The anterior cruciate ligament (ACL) is the most commonly injured ligament of the knee. Every year, there are around 30 cases of ACL injuries for every 100,000 people. If the ACL is torn, surgery is commonly used to reconstruct the damaged ligament.
The ACL runs diagonally through the inside of the knee. It connects the underside of the femur to the top of the tibia. The ACL prevents the tibia from moving in front of the femur and gives the knee joint stability. It also helps control the back and forth movement of the knee.
Most injuries to the ACL happen when playing sports, such as football, squash, tennis or skiing (see the box, left). ACL injuries account for 40% of all sports injuries.
If the ACL is torn, the knee becomes very unstable and loses its full range of movement. This can make it difficult to perform certain movements, such as turning on the spot, and could make some sports impossible to play.