The knee x-ray shows Segond fracture.

The Segond fracture is cortical avulsion of the proximal tibia located just below the level of lateral plateau. The tiny avulsion fracture has a high association with significant internal knee derangement including tear of the anterior cruciate ligament. Mechanism is related to high internal rotation and varus knee stress.

Segond fracture is usually associated with lipohaemarthrosis. So, if the lateral knee x-ray shows significant knee joint effusion or a lipohaemarthrosis, carefully scrutinize the AP view for the presence of Segond fracture or other intraarticular knee injuries (tibial plateau, patellar fracture).

Further reading https://www.ajronline.org/doi/10.2214/AJR.14.12693