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| IMAGE QUIZ ARCHIVE | ||||||
| Image Quiz | ||
| Painless Effusion of the Knee (continued) |
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| Answer: Lipoma arborescens | ||
 Fig. 3-A |  Fig. 3-B | For larger view, click on image |
| Fig. 3-A. Photomicrograph of a cross section of one of the villous projections, demonstrating mature, benign adipose tissue (× 40). Fig. 3-B. Photomicrograph demonstrating superficial inflammation and mature adipose tissue in the subsynovial layer (× 200). |
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| Discussion | ||
| Lipoma arborescens is a rare intra-articular condition that can result in painless swelling of a joint2,3. It almost always affects the knee, although there have been case reports of involvement of the wrist, hip, and ankle2-5. The lesion is a benign hyperplastic process that has been defined as a diffuse collection of fat replacing the subsynovial layer and resulting in the formation of villous projections6,7. | ||
| Patients typically are seen with a progressive, painless swelling of the knee. As the volume of the effusion increases, pain and limitation of the range of motion often develop8. Many patients are symptomatic for several years. The clinical course often is marked by intermittent exacerbations, with tense effusions lasting for several days. These exacerbations are believed to be caused mechanically, when hypertrophic villi become trapped between joint surfaces2. | ||
| The laboratory findings are generally unremarkable2,8,9. Joint fluid typically is negative for crystals, and cultures of the fluid are negative as well. Plain radiographs frequently reveal an extra soft-tissue lesion in the region of the suprapatellar pouch and often show osteoarthrotic changes8,10. | ||
| Computed tomography typically demonstrates a low-attenuation mass that is consistent with fat, and it may outline the synovial fronds6,8. The lesion is not enhanced after intravenous injection of contrast medium6,8. These features can help to differentiate lipoma arborescens from pigmented villonodular synovitis, which produces a high-attenuation lesion that is enhanced by contrast medium. The location of the lesion is also important: lipoma arborescens typically involves the suprapatellar recess, whereas pigmented villonodular synovitis tends to extend into the semimembranosus-gastrocnemius bursa9. | ||
| The appearance of lipoma arborescens on magnetic resonance imaging is believed to be pathognomonic11. Magnetic resonance imaging outlines the synovial mass and readily reveals its frond-like appearance8,11. The lesion demonstrates the same signal intensity as fat on all sequences8. In contrast, the lesion that is associated with pigmented villonodular synovitis typically demonstrates low signal intensity, secondary to hemosiderin, on both T1 and T2-weighted images8,9. | ||
| References | ||
1. Sola JB, Wright RW. Arthroscopic treatment for lipoma arborescens of the knee. A case report. J Bone Joint Surg Am. 1998;80:99-103. |
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