Primary lipoma arborescens of the knee may involve the development of early osteoarthritis if prompt synovectomy is not performed
© The Author(s) 2014
Received: 27 November 2013
Accepted: 2 April 2014
Published: 6 May 2014
Primary lipoma arborescens (LA) is a rare, benign intra-articular hyperplastic tumor that has been associated with osteoarthritis (OA). The aim of this study was to determine whether prompt synovectomy could avoid progressive joint degeneration in cases of primary LA of the knee.
Materials and methods
A review of currently available literature about the disease was carried out. The clinical, histological and radiological records of a series of nine knees with primary LA diagnosed and treated between 2002 and 2012 were retrospectively reviewed. Eight of the knees had histological confirmation of LA and none had evidence of condropathy on the initial magnetic resonance image or degenerative changes at the initial radiographic examination.
At the final follow-up no evidence of OA was found in the three knees that underwent synovectomy when symptoms did not last more than 1 year. The five knees in which synovectomy was delayed developed progressive joint degeneration.
In this series, primary LA of the knee involved the development of early osteoarthritis when prompt synovectomy was not performed. Timely synovectomy is strongly recommended, if not mandatory.
Level of evidence
Details of seven patients with lipoma arborescens of the knee
-High blood pressure
Trigger or particular debut
After a twist playing soccer
Sudden, oppressive and atraumatic left knee pain with a large effusion
Prior to surgery
11 months in the right knee
Prior to surgery Physical symptoms and Findings
Recurrent effusions and discomfort
Recurrent effusions, swelling and a large palpable supra-patellar mass
Recurrent effusions and anterior knee pain
Soft tissue palpable mass above the superior aspect of the patella and anterior knee pain
Recurrent effusions, pain and swelling in both knees
Oppressive left knee pain.
Pain, swelling and recurrent effusions
2 AS. PFA*. 1 AS after PFA
AS in both knees
OS in the left knee, no treatment in the right knee
Pain free. No effusions
Pain free. No effusions
Still pain. Still effusions
Pain free. No effusions
Asymptomatic in both knees
The symptoms of the left knee subsided almost completely. Still pain in the right knee
Pain free. No effusions
Bright yellow villi in the whole suprapatellar pouch
Large, pedicled and encapsulated tumor
Synovitic villi in the whole knee
Synovitic villi in the suprapatellar pouch
Synovitic villi at the suprapatellar pouch of both knees
An elongated diffuse synovial proliferation in the whole knee
Synovitic villi in the whole knee
27 months in the right knee
Evidence of osteoarthritis
Yes, in the left knee
Yes, in both knees
Materials and methods
Synovectomy was performed in 88.89 % (8/9) of the knees. At the latest follow-up, 12.5 % (1/8) of the knees in which synovectomy was performed were pain and swelling free. Of the operated knees, 12.5 % (1/8) had recurrent effusions despite synovectomy. One of the patients with bilateral LA rejected synovectomy in the right knee, so it remained symptomatic. Of the knees that underwent synovectomy, 62.5 % (5/8) developed OA. One of the patients with bilateral LA, whose initial MRIs showed no evidence of condropathy in either knee, eventually developed OA in the knee that was not surgically treated. Of the operated knees, 37.5 % (3/8) underwent synovectomy in the first year after the onset of symptoms and 62.5 % (5/8) after the first year. All (3/3) of the knees that had undergone synovectomy when symptoms did not last more than 1 year showed no evidence of OA. All (5/5) of the knees in which synovectomy was delayed more than 1 year after clinical debut developed progressive joint degeneration.
This series of seven patients and nine knees diagnosed with primary LA with a variety of presentations of the same disease showed different prognosis conditions. The two most common complications were recurrence and development of secondary degenerative changes. The fact that the three cases where synovectomy was promptly performed did not develop degenerative changes, makes us conclude that timely excision of the tumor is strongly recommended, if not mandatory.
LA was first described in detail in 1957 . It is an uncommon, benign intra-articular lesion of unknown etiology in which there is diffuse replacement of the subsynovial tissue by mature fat cells along with prominent villous transformation of the synovium. Although the knee is the joint most commonly involved, it has been described in other locations [3–6]. It most commonly affects people in the fourth and fifth decade of life . Once considered more frequent in the male population, it is currently considered to be equally distributed in both genders . The majority of cases are monoarticular . In fact, there have been only a few cases of bilateral LA described in the literature [2, 11, 14–17], as were cases five and six of this series. A few reports have described polyarticular involvement [6, 17].
Almost all reports on primary LA of the knee have been single-case reports. Two short series previously published had six  and eight cases , respectively. A recent publication describes a group of 39 cases of secondary LA, as the consequence of chronic reactive changes in patients with OA . In their series, only three of the cases had no evidence of OA. All of the patients in our series had an initial MRI with evidence of LA and no evidence of condropathy, as well as an initial radiographic examination without evidence of OA. We believe that primary and secondary LA should be considered as different entities.
Patients with LA usually have long-standing, slowly progressive swelling of the involved knee, which may be associated with effusion, decreased range of motion and pain. However, two patients in this series had started with a sudden onset of pain and effusion. A soft, painless, boggy swelling in the suprapatellar pouch can frequently be palpated. Due to the fact that the tumor is painless, patients usually seek medical evaluation after several years of mechanical symptoms, as was the case in some of the patients in this series. The laboratory tests are usually unremarkable and negative for HLA B27 and rheumatoid factor . The joint aspirate is negative for crystals and cultures of the fluid are sterile .
Plain radiographs are generally normal during the first stages of the disease , but a soft-tissue density in the suprapatellar pouch can be observed if it is meticulously evaluated. In more advanced stages, subchondral bone erosions suggesting synovial invasion, cyst formation or secondary osteoarthritic changes can be seen.
The MRI is considered the gold standard for the diagnosis of LA . It has a pathognomonic aspect consisting of an intra-articular synovial mass with frond-like architecture and a high signal intensity which is suppressed using fat-selective presaturation . There is lack of enhancement after injection of gadolinium, which helps to exclude synovial inflammatory or neoplastic processes, and there are no magnetic susceptibility effects due to hemosiderin or calcification. MRI also allows for the correct evaluation of the size and extension of the tumor, accurate preoperative planning, evaluation of the state of the cartilage and effective follow-up while avoiding the need for synovial biopsy . These findings in the MRI should be correlated with the histology that should be performed after resection, to confirm the diagnosis, and then medical professionals should be aware of recurrences that could have a clear influence on prognosis.
Macroscopically, LA has a frond-like appearance with numerous broad-based polypoid or thin papillary villi composed of fatty yellow tissue . Histologically, the villi are composed of mature adipose tissue, and enlarged or congested hyperemic capillaries may be present . The overlying synovial membrane may contain mononuclear chronic inflammatory cells and the synovial cells may seem to be enlarged and reactive with abundant eosinophilic cytoplasm.
An association between LA and OA has been postulated [14, 23], but the causal relationship between these two entities has not been fully clarified. It has been suggested that the long-standing synovial thickening effusions caused by repeated mechanical injury to the proliferated villi eventually lead to OA . Subchondral cysts and bone erosions can also be observed in some patients , as were seen in case six of this report.
The severity of the degenerative changes might have some relationship with the duration of symptoms. Conversely, LA has been suggested to be secondary to OA in elderly patients . Authors have classified this as a secondary type, which is much more common than primary cases. This secondary LA is thought to be a lipomatosis secondary to chronic irritation . The same mechanism could be observed in cases of meniscal injuries, trauma and chronic synovitis. However, this is not an actual neoplasm, so it should not be considered as LA. Although the patient in case four related the onset of symptoms to a traumatic episode, due to the results of the MRI and histology compatible with LA, the traumatic injury was considered only a coincidence.
The recommended treatment for symptomatic LA is open or arthroscopic synovectomy [9, 20]. The election of one technique over the other mainly relies on the size of the tumor and on the personal experience and preferences of the surgeon. Those few previous reports that had performed the synovectomy arthroscopically reported favorable outcomes at 1 year  and 2 years [21, 25]. In this series, synovectomy was performed in 8 of the 9 knees. Recurrence of LA after surgical treatment is considered very rare . In this scenario, the patient in case three, with several recurrences and who had undergone three arthroscopic synovectomies and a patellofemoral arthroplasty, is a very atypical case. In fact, despite the four surgical procedures, she is still symptomatic.
This retrospective case series of a low number of patients is, however, one of the largest series ever reported on primary LA. These patients were seen at different stages of the disease and treated with different surgical techniques. This heterogeneity might be a logical consequence of the different clinical expressions and the evolution of the disease observed among these patients.
We conclude that progressive joint degeneration could be prevented or at least delayed, if prompt synovectomy is performed.
We want to thank Eric Goode for his help in correcting the manuscript.
Conflict of interest
The patients gave their informed consent prior to being included in the study. The study was authorized by the local ethical committee and performed in accordance with the ethical standards of the 1964 Declaration of Helsinki as revised in 2000.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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