Bilateral knee lipoma arborescens mimicking persistent inflammatory arthritis in a patient with psoriatic arthritis.

Lipoma arborescens is a rare intra-articular lesion typically associated with chronic monoarthritis. We report on a man in his 40s with psoriatic arthritis, well controlled with secukinumab, who developed chronic bilateral knee effusion and soft, non-tender suprapatellar masses over several years despite low systemic disease activity. Magnetic resonance (MR) imaging revealed frond-like fatty proliferation with effusion, which was confirmed as lipoma arborescens on histopathology. Bilateral synovectomy restored full knee extension and improved mobility and function. MR imaging is an excellent non-invasive tool to diagnose persistent knee swelling, guide management and prevent long-term complications.