The frontal knee x-ray shows a large osteochondral lesion of the lateral femoral condyle, whereas the lateral knee x-ray indicates a joint effusion.

An MRI scan of the knee confirmed osteochondritis dissecans (OCD) with intact overlying cartilage.

OCD is due to avascular necrosis of an osteochondral fragment. The average age of presentation is 10-20 years, and it can cause chronic knee pain in children. In the emergency setting, it can present as a locked joint due to separation of the osteochondral fragment into the knee, forming a loose body.