Diagnosis & Immediate Assessment
Understanding the injury is the first step towards appropriate management. ACL injuries can affect knee stability and may occur alongside damage to the meniscus, cartilage or other ligaments.
What is the ACL?
The anterior cruciate ligament (ACL) is one of the major ligaments inside the knee. It connects the thigh bone (femur) to the shin bone (tibia) and helps control forward movement and rotation of the knee.
The ACL is particularly important during turning, pivoting, jumping, landing and sudden changes of direction.
For more information about the ligament, injury mechanisms and associated damage, see ACL Tear.
How do ACL injuries occur?
ACL injuries commonly occur when the knee is subjected to sudden twisting, pivoting or landing forces. They may occur during sport, workplace activities, falls or accidents.
Common injury mechanisms include:
- Sudden changes of direction
- Pivoting with the foot planted
- Landing awkwardly after a jump
- Sudden stopping or deceleration
- Hyperextension of the knee
- Direct impact or collision
Many sporting ACL injuries occur without direct contact with another player.
What are the signs of an ACL tear?
Common symptoms include a popping sensation at the time of injury, knee pain, rapid swelling, reduced movement and difficulty continuing activity.
Some people experience instability or episodes where the knee gives way during turning or pivoting.
Being able to walk after an injury does not necessarily rule out an ACL tear.
For a more detailed explanation of immediate and delayed symptoms, see ACL Tear Symptoms.
What should you do immediately after an ACL injury?
Stop the activity and avoid movements that cause further pain or instability.
Early management may include relative rest, ice for comfort, compression, elevation and crutches if walking is painful or difficult.
Medical assessment is recommended when there is significant swelling, restricted movement, pain or instability.
Seek urgent medical assessment if the knee is visibly deformed, completely locked, severely painful or cannot safely bear weight. Numbness, altered circulation or a cold foot also warrant urgent assessment.
How is an ACL tear diagnosed?
Diagnosis usually involves discussing how the injury occurred, examining knee movement and stability, and considering imaging when appropriate.
MRI can help confirm an ACL injury and identify associated damage to the meniscus, cartilage or other knee ligaments. X-rays may be used to assess for fractures or other bone injuries.
For more information about clinical examination, imaging and MRI findings, see ACL Injury Diagnosis and MRI.
Partial versus complete ACL tears
An ACL injury may involve only some ligament fibres or result in complete rupture.
| Injury | Description |
|---|---|
| Partial ACL tear | Some fibres remain intact. The knee may retain adequate stability or may experience instability during activity. |
| Complete ACL tear | The ligament is fully disrupted and may no longer adequately control abnormal knee movement. |
The MRI appearance alone does not determine treatment. Functional stability, symptoms, associated injuries and activity goals are also important.
For more information about injury severity and management, see Partial ACL Tear.
Associated meniscus and cartilage injuries
ACL tears may occur together with damage to the meniscus, articular cartilage or other knee ligaments.
The menisci are fibrocartilage structures that help distribute load within the knee. Articular cartilage is the smooth tissue covering the joint surfaces.
Identifying associated injuries is important because they can influence treatment recommendations, rehabilitation and long-term knee function.
For more information, see
Meniscus Tears.
