Clinical History: A 78 year-old male complains of a distal thigh mass for six months. MRI of the distal thigh was performed. Corresponding (1a) sagittal fat-suppressed proton density (PD FS) and (1b) T1-weighted images are presented. What are the findings? What is your diagnosis?
Villous lipomatous proliferation of the synovial membrane (lipoma arborescens).
Lipoma arborescens is a “lipoma-like” benign villous proliferation of synovium with replacement of the subsynovial connective tissue by mature fat cells. It is not a neoplasm, but rather thought to be a nonspecific reactive response to chronic synovial irritation, whether from mechanical or inflammatory insults. Other terms that have been used in the literature include diffuse lipoma of the joint, diffuse synovial lipoma, diffuse articular lipomatosis, fatty infiltration of the synovial membrane, or villous lipomatous proliferation of the synovial membrane. The latter two emphasize the non-neoplastic nature of this lesion.
The condition is frequently described as rare, but imaging diagnosis was very difficult prior to the advent of MRI2. It turns out that mild subsynovial fatty infiltration is not uncommon, although more extensive lesions which acquire the term lipoma arborescens are less common. Patients are typically in the 5th – 7th decades, but lipoma arborescens also occurs in young patients with chronic arthritis. One report described men affected more often than women7, but a more extensive series showed no significant gender predilection11.
Lipoma arborescens is nearly always associated with underlying degenerative joint disease, chronic rheumatoid arthritis2,10, or prior trauma. An effusion is typically present, and can be produced by the lesion or from the underlying arthritis2. Rare cases of a primary lesion without an underlying chronic articular process have been reported. Some authors have proposed two separate categories of lipoma arborescens, an idiopathic “primary” type occurring in younger patients, and a much more common “secondary” type most often seen in elderly patients11. Lipoma arborescens is unilateral in most cases11 but bilateral lesions may be found in patients with underlying bilateral chronic arthropathy.
The name lipoma arborescens reflects its gross appearance, that of a fatty mass with a branching villous growth pattern. Microscopically, there is synovial hyperplasia with scattered chronic inflammatory cells and abundant deposition of mature adipocytes in the subsynovial tissue5. The adipocytes are accompanied by nutrient blood vessels and are covered by several layers of synovial cells. Synovium responds to irritation by increased capillary blood flow, influx of inflammatory cells, and synovial proliferation. As this becomes chronic, fat cells may also infiltrate the subsynovial tissue.
MRI demonstrates frond-like areas projecting inward from the synovium with signal equal to fat on all imaging sequences. If intravenous Gadolinium contrast is administered, enhancement can be seen of the chronically inflamed overlying synovium, but no enhancement is seen in the underlying fat. The fatty proliferation may be of such degree that it forms a mass, but mild cases with a pattern of diffuse villous proliferation are much more common. Villi may be small and feathery in appearance or they may be more bulky fatty lobules. Osseous erosions have been reported secondary to lipoma arborescens8, although when present they are more often due to underlying arthritis.
Differential diagnosis on MRI
Lipoma arborescens has characteristic features that usually make it a straightforward MRI diagnosis. However, other entities occurring as filling defects in the suprapatellar recess may cause confusion.
Small filling defects outlined by effusion in the suprapatellar bursa are most obvious on sequences where fluid is bright (such as T2-weighted or fat-suppressed proton-density imaging sequences). Correlation with T1-weighted images allows us to better assess the nature of this tissue. Findings which remain isointense to fat on all imaging sequences are reliably confirmed as fat. If in addition a sclerotic peripheral rim is present, the appearance is typical for osseous loose bodies.
If signal is intermediate on T1-weighted images, the tissue may be hyaline cartilage, such as from synovial chondromatosis. In this condition, multiple nodules of hyaline cartilage form in subsynovial connective tissue. The small lobules of hyaline cartilage may calcify centrally, forming areas with numerous punctate foci of very low signal. The cartilage is distributed throughout the joint, within the suprapatellar recess as well as low anterior and posterior locations, including within Baker’s cysts. Osseous erosions are often present.
Intermediate signal intensity can also be seen with fibrinous tissue, such as “rice bodies” which may accompany rheumatoid arthritis. These are typically very small in size and numerous. Dependent positioning reflects that they are free in the joint rather than fixed to the synovium.
Very low signal on all imaging sequences may indicate the presence of calcium. Numerous small foci of calcification are often seen in synovial chondromatosis.
Low to intermediate signal intensity on T1-weighted images and low signal intensity on T2-weighted or gradient echo sequences is more often seen with hemosiderin deposition. Variable quantities of hemosiderin are deposited heterogeneously in larger areas in pigmented villonodular synovitis (PVNS) or hemosiderotic synovitis (such as from trauma or hemophilia). Hemosiderin deposition can also often be found in rheumatoid arthritis.
It may be difficult to determine if an edematous fatty mass located anteriorly is projecting into the suprapatellar recess or if it is the quadriceps fat pad itself. An edematous fat pad may develop from quadriceps fat pad impingement (see prior MRI Web Clinic of September 2008). With chronic mechanical impingement and inflammation, the fat pad may hypertrophy, causing an appearance compatible with lipoma arborescens.
Synovial hemangioma (venous malformation) is an uncommon lesion which may be intraarticular or extraarticular. It has a characteristic appearance that may at first glance be confused with lipoma arborescens. However, the signal intensities are reversed – the lobulated lesions are bright on fat-suppressed images, but lower in signal on T1-weighted images. Venous malformations are comprised of low-flow serpiginous vessels with signal intensity typical of fluid. These are separated by small amounts of fat. Calcified phleboliths may be also present.
A true intraarticular lipoma is a rare lesion characterized by a localized rounded mass of adipose tissue rather than multiple villi. There is no associated synovial proliferation, but rather a thin fibrous capsule. Intraarticular lipomas can be sessile or pedunculated.
Intraarticular liposarcoma is a rare lesion which we have not encountered in our practice. Liposarcomas occur in two main forms, most commonly low grade myxoid liposarcoma, found in young adults, and less commonly high grade pleomorphic liposarcoma, found in older patients. Liposarcomas are typically found proximal to the knee or elbow, with high grade lesions most often found in the buttocks, abdomen, or groin. Low grade lesions are well-differentiated and have high fat content. One case report3 described a mass in the knee which resembled a lipoma on MRI and at arthroscopy. Histologically they found a low grade myxoid liposarcoma. They concluded that all lipomas at unusual sites, such as the knee, should be investigated for possible malignancy. Another case report9 described a high-grade pleomorphic intra-articular liposarcoma of the knee. The MRI showed a lesion more closely resembling PVNS, with multifocal nodules anteriorly and posteriorly. These had intermediate signal, not matching fat, and thus should not be mistakenly diagnosed as lipoma arborescens.
Although lipoma arborescens is most often found at the suprapatellar recess of the knee, it can occur wherever there is synovium. It occurs in other joints and bursae (hip, shoulder, wrist, elbow), and more rarely in the tendon sheaths (a few reports in tendon sheaths at the ankle and one at the wrist – usually with accompanying involvement of the adjacent joint)4. We have encountered several in the shoulder, both in the joint and in the subacromial-subdeltoid bursa. As with previously reported cases in the shoulder6, nearly all of our patients had chronic rotator cuff tears.
Lipoma arborescens most often develops in the setting of chronic underlying arthritis. The underlying arthritis is the primary problem requiring treatment and the subsynovial fatty proliferation is a usually a secondary feature. In patients without underlying arthritis, this may rarely be a primary process and the cause of recurring effusions, or a marker of underlying chronic synovitis. If large or mass-like, lipoma arborescens can cause pain or limit range of motion. When found on MRI exams, this benign process should not be mistaken for liposarcoma.
When indicated, synovectomy can be curative1. However, if underlying chronic arthritis remains, recurrence is possible.
Villous subsynovial lipomatous proliferation is an uncommon response to chronic synovial irritation, whether mechanical or inflammatory in nature. It is typically found in chronically arthritic joints, most often in the suprapatellar recess of the knee, but also in other joints, bursae, and even tendon sheaths. Occasionally this process becomes mass-like and more deserving of the term lipoma arborescens. Features on MRI are distinctive. Many radiologists, rheumatologists, and orthopedic surgeons will encounter this in their practice and should not mistake it for neoplasm or other pathology.
1 Blais RE, LaPrade RF, Chaljub G, Adesokan A. The arthroscopic appearance of lipoma arborescens of the knee. Arthroscopy 1995;11(5):623-627.
2 Coll JP, Ragsdale BD, Chow B, Daughters TC. Best Cases from the AFIP: Lipoma Arborescens of the Knees in a Patient with Rheumatoid Arthritis. Radiographics 2011 Mar-Apr;31(2):333-7.
3 Khan AM, Cannon S, Levack B. Primary Intra-Articular Liposarcoma of the Knee. Case Report. J Knee Surg 2003 Apr;16(2):107-9.
4 Moukaddam H, Smitaman E, Haims AH. Lipoma arborescens of the peroneal tendon sheath. J Magn Reson Imaging 2011 Jan;33(1):221-4.
5 Murphey MD, Carroll JF, Flemming DJ, Pope TL, Gannon FH, Kransdorf MJ. From the archives of the AFIP: benign musculoskeletal lipomatous lesions. Radiographics 2004 Sep-Oct;24(5):1433-66.
6 Nisolle J, Blouard E, Bardrez V, Boutsen Y, De Cloedt P, Esselinckx W. Subacromial-subdeltoid lipoma arborescens associated with a rotator cuff tear. Skeletal Radiol 1999;28(5):283-285.
7 O’Connell JX. Pathology of the Synovium. Am J Clin Pathol 2000;114:773-784.
8 Ryu KN, Jaovisidha S, Schweitzer M, Motta AO, Resnick D. MR imaging of lipoma arborescens of the knee joint. AJR Am J Roentgenol 1996 Nov;167(5):1229-32.
9 Shaerf DA, Mann B, Alorjani M, Aston W, Saifuddin A. High-grade intra-articular liposarcoma of the knee. Skeletal Radiol 2011 Mar; 40(3):363-5.
10 Weston WJ. The intra-synovial fatty masses in chronic rheumatoid arthritis. Br J Radiol 1973;46:213-216.
11 Vilanova JC, Barceló J, Villalón M, Aldomà J, Delgado E, Zapater I. MR imaging of lipoma arborescens and the associated lesions. Skeletal Radiol 2003 Sep;32(9):504-9. Epub 2003 Jun 17.