Anterior cruciate ligament

At a glance

English

Anterior cruciate ligament

Latin (TA2)

Ligamentum cruciatum anterius

System

Ligament

Region

Lower limb

Side

Paired, left and right

About the anterior cruciate ligament

The anterior cruciate ligament, or ACL, is one of the two cruciate ligaments that lie in the centre of the knee. The pair cross each other like the arms of an X, which is where the name comes from (cruciate means cross-shaped). Both lie inside the fibrous capsule but outside the synovial cavity, because the synovial membrane folds around them from the front. In four-legged animals the same ligament is called the cranial cruciate ligament.

Below, the ACL is fixed to the anterior intercondylar area of the tibia, in front of the intercondylar eminence and between the anterior horns of the two menisci, with some fibres blending into a meniscal horn. From there it runs upwards, backwards and laterally into the intercondylar notch of the femur, where its fibres fan out over the rear of the lateral condyle's inner, notch-facing surface. It is usually described as two bundles named by their tibial attachments: the anteromedial bundle, which is tightest when the knee is bent, and the posterolateral bundle, which is tightest as the knee straightens.

Its main task is to stop the tibia sliding forwards under the femur, and it provides most of the resistance to that movement. It also limits inward rotation of the tibia and hyperextension, and it guides the small locking rotation at the end of extension. The ligament contains mechanoreceptors that report joint position, speed and strain to the nervous system, so it helps to coordinate the muscles around the knee.

The ACL is the knee ligament most often torn. The typical story is a non-contact pivot or sudden change of direction with the foot planted, often with a felt pop and rapid swelling from bleeding into the joint. It may be torn together with the medial collateral ligament and the medial meniscus. Clinicians test it with the Lachman, anterior drawer and pivot-shift tests. A torn ACL does not heal well because synovial fluid washes away the clot, so active people usually have it reconstructed with a graft of their own tendon, such as a hamstring or patellar tendon, or with donor tissue.

Attachments and function

Attaches from

The anterior intercondylar area of the tibia, in front of the intercondylar eminence and between the anterior horns of the menisci.

Attaches to

The posterior part of the medial (inner) surface of the lateral femoral condyle in the intercondylar fossa.

Function

Resists forward displacement of the tibia on the femur, limits inward rotation and hyperextension, and supplies proprioceptive information.

Nerve supply

Posterior articular branches of the tibial nerve.

Blood supply

Mainly the middle genicular artery, through the synovial covering; the ligament itself is poorly vascularised.