A fifty-eight-year-old man was seen because of a three-month history of recurrent painless effusion in the left knee. Aspiration of the knee and injection of cortisone had been performed at another institution, but the effusion had recurred after one to two weeks. The effusion was not relieved with use of non-steroidal anti-inflammatory agents. The patient had not noted catching, locking, or giving-way of the knee, but the swelling seemed to be worse after activities that involved twisting.
Physical examination revealed a large effusion in the left knee. The active range of motion was from 5 to 125 degrees of flexion. Tests for instability were negative, and there was no tenderness at the joint line. Anteroposterior radiographs (made with the patient bearing weight), lateral radiographs, and Merchant radiographs revealed normal tibiofemoral joint spaces and mild patellofemoral osteoarthrosis.
Because the patient had a history of recurrent effusion, magnetic resonance imaging of the left knee was performed to rule out a meniscal tear. The magnetic resonance images revealed a small vertical tear of the posterior horn of the medial meniscus and a moderate joint effusion. T1 and T2-weighted axial and sagittal magnetic resonance images showed a high-signal-intensity lesion consisting of lobulated material projecting into the suprapatellar pouch (Figs. 1-A and 1-B). The lesion had the same signal intensity as fat on all sequences. These findings were thought to be consistent with lipoma arborescens.
After the diagnostic studies had been performed, the patient continued to have recurrent effusion. He was taken to the operating room for arthroscopic débridement of the meniscal tear and resection of the lesion. At the time of arthroscopy, a two by one-centimeter mass was seen just proximal to the trochlear groove on the extra-articular portion of the femur, in the suprapatellar pouch (Figs. 2-A and 2-B). Mild fibrillation of the cartilage was noted on the undersurface of the patella and in the trochlear groove, and some partial-thickness loss of cartilage was observed on the medial femoral condyle. A small tear of the medial meniscus also was seen. After specimens had been obtained for pathological analysis, the fatty areas were resected and the medial meniscus was debrided.
Histological examination revealed superficial inflammation and mature adipose tissue in the subsynovial layer, findings that were thought to be consistent with lipoma arborescens. Low and high-power photomicrographs of cross sections of the specimen showed villous projections that were filled with mature, benign adipose tissue (Figs. 3-A and 3-B).
One month after the operation, the swelling of the left knee had resolved. However, the patient reported a two-week history of painless swelling of the right knee. He did not recall any episode of trauma involving the knee. The active range of motion of the right knee was from 0 to 115 degrees of flexion. No instability or tenderness at the joint line was noted. One hundred and five milliliters of clear yellow fluid was aspirated from the knee, and cortisone was injected into the joint.
The effusion in the right knee recurred within six weeks. Magnetic resonance images demonstrated mild osteoarthrotic changes involving the medial compartment and a moderate effusion of the joint. A lesion with a slight frond-like appearance was noted in the posterior aspect of the suprapatellar pouch. The lesion had the same signal intensity as fat on all sequences.
Arthroscopy of the right knee then was performed. At the time of the procedure, 120 milliliters of clear joint fluid was aspirated from the knee. Arthroscopy revealed a fatty mass extending across the femur, beginning just proximal to the articular cartilage. Small fissures and fibrillation were seen in the trochlear groove and on the undersurface of the patella as well as in the medial compartment. The prepatellar fat pad was noted to be extending into the notch. Biopsy specimens were taken from the fat pad as well as from the mass in the suprapatellar pouch. Arthroscopic synovectomy of the knee and débridement of the fat pad then was performed.
Histological examination revealed synovial hyperplasia and mature adipose tissue in the subsynovial layer, findings that were thought to be consistent with lipoma arborescens. Non-specific synovitis also was noted. The results of biopsies of tissue from the prepatellar fat pad also were consistent with lipoma arborescens.
Two years after the arthroscopic synovectomy, the patient was asymptomatic.
Lipoma arborescens is an uncommon condition that usually affects the knee, but it also may occur in other joints. The condition usually occurs unilaterally and rarely is seen bilaterally. In the knee, it has a predilection for the suprapatellar pouch. Only twenty patients have been reported to have lipoma arborescens of the knee, to our knowledge, and four of these patients had bilateral involvement1-4,6-10,13,17. The lesion is more common in men and has been reported to occur over a wide age-range (nine to sixty-eight years)1.
The etiology of lipoma arborescens is unknown, but developmental, traumatic, inflammatory, and neoplastic origins have been postulated2,10. A synovial reaction to a traumatic injury has been proposed11, but most patients who have lipoma arborescens do not have a history of trauma. Hallel et al. suggested that the term villous lipomatous proliferation of the synovial membrane should be used because the term lipoma implies a tumorous process. Those authors also suggested that the lesion may predispose to osteoarthrosis, as the severity of the degenerative changes in their patients seemed to be related to the length of time that the patients had been symptomatic. This hypothesis is supported by the findings of Donnelly et al., who reported chondromalacia of the lateral tibial plateau in a nine-year-old girl who had lipoma arborescens.
Patients typically are seen with a progressive, painless swelling of the knee. As the volume of the effusion increases, pain and limitation of the range of motion often develop9. Many patients are symptomatic for several years. To our knowledge, the longest duration of symptoms was reported by Hallel et al., who described the case of a sixty-six-year-old man who had a thirty-year history of painless swelling and effusion. The clinical course often is marked by intermittent exacerbations, with tense effusions lasting for several days. These exacerbations are believed to be caused mechanically, when hypertrophic villi become trapped between joint surfaces10.
The laboratory findings are generally unremarkable6,9,10. Joint fluid typically is negative for crystals, and cultures of the fluid are negative as well. Plain radiographs frequently reveal an extra soft-tissue lesion in the region of the suprapatellar pouch and often show osteoarthrotic changes4,9.
Computed tomography typically demonstrates a low-attenuation mass that is consistent with fat, and it may outline the synovial fronds9,13. The lesion is not enhanced after intravenous injection of contrast medium9,13. These features can help to differentiate lipoma arborescens from pigmented villonodular synovitis, which produces a high-attenuation lesion that is enhanced by contrast medium. The location of the lesion is also important: lipoma arborescens typically involves the suprapatellar recess, whereas pigmented villonodular synovitis tends to extend into the semimembranosus-gastrocnemius bursa6.
The appearance of lipoma arborescens on magnetic resonance imaging is believed to be pathognomonic8. Magnetic resonance imaging outlines the synovial mass and readily reveals its frond-like appearance8,9. The lesion demonstrates the same signal intensity as fat on all sequences9. In contrast, the lesion that is associated with pigmented villonodular synovitis typically demonstrates low signal intensity, secondary to hemosiderin, on both T1 and T2-weighted images6,9.
On gross examination, the lesion is yellowish-white and shows villous proliferation10. Histological examination reveals diffuse replacement of the subsynovial layer by mature fat cells, which form villous projections7,15. A moderate infiltration of mononuclear inflammatory cells may be evident, and this may be a response to mechanical irritation caused by the mass12,17.
The recommended treatment for lipoma arborescens is arthrotomy and synovectomy. We are aware of only one patient, a nine-year-old girl with multiple lipomata, who had a recurrence after anterior synovectomy5. Hallel et al. reported no recurrences in three patients who were followed for eight, ten, and eleven years after synovectomy. One of these three patients had a revision of a loose total knee implant seven years after the synovectomy; histological examination of an intraoperative specimen demonstrated no evidence of lipoma arborescens.
The arthroscopic appearance of lipoma arborescens has been described previously3, but, to our knowledge, we are the first to report on the use of arthroscopic synovectomy for the treatement of the lesion. At the time of writing, our patient had been followed for two years and had had no recurrence of the symptoms. If the lesion is not extensive, it is amenable to arthroscopic resection.
In summary, lipoma arborescens is a rare intra-articular lesion, occurring most often in the knee, that should be considered in the differential diagnosis of a patient who has painless swelling of a joint. The lesion has a characteristic appearance on magnetic resonance imaging, which allows the diagnosis to be made preoperatively. Anterior synovectomy is curative, and, if the lesion is small, this procedure can be performed arthroscopically.