Lipoma arborescens of the hip.

Lipoma arborescens is a rare intra-articular lesion characterized by extensive villous proliferation of the synovial membrane and hyperplasia of subsynovial fat. Although it has been described as arising from traumatic, inflammatory, rheumatologic, developmental, and neoplastic causes, its etiology remains unknown. This article describes in detail--for the first time in the orthopedic literature-a case of lipoma arborescens of the hip. Clinical presentation, histologic and radiographic findings, and treatment of this rare lesion are discussed. Based on clinical, radiologic, and surgical evaluation, differential diagnosis for this lesion should include other conditions that cause synovial thickening, proliferation, and joint effusion--for example, synovial chondromatosis, pigmented villonodular synovitis, synovial hemangiomatosis, and rheumatoid arthritis. Description of this case covers the full radiologic evaluation, including magnetic resonance imaging (MRI), diagnostic pathologic description, and minimum 24-month follow-up. Although lipoma arborescens of the hip is a rare condition, it should be considered in the differential diagnosis of patients with its characteristic clinical prodrome and MRI findings confirming a periarticular fatty mass.