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

Fibular Styloid Fracture/Arcuate Sign:

• Mechanism: Direct trauma to the anteromedial knee while in extension or sudden hyperextension. Case reports of this pathology 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.

References:
1. Knipe, D. "Arcuate Sign." Radiopeda. https://radiopaedia.org/articles/arcuate-sign
2. Huang et al. "Avulsion Fracute of the Head of the Fibula (The "Arcuate" Sign). American Journal of Radiology. February 2003.