Anterior cruciate ligament tear

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

Clinical Presentation 

Patients usually present complaining that their knee "gives way" especially during pivoting movements. They also describe apprehension when attempting to do non linear tasks. They usually describe a popping sensation at the time of injury combined with swelling and the initial inability to weight wear which improves in a short period. The combination of the Lachman, pivot shift and anterior drawer tests is used to clinically confirm the diagnosis ⁹. A positive pivot shift is considered the most sensitive clinical sign ¹⁰. 

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 characterizing 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
• fiber 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.

ACL tear may only involve one bundle. Imaging signs of isolated posterolateral bundle tear are as follows: 

• gap sign: fluid signal and/or a gap between the medial aspect of the lateral femoral condyle and the lateral aspect of the mid-ACL,can be seen on either axial or coronal MRI images.
• footprint sign: incomplete coverage of the lateral aspect of the tibial spine of the tibia by the distal ACL attachment, seen only on coronal MRI images ⁸

Secondary signs

Secondary signs include ⁷:

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

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