Arcuate sign (knee)

The arcuate sign is often a subtle but important finding on knee x-rays and represents an avulsion fracture of the proximal fibula at the site of insertion of the arcuate ligament complex, and is usually associated with cruciate ligament injury (~90% of cases) ². The fracture fragment is attached to the lateral (fibular) collateral ligament, the biceps femoris tendon, or both.

Clinical presentation

The fracture results most often from a direct blow to the anteromedial tibia when the knee is extended, resulting in posterolateral subluxation of the tibia in external rotation ^1,3. Alternatively, sudden hyperextension of the knee with the tibia internally rotated may cause the same injury ¹.

The importance of this injury is that if it is not diagnosed acutely, posterolateral instability may develop which is challenging to correct, and may result in failed cruciate ligament reconstruction ^1,3.

Radiographic features

The shape and size of the avulsed fracture vary according to which ligaments are responsible.

Plain radiograph

The avulsion fracture is usually small, <1 cm in size, and involves the styloid process of the fibula. It is displaced superiorly and medially. Occasionally the fracture may be larger and extends to include the lateral aspect of the proximal fibula.

Slight internal rotation AP films are usually best to radiographically demonstrate this injury ³.

MRI

MRI can delineate the soft tissue component of the injury as well as evaluate associated injuries that are common. Associated findings include ²:

• cruciate ligament injuries

  • especially posterior cruciate ligament ³

• bone bruises

  • anteromedial femoral condyle: 50%

  • anteromedial tibial plateau: 28%

• meniscal tears

  • medial meniscus: 28%

  • lateral meniscus: 22%

• popliteus muscle injury: 33%

• medial collateral ligament injury: common ³

Ultrasound

During ultrasound evaluation of the proximal fibula the arcuate sign is visible as a hypoechogenic gap in the hyperechogenic cortex of the bone. The transducer should be orientated along with the long axis of the fibula to obtain the optimal visualisation ⁴.

Differential diagnosis

• Segond fracture

  • avulsion is from the tibia

  • fragment more medial and anterior