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Anatomy · Joints & cartilage

Anterior cruciate ligament

The knee's most famous ligament

Lower limb · inside the knee

The anterior cruciate ligament (ACL) is one of two strong cords that cross inside the knee. It runs from the front of the tibia's intercondylar area upwards, backwards and outwards to the inner surface of the lateral femoral condyle. It stops the tibia sliding forwards on the femur and resists hyperextension and rotation, and it is the knee ligament most famously ruptured in sports that involve sudden pivoting and landing.

How to spot them

  • ▸Attaches to the anterior part of the tibial plateau between the condyles, in front of its posterior partner
  • ▸Runs upwards, backwards and laterally to the medial face of the lateral femoral condyle
  • ▸Lies inside the joint capsule but outside the synovial cavity, so it heals poorly
  • ▸Tight in full extension; tested by the Lachman and anterior drawer tests

What to know

It stops the shin bone sliding forward on the thigh bone

Anterior cruciate ligament — the anterior drawer test looks for its failure.

Rupture is a feared sports injury: a sudden pivot or awkward landing, often with an audible pop and a knee that gives way

Anterior cruciate ligament — non-contact tears in football, skiing and netball.

A blow to the outside of the extended limb can tear it after the medial collateral ligament and medial meniscus, in a triad

Anterior cruciate ligament — the 'unhappy triad'.

It tightens in full extension and resists hyperextension

Anterior cruciate ligament — OpenStax.

Reconstruction replaces it with a graft, commonly from the patellar or hamstring tendon

Anterior cruciate ligament — patellar tendon or hamstring autograft.

Key ideas

  • Named for what it does at the tibia: 'anterior' refers to its tibial attachment, in front of the posterior cruciate. 'Cruciate' means cross-shaped, as the two ligaments cross inside the joint.
  • Why it is so famous: it is the most commonly injured ligament of the knee in athletes, with a long recovery after reconstruction.

Quick-fire facts

  • •It is intracapsular but extrasynovial: inside the joint capsule but outside the synovial membrane.
  • •OpenStax: the ACL becomes tight when the knee is fully extended and resists hyperextension.
  • •It has two functional bundles: anteromedial and posterolateral.
  • •Women's ACL injury rates in comparable sports are higher than men's.
  • •Poor healing follows rupture: it is bathed in synovial fluid and has a limited blood supply, so surgical reconstruction is usually needed for active people.
  • •The Lachman test (knee flexed about 20 to 30 degrees) is the most sensitive clinical test of ACL integrity.
  • •It is supplied mainly by the middle genicular artery.
  • •The 'unhappy triad' (O'Donoghue) combines ACL, medial collateral ligament and medial meniscus injury.

Drill it — 7 cards

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Sources