Symptoms of a torn anterior cruciate ligament (ACL)

Maybe you've heard a disconcerting 'pop' sound, followed by a feeling of weakness in your knee? These may be signs of a torn anterior cruciate ligament (ACL), a common but potentially serious knee injury that affects many people each year.

The ACL plays a crucial role in your knee's stability, and when it's damaged, it can have a big impact on your mobility and quality of life. Therefore, understanding the symptoms of a torn ACL is vital for early diagnosis and proper treatment.

Understanding your ACL

Before we go into the symptoms of a torn ACL, it's essential to understand what the ACL is and its role in your knee's function. This knowledge will help you appreciate why an ACL injury can have such a significant impact on your mobility and overall knee health.

What is your ACL?

The anterior cruciate ligament (ACL) is one of four major ligaments in your knee joint. Ligaments are tough bands of tissue that link bones together, providing stability and controlling movement. The ACL is a key structure in your knee, running diagonally through the centre of your joint.

The ACL connects your thigh bone (femur) to your shinbone (tibia). It's not just a simple connective tissue; the ACL is a complex structure made up of densely packed collagen fibres. These fibres are organised in a specific way that allows your ligament to bear significant stress and provide crucial support to your knee joint.

Immediate symptoms of a torn ACL

When you tear your ACL, the signs often show up immediately and can be quite alarming. Understanding these symptoms can help you recognise when you might have suffered an ACL injury and need to seek prompt medical attention.

The ‘popping’ sound

One of the most distinctive symptoms of a torn ACL is a loud ‘pop’ or ‘popping’ sensation in the knee at the moment of injury. This sound isn't always audible to others, but many people claim to hear or feel this pop when their ACL tears. However, it's important to know that while this symptom is common, not everyone experiences it. Some people might describe it more as a ‘snapping’ or ‘cracking’ sensation.

The pop occurs because the ACL, which is under tension, suddenly gives way. Imagine the sound of a taut rubber band snapping - that's similar to what happens when your ACL tears. This sound is often the first indication that a significant injury has occurred.

Sudden pain and discomfort

Immediately after the popping sensation, you're likely to experience severe pain in your knee right away. This pain can range from a sharp, intense pain to a deep, aching discomfort. The intensity of the pain can differ by person and may depend on the degree of the tear.
The pain is usually localised to the knee joint, but some patients may experience it throughout the whole leg. However, sometimes the pain may not be as severe as you might expect for such an injury, especially in the case of partial tears. But don’t be fooled– even if the pain appears to be mild at the beginning, it is vital to take the injury seriously.

Rapid swelling

Another hallmark symptom of a torn ACL is rapid swelling of your knee. This swelling normally occurs in the first few hours after the injury, often within just 30 minutes to two hours. The swelling is caused by bleeding within the joint, as the torn ACL bleeds into the knee space.

The degree of inflammation can be quite dramatic, causing your knee to balloon noticeably. You might see that your injured knee looks much bigger than your uninjured knee. This swelling can be uncomfortable and may contribute to stiffness and difficulty moving the joint.

Loss of range of motion

As the knee swells and pain sets in, you'll likely experience a significant loss of range of motion in your knee. You may find it difficult or impossible to fully straighten your leg or bend it completely.

This restriction in movement is partly due to the swelling, which physically limits the joint's ability to move, and partly due to pain, which makes movement uncomfortable.

Trying to move your knee might also cause a feeling of instability or like something is ‘catching’ inside the joint. This sensation can be quite disconcerting and is another sign that something is seriously wrong with your knee's internal structure.

Ongoing symptoms of a torn ACL

While the immediate symptoms of a torn ACL can be quite dramatic, there are also longer-lasting symptoms that persist beyond the initial injury. These ongoing symptoms can significantly impact your daily life and activities.

Instability and ‘giving way’ sensation

The most common symptom that is usually present even after the injury of the ACL is the sensation of the knee buckling or ‘giving way’. This occurs because the ACL, which normally prevents the shinbone from sliding too far forward, is no longer functioning properly.

You might experience this instability when trying to change direction quickly, or even everyday movements like walking downstairs. Your knee might suddenly buckle or feel like it's going to collapse under you. This instability can be alarming and may cause you to lose confidence in your knee's ability to bear your weight.

Persistent pain and tenderness

While the intense, acute pain of the initial injury may subside, many people with torn ACLs experience ongoing pain and tenderness in the knee. This pain can vary in intensity and may be aggravated by certain activities or movements.

You might feel a dull ache throughout the day, which worsens with activity. The knee may also be tender to the touch, while some people describe a feeling of pressure or fullness in the knee, which can be uncomfortable.

Difficulty with weight-bearing activities

A torn ACL can make it challenging to bear weight on the affected leg. You might find it painful or unstable when you stand for long periods, walk, or climb stairs. Running, jumping, or any high-impact activities are likely to be particularly difficult and may exacerbate your symptoms.

This difficulty with weight-bearing can be quite a problem in your day-to-day life, as you may find it difficult to carry out routine activities or engage in sports and exercise. You may notice that you are walking with a limp or overcompensating for the knee by shifting weight onto the uninjured leg.

Changes in gait and movement patterns

As you try to compensate for the pain and instability in your knee, you might develop altered movement patterns. You may start to walk differently, perhaps with a limp or by keeping your knee slightly bent to avoid fully extending it.

These changes in your gait can lead to secondary issues, such as pain in other parts of your body. For example, you might develop hip or back pain as your body tries to adapt to the new way of moving. It's important to address these altered movement patterns during rehabilitation to prevent long-term complications.

Differentiating between partial and complete ACL tears

ACL tears can range from minor, partial tears to complete ruptures of the ligament. The symptoms can vary depending on the severity of the tear, although it's important to note that even a partial tear can cause significant symptoms and instability.

Symptoms of partial ACL tears

In a partial ACL tear, some of the ligament fibres remain intact. The symptoms of a partial tear can be like those of a complete tear, but they may be less severe. You might experience:

• Less dramatic swelling
• Some instability, but perhaps not as pronounced as with a complete tear
• Pain, particularly with certain movements
• Ability to bear some weight on the affected leg, although it might be uncomfortable

It's worth knowing that even with a partial tear, you may still hear or feel a ‘pop’ at the time of injury. The knee might also still feel unstable, especially during activities that put stress on the ACL.

Symptoms of complete ACL tears

To remind us, a complete ACL tear involves a full rupture of the ligament, and the symptoms are typically more severe and may include:

• A loud ‘pop’ or "popping" sensation at the time of injury
• Severe pain and rapid, significant swelling
• Marked instability of the knee, with a feeling that your knee might ‘give way’ completely
• Difficulty bearing any weight on your affected leg
• Possible visible deformity of the knee joint

With a complete tear, you're more likely to experience a dramatic onset of symptoms and may find it very difficult to continue any activity after the injury occurs.

Don’t hesitate to get treatment for a torn ACL

Recognising the symptoms of a torn ACL is crucial for ensuring timely and appropriate treatment. Remember, these symptoms may be mild or severe, and not everyone will have all the symptoms. However, if you think you've torn your ACL, it's vital to seek prompt medical attention. Early diagnosis and treatment can significantly improve your outcomes, helping you return to your normal activities and reducing the risk of long-term complications.

Concerned you might have torn your ACL? Don't wait for symptoms to worsen.

Book an appointment online today with Circle Health Group or give us a call directly and we'll connect you with a specialist who can provide expert diagnosis and treatment options for your knee injury.

Our team of experienced surgeons and physiotherapists are here to guide you through every step of your recovery, from the initial diagnosis to personalised treatment and comprehensive rehabilitation.