What does the ACL actually do in my knee?

Where the ACL sits

The ACL runs diagonally inside the knee joint, from the back of the thigh bone (femur) to the front of the shin bone (tibia). It crosses the posterior cruciate ligament (PCL) in an X shape, which is where the word cruciate, meaning cross-shaped, comes from. It is about the width of a finger and is covered by a thin lining that carries its blood supply.

What it does

• Stops the tibia sliding forward on the femur
• Controls twisting of the shin bone, especially when the foot is planted
• Works with the menisci and the muscles around the knee to keep the joint centred during sudden changes of direction
• Contains nerve endings that help the brain sense the position of the knee

In straight-line activities such as walking or cycling, the muscles do much of the work. In football, padel, basketball and handball, the ACL is under heavy load every time you cut or land.

What happens when it tears

A torn ACL usually does not heal back to the bone because it sits in joint fluid and its two ends separate. Many people can walk normally once the swelling settles, but the knee may shift or give way with pivoting. Repeated giving way can damage the meniscus and cartilage over time.

When to have it checked

If you twisted your knee, felt a pop, or the knee swelled within hours, an examination is worthwhile even if you can walk. At Aman Hospital in Doha, Dr. Gelber tests the stability of the ligament by hand and arranges an MRI when it will change the plan.