US and MRI findings of Synovial Lipoma Arborescens of the hip: a case report

Abstract

Lipoma arborescens is a rare intra-articular lesion consisting of villous lipomatous proliferation of the synovium that tends to occur in the knee joint, especially in the suprapatellar pouch. The lesion has been observed in other locations, including the glenohumeral joint, subdeltoid bursa, hip, and elbow. We report a case of a lipoma arborescens of a unilateral hip joint in a 62-year-old man. Ultrasonography and MRI were performed. In addition, a surgical synovectomy was performed and microscopic examination of the tumor revealed multiple variable-sized villous masses composed of mature fat tissue and lined by synovial cells which confirmed the diagnosis of lipoma arborescens. Synovial lipoma arborescens is a rare disorder of unknown etiology and is characterized by a villolipomatous proliferation of the synovial tissue. However, its association with chronic inflammatory disease such as a rheumatoid arthritis, degenerative lesions, or trauma is frequently reported. This case of lipoma arborescens was diagnosed by ultrasonography because MRI showed no difference between synovial osteochondromatosis and lipoma arborescens. On MRI, the signal of the lesion was similar to that of fat on T1-weighted, T2-weighted, and fat saturated images. The differential diagnosis includes intra-articular lipoma, synoval hemangioma, and synovial osetochondroma. On ultrasonography, the villous fatty projections of lipoma arborescens typically display echogenic pattern, similar to that of adjacent subcutaneous fat, and may undulate in real time within the surrounding effusion. The mass is usually soft in consistency and compressible.