ACL injury

Learn about this injury that affects one of the main ligaments in your knee and most commonly occurs during sports such as soccer, basketball and football.

Update Date: 14.08.2026

Overview
Symptoms
Causes
Risk factors
Complications
Prevention
Diagnosis
Treatment
Preparing for an appointment

Overview

An anterior cruciate ligament (ACL) injury is a tear or sprain of one of the main ligaments in your knee. Ligaments are strong bands of tissue that connect one bone to another. The ACL is one of two ligaments that cross in the middle of the knee. It connects your thigh bone to your shinbone and helps keep the knee joint stable.

ACL injuries often happen during sports or activities that involve sudden stops, jumps, pivots or changes in direction. Examples include soccer, basketball, football and downhill skiing. Many people hear or feel a pop in the knee when an ACL injury happens. The knee may swell, feel unstable and become too painful to put weight on it.

Treatment depends on how serious your ACL injury is. Treatment may include rest and rehab exercises to help you regain strength and stability. Or it may include surgery to replace the torn ligament followed by rehab. A proper training program may help reduce the risk of an ACL injury.

ACL injury

The anterior cruciate ligament (ACL) is one of the key ligaments that help stabilize the knee joint. The ACL connects the thigh bone, called a femur, to the shinbone, called a tibia. ACL tears happen most commonly during sports or activities that involve sudden stops, jumps, pivots or changes in direction.

Symptoms

An ACL injury can happen suddenly. Common symptoms of an ACL injury may include:

  • A loud pop or a popping feeling in the knee.
  • Serious pain.
  • Rapid swelling.
  • Loss of range of motion.
  • Knee instability or the feeling that the knee "gives out."

When to see a doctor

Seek care right away if any injury to your knee causes symptoms of an ACL injury. The knee joint is a complex structure of bones, ligaments, tendons and other tissues that work together. Often, other parts of the knee such as the cartilage, meniscus or other ligaments are injured along with the ACL. It is important to get a prompt and accurate diagnosis to determine how serious the injury is and get proper treatment.

Causes

ACL injuries can happen in both contact and noncontact sports or activities. ACL injuries often happen during movements that can put stress on the knee, including:

  • Suddenly slowing down and changing direction.
  • Pivoting with your foot firmly planted.
  • Landing awkwardly from a jump.
  • Stopping suddenly.
  • Receiving a direct blow to the knee or having a collision, such as a football tackle.

When the ligament is damaged, there is usually a partial or complete tear of the tissue. A mild injury may stretch the ligament but leave it intact.

Risk factors

Certain factors can increase your risk of an ACL injury, including:

  • Being female. Women and girls are at higher risk of ACL injury than males, possibly due to differences in body structure, muscle strength and hormones.
  • Playing certain sports. Sports such as soccer, basketball, football, volleyball, gymnastics and downhill skiing have a higher risk of ACL injury.
  • Poor conditioning or fatigue.
  • Incorrect technique or muscle imbalances when jumping or landing.
  • Playing on artificial turf.
  • Using footwear that does not fit properly.
  • Using poorly maintained sports equipment, such as ski bindings that are not adjusted properly.
  • Having a previous ACL injury. If you have hurt your ACL before, you are more likely to injure it again.

Complications

People who have an ACL injury are more likely to develop osteoarthritis in the knee. Arthritis may happen even if you have surgery to reconstruct the ligament. The risk of arthritis depends on many factors, including how serious the original injury was, other related injuries in the knee, and how active you are after treatment.

Prevention

You can lower your risk of ACL injury with proper training and exercise. A sports medicine physician, physical therapist, athletic trainer or other specialist in sports medicine can assess how you move and offer instruction and feedback that can help you reduce your risk of a knee injury.

Programs to reduce ACL injury include:

  • Core exercises. Strengthening the hips, pelvis and lower abdomen to avoid moving the knee inward during a squat.
  • Leg exercises. Strengthening leg muscles, especially the hamstrings, to keep muscle strength balanced.
  • Jumping and landing training. Learning the right way to jump and land to protect the knees.
  • Pivoting and cutting training. Improving technique when making pivoting and cutting movements.

Training to strengthen muscles and to improve jumping and landing techniques may help to reduce the higher ACL injury risk in female athletes.

Gear

Wear the right shoes and padding for your sport to help prevent injury. If you downhill ski, make sure a trained professional adjusts your ski bindings so that your skis release properly if you fall.

Wearing a knee brace may not prevent ACL injury or reduce the risk of getting hurt again after surgery.

Diagnosis

Diagnosis of ACL injury begins with a physical exam and detailed history. During the exam, your healthcare professional may check your knee for swelling and tenderness and compares it with your other knee. Your healthcare professional also may move your knee into a variety of positions. This is done to check your range of motion and see how well the knee moves and works.

Often the physical exam is enough to diagnosis an ACL injury. But you may need more tests to rule out other issues and to see how serious the injury is. These tests may include:

  • X-rays. X-rays may be needed to rule out a bone fracture. However, X-rays do not show soft tissues, such as ligaments and tendons.
  • Magnetic resonance imaging (MRI). An MRI uses radio waves and a strong magnetic field to create images of both hard and soft tissues in your body. An MRI can show the extent of an ACL injury and any damage to other tissues in the knee, including other ligaments, cartilage and the meniscus.
  • Ultrasound. This test uses sound waves to visualize internal structures. Ultrasound may be used to check for injuries in the ligaments, tendons and muscles of the knee.

Treatment

Treatment for ACL injury depends on how serious the injury is, your activity level and the stability of your knee. Some injuries can be treated without surgery. Others need surgery to reconstruct the ligament.

Nonsurgical care

For partial tears or for people who are less active, treatment may include the R.I.C.E. method to lessen pain and swelling:

  • Rest. Take it easy and avoid putting weight on your knee as you heal.
  • Ice. Put ice on your knee for 20 minutes every two hours while you are awake.
  • Compression. Wrap your knee with an elastic bandage or compression wrap.
  • Elevation. Lie down and prop your knee up on pillows.

Treatment without surgery also may include:

  • Bracing. Wearing a brace to keep your knee stable.
  • Crutches. Using crutches to avoid putting weight on your knee while it heals.
  • Physical therapy. Doing exercises to restore motion and strengthen supporting muscles.
  • Activity modification. Avoiding movements that involve cutting or pivoting.

The goal of rehabilitation is to lessen pain and swelling, restore your knee's full range of motion, and strengthen your muscles. This approach can work well to treat an ACL injury for people who do not have instability in the knee. Nonsurgical treatment also can work well for people who are not very active, do moderate exercise, or play sports or do activities that put less stress on the knees.

Surgical reconstruction

If you have a complete ACL tear or are very active, you may need surgery. Your healthcare professional may recommend surgery if:

  • You are an athlete and want to keep playing sports, especially those that involve jumping, cutting or pivoting.
  • More than one ligament or the meniscus or cartilage in your knee also is injured.
  • The injury is causing your knee to give out during everyday activities.

Before surgery, you likely will do some physical therapy to strengthen your knee and improve your range of motion.

During ACL reconstruction, the surgeon removes the damaged ligament and replaces it with a piece of tendon. This is tissue similar to a ligament that connects muscle to bone. This replacement tissue is called a graft. Your surgeon uses a piece of tendon from another part of your knee or a tendon from a deceased donor.

Physical therapy

Physical therapy after surgery is important. Successful ACL reconstruction combined with rigorous rehabilitation can usually restore stability and function to your knee. A physical therapist teaches you exercises that you can do either with the therapist's help or at home. You also may wear a brace to stabilize your knee and use crutches for a while to avoid putting weight on your knee. Rehabilitation focuses on:

  • Early range of motion and pain control.
  • Gradual strengthening of leg and core muscles.
  • Balance and neuromuscular training to restore knee control.

There is no set time frame for athletes to return to play. Research shows that up to one-third of athletes may have another knee injury in the same or opposite knee within two years, especially with high-risk sports such as soccer. A longer recovery period may reduce the risk of reinjury.

In general, it can take a year or more before athletes can safely return to play. Your healthcare team and physical therapists test your knee's stability, strength, function and readiness to return to sports activities at various points during your rehabilitation. It is important to ensure that strength, stability and movement patterns are well-developed before you return to an activity with a risk of ACL injury.

Preparing for an appointment

Because ACL injury causes a lot of pain and makes the knee hard to move, many people seek medical attention right away. Others may make an appointment with their family healthcare professional. Depending on how serious your injury is, you may be referred to a healthcare professional specializing in sports medicine or an orthopedic surgeon, who specializes in bone and joint surgery.

What you can do

Before an appointment, be prepared to answer the following questions:

  • When did the injury happen?
  • What were you doing at the time?
  • Did you hear a loud pop or feel a popping sensation?
  • Was there much swelling afterward?
  • Have you injured your knee before?
  • Have your symptoms been constant or do they come and go?
  • Do any specific movements seem to improve or worsen your symptoms?
  • Does your knee ever lock or feel blocked when you are trying to move it?
  • Do you ever feel that your knee is not stable or not able to support your weight?

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