Anatomy
The anterior cruciate ligament (ACL) runs from a semicircular origin on the medial aspect of the lateral femoral condyle, in a spiral course forwards and laterally to a fan shaped insertion on the anterior tibial eminence.

  • uns parallel to the intercondylar roof
  • Two bundles, the largest is the anteromedial bundle
  • anteromedial band becomes taut in flexion
  • Insertion can be ill defined as it fans out and blends with the ant horn of lateral meniscus
  • AMB is low signal on most sequences PLB is brighter and less well defined


Its neurovascular supply is from the lateral geniculate artery and tibial nerve branches

In children the lower third of the ligament in particular can be poorly defined. Indeed in very young children the anteromedial bundle in its entirety may be poorly demarcated making interpretation of anterior cruciate ligament rupture more difficult. Fortunately, in children it is rarely injured and when it occurs it is most commonly an avulsion injury from the tibial insertion.
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Imaging
A synovial fold called the Ligamentum Mucosum runs parallel to the ACL and in front of it. A prominent fold may mimic the appearance of an intact ACL.

A minimum slice thickness of 4mm is recommended to ensure that the ligament is properly seen. Orientation of images along the axis of the ACL can be helpful, . Sagittal images are supported by coronal and axial sections. These can be particularly helpful in providing alternative visualisation of the femoral origin which can sometimes be difficult to depict on sagittal images.
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Injury
Sports injuries account for a high proportion of injuries to the ACL. The injury can occur by a variety of mechanisms but most frequently occurs with tibial internal rotation and abduction (Pivot Shift) Valgus results in distraction of the medial joint compartment and impaction of the lateral femoral condyle with the lateral tibia plateau. The injury is common in skiing, football and netball There is a 6% 5 year incidence in professional soccer players , 4% of australian rules football and 2% of NFL footballers. Typically the patient will describe an instantaneous moment of injury, where indeed an audible “pop” may be heard. This is followed by slow onset haemarthrosis occurring two to three hours after the event. Signs of injury are divided into primary (those that directly depict the tear) and secondary. Subsequent sections will describe the primary imaging findings in complete ACL rupture, changes that might suggest partial rupture, secondary signs of ACL rupture and their usefulness and associated injuries. The primary signs of ACL injury, non visualisation or definite loss of continuity, carries a strong predictive values for ACL rupture. When the primary signs of non visualisation of fibre bundles or focal disruption and intrinsic oedema are also applied to coronal and axial images, the diagnosis can usually be established. A variety of secondary signs of anterior cruciate ligament disruption have been described. In the majority of cases, the primary signs have a reliability such that the secondary signs are rarely if ever of practical value. They may however be helpful in differentiating a partial or low grade tear from a complete or high grade injury. Secondary signs of anterior cruciate ligament rupture can be divided into three groups 1) those that involve a bony injury, 2) those reflecting soft tissue injury and 3) those that reflect anterior tibial translation.
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Three separate bony injuries are typically associated with ACL rupture. The common of these is microfracture of the posterior and lateral margin of the tibial plateau.Microfracture will be more usually encountered in the acute phase following injury, although the findings can persist for many months. Posterolateral tibial microfracture may be associated with lateral femoral condylar microfracture in a proportion of patients. Another common pattern is a compression injury to the anterior aspect of the lateral femoral condyle, which causes deepening of the femoral notch and injury to the overlying articular cartilage. There is a plain radiographic correlate to this injury, deepening of the femoral notch by more than 1.5mm on a lateral plain film is strongly associated with ACL rupture . Less common are bony injuries to the posteromedial tibia with avulsion of the central slip of semimembranosis from the infra glenoid tubercle. Whether this is a true avulsion due to external rotation impaction as a result of anterior subluxation of the medial tibia during varus and external rotation is debated. The Segond fracture is an avulsion fracture of the anterolateral margin of the proximal tibial plateau at the site of attachment of the lateral capsular ligament. Although uncommon, there is a very high association with ACL rupture. Abnormal angulation of the anterior cruciate ligament measured either with respect to the intercondylar roof or the tibial attachment is a sign of rupture of the anterior cruciate ligament. Several different methods have been described to measure these angles. Signs of abnormal tibial translation are more reliable but not always present. Anterior cruciate ligament angles and therefore generally only measured when translation is not present.
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