ACL Injury

The anterior cruciate ligament, or ACL, is one of the strong bands of tissue that connect the thigh bone to the shin bone inside the knee. It runs diagonally through the middle of the knee and helps keep the joint stable, particularly when you stop suddenly, change direction or pivot.

An ACL injury means this ligament has been stretched, partly torn or completely torn. Injuries are graded by severity, from a mild sprain that overstretches the ligament to a complete tear that splits it into two pieces. ACL tears are among the most common serious knee injuries.

Because the ACL is important for controlling rotation and forward movement of the shin bone, a significant tear can leave the knee feeling unstable, especially during sports that involve cutting, jumping and rapid changes of direction.

At the moment of injury many people feel or hear a popping sensation in the knee. This is usually followed quickly by pain and noticeable swelling that develops within a few hours, as bleeding occurs inside the joint.

Other common symptoms include difficulty bearing weight, a reduced range of movement, tenderness around the joint, and a feeling that the knee is loose or gives way, particularly when trying to turn or change direction. Some people can still walk after the injury, which does not rule out a serious tear.

ACL injuries are often non-contact, happening when a person suddenly slows down and changes direction, pivots with the foot planted, lands awkwardly from a jump, or stops abruptly. They can also result from a direct blow to the knee, such as a tackle in sport.

Sports that involve frequent twisting, jumping and rapid changes of direction, such as football, basketball, skiing and netball, carry a higher risk. Factors such as movement technique, muscle strength and the surface or footwear used can influence the chance of injury.

A doctor performs physical stability tests (such as the Lachman test and pivot-shift test) to assess ACL integrity.

Imaging helps confirm the diagnosis and identify any other damage. An MRI scan shows the soft tissues of the knee in detail and can reveal the extent of the ACL tear as well as injuries to other ligaments, the menisci (the cartilage cushions in the knee) or the joint surface. X-rays may be taken to check for any associated bone injury.

Initial care focuses on settling pain and swelling, typically with rest, ice, gentle compression, elevation and pain relief, often alongside early physiotherapy to restore movement and muscle control.

Treatment then depends on the severity of the tear, the stability of the knee and your activity goals. Some people, particularly those with a less active lifestyle or a partial tear, do well with a structured rehabilitation programme that strengthens the muscles around the knee to compensate for the injured ligament. A torn ACL does not heal back together on its own.

Surgical reconstruction is often recommended for people who are very active, take part in pivoting sports, or have ongoing instability where the knee gives way. The procedure replaces the torn ligament with a graft of tissue, after which an extended period of rehabilitation is needed to rebuild strength, movement and confidence before returning to sport.

Not all ACL injuries can be prevented, but the risk can be reduced. Training programmes that improve jumping and landing technique, strengthen the leg and core muscles, and develop balance and control have been shown to lower the chance of injury, particularly in sports with a lot of cutting and jumping. Good footwear and a proper warm-up also help.

The outlook after an ACL injury is generally good. With appropriate rehabilitation, and surgery where needed, many people regain a stable knee and return to their previous activities, although recovery from a tear takes time and commitment to a rehabilitation programme. Addressing other knee damage and following the full recovery plan help reduce the risk of further problems later.