The Anterior Cruciate Ligament (ACL) is a major stabiliser of the knee joint, preventing the lower leg from slipping forward, away from the knee.
Injury to this ligament usually manifests as a full thickness tear and often requires surgical reconstruction, particularly if the affected individual wishes to participate in sporting activities.
The normal ACL on MRI is a taut, thick dark structure that takes an oblique path through the knee joint (figure A). A typical ACL tear is detected by the loss of the normal dark signal (figure B) and is replaced by bright signal on MRI (figure C). Also, the ligament is absent where it arises from the femur, a finding known as the "empty notch sign". As the injury occurs due to a twisting injury, bone bruises may occur (figure D), with the bruises most commonly indicative of a pivot-shift mechanism of injury.
ACL tears have a high association with meniscal injury, which can present with inability to straighten the knee - a "locked knee" (figure E). This is due to a meniscal tear that is flipped from its expected position, preventing further movement of the two bones (femur and tibia) that comprise the knee joint.
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