Answer: Subacute Osteomyelitis

Histology: Subacute osteomyelitis, also known as a Brodie’s abscess, is one manifestation of osteomyelitis where the body has attempted to heal the infection without medical intervention. The radiograph shows are a well-circumscribed lytic lesion in the metaphysis adjacent to the epiphyseal plate. Radiologically this may be confused with other inactive metaphyseal lesions such as non-ossifying fibroma, eosinophilic granuloma, and a chondromyxoid fibroma. The histologic features of osteomyelitis are typical of infection anywhere. These include granulation tissue and varying amounts of acute purulent exudate. The type of osteomyelitis, whether acute, subacute, or chronic, cannot be made on histologic grounds. This distinction is made on radiographic and clinical features. In this case, the radiology is that of subacute osteomyelitis.

Discussion: Histologically, an important differential is eosinophilic granuloma. Eosinophilic granuloma may present a very similar radiographic appearance and occurs in patients in the same age group as those with osteomyelitis. An important tool in differentiating these two entities is immunhistochemistry for S100 protein and CD1A. The Langerhans’ cells of eosinophilic granuloma stain positively with these stains whereas the histiocytes in osteomyelitis do not.