The frontal left knee x-ray shows an avulsion fracture of the head of the fibula. This is the arcuate sign, and is indicative of an injury to the arcuate ligament complex, a major stabilising force to the postero-lateral aspect of the knee joint.

The HBL view of the knee shows a lipohaemarthrosis.

The arcuate sign is a subtle injury, but it carries a significant risk to the postero-lateral stability of the knee if missed.

The arcuate ligament complex inserts at the head of the fibula and consists of:

• PCL (posterior cruciate ligament)
• Lateral collateral ligament
• Biceps femoris tendon

Link to the reference article can be found here.