The anterior cruciate ligament (ACL) is one of the most commonly injured structures in the knee. Athletes and active individuals are especially at risk, but ACL injuries can happen to anyone—from a weekend jogger to someone who slips while walking on uneven ground.
Because the ACL is important for knee stability, an injury can greatly affect daily activities and sports performance. Yet many people struggle to recognise whether their knee pain or instability points to an ACL tear or something else.
This article explains the role of the ACL, signs and symptoms of a tear, how it’s diagnosed, and what to do if you suspect this type of injury.
What is the ACL?
The ACL is one of four major ligaments that stabilise the knee. It runs diagonally through the middle of the knee, connecting the thighbone (femur) to the shinbone (tibia). Its key functions are:
- Preventing the shinbone from sliding forward excessively
- Providing rotational stability during movements like cutting, pivoting, or twisting
- Helping coordinate complex knee motions during running, jumping, and landing
How Do ACL Tears Happen?
ACL injuries are usually the result of sudden, high-stress movements rather than gradual wear and tear. Common scenarios include:
- Pivoting sharply while running or changing direction
- Landing awkwardly from a jump
- Sudden deceleration, especially in sports like football, basketball, or netball
- Direct trauma to the knee, such as during a tackle or collision
Although athletes face the highest risk, ACL tears can also occur outside sports. Slipping on a wet floor, missing a step, or turning suddenly with the foot planted can all cause this injury.
Signs and Symptoms of an ACL Tear
Recognising the signs of an ACL tear is important, as it allows for prompt assessment and treatment to prevent further damage to the knee, particularly to the meniscus or cartilage.
1. A “Pop” at the Time of Injury
Many patients report hearing or feeling a popping sound when the ACL tears. This distinctive sensation often occurs at the moment of injury and is one of the hallmark indicators.
2. Immediate Pain
Pain sets in quickly, usually severe enough to stop activity right away. While the intensity may decrease after the initial incident, discomfort tends to persist with movement.
3. Rapid Swelling
The knee often swells within the first few hours due to bleeding inside the joint. Unlike mild sprains where swelling develops more slowly, an ACL tear typically leads to noticeable swelling very quickly.
4. Instability or “Giving Way”
An injured ACL compromises knee stability. People often describe their knee as “giving way,” especially when trying to pivot, turn, or walk on uneven ground.
5. Limited Range of Motion
Swelling and pain can make it difficult to fully bend or straighten the knee. Activities like squatting or kneeling may feel particularly restricted.
6. Difficulty Bearing Weight
Some patients find they cannot comfortably put weight on the affected leg immediately after the injury. Others may be able to walk but feel unstable and hesitant.
Partial vs. Complete ACL Tears
ACL tears can be broadly classified into complete tears and partial tears:
- Partial tears: Only part of the ligament fibres are damaged. Pain, swelling, and instability may be present but to a lesser degree.
- Complete tears: The ligament is fully torn. These typically cause significant instability, pronounced swelling, and the classic “pop” at the time of injury.
Regardless of severity, an orthopaedic evaluation is strongly recommended because partial tears can deteriorate into complete tears if the knee remains unstable.
What Conditions Can Mimic an ACL Tear?
Besides an ACL tear, other injuries can produce similar symptoms, which can make it difficult to tell exactly what injury you may have. These include:
- Meniscus tears: This can also cause pain, swelling, and locking
- Collateral ligament injuries: These usually cause pain on the inner or outer sides of the knee
- Patellar dislocation: This may cause a pop and swelling but often presents with kneecap misalignment
This overlap in symptoms underscores why professional assessment by a doctor is important instead of relying solely on self-diagnosis.
How an Orthopaedic Specialist Confirms an ACL Tear
If you suspect an ACL injury, the next step is an evaluation by an orthopaedic surgeon or sports medicine specialist. Common approaches include:
Clinical Examination
The doctor will assess swelling, stability and range of motion. Specific tests can help determine whether the ACL is intact.
Imaging Tests
- MRI scans provide a detailed view of soft tissues, confirming whether the ACL is torn and whether other structures like the meniscus are also damaged.
- X-rays may be performed to rule out fractures, though they cannot directly visualise ligaments.
These ensure that the diagnosis is accurate and guides the most appropriate treatment plan.
What To Do If You Suspect an ACL Tear
If you believe you may have torn your ACL, here are some immediate steps to take:
- Rest the knee and avoid putting weight on it
- Ice the area to help reduce pain and swelling
- Compression with a bandage can limit swelling
- Elevation of the leg above heart level can also help
These measures, however, are temporary. Medical evaluation is necessary to confirm the injury and discuss proper treatment options.
When To Seek Help From an ACL Doctor
If you experience knee pain, swelling or instability following an injury, don’t wait for symptoms to “go away” or “settle”. Prompt medical assessment can go a long way in:
- Confirming whether the ACL or another structure is injured
- Preventing further damage to the meniscus or cartilage
- Initiating early treatment that supports better long-term outcomes
Whether managed non-surgically or with reconstruction surgery, recovery from an ACL tear requires patience, rehabilitation, and guidance from an orthopaedic specialist. With the right approach, many patients are able to return to their former activity levels.
