Brodie's abcess (subacute osteomyelitis)

Definition Brodie's abscess, a chronic abscess of bone surrounded by dense fibrous tissue and sclerotic bone.

Radiographic Appearance There is quite a well demarcated lucent area in the metaphysis of the humorous of the tibia. This lucent area lies immediately underneath the epiphyseal plate. There is no evidence of infiltrative type destruction of the bone adjacent to it and there is no periosteal reaction. The appearances are relatively non-specific, although this is a typical location for a Brodies abscess.In the early stages of osteomyelitis the x-rays are normal or may show some minor soft tissue changes. A nuclear bone scan is a much more sensitive test for detecting early disease. Generally osteomyelitis has to be present for at least 4 to 7 days before there are significant radiological changes. If there are radiological changes, the most common appearance is irregular bone destruction. Later periosteal reaction is seen, and even later this much better defined stage, known as a Brodies Abscess, is seen often when the osteomyelitis is inadequately or only partially treated.

Pathology Acute hematogenous osteomyelitis is most commonly seen in children and characterized by accumulation of the pathogenic organisms in the terminal arterioles and capillars of the bone metaphysis. In children a boy to girl ratio of 3/1 is seen. As edema and granulation occur, the intraosseous pressure may increase and result in bone necrosis due to compression of the vascular structures. These may lead to formation of a Brodie's abscess. In adults other pathogenic mechanisms of osteomyelitis are more common and include traumatic inoculation and spread from a nearby infected focus. Brodie's abscess as located form of chronic osteomyelitis is very common in children, due to high vascularity of the metaphysis and growth plates. Metaphyseal locations are most common before closure of the growth plates. After closure, a metaepiphyseal abscess is most frequent. When not hematogeneous in etiology, they occur most frequently in young adults at the long bones of the lower extremities. Pathologically, the wall of the abscess contains large amounts of granulation tissue, accounting for pronounced rim enhancement on contrast-enhanced MRI or CT scans. The central portions are mainly constituted by necrotic fluid and pathologic organisms. Staphylococcus aureus is cultured in half of the cases. The abscess is commonly surrounded by inflammatory changes and edema of adjacent bone marrow. Transcortical fistulization may lead to soft tissue spread. Until recently, early detection of bone abscedation was only possible by bone scintigraphy. This technique however is non-specific, as neoplastic changes or avascular necrosis revealed similar changes. MRI is considered more specific and furthermore allows better anatomical and topographical evaluation of disease extent. Only advanced stages of bone abscess are seen on conventional radiographs as areas of bone sclerosis with central radiolucency and eventually periosteal reaction and bone sequestration within the abscess

Treatment: Patients are initially treated with two days of intravenous antibiotics and then were switched to oral antibiotics to complete a six week course.

Image 1 Image 2 MRI Knee  (http://www.rbrs.org)A cortical fistula (arrow) is clearly demonstrated. Axial TSE T2-weighted MR-image

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