Lipoma arborescens is a rare, benign intra-articular lesion of unknown etiology in which there is diffuse replacement of the subsynovial tissue by mature fat cells, with prominent villous transformation of the synovium. Associated conditions have included osteoarthritis, joint trauma, diabetes mellitus; in 20% of the cases, popliteal cysts were noted. The knee is most commonly involved, but the condition has also been described in the wrist, shoulder, and hip.

The appearances on MRI are diagnostic and include a synovial mass with a frond-like architecture and a fat-signal intensity on all pulse sequences, which is suppressed using fat-selective presaturation. There is no evidence of magnetic susceptibility effects from hemosiderin; there are an associated effusion and a potential chemical shift artifact.