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.
Ligaments
Ligaments are tough bands of connective tissue (fibres that support other tissues and organs in your body). Ligaments link two bones together at a joint. They strengthen the joint and limit its movement in certain directions.
The knee
There are three bones that meet in the knee:
- thigh bone (femur),
- shin bone (tibia), and
- knee cap (patella).
The three bones are connected by four ligaments. The ligaments hold the bones together and keep the knee stable. There are two collateral ligaments on the sides of the knee, and two cruciate ligaments inside the knee.
The anterior cruciate 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.
Reconstructive surgery
It is not possible to repair the torn ACL by stitching it back together. However, it can be reconstructed by grafting (attaching) new tissue onto it. This will encourage new ligament to grow over the grafted tissue. Once new ligament has grown, the knee will be more stable.
Reconstructive surgery is successful in around 90% of cases. A few people may still have knee pain or instability after the operation. Recovery usually takes around six months, although it could be up to a year before athletes can fully return to their sport.
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