Intra-Articular Lipoma of the Knee Joint

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Abstract
Although lipomas are common soft-tissue tumors, intra-articular occurrence is unusual1-4. Occasional cases of intra-articular lipoma arborescens have been reported5-11; however, true intra-articular lipoma is extremely rare3,4. These two entities have been considered as one in some recent reports12-14, but true lipoma and lipoma arborescens are different pathological entities3,4, with different clinical presentation and pathogenesis. Stressing the differences between true lipoma and lipoma arborescens, we describe the case of a patient who had a large intra-articular lipoma and no joint symptoms. A seventy-three-year-old woman who had been previously healthy was admitted to our hospital because of a soft-tissue mass involving the right knee joint. She had first noted the mass ten years earlier, when it appeared to be much smaller. Because she had no pain or limitation of activities, she had not sought medical attention. Upon discovering the mass, the patient's local physician referred her to us in September 1999. The medical history and the family history were unremarkable, and the patient remembered no trauma to the knee. Physical examination revealed a soft-tissue mass measuring 6 by 9 cm on the anteromedial aspect of the right knee joint (Fig. 1). On palpation, the mass was found to be adherent to the joint, soft, nontender, and similar in temperature to the surrounding tissue. The mass became more prominent and firm with contraction of the quadriceps muscle. No joint effusion was present. Full extension and 140 degrees of flexion of the knee were possible, without pain or locking. No muscle atrophy was evident. The results of blood tests, including erythrocyte sedimentation rate, rheumatoid factor, and C-reactive protein level, were essentially normal. Plain radiographs of the right knee joint showed no abnormalities except for soft-tissue swelling (Fig. 2). No osteoarthritic change or bone erosion was present. T1 and T2-weighted axial magnetic resonance images showed a soft-tissue tumor with high signal intensity containing linear structures of low signal intensity (Figs. 3-A and 3-B). The lateral part of the mass was located between the femur and the patella. A small amount of joint fluid also was demonstrated. The capsule of the mass was enhanced strongly on T1-weighted, fat-suppressed images after injection of gadolinium-diethylenetriamine penta-acetic acid (Fig. 3-C). The linear structures within the tumor were not enhanced. A gallium scan showed no uptake in the lesion. The preoperative differential diagnosis included intra-articular lipoma, extra-articular lipoma protruding into the joint, lipoma arborescens, and liposarcoma. An excision was performed. The mass was not directly apparent following the skin incision, but arthrotomy by means of a medial parapatellar incision revealed an ovoid soft-tissue mass within a white fibrous capsule (Fig. 4). The mass was not directly adherent to the joint capsule but was attached by a fibrous stalk to the posterior part of the joint capsule in the area of the suprapatellar bursa. The lateral portion of the lesion was located within the patellofemoral joint. No villous synovial proliferation was observed either over the surface of the tumor or elsewhere in the joint. The lesion was easily and completely removed after the stalk was cut. Slight erythema of the synovial tissue overlying the anterior surface of the femur was observed after the excision. Neither degenerative changes of the joint nor meniscal tears were seen. The cut surface of the mass showed lipomatous tissue.

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