What lesion should be considered in this case?
There are only a small number of common epiphyseal lesions, which include: chondroblastoma, giant cell tumour (usually in patients with closed growth plates), osteomyelitis, clear cell chondrosarcoma and eosinophilic granuloma.
A sharply defined lucent lesion is centered on the epiphysis of the distal femur, and appears to transgress the growth plate (which remains open). It has a narrow zone of transition and no convincing matrix calcification. Anteriorly it appears to abut the articular surface, possibly breaching it. A joint effusion is present. No periosteal reaction is present.