Introduction:
Most common lipid lesion of bone
40-50 years with slight male prediliction
Usually present with pain
Most common region is intertrochanteric region of femur (1/3rd), followed by calcaneus (8%). Other bones include ilium (adjacent to SIJ), tibia, fibula, humerus, ribs.
Long bone: usually metaphyseal, but can be diaphyseal and epiphyseal involvement is rare. Usually intramedullary, rarely intracortical.
Typically single
As the lesion ages, undegoes fat necrosis, which inturn undergoes dystrophic calcification, cyst formation, reactive new bone formation
Contain varying amounts of fat, bone, fibrous tissue, cystic degeneration
Radiograph:
Intraosseous lipomas composed solely of fat (Milgram stage 1) are radiolucent, well-circumscribed lesions that frequently are associated with mild focal expansile remodeling of the affected bone (50%), may have septated appearance. Expansile in thin long bones (fibula and ulna). Differentials include SBC, FD, plasmacytoma.
Central or peripheral ossification or calcification (Milgram stage 2 or 3) may be seen, which is helpful sign in narrowing differentials
Severely involuted lesions show thick peripheral ossification with variable central ossification-calcification.
Expansile rounded lucency rather is feature of lipoma, where as bone necrosis is seen as irregular serpentine margin
CT:
less than –60 to –100 HU. Expansile remodeling of medulla
The attenuation of normal marrow is slightly higher than lipoma (due to cellular elements in marrow)
Central cystic areas, surrounded by rim of ossification, surrounded by fat, surrounded by a rim of ossification or fibrous capsule (bull’s-eye appearance) is characteristic for lipoma
MR :
May be difficult to differentiate from surrounding normal yellow marrow, but usually shows slightly lower signal than lipoma on T1.
Bone scintigraphy:
Absent to moderate uptake
Treatment:
Asymptomatic lesions are not treated. Symptomatic one with curettage / bone graft placement. Recurrence and malignant transformation are rare
References:
Murphey MD et al. Benign Musculoskeletal Lipomatous Lesions. RadioGraphics 2004;24:1433-1466
