The anterior cruciate ligament, or ACL, is part of the knee. Damage to the ACL is one of the more common, serious, and painful knee injuries.

Three bones — the shinbone, thighbone, and the patella — come together at the knee. Four ligaments help hold these bones in the right alignment, while tendons connect the bones to muscle. Cartilage acts as a shock absorber and promotes easy movement.

The ACL is one of two cross-shaped ligaments that connect the thighbone to the shinbone and help stabilize the knee.

Learn about the causes and symptoms of an ACL injury, as well as treatment options, here.

a soccer player suffering from an ACL injuryShare on Pinterest
An ACL injury frequently occurs during active sports.

An ACL injury develops when the ACL, a ligament that links the thighbone and shinbone, stretches beyond its capacity and tears.

This type of injury frequently occurs during active sports that involve a lot of jumping and rapid starts and stops. More than 70% of ACL injuries occur without any contact or blow to the knee.

ACL injuries affect roughly 1 in 3,000 people in the United States. Many people who sustain this type of injury are young and active. ACL injuries are also more common in females than in males, even among those who play the same sport.

When a person has an ACL injury, it is fairly common for there to be injuries to other parts of the knee, including the cartilage, tendons, and bones.

an infographic of an ACL injury

Along with significant pain, a key sign of an ACL injury is a "popping" sound, which occurs at the moment that the ACL is torn or stressed.

The symptoms of an ACL injury can include:

  • pain
  • swelling
  • difficulty or inability to extend the knee
  • discomfort when walking
  • soreness around the knee
  • an inability to put weight on the leg
  • a feeling that the knee may give out

Although an individual of any age and fitness level can injure their ACL, these injuries usually occur during activity and in connection with:

  • sudden starts, stops, or changes of direction while moving
  • a blow to the knee, especially from the side
  • overstretching the knee

ACL injuries are more common during sports that involve a lot of twisting and turning, such as:

  • soccer
  • tennis
  • football
  • skiing
  • basketball

Doctors categorize ACL injuries by their severity, as follows:

  • Grade 1 sprains: At this level, the ACL can still keep the knee stable, but the ligament is overstretched.
  • Grade 2 sprains: Also called a partial tear, this involves the ACL stretching to the point that it becomes loose.
  • Grade 3 sprains: Also known as a complete tear of the ligament, this involves the ACL tearing in two and no longer controlling the kneecap.

According to the American Academy of Orthopaedic Surgeons, a partial tear is more rare than a complete tear.

A physician will assess the extent of the injury, evaluate the range of motion of the affected knee, and compare it to the other knee before making a diagnosis.

They may use X-rays to look for signs of bone damage or an MRI to conclusively identify a soft tissue injury, such as damage to the ACL.

After an ACL injury, a doctor may recommend:

  • elevating the foot above the level of the head
  • applying an ice pack (wrapped in a towel) to the knee
  • taking ibuprofen to reduce pain and swelling
  • seeing a doctor for evaluation and treatment

Approaches to treatment vary, depending on the severity of the injury and the age and fitness of the person.

For young and otherwise healthy people, a doctor may recommend surgery to ensure that the person can fully resume their activities.

Surgery for an ACL injury involves miniature cameras and small incisions. This less invasive procedure is called an arthroscopy.

Usually, a surgeon removes the damaged portion of the ACL and grafts a new ligament into place. The graft can come from the injured individual's body, from a donor, or it may be synthetic.

Researchers are studying new methods, such as bio-enhanced repair, to improve ACL surgery and reduce any associated risk of developing arthritis later.

If the doctor does not recommend surgery and the knee has retained stability, treatment may involve:

  • using crutches to keep weight off the knee
  • wearing a brace to support and further stabilize the knee
  • doing physical therapy to strengthen muscles in the legs and restore the full range of motion

For people who play sports, it can take 7–9 months before they are ready to return to their activities. During the recovery process, a person may need to use crutches or a knee brace.

Physical therapy is crucial for recovery from an ACL injury. Gentle stretches and strengthening exercises can help people:

  • ease pain and swelling
  • become more flexible and expand their range of motion
  • build strength around their knees and in their upper and lower legs
  • renew their sense of balance

While it is not always possible to prevent an ACL injury, the following tips may help:

  • learning the best techniques for jumping, landing, pivoting, and cutting
  • strengthening the leg muscles
  • strengthening and stabilizing the core, hips, and pelvis

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Physical therapy can help prevent future ACL injuries.

Although it can take many months to recover from an ACL injury, with the right treatment and follow-up care, most people are able to return to their regular activities.

However, anyone who has experienced an ACL tear has a greater risk of developing osteoarthritis in the injured knee.

To reduce the risk of future knee injuries, people can do:

  • physical therapy
  • strength training
  • neuromuscular training in jumping and turning, as well as balance and agility

ACL injuries are common but serious. Most of these injuries take place during sports that involve sudden starts and stops, pivoting, and jumping. Females are more at risk of ACL injuries than males.

Treatment for an ACL injury may be limited to physical therapy and knee support, but a doctor may recommend surgery for more active people.

Recovery from an ACL injury can take months, and it may be more than 7 months before a person can play sports again.

A person who has had an ACL injury faces a greater risk of knee injuries in the future. It may help to take precautions and speak to a physical therapist about ways to reduce this risk.