ACL Injury

The anterior cruciate ligament runs through the centre of the knee and resists forward shift and rotation of the tibia. It is most often torn in sport, in a twisting, decelerating or landing movement, frequently without contact.

How it presents

At the time of injury: a pop or snap, immediate pain, a sense of the knee giving way, and usually an inability to continue. Swelling develops within a few hours, which distinguishes it from many lesser injuries where swelling appears the next day.

Afterwards, the characteristic symptom is instability: the knee giving way when turning, changing direction, or stepping off a kerb.

How it is assessed

Examination first. Lachman and pivot shift testing in experienced hands is highly informative. MRI confirms the diagnosis and, more importantly, identifies associated injuries: meniscal tears, cartilage damage, and other ligament injury, all of which change the plan.

X-rays are taken to exclude fracture, particularly in younger patients.

Surgery or not

This is genuinely a decision, not a formality. Reconstruction is generally indicated where:

• the knee gives way during ordinary daily activity
• you intend to return to a sport involving cutting, pivoting or landing
• there is a repairable meniscal tear
• there is significant additional ligament injury

Non-surgical management is reasonable where the knee is stable in straight-line activity, demands are low, and structured rehabilitation restores confident function. It requires proper rehabilitation, not just avoidance.

The reconstruction page covers graft choices, what the operation involves, and the realistic return-to-sport timeline.

The first few weeks matter

Whatever the eventual decision, early management is the same and it is often neglected:

• Control the swelling
• Restore full extension of the knee
• Get the quadriceps working again
• Regain normal walking

A knee that is stiff and swollen at the time of surgery does worse than one that is settled and moving, which is why reconstruction is usually not performed immediately.

In younger patients

Where the growth plates are still open, the surgical approach differs and the timing is a specific consideration. This should be discussed with a surgeon familiar with paediatric and adolescent ACL injury.

Common questions

How do I know if I have torn my ACL?

The typical story is a twisting or landing injury with a pop, rapid swelling within a few hours, and a feeling that the knee gave way. Examination by an experienced clinician is more informative than most patients expect, and MRI confirms it and looks for associated meniscal or cartilage injury. A knee that swells rapidly after a twisting injury should be assessed.

Does a torn ACL always need surgery?

No. The decision turns on what you need the knee to do. Instability during everyday activity, or a return to pivoting sport, generally means reconstruction. A knee that is stable in straight-line activity, in someone who does not play cutting sports and who rehabilitates well, can do perfectly adequately without it. An associated repairable meniscal tear shifts the balance toward surgery.

Can an ACL heal on its own?

A completely torn ACL does not heal back into a functional ligament, which is why the operation replaces it with a graft rather than repairing it. Partial tears sometimes function adequately, which is a matter of assessment rather than assumption.

What happens if I leave it?

Some people cope well. The concern with an unstable knee is repeated episodes of giving way, each of which risks meniscal and cartilage damage, and over years that raises the risk of arthritis. That is the main argument for reconstruction in an active patient, alongside function.

Related reading

ACL Reconstruction

ACL reconstruction: who actually needs it, graft choices, the realistic return-to-sport timeline, and why some torn ACLs are managed without surgery.

Meniscus Tear

The difference between a traumatic and a degenerate meniscal tear, why that difference decides the treatment, and when surgery genuinely helps.