Classic Signs of an ACL Injury: What You Need to Know

Anterior cruciate ligament (ACL) injuries are among the most common and serious knee injuries, particularly in athletes and physically active individuals. The ACL is a key ligament that stabilises the knee joint, especially during activities that involve sudden stops, changes in direction, or jumping. Recognizing the classic signs of an ACL injury early is critical for prompt diagnosis, appropriate treatment, and optimal recovery.

What Is the ACL and Why Is It Important?

The ACL runs diagonally through the centre of the knee, connecting the femur (thigh bone) to the tibia (shin bone). Its primary role is to prevent the tibia from sliding forward and to provide rotational stability to the knee. When the ACL is injured—either partially or completely torn—knee stability is compromised.

1. A Popping Sound or Sensation

One of the hallmark signs of an ACL injury is a sudden “pop” felt or heard at the moment of injury. Many individuals report an audible popping sound or a distinct sensation within the knee. This often occurs during non-contact movements such as pivoting, cutting, or landing awkwardly from a jump.

2. Immediate Pain and Inability to Continue Activity

ACL injuries are typically associated with immediate pain, although the intensity can vary. In many cases, the pain is significant enough to prevent the individual from continuing the activity or sport. The knee may feel unstable or unable to support body weight.

3. Rapid Swelling of the Knee

Swelling that develops quickly—often within one to two hours—is a classic sign of an ACL tear. This rapid swelling is usually due to bleeding within the joint (haemarthrosis) and distinguishes ACL injuries from more minor knee strains, which tend to swell more gradually.

4. Knee Instability or “Giving Way”

A feeling that the knee is unstable or “giving out” is a common symptom following an ACL injury. This instability is particularly noticeable during activities that involve turning, pivoting, or descending stairs. Even during normal walking, some individuals feel a lack of confidence in the knee’s support.

5. Reduced Range of Motion

Following an ACL injury, knee stiffness and limited range of motion are common. Swelling, pain, and joint irritation can make it difficult to fully bend or straighten the knee. This limitation may persist until inflammation is addressed and proper treatment begins.

6. Difficulty Bearing Weight

While not universal, many individuals experience difficulty bearing weight on the affected leg immediately after injury. This may be due to pain, swelling, or the sensation of instability within the joint.

When to Seek Medical Evaluation

Any combination of popping, immediate swelling, pain, or instability following a knee injury warrants prompt medical evaluation. A healthcare professional may use a physical examination, imaging studies such as MRI, and clinical history to confirm an ACL injury and rule out associated damage to the meniscus or other ligaments.

The Importance of Early Diagnosis

Early recognition and diagnosis of an ACL injury allow for timely management, whether through physical therapy, bracing, or surgical intervention. Delayed treatment can increase the risk of further knee damage, including cartilage wear and meniscal tears.