Anterior cruciate ligament tear

Anterior cruciate ligament (ACL) tears are the most common knee ligament injury encountered in radiology and orthopaedic practice.

Pathology

The ACL is the most commonly disrupted ligament of the knee, especially in athletes who participate in sports that involve rapid starting, stopping, and pivoting (e.g. soccer, basketball, tennis, netball and snow skiing).

Associations

• O'Donoghue's unhappy triad
• Segond fracture
• posteromedial corner injury of the knee

Radiographic features

An avulsion of the tibial attachment may be seen in younger patients.

Plain radiograph

• deep lateral sulcus sign 
• anterior tibial translocation sign
• Segond fracture 
• arcuate fracture
• joint effusion

CT

Considered to have high specificity and sensitivity in detecting ACL disruption ⁶. CT is helpful in characterising the avulsion bone fragment when it is present. 

MRI

Imaging of ACL tears should be divided into primary and secondary signs.

Primary signs are those that pertain to the ligament itself. Secondary signs are those which are closely related to ACL injuries.

Primary signs

• swelling
• increased signal on T2 or PDFS
• fibre discontinuity
• change in the expected course of ACL: ACL angle that is less steep than Blumensaat's line: when drawing a line in the course of a normal ACL on the sagittal image the angle should be as steep or steeper than the intercondylar roof, so the apex is pointing posteriorly. If the ACL is less steep than the intercondylar roof (i.e. the apex of the angle points anteriorly) means that ACL is completely torn and collapsed
• empty notch sign, in case of avulsion at the femoral attachment

ACL tears typically occur in the middle portion of the ligament (midsubstance tears) and appear as discontinuity of the ligament or abnormal contour. The signal of the ACL can be more hyperintense on T2. If the angle is still normal and there is a hyperintense signal, a partial rupture is more likely than a complete rupture.

Secondary signs

Secondary signs include:

• bone contusion in lateral femoral condyle and posterolateral tibial plateau ⁷
• >7 mm of anterior tibial translation, also known as anterior tibial translocation sign or anterior drawer sign ⁷
• uncovered posterior horn of the lateral meniscus
• Segond fracture, and to a lesser degree arcuate sign
• reduced PCL angle due to buckling of PCL ⁷
• positive PCL line sign ⁷
• medial or lateral collateral ligament injury

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