ACL tear or knee instability

An anterior cruciate ligament (ACL) tear is a common knee injury that affects stability and confidence during movement. The ACL is one of the primary ligaments that support the knee, helping control forward motion and rotation of the lower leg. When it's stretched or torn, the knee can feel unstable, weak or prone to giving way, especially during pivoting, cutting or sudden changes in direction.

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Illustration of a human knee joint with labeled parts including quadriceps, femur, patella, and ligaments.

Anatomy of the knee

The knee joint is supported by four major ligaments that connect the femur (thighbone) to the tibia (shinbone). The ACL runs diagonally through the center of the knee and works closely with the posterior cruciate ligament (PCL) to stabilize forward movement and rotational forces.

The ACL plays a critical role in activities such as running, jumping, landing and changing direction. When it's damaged, the knee can lose its ability to remain stable during both sports and everyday movement. ACL injuries range in severity, from mild stretching to a complete tear, and without proper treatment and rehabilitation, an ACL tear can lead to ongoing knee instability and additional joint damage.

Overview

Symptoms

You may need evaluation for an ACL injury if you experience:

A popping sensation at the time of injury
Immediate swelling within the first few hours
Pain or limited range of motion
A feeling that the knee may give out during movement
Difficulty returning to sports or physical activity

Persistent instability or swelling after an injury is a sign that specialized orthopedic care may be needed.

Causes

ACL injuries most often occur during sports or physical activity. Common causes include:

Sudden changes in direction or pivoting with the foot planted
Rapid deceleration or stopping while running
Incorrect landing from a jump
Twisting or hyperextending the knee
Direct contact or collision, such as during football or soccer

While athletes are at higher risk, ACL tears can also result from falls or accidents during daily activities.

Risk factors

Certain factors can raise your risk of an ACL tear, though having one or more of these does not mean an injury will happen:

Participation in pivoting or cutting sports, such as soccer, basketball or football
Sudden increases in activity level or intensity
Previous knee injury or instability
Muscle imbalances or limited conditioning around the knee

Knowing your personal risk factors can help you and your care team make informed decisions about activity and prevention.

Diagnosis

Diagnosis begins with a detailed discussion of your symptoms, activity level and medical history, followed by a thorough physical exam to assess knee stability, swelling and motion.

A variety of imaging studies can help confirm the diagnosis. X-rays help rule out fractures, while MRI scans provide detailed images of the ACL and surrounding cartilage, menisci and ligaments. In select cases, additional testing or arthroscopy may be recommended to fully evaluate the knee.

A medical illustration of a knee joint with an ACL tear.

An overview of ACL injuries

See the different types of knee injuries that can cause an ACL tear, and how those injuries can affect your knee joint.

Videos in our patient education library are available to help you gain a general understanding of the condition or procedure. Read disclaimer.

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