Radiology for the surgeon: Musculoskeletal case 41

The MRI of the knee shows an intraarticular soft tissue mass with polypoid frond-like projections located in the suprapatellar bursa. Areas within the mass follow the signal intensity of fat on all imaging sequences: high T1 signal intensity (Fig. 1A), intermediate-to-high T2 signal intensity (not shown) and signal suppression on fat saturation sequences (Fig. 1B). The mass enhances on postgadolinium images, and there is also diffuse enhancement of the synovium (Fig. 1C, 1D). A moderate-sized joint effusion is seen. There is no low signal intensity within the synovium on any of the imaging sequences to suggest the presence of hemosiderin. [Lipoma] arborescens (LA) is a rare, benign intra-articular lesion characterized by lipomatous villous proliferation of the synovium secondary to replacement of the subsynovial tissue by mature fat cells.1-8 There is associated chronic inflammation of the synovium.1-4 The disorder typically affects the knee as a monoarticular process and is most commonly located in the suprapatellar pouch; however, other joints and bilateral involvement have been described.1,4,5 MRI is the best imaging modality and the cornerstone of the preoperative diagnosis of LA. The typical MRI features have been well described in the literature. These features include 1) a frond-like synovial mass that is similar to fat signal intensity on all imaging sequences, 2) potential chemical shift artifact at fat-fluid interfaces, 3) absence of susceptibility artifacts owing to absence of hemosiderin deposition, which helps to exclude pigmented villonodular synovitis, and 4) associated joint effusion.1-8