Musculoskeletal Imaging

Abscess

a localized collection of pus contained within tissues or confined spaces. For a general description, see abscess.

Infecting organisms in abscesses affecting the musculoskeletal system are usually Staphylococcus aureus (in bone abscesses) or Mycobacterium tuberculosis (in tuberculous abscesses).

A sharply delineated focus of infection in bone (Brodies abscess) is lined by inflammatory granulation tissue and may be rimmed by spongy eburnated bone. Such lesions are especially common in children and may localize to the metaphysis of tubular bones (Fig.1). Abscesses in the metaphysis, which can be single or multiple, occur during the subacute or chronic stages of osteomyelitis. Radiographically the abscesses appear as radiolucent areas with adjacent sclerosis. In the metaphysis, a tortuous channel may connect the lucent region with the growth plate. Abscess cavities in the diaphysis can be located centrally, in subcortical areas, or in the cortex itself and sometimes contain a central sequestrum. In an epiphysis, a circular, well-defined osteolytic lesion is seen. Cortical abscesses consist of a lucent lesion with surrounding sclerosis and periostitis, resembling an osteoid osteoma or a stress fracture. Cortical abscesses tend to be smaller or larger than approximately 2 cm and circular or elliptical, and without calcification; in contrast, an osteoid osteoma is characteristically smaller than 2 cm, circular, and may or may not have calcification; and a stress fracture is linear and without internal matrix.

Abscesses in tuberculosis frequently extend paraspinally from vertebral and discal sites to the adjacent ligaments and soft tissues. Subligamentous spread can allow invasion of bone and intervertebral discs at distant sites. Abscesses may burrow for extraordinary distances before perforating an internal viscus or the body surface. Tuberculous abscesses can produce soft tissue swelling on radiographs that appears out of proportion to the degree of osseous and discal destruction. In addition, tuberculous psoas abscesses may show calcification, although nontuberculous psoas abscesses rarely calcify.

More detailed assessment of Brodie's, psoas or paraspinal abscesses can be provided by CT scanning or MR imaging.

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Fig.1

a. Lateral radiograph of the knee in a 17-year old girl demonstrates an osteolytic lesion in the femur with a sclerotic border. b, c. Fat-suppressed T1-weighted gadolinium-enhanced sagittal (b) and axial (c) MR images of the distal femur demonstrate an abscess with peripheral enhancement and extension posteriorly into the soft tissues. (Courtesy of Doug Goodwin, MD, Lebanon, New Hampshire)
Abscess, Fig.1 (a)
Abscess, Fig.1 (b)
Abscess, Fig.1 (c)