Overview

The ACL arises from the anterior intercondylar area of the tibia, behind the attachment of the medial meniscus, and passes posterosuperiorly and laterally to attach to the posteromedial aspect of the lateral femoral condyle in the intercondylar fossa. It has an anteromedial bundle (tight in flexion) and a posterolateral bundle (tight in extension). Being enclosed by synovial membrane, which reflects around it, it lies inside the capsule but outside the synovial cavity.

It is the primary restraint to anterior translation of the tibia relative to the femur (≈85% of the resistance at 30–90° flexion) and also resists hyperextension and internal rotation of the tibia. It is weaker than the PCL and has a relatively poor blood supply (middle genicular artery), so complete tears do not heal spontaneously.

Location: Intercondylar notch of the knee.

Anatomy

Origin: Anterior intercondylar area of the tibia.
Insertion: Posteromedial aspect of the lateral femoral condyle (intercondylar fossa).
Innervation: Articular branches of the tibial nerve.
Arterial supply: Middle genicular artery.

Function

Prevents anterior translation of the tibia on the femur (posterior translation of the femur on the fixed tibia)
Limits hyperextension and internal rotation of the tibia
Contributes proprioceptive feedback from the knee

Clinical significance

ACL rupture

Typically non-contact pivoting or landing with the knee near extension and in valgus (football, skiing, netball), with an audible "pop", inability to continue, and rapid haemarthrosis (within ≈2 h). Lachman test is the most sensitive clinical test; pivot-shift is the most specific. A Segond fracture (avulsion of the lateral tibial plateau) is pathognomonic. Women are at 2–8 times higher risk. Young athletes usually undergo reconstruction (hamstring or patellar tendon graft).

Mnemonics

Direction of the ACL

"Hands in pockets": the ACL runs like your hands going into the front pockets — from anterior tibia to the lateral side (lateral femoral condyle).

Test yourself

Which test is most sensitive for an anterior cruciate ligament tear?

Posterior drawer test
McMurray test
Lachman test
Valgus stress test

Answer: Lachman test

The Lachman test (anterior translation of the tibia with the knee in ≈20–30° flexion) is the most sensitive clinical test for ACL rupture. The posterior drawer tests the PCL; McMurray the menisci; valgus stress the MCL.