The knee x-rays show an avulsion fracture of the head of the fibula and the lateral view of the knee shows mild effusion.
This avulsion fracture of the fibular head is called the arcuate sign and is due to an injury to the arcuate ligament complex which consists of the lateral collateral ligament, the popliteofibular ligament, the biceps femoris and the popliteus tendon. These fractures are also associated with injury to the cruciate ligaments.
Clinically, knee instability and peroneal nerve function needs to be checked. As such, patients with this type of injury need a referral to an orthopaedician from the ED.