[True intra-articular lipoma in a rheumatoid knee]

Lipoma is a frequent benign tumor of the soft tissue, but intra-articular locations are rare. We report a case that occurred in a rheumatoid knee.

A 50-year-old woman had been treated for 17 years for seronegative deforming rheumatoid arthritis. Disease course under corticotherapy (7.5 mg/day) proceeded by flares and remissions. She had reported arthritis of the left knee for the past two years; concern about infection led to aspiration of the knee joint, which found inflammatory aseptic fluid, and radiography of the knee was normal. The patient was unable to afford magnetic resonance imaging or computed tomography. Synovial biopsy showed nonspecific chronic synovitis. Repeated corticosteroid injections produced no improvement. A second synovial biopsy was performed without specific results. Surgical biopsy followed and identified an intra-articular fatty mass, which was then excised surgically. Histologic examination showed a true synovial lipoma.

True intra-articular lipoma, found mostly in the knee, is extremely rare and usually occurs de novo. For our patient its appearance in an arthritic knee required that lipoma arborescens be ruled out. MRI can provide a positive and differential diagnosis, but nonetheless requires histologic confirmation. It shows an encapsulated adipose mass, surrounded by synovial membrane, while lipoma arborescens is a villous proliferation in which fat cells infiltrate the synovium. Treatment is surgical or arthroscopic.