ACL Tear: Symptoms, MRI, Treatment and Recovery

An anterior cruciate ligament, or ACL, tear is one of the best-known sporting knee injuries — but you do not have to be a professional athlete to sustain one.

It commonly happens during a sudden twist, rapid change of direction or awkward landing, particularly when the foot stays planted while the knee rotates. Football, skiing, netball and basketball are classic examples.

Many people describe hearing or feeling a “pop” at the moment of injury, followed by the knee giving way and significant swelling over the next few hours.^[1,2]

An ACL injury can be painful and disruptive, but treatment is not the same for everyone. Some people recover well with structured rehabilitation, while others benefit from ACL reconstruction. The right approach depends on the injury, the stability of the knee, associated damage, activity level and the individual's goals.^[1,2]

What is the ACL?

The knee is formed by three main bones:

• —the femur, or thigh bone
• —the tibia, or shin bone
• —the patella, or kneecap

The smaller fibula sits alongside the tibia.

Inside the knee are two cruciate ligaments: the anterior cruciate ligament and posterior cruciate ligament.

The ACL runs diagonally through the centre of the knee, connecting the femur to the tibia.

Its main roles include helping to prevent the tibia moving too far forwards relative to the femur and contributing to rotational stability of the knee.

This becomes particularly important during movements involving pivoting, cutting, landing and sudden changes of direction.

How does an ACL tear happen?

A large proportion of ACL injuries occur without another person directly hitting the knee.

Imagine a footballer running towards the ball. They plant one foot into the ground and suddenly turn in another direction. The foot remains relatively fixed, while the body and knee rotate above it.

That combination of rotation, force and rapid deceleration can place a significant load through the ACL.

Typical mechanisms include:

• —suddenly changing direction
• —twisting with the foot planted
• —rapidly slowing down
• —landing awkwardly after jumping
• —the knee collapsing inwards during landing or cutting
• —direct trauma to the knee in some cases

That is why ACL injuries are particularly associated with sports such as football, rugby, netball, basketball and skiing.^[1]

What does an ACL tear feel like?

The injury can feel dramatic.

Common symptoms include:

A loud or noticeable pop

Many people report hearing or feeling a pop inside the knee when the injury occurs. Not everyone experiences this, however, and a pop does not automatically mean the ACL has torn.

The knee gives way

Because the ACL contributes to knee stability, people may describe the knee suddenly buckling or giving way. After the initial injury, this instability can sometimes continue — particularly during twisting or pivoting movements.

Rapid swelling

A significant ACL tear may cause bleeding within the knee joint, known as a haemarthrosis. This can cause the knee to become noticeably swollen over the first few hours following injury.

Pain and reduced movement

Pain varies considerably between people. Some people initially have severe pain, while others are surprised that the pain settles relatively quickly despite having sustained a major ligament injury.

The swelling may make it difficult to fully bend or straighten the knee.

Can you still walk with a torn ACL?

Yes.

Once the immediate pain and swelling begin to improve, some people with a complete ACL rupture can walk reasonably normally.

The problem may become much more obvious when they attempt to run, turn, pivot or return to sport, when the knee may feel unstable.

This is one reason why the history of the injury and a proper knee examination are so important.

How is an ACL tear diagnosed?

Diagnosis usually involves a combination of:

• —what happened during the injury
• —the symptoms afterwards
• —examination of the knee
• —imaging when appropriate

Clinicians can perform specific tests that assess the amount of movement between the tibia and femur and the rotational stability of the knee.

MRI is frequently used to confirm the injury and, importantly, to assess the rest of the knee.^[2,3]

What does an ACL tear look like on MRI?

This is where MRI becomes particularly useful.

On a normal MRI, the ACL usually appears as a continuous low-signal band of fibres extending between the femur and tibia.

When it tears, a radiologist may see:

• —disruption of the ACL fibres
• —abnormal orientation or waviness of the ligament
• —increased signal within the injured ligament
• —loss of normal fibre continuity
• —surrounding fluid or swelling
• —secondary signs suggesting abnormal movement of the tibia

The ligament can be partially injured or completely ruptured.

However, the ACL is only part of what we are assessing.

An ACL injury involves a significant force passing through the knee, so on MRI we carefully examine the menisci, cartilage, other ligaments and bones as well.

MRI also allows assessment for associated injuries such as:

• —meniscal tears
• —bone bruising
• —collateral ligament injury
• —cartilage injury
• —other internal knee injuries

MRI has good diagnostic performance for identifying ACL tears, although imaging is always interpreted alongside the clinical history and examination.^[3]

Why do ACL tears cause bone bruising?

One of the fascinating things we often see on MRI after an acute ACL injury is a characteristic pattern of bone bruising.

When the ACL fails during a twisting injury, the femur and tibia can impact against one another.

The bone is not necessarily fractured, but the force can produce microscopic injury within the bone marrow.

On MRI this appears as increased fluid-sensitive signal — commonly called a bone bruise or bone contusion.

Bone bruises are particularly common within the lateral part of the knee following an acute ACL injury.

Their distribution can help us understand the mechanism of injury and can also alert us to look particularly carefully for associated meniscal or cartilage damage.^[9]

What other injuries can occur with an ACL tear?

An MRI report for an ACL injury should not simply stop at: “ACL tear.”

Associated injuries can include:

• —meniscal tears
• —cartilage damage
• —medial collateral ligament injury
• —other ligament injuries
• —bone bruising
• —osteochondral injury

Particular attention is paid to the menisci.

These are the shock-absorbing pieces of fibrocartilage that sit between the femur and tibia.

Certain meniscal injuries can occur alongside an ACL tear and may influence treatment decisions.

That is one reason why MRI is so valuable: it allows the whole knee to be assessed rather than simply confirming whether the ACL is intact.^[3,9]

Does every ACL tear need surgery?

No.

This is probably one of the biggest misconceptions surrounding ACL injuries.

An ACL tear does not automatically mean you need an operation.

Treatment needs to be individualised.^[1,2,4,5,6]

Some people can achieve an excellent level of function with a structured rehabilitation programme, particularly if the knee becomes stable and their activities do not require large amounts of pivoting or cutting.

Others continue to experience instability despite rehabilitation or want to return to sports that place very high rotational demands on the knee.

For those patients, reconstruction may be considered.

The decision can depend on:

• —age
• —sporting and activity goals
• —occupation
• —whether the knee continues to give way
• —associated meniscal or cartilage injuries
• —other ligament injuries
• —response to physiotherapy
• —the individual's preferences

What does ACL rehabilitation involve?

Whether someone ultimately undergoes surgery or not, rehabilitation is a major part of ACL treatment.

Initially, the priorities often include controlling swelling, restoring knee movement and reactivating the muscles around the knee.

Rehabilitation then progresses towards improving:

• —quadriceps strength
• —hamstring strength
• —balance
• —proprioception
• —neuromuscular control
• —movement quality
• —running capacity
• —jumping and landing mechanics
• —sport-specific movements

This is not simply about making the leg stronger.

The aim is to retrain how the entire lower limb responds during demanding movements.

Structured rehabilitation can produce good outcomes for selected patients with ACL rupture.^[4,5,6]

Rehabilitation or surgery: what does the evidence show?

This is more complicated than saying one option is always better.

In the COMPARE randomised trial, patients with an acute ACL rupture were allocated either to early reconstruction or rehabilitation with the option of delayed surgery.

At two years, the early surgery group had slightly better patient-reported outcomes, but the difference was of uncertain clinical importance. Importantly, around half of those initially allocated to rehabilitation did not ultimately undergo ACL reconstruction.^[4]

That means a rehabilitation-first approach can allow some people to avoid surgery.

The situation is somewhat different for people with a long-standing ACL-deficient knee that continues to feel unstable.

The UK ACL SNNAP randomised trial studied people with non-acute ACL injuries and persistent symptoms of instability.

Both rehabilitation and surgery improved symptoms, but surgical reconstruction produced better knee function and stability overall. Around 41% of people allocated to rehabilitation subsequently underwent reconstruction because of continuing symptoms.^[5]

What is ACL reconstruction?

When surgery is required, the ACL is most commonly reconstructed using a graft rather than simply stitched back together.

The graft acts as a replacement for the damaged ACL.

Different grafts can be used, including tissue from the patient's own hamstring, patellar or quadriceps tendon.

The exact choice depends on factors such as age, sporting demands, individual anatomy and surgeon preference.

Current evidence-based guidance generally favours ACL reconstruction rather than primary repair for typical midsubstance ACL tears when surgical treatment is indicated.^[2]

When should ACL surgery happen?

There is no single correct date for everyone.

The knee often needs time for swelling to improve and movement to return before reconstruction.

However, if reconstruction has been selected for an acute isolated ACL tear, prolonged delays may increase the opportunity for episodes of instability and additional damage to the meniscus or cartilage.

The American Academy of Orthopaedic Surgeons recommends that, when surgery is indicated for an acute isolated ACL tear, reconstruction is generally favoured within approximately three months of injury because the risk of additional meniscal and cartilage injury begins to increase with delay.^[2]

Individual circumstances can be very different, so timing should be decided with the treating orthopaedic team.

Can a torn ACL heal itself?

This is an interesting area of ongoing research.

Traditionally, complete ACL ruptures were thought to have very limited capacity to heal.

More recent research suggests that some ACLs treated without surgery can demonstrate restoration of ligament continuity on MRI.

A secondary analysis of the KANON trial found MRI evidence of ACL healing in a proportion of patients managed initially with rehabilitation, and those showing healing had favourable outcomes.^[7]

That does not mean every ACL tear will heal naturally, nor does an MRI that appears more continuous necessarily guarantee normal mechanical stability.

But it does reinforce the idea that ACL injuries are not identical and that treatment should not automatically be determined by the word “tear” on an MRI report.

How long does recovery take?

Recovery from an ACL injury is measured in months rather than weeks.

The timeline depends on whether the injury is treated surgically or non-surgically, the associated injuries, the person's starting level of fitness and how rehabilitation progresses.

Following ACL reconstruction, returning to competitive pivoting sport commonly takes many months.

Importantly, return to sport should not be based on the calendar alone.

Assessments may include:

• —quadriceps and hamstring strength
• —hop testing
• —balance and movement control
• —sport-specific drills
• —confidence in the knee
• —psychological readiness

British Orthopaedic Association guidance recommends a criteria-based approach to return to sport rather than relying purely on time since surgery.^[8]

Research has also shown that returning too early to high-demand sport can be associated with a higher risk of another knee injury.^[10]

Can you tear the ACL again?

Unfortunately, yes.

After ACL reconstruction, the graft itself can rupture.

There is also a risk of tearing the ACL in the opposite knee.

This is particularly important for younger athletes returning to high-level pivoting sports.

That is why rehabilitation should not stop simply because someone is able to run again.

Strength, movement quality, landing technique, confidence and sport-specific control all matter when deciding whether someone is ready to return.

Does an ACL tear increase the risk of arthritis?

ACL injury is associated with an increased long-term risk of post-traumatic osteoarthritis.

Part of this risk is likely related to the initial trauma itself, altered knee mechanics and associated injuries to structures such as the menisci and cartilage.

ACL reconstruction restores stability for many people, but it does not completely eliminate the future risk of osteoarthritis.^[2]

Preserving the meniscus where possible, restoring movement and strength, avoiding repeated instability and managing associated injuries are therefore important aspects of long-term knee health.

Can ACL injuries be prevented?

Not every ACL injury is preventable.

However, structured neuromuscular injury-prevention programmes can reduce the risk of ACL injury, particularly in athletes participating in higher-risk sports.^[2,11]

These programmes often include:

• —landing technique
• —balance exercises
• —strength training
• —plyometrics
• —cutting and change-of-direction technique
• —trunk and hip control

They are particularly useful when incorporated into regular warm-up programmes rather than performed only occasionally.

When should you seek medical assessment?

A significant twisting knee injury should be assessed if there is:

• —rapid swelling
• —inability to fully straighten the knee
• —significant difficulty putting weight through the leg
• —repeated giving way
• —locking of the knee
• —severe persistent pain
• —obvious deformity
• —concern about a major ligament injury

A locked knee can sometimes indicate a displaced meniscal tear and warrants timely assessment.

The key message

An ACL tear is much more than simply a damaged ligament on an MRI.

The mechanism of injury, stability of the knee, associated damage, sporting goals and response to rehabilitation all influence what happens next.

MRI can confirm the ACL injury, but its other major role is identifying the bone bruises, meniscal tears, cartilage damage and additional ligament injuries that may accompany it.

For some people, structured rehabilitation can restore excellent function.

For others — particularly when instability persists or when returning to demanding pivoting sports is important — reconstruction may offer the best route back to the activities they want to do.

The decision should be individualised and made together with an appropriately experienced healthcare team.