Answer: Avulsion fracture of the fibular styloid process, compatible with an arcuate sign.

 Fibular Styloid Fracture/Arcuate Sign

• Mechanism is direct trauma to the anteromedial knee while in extension or sudden hyperextension; also seen in knee dislocations.
• Avulsion fracture at the proximal fibula at the insertion site of the arcuate ligament; 90% of fractures have disruption of the lateral collateral ligament
• Injury to other structures of the posterolateral ligamentous complex is also common. 
• This injury is highly suspicious of an unstable knee joint and needs emergent orthopedic evaluation.
• Due to anatomic location of fracture, can also involve injury to peroneal nerve.