ACL Tear Symptoms

An anterior cruciate ligament (ACL) tear can cause a popping sensation, sudden knee pain, swelling, difficulty continuing activity and a feeling that the knee is unstable or may give way. However, symptoms vary, and you can't confirm an ACL tear from symptoms alone.

Some people have dramatic symptoms immediately after injury, while others can walk and notice instability only later when turning, pivoting or returning to sport.

For an overview of the injury, causes, diagnosis and treatment options, see ACL Tear.

Key Points

• A pop or tearing sensation at the time of injury is common but does not occur in everyone.
• Knee swelling may develop within the first few hours.
• Many people cannot continue playing or exercising after the injury.
• The knee may later feel unstable or give way, especially when pivoting or changing direction.
• Being able to walk does not rule out an ACL tear.
• Symptoms alone cannot confirm whether the ACL is partially or completely torn.

Did I tear my ACL?

You may have torn your ACL if your knee twisted or changed direction suddenly and you experienced a pop, rapid swelling, pain or difficulty continuing the activity. Later, the knee may feel unstable when turning or pivoting.

The anterior cruciate ligament, usually shortened to ACL, is one of the main ligaments inside the knee. It helps control forward movement and rotation of the shin bone (tibia) relative to the thigh bone (femur).

A typical ACL injury may involve several of the following:

• A sudden twisting or pivoting knee injury
• Hearing or feeling a pop
• Immediate pain
• Swelling developing relatively quickly
• Difficulty bearing weight
• Reduced knee movement
• Inability to continue playing sport
• A feeling that the knee has shifted
• Giving way during later activity

No single symptom proves that the ACL has torn. Other injuries, including meniscus tears and injuries to other knee ligaments, can cause similar symptoms.

What does an ACL tear feel like when it happens?

An ACL tear often feels sudden rather than gradually developing pain. Some people describe a pop, snap, or tearing sensation deep inside the knee, followed by pain, swelling, or a feeling that the knee has shifted.

The injury commonly occurs during movements such as:

• Changing direction suddenly
• Pivoting with the foot planted
• Landing awkwardly
• Stopping quickly
• Twisting the knee
• A collision or tackle

Many ACL injuries occur without another player making contact with the knee.

A person may immediately know that something significant has happened, but the severity of the ACL injury cannot reliably be determined from the sensation alone.

Is a popping sound a sign of an ACL tear?

A pop at the time of injury is a common symptom of an ACL tear, but not everyone with a torn ACL hears or feels one.

The pop may occur as the ligament is injured, and some people describe a sensation that the knee briefly moved out of position.

However, a popping sensation can also occur with other knee injuries. Conversely, the absence of a pop does not rule out ACL damage.

Does an ACL tear cause immediate swelling?

An ACL tear commonly causes knee swelling within the first few hours of injury, although the amount and timing of swelling vary.

A substantial ACL injury can cause bleeding within the knee joint, resulting in relatively rapid swelling. The knee may become tight, stiff and difficult to bend or straighten.

Swelling that develops more gradually can also occur with other knee injuries, so the timing of swelling is helpful but not diagnostic on its own.

Where is the pain from an ACL tear?

ACL pain is commonly felt within or around the knee rather than at one easily identifiable point. Pain may be more widespread immediately after the injury because of swelling and associated damage.

Pain location and severity vary considerably.

Some people experience significant pain initially, while others find that the pain improves relatively quickly but instability becomes the more important problem.

Pain in a particular area of the knee can also reflect an associated injury—for example, damage to the meniscus or another ligament.

Can you walk with a torn ACL?

Yes. Some people can walk with a torn ACL, particularly after the initial pain and swelling begin to settle.

Being able to walk therefore does not mean that the ACL is intact.

Immediately after injury, walking may be difficult because of:

• Pain
• Swelling
• Reduced knee movement
• Muscle inhibition
• A feeling of instability

Later, walking in a straight line may feel relatively normal. The problem may become more obvious when the person tries to pivot, turn quickly, run or return to sport.

What does ACL instability feel like?

ACL instability may feel as though the knee shifts, buckles, or gives way unexpectedly, particularly during turning or pivoting.

Instability means the knee does not feel mechanically secure during certain movements.

People may describe:

• The knee "giving way"
• The knee "buckling"
• A sensation that the knee slips
• Lack of confidence when changing direction
• Avoiding particular movements because the knee feels unreliable

This symptom may become more noticeable after the initial pain and swelling have improved.

Episodes of giving way are particularly important to mention during specialist assessment because they provide information about how the injured knee functions.

Can ACL symptoms appear after the initial injury?

Yes. Some symptoms are most noticeable immediately after an ACL injury, while others become more apparent days or weeks later.

Early Symptoms

Early features may include:

• A pop or tearing sensation
• Pain
• Rapid swelling
• Difficulty bearing weight
• Restricted movement
• Inability to continue sport

Later Symptoms

Once swelling and pain settle, a person may notice:

• Giving way
• Instability while turning
• Difficulty running or changing direction
• Reduced confidence in the knee
• Recurrent swelling after activity
• Difficulty returning to sport

This explains why someone may think the knee has recovered at first but later have problems with more demanding movements.



Are partial ACL tear symptoms different from a complete tear?

Partial and complete ACL tears can cause similar symptoms, so symptoms alone cannot reliably determine the extent of ligament damage.

A partial ACL tear means that some of the ligament fibres remain intact. A complete tear, or rupture, means the ligament has been fully disrupted.

The important issue is not simply whether some fibres remain intact, but whether the ACL continues to provide adequate functional stability.

For more information, see Partial ACL Tear.



ACL Tear versus Meniscus Tear: are the symptoms different?

ACL and meniscus injuries can both cause pain and swelling after a twisting knee injury, but instability is more characteristic of ACL deficiency while catching or locking may suggest meniscus involvement.

A meniscus is a C-shaped piece of fibrocartilage between the thigh bone and shin bone that helps distribute load within the knee.