Common Signs of an ACL Injury and When to Seek Medical Attention

Knee injuries are common among individuals who play sports, exercise regularly, or have physically demanding jobs. One of the structures most frequently affected is the anterior cruciate ligament (ACL) — a key ligament that helps stabilise the knee. Recognising the early signs of an ACL injury can help prevent further damage and support appropriate treatment planning.

Understanding the ACL

The anterior cruciate ligament connects the thighbone (femur) to the shinbone (tibia). It plays a crucial role in keeping the knee stable during sudden movements such as pivoting, jumping, or changing direction.
 

ACL injuries often occur in sports like football, basketball, netball, or skiing — activities that involve quick stops or turns. However, they can also happen during routine movements if the knee twists unexpectedly or absorbs too much force.
 

When the ACL is overstretched or torn, the stability of the knee joint is affected, leading to pain, swelling, and reduced mobility.

Common Signs and Symptoms of an ACL Injury

1. A “Popping” Sound or Sensation

Many people report hearing or feeling a pop in the knee at the moment of injury. This is often accompanied by a sudden onset of pain and an immediate sense that something has gone wrong.

2. Rapid Swelling

Swelling typically develops within a few hours after the injury. This occurs due to bleeding inside the joint. The knee may feel tight, warm, and appear visibly swollen.

3. Pain and Discomfort

The pain may vary depending on the severity of the tear. It can be sharp initially, followed by a persistent ache that worsens when walking or attempting to bend the knee.

4. Instability or “Giving Way”

A common symptom of an ACL injury is a feeling that the knee cannot support body weight or may “give way” unexpectedly. This instability can make walking or turning difficult.

5. Limited Range of Motion

After injury, it may be difficult to fully straighten or bend the knee. Stiffness and discomfort can persist even when resting.

When to Seek Medical Attention

It is important to seek medical advice if knee pain, swelling, or instability does not improve after rest or self-care measures.
 

Medical assessment is recommended if you experience:

• Difficulty bearing weight on the affected leg
• Persistent swelling that develops soon after injury
• A “locking” or “catching” sensation when moving the knee
• Ongoing instability or loss of confidence in the joint
   

Prompt evaluation helps determine whether the ACL or other knee structures, such as the meniscus or cartilage, are involved. Delaying treatment may increase the risk of additional injury.

How ACL Injuries Are Diagnosed

An orthopaedic specialist will usually begin with a detailed discussion about how the injury occurred and the symptoms that followed.
 

A physical examination is performed to assess knee stability, range of motion, and tenderness. Specific tests — such as the Lachman or pivot shift test — help identify ACL instability.
 

If a tear is suspected, imaging studies such as X-rays or an MRI may be recommended.

• X-rays help rule out fractures or bone injury.
• MRI scans provide a detailed view of ligaments, tendons, and cartilage, confirming the extent of ACL damage.

Treatment Options

Treatment depends on the severity of the injury, level of physical activity, and lifestyle factors.
 

Options generally include:

Non-surgical Management

• Rest, Ice, Compression, and Elevation (RICE): Helps reduce pain and swelling.
• Physiotherapy: Focuses on strengthening surrounding muscles and improving flexibility.
• Knee Bracing: May be used temporarily to support the joint during recovery.
   

This approach may be suitable for individuals with partial tears or lower physical activity demands.

Surgical Reconstruction

When the ACL is completely torn or knee instability remains despite rehabilitation, ACL surgical reconstruction may be discussed.
 

This procedure involves replacing the damaged ligament with a graft, followed by a structured rehabilitation programme.

Recovery and Prevention

Rehabilitation is an important part of recovery after an ACL injury. Physiotherapy helps restore muscle strength, balance, and movement control. A gradual return to normal activities is encouraged under medical supervision.
 

To help reduce the risk of future injury:

• Warm up properly before physical activity
• Strengthen thigh and core muscles
• Practise correct jumping and landing techniques
• Use appropriate footwear for your sport or activity

An ACL injury can significantly affect knee stability and movement. Recognising early signs — such as pain, swelling, and a “popping” sensation — allows for timely medical assessment and appropriate management.
 

If knee discomfort or instability persists, consulting an orthopaedic specialist can help determine the cause and guide a suitable treatment plan.