Imaging of the anterior cruciate ligament


The ACL resists anterior tibial translation during extension and provides rotational stability[6-8]. The anteromedial bundle is taut when the knee is flexed and the posterolateral bundle is taut when the knee is extended[3]. The anteromedial bundle is longest in flexion and may be the primary component that resists anterior displacement of the tibia in flexion. The posterolateral bundle seems primarily to resist anterior tibial translation in extension and also contributes to rotatory stability of the knee joint[8] being employed in the “screw home” phenomenon i.e. during terminal extension of the knee, the tibia externally rotates relative to the femur serving to “lock” the knee in extension. The anteromedial and posterolateral bundles stabilize the knee joint in response to anterior tibial loads and combined rotatory loads in a synergistic way[9].

ACL tears may be partial or complete. Partial tears can range from a minor tear involving just a few fibres to a high grade near-complete tear involving almost all of the ACL fibres. A partial tear can involve both or only a single bundle to varying degree. Sometimes plastic deformity of the ACL without fibre discontinuity can occur causing ACL insufficiency[10].

The mechanism of the ACL injury includes internal rotation of the tibia relative to the femur. This commonly occurs during falls while skiing, as well as in contact sports such as football. With valgus stress, the medial femorotibial joint compartment is distracted producing medial collateral injury and medial meniscal injury (O’Donoghue’s triad). Another mechanism of ACL injury is hyperextension such as occurs during jumping or high kick maneuvers and will lead to contra-coup bone contusion on the anterior tibia and femoral condyle. ACL tears resulting from hyperextension frequently occur without concomitant collateral ligament or meniscal injury[11]. The third mechanism is external rotation of the tibia relative to the femur with varus stress leading to impaction and bone oedema medially and distraction laterally resulting in avulsion of the lateral tibial rim (Segond fracture) and tear of the lateral collateral ligament.

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The majority of the ACL injuries can be diagnosed by history and clinical examination. The anterior drawer test, Lachman test and pivot shift test are the most commonly applied clinical tests to diagnose ACL tear though they do rely both on the experience of the clinician and the degree of patient cooperation. In chronic ACL insufficiency, the pivot shift test has reported high sensitivities for detecting the ACL injury ranging from 84% to 98.4%. The test’s specificity has been shown to vary more widely, with reported values from as low as 35% in the alert patient to as high as 98.4% in the anesthetized patient[12]. Anterior drawer and Lachman tests have similar sensitivity but lower specificity. However, in acute injury, if the patient is in pain or swelling, the examination may be limited and the sensitivity and specificity of the clinical tests are limited[12]. Association injury such as meniscal tear or chondral injury may also limit a full clinical examination. As a result, magnetic resonance imaging (MRI) is helpful in the assessment of suspected ACL injury.

Most ACL tears (approximately 80%) are complete, occurring around the middle one-third of the ACL (90%) or less frequently close to the femoral (7%) or tibial (3%) attachments. Less frequently (approximately 20%), ACL tears are incomplete with partial disruption of the ACL fibres[13]. Partial tears may involve only one or both bundles to a varying degree though the anteromedial band does tend to be the more commonly affected. Imaging, and in particular MRI, is very helpful in the assessment of suspected ACL injury.