Lipoma Arborescens

Frond-like, tree-like projections from the synovium that follow fat signal - bright on T1 and suppressing on fat-saturated sequences - in a chronically, usually painlessly swollen knee with a recurrent effusion is the near-pathognomonic appearance of lipoma arborescens. This is a rare benign intra-articular lesion, a villous proliferation of mature adipose tissue that replaces the synovial lining, best regarded as a reactive change rather than a true neoplasm. It most commonly affects the knee, particularly the suprapatellar pouch, and it is often associated with chronic synovial irritation or inflammation, such as osteoarthritis, rheumatoid arthritis or prior trauma, although a primary idiopathic form also occurs. MRI is the gold standard, and the fat signal is the key to the diagnosis - it distinguishes lipoma arborescens from the main mimics: pigmented villonodular synovitis, which contains haemosiderin and shows blooming, low signal, on gradient-echo sequences, and synovial chondromatosis, which produces cartilaginous or calcified loose bodies. The treatment of choice is synovectomy, which can be open or arthroscopic, and it gives excellent outcomes with low recurrence when the abnormal synovium is completely excised, resolving the swelling and effusion. I would also address any associated joint pathology, such as osteoarthritis, as appropriate.