PMC4901022_01
25-year-old woman with an extraskeletal para-articular osteochondroma. Lateral radiograph of the right knee demonstrates an ovoid calcified mass in the Hoffa's fat pad (arrow).
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There was no joint effusion, and no evidence of bony attachment to the patella, tibia, or femur (Fig. 1).
25-year-old woman with an extraskeletal para-articular osteochondroma. Precontrast coronal T1-weighted (A), sagittal T2-weighted (B), and axial gradient-echo (C) MRI images of the right knee. There is an ovoid mass in the Hoffa's fat pad. The mass shows iso-signal intensity to the muscle peripherally, and there is a lobulated central osseous lesion that shows similar signal intensity to the bone on T1-weighted image (arrow in A). It shows slightly heterogeneous, diffuse, high signal intensity, with the central lesion showing a low signal intensity rim on T2-weighted image (arrow in B). Axial precontrast gradient-echo (GRE) image shows iso-signal intensity of the lesion, with the central portion showing slightly lower signal intensity than the periphery (arrow in C).
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An axial, precontrast, gradient-echo (GRE) image showed iso-signal intensity of the lesion, with the central portion showing slightly lower signal intensity than the periphery (Fig. 2).
25-year-old woman with an extraskeletal paraarticular osteochondroma. Post-contrast axial (A) and sagittal (B) gradient-echo images of the right knee. The mass shows mainly rim enhancement (arrow in A and B) with scattered nodular enhancing portion (arrowhead in B) internally, and the central portion shows bright diffuse contrast enhancement.
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Axial and sagittal postcontrast GRE images demonstrated mainly rim enhancement of the mass, with a scattered nodular-enhancing portion internally, and the central portion showed bright diffuse contrast enhancement (Fig. 3).
25-year-old woman with an extraskeletal paraarticular osteochondroma. The cartilage matrix of the lesion was composed of hyaline cartilage or fibrocartilage as evidenced by eosinophilic collagen fibers within the matrix. Some areas had poorly formed myxoid cartilage (A). Endochondral ossification of the peripheral rim of cartilage (top) to mature trabecular bone with fatty marrow is characteristic of para-articular osteochondroma (B).
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There was extensive endochondral ossification of the cartilage to mature-appearing trabecular bone (Fig. 4).