Musculoskeletal Imaging

Lipoma

a benign tumour composed of mature fat cells, which may occur in any tissue that contains fat. Lipomas are very common soft tissue lesions but are unusual in bone. Deep within the limb lipomas may grow within muscles (intramuscular type) or between muscles (intermuscular type). Occasionally they may be associated with nerve paralysis, macrodactyly, osseous deformity and carpal tunnel syndrome.

Lipomas of soft tissue show predilection for the subcutaneous tissues of the back, extremities and thorax. Ossification is observed occasionally, and lipomas located close to a bone may incite cortical hyperostosis (Fig.1). Other varieties of fatty tumours include lipomatosis, hibernoma, lipoblastomatosis, lipoma arborescens, macrodystrophia lipomatosa, fibrolipomatous hamartoma and mesenchymoma.

The radiographic appearance of lipomas is of a homogeneous radiolucent mass with sharp margins. Muscle contraction may change the lesion's shape, however. A tumour with a nonhomogeneous appearance is of concern because this pattern is typical of liposarcoma. On arteriograms lipomas show displacement of normal vessels and absent neovascularity. On CT scans the tumours usually appear as homogeneous, sharply marginated as low density masses, which do not enhance after administration of contrast agent. MR imaging also provides accurate assessment of lipomas.

Synovial lipomas are solitary round or oval masses of mature fat, covered with synovium, occurring almost exclusively in the knee joint. Lipomas originating in the periosteum are rare. On radiograph a radiolucent mass may be observed adjacent to bone, frequently with associated cortical hyperostosis. Sometimes calcifications are present.

Lipomas of bone may be intraosseous, cortical or parosteal. Intraosseous tumours are found most frequently in the fibula, femur, tibia and calcaneus. On radiographs they appear as osteolytic lesions with a thin but well defined sclerotic border. Commonly internal bone ridges or lobulation is present. In the calcaneus and proximal end of the femur this appearance is nearly diagnostic (Fig.2). Calcaneal lipomas frequently have a central radiodense calcified or ossified focus (see calcaneus (III:1), Fig. 3). Intracortical lipomas are characterized by nonspecific radiolucent lesions in the cortex. Parosteal tumours generally occur in long tubular bones and cannot easily be distinguished from a lipoma arising in the periosteal membrane.

CT scanning and MR imaging are able to identify the tissue characteristics (i.e. fat) in lipomas of bone.

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Fig.1

a. Radiograph of the hip demonstrates irregular calcifications in the soft tissues adjacent to the hip joint. b. Sagittal T1-weighted MR image shows a large mass of predominantly high signal intensity (owing to the presence of fat) that also contains irregular areas of low signal intensity (owing to the presence of calcifications). Most of the lipoma is isointense with the adjacent subcutaneous fat. c. Axial fat-suppressed T1-weighted MR image demonstrates suppression of the signal intensity in most of the lipoma. The central low signal intensity represents calcification (seen on the radiograph).
Lipoma, Fig.1 (a)
Lipoma, Fig.1 (b)
Lipoma, Fig.1 (c)
Lipoma, Fig.2 (a)
Lipoma, Fig.2 (b)
Lipoma, Fig.2 (c)