Lipoma arborescens: An unusual cause of swelling of the knee.

We report a patient with a lipoma arborescens in the knee, a chance finding discovered on MRI. This is an unusual cause of swelling of the knee joint; if this condition is present, it is almost always located in the suprapatellar pouch. In this case, the lipoma arborescens was found in the popliteal space.

Case report

A 71-year-old female patient presented at our outpatient orthopedic department because of pain and swelling at the medial and posterior aspect of the right knee. These complaints appeared after a long walk. On clinical examination, we found some diffuse swelling of the knee joint as well as a localized swelling in the popliteal space. Flexion measured 130 degrees, with 10 degrees of extension deficit. There was tenderness around the medial joint space, with a positive McMurray sign. Collateral and cruciate ligaments were stable. Conventional radiographs showed no abnormalities.

MRI of the right knee revealed a medial and lateral meniscal tear, a Baker’s cyst in the popliteal space, and a focus of lipoma arborescens (Figure 1, Figure 2, Figure 3). We advised the patient to undergo arthroscopy, but because of diminishing symptoms, this patient preferred to be treated expectantly.

Figure 1

71-year-old female with lipoma arborescens. Sagittal T1-weighted MRI of the right knee shows a focus of lipoma arborescens (1) of 26.9 mm length. A Baker's cyst (2) was also found in the popliteal space (2).

Figure 2

71-year-old female with lipoma arborescens. Coronal T1-weighted MRI of the right knee shows a focus of lipoma arborescens (1).

Figure 3

71-year-old female with lipoma arborescens. Axial, T2-weighted, fat-suppressed MRI image of the right knee shows a focus of lipoma arborescens (1) and a Baker's cyst (2).

Discussion

Lipoma arborescens is a rare, primary, benign tumor of the synovium of diarthrodial joints. It is characterized by villo-lipomatous proliferation of the synovium and diffuse replacement of the subsynovial tissue by mature fat cells (1, 2). It was first described by Arzimanoglu in 1957 (3). The proliferating cells resemble a tree, which explains the name “arborescens,”from the Latin word “arbor” (meaning tree) (4, 5).

Generally, one joint is affected, with the highest prevalence in the suprapatellar pouch of the knee joint (1, 2, 6, 7). Other joints that can be affected are the hip, shoulder, elbow, and ankle (8, 9, 10, 11, 12). The sex ratio is almost equal (4). The definite etiology of lipoma arborescens is unknown (1, 3), but the frequent occurrence of associated conditions (including local trauma, osteoarthritis, diabetes mellitus, and rheumatoid arthritis) suggests that this may be a reactive disease (1, 4, 5, 6, 13, 14). Patients usually present with a longstanding, slowly progressive swelling of the involved joint and effusion. With increasing effusion, pain and restriction of the range of motion can develop (7, 15, 16). The condition can also present as a recurrent hemarthrosis in patients with knee osteoarthritis (17).

The differential diagnosis of synovial proliferative disorders should include pigmented villonodular synovitis, rheumatoid arthritis, synovial chondromatosis, synovial hemangioma, intra-articular lipoma, xanthomata, and lipoma arborescens (4, 7, 15, 16, 18, 19).

The diagnosis can be made by MRI, which can be considered to be the gold standard (4, 15, 20). Characteristic MRI findings include the following:

A localized synovial mass with a frondlike architecture

Fat-signal intensity on all pulse sequences

Suppression of signal with fat-selective presaturation

Associated joint effusion

Potential chemical-shift artifact (a thin, low-intensity line that partially surrounds structures on MRI; this artifact is seen at the interface of tissues with different chemical shift properties, like water and fat, due to the phenomenon of chemical shift)

Absence of magnetic susceptibility effects from hemosiderin (for differentiation from pigmented villonodular synovitis)

Treatment consists of synovectomy. Arthroscopic partial synovectomy can be performed for lipoma arborescens in the anterior compartment of the knee (14). In more extensive cases, a total synovectomy should be performed by arthrotomy (1, 7, 17, 21). The recurrence rate is low (1, 7, 14, 17, 21). No malignant degeneration has been described.