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Illustration 1. The patient, an 18-year-old man who had had intermittent swelling of his left knee for several years, presented with recent onset of pain. Clinical examination showed the left knee to be diffusely swollen and warm. All other joints were clinically normal. Aspiration of the knee yielded 100 cc of clear yellow fluid. Magnetic resonance imaging (MRI) revealed an effusion with frond-like proliferation of synovium filling the entire knee joint, visible on a sagittal fat-suppressed fast spin-echo T2-weighted image (left) but more prominent on a sagittal T1-weighted image of the lateral aspect of the suprapatellar pouch (right). The signal intensity of the intraarticular mass-like lesion (arrow) followed that of signal intensity of subcutaneous fat (arrowhead) on all sequences, a key diagnostic clue. No bone erosions were present. Based upon these MRI results, the main differential diagnosis was lipoma arborescens. Considered to be a rare idiopathic joint abnormality, lipoma arborescens manifests as non-neoplastic lipid deposits that replace and distend the synovium, most often in the suprapatellar pouch of the knee. The synovial involvement leads to mechanical and inflammatory insult. The location and signal characteristics of lipoma arborescens are easily recognized by MRI. Although considered to be less likely, other diagnostic possibilities included tumefactive synovial osteochondromatosis, pigmented villonodular synovitis, and synovial hemangioma. Synovial osteochondromatosis typically presents with an effusion with multiple round loose bodies. If there is fat within the bodies, this indicates bone marrow, and ossification will be evident on the plain radiography. Due to hemosiderin deposition, pigmented villonodular synovitis demonstrates low signal intensity on T1-weighted images, variable signal on T2-weighted images, and typical dark blooming effects on gradient-echo images. Synovial hemangioma, a vascular benign tumor, is a rare cause of intraarticular mass that may contain phleboliths. Synovial biopsy of this patient's mass confirmed the lipoma arborescens. Arthroscopic synovectomy was performed, but after 1 year the lesion recurred.

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