic career. Unfortunately, these numbers are increasing due to the increased intensity of sports.

What causes an ACL tear?

Impact to the outside of the knee or sudden changes in direction can stretch, fray or tear the ACL completely. Too much strain on the knee joint from sharp pivots or quick stops can also injure the ACL. An ACL injury can also occur without any contact.

Can ACL tears be prevented?

Prevention starts with understanding the importance of balancing the strength of the muscles around your knee and core. The ACL is at highest risk when the hamstrings do not fire with enough force or at the right timing to protect the ACL. Good muscle strength and coordination minimizes injuries – this includes strength in your hamstrings, quadriceps and your core (abdominal and gluteal muscles).

Teenage athlete Erin, who tore her ACL, hears about what she can expect on her ACL surgery and recovery journey.

The Connection Journey: Healing the Whole Athlete

Carolyn knows the challenge of sports injury recovery. She tore her ACL last year, but is now back on the field and sharing her journey with another family.

What are signs and symptoms of an ACL injury?

Many anterior cruciate ligament (ACL) injuries happen while playing sports, but not necessarily from a hard collision. In fact, most ACL injuries are what we call "non-contact," meaning they occur during a sudden change in direction, not when struck by another player.

Outside of traditional sports, ACL injuries can occur whenever someone is doing something active – at a playground, riding a bike or playing tag. ACL tears are also often caused by from car accidents or other traumatic events.

Athletes who have suffered an ACL injury typically experience rapid swelling of the knee within 24 hours of the injury. Some may be able to walk after the injury, or even continue to play their sport. Many people report that they hear or feel a "pop" in their knee when the injury occurs. As the swelling subsides over the following days or weeks, most people notice that their knee feels unstable or "gives out."

How is an ACL tear diagnosed?

ACL injuries are diagnosed by a physical examination to assess the stability of the knee. X-rays are important to distinguish between a torn ACL or a fracture, as they can have similar symptoms. Magnetic resource imaging (MRI) is also helpful because it helps check for damage to other ligaments and cartilage in the knee.

Learn more about knee X-rays and other common orthopedic tests.

How is an ACL injury treated

Although non-surgical treatment – such as knee-bracing and physical therapy – is an option, it is generally not the treatment of choice for young people. Young people who choose not to undergo surgical reconstruction tend to experience continued episodes of their knee "giving out" and long-term knee problems.

In general, doctors at Children's Hospital Colorado recommend a knee operation to make a new ACL out of another tendon. There are several different choices for the new ACL, which your child's surgeon will discuss with you.

What is ACL surgery?

The goal of ACL surgery is to return the knee to normal function and stability. During ACL surgery, an orthopedic surgeon will create a new ligament from your child's own tissue (called a graft) to reconstruct your child's ACL. Our pediatric specialists will use both local and general anesthesia to keep your child comfortable during the procedure.

Your child will likely be cleared to return home a few hours after the ACL procedure. They will need to be on crutches for 2 to 6 weeks after surgery until they gain muscle control and are able to walk without pain in the brace. Your child's physical therapist or physician will let you know when they are ready to walk without the brace and crutches.

Physical therapy (PT) is an important component of your child's recovery before returning to sports. PT should start 4 to 5 days after surgery and will last at least 9 to twelve months. Learn more about ACL injury rehabilitation.

Your child's care team will also work with you to schedule a series of follow-up appointments after surgery.

Watch this video to learn about ACL surgery at Children's Colorado.



What are the different graft options for ACL surgery

Our sports medicine specialists use tissue from your child's own body to reconstruct their ACL. This is called an autograft, which research shows adapts more quickly, predictably and with a much higher rate of success compared to alternatives.

The two most common autograft options use either two of the hamstring tendons or a part of the patella (a fancy word for knee) tendon. At Children's Colorado, we also offer a third option, called the quadriceps tendon autograft (also called the quad-tendon graft for short). Learn more about the different ACL graft options.

Is ACL surgery different for children and teens?

An important consideration for ACL surgery in children and adolescents is that their bones are still growing. Bone growth comes from the growth plates, which are at the ends of the bones and increase the length of the bone.

Traditional ACL surgery puts these structures at risk for causing a limb length discrepancy or a deformity. At Children's Colorado, we specialize in surgical techniques to minimize this risk and maximize a young athlete's return to competitive sports and recreation.

What is the recovery process after ACL surgery?

To complete ACL rehabilitation thoroughly and safely, our sports medicine experts use a 9 to 12 month average timeframe for returning an athlete to sport. However, this process and timing will vary for each athlete.

If you break our rehabilitation approach down to a simple process, the physical components of ACL recovery and preparation for return to sport are:

  1. Get rid of the swelling in the injured knee
  2. Get the motion back in the joint
  3. Make the quadriceps work again
  4. Walk normally
  5. Recover the strength in the entire leg to be comparable with the other leg
  6. Re-train balance, agility, symmetry of movement and the ability to move with explosive, quick patterns
  7. Recover foot speed and every other skill your particular sport requires

See a full summary of our ACL rehabilitation timeline to learn more about the recovery process.

Helpful resources for an ACL injury:


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