Imaging description

Radiography shows evidence of a joint effusion, and variable degrees of osteoarthritis. Erosions have been described in about 25% of patients with lipoma arborescens but they are less common compared with pigmented villonodular synovitis. CT shows the fatty mass and the joint effusion. Attenuation measurements demonstrate the low density within the mass consistent with fat. Ultrasound is also very effective in demonstrating the joint effusion and the villous nature of the mass. In addition ultrasound demonstrates the hyperechoic appearance of the mass suggesting the presence of fat. The appearance of lipoma arborescens on MRI is believed to be pathognomonic. MRI can readily reveal the joint effusion, the mass, and the frond-like projections. The fatty components of the tumor follow the signal characteristics of the subcutaneous fat on all pulse sequences (Figure 76.1). MRI can differentiate lipoma arborescens from other intra-articular diseases like pigmented villonodular synovitis (PVNS) and synovial chondromatosis.

Importance

Lipoma arborescens is a rare intra-articular lesion where there is replacement of the subsynovial tissue with mature fat cells and the formation of proliferative villous projections. The exact etiology of lipoma arborescens is not known but it has been suggested that it represents a reactive process of the synovial membrane due to a variety of insults. Lipoma arborescens occurs in association with degenerative joint disease, rheumatoid arthritis, and trauma. The condition commonly affects men, and the knee is the most commonly involved joint although the disease has been described in the wrist, hip, glenohumeral joint, subachromal subdeltoid bursa, and tendon sheath of the peroneal tendons. In the knee, which is the most commonly involved joint, lipoma arborescens has a predilection for the suprapatellar pouch. Bilateral knee involvement has been described in about 20% of the patients. Lipoma arborescens is often associated with a joint effusion and aspiration of the effusion reveals serosanguinous fluid without crystals or microorganisms.